Electrical Q&A Part-1docshare04.docshare.tips/files/23711/237117520.pdf · DOL:direct online...

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Electrical Q&A Part-1 1) Why ELCB cannot work if Neutral input of ELCB does not connect to ground? ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that same current will have to return neutral so resultant current is zero. Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB will trip and it will safe guard the other circuits from faulty loads. If the neutral is not grounded fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current. 2) What is the difference between MCB & MCCB, Where it can be used? MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit. Normally it is used where normal current is less than 100A. MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. Under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A. 3) Why in a three pin plug the earth pin is thicker and longer than the other pins? It depends upon R=ρL/A where area (A) is inversely proportional to resistance (R), so if area (A) increases, R decreases & if R is less the leakage current will take low resistance path so the earth pin should be thicker. It is longer because the The First to make the connection and last to disconnect should be earth Pin. This assures Safety for the person who uses the electrical instrument. 4) Why Delta Star Transformers are used for Lighting Loads? For lighting loads, neutral conductor is must and hence the secondary must be star winding and this lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary So delta / star transformer is used for lighting loads. 5) What are the advantages of star-delta starter with induction motor? The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times of current of Direct online starting Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced. 6) What is meant by regenerative braking? When the supply is cut off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load (resistor) across the armature winding and the motor should have maintained continuous field supply so that back e.m.f voltage is made to apply across the resistor and due to load the motor stops quickly. This type of breaking is called as ―Regenerative Breaking‖. 7) When voltage increases then current also increases then why we need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only? No. We cannot sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage (As most of the loads are non-linear in nature).So, the over voltage protection & over current protection are completely different. Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition, earth fault thereby reducing

Transcript of Electrical Q&A Part-1docshare04.docshare.tips/files/23711/237117520.pdf · DOL:direct online...

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Electrical Q&A Part-1

1) Why ELCB cannot work if Neutral input of ELCB does not connect to ground?

ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current

will flow through phase and that same current will have to return neutral so resultant current is zero.

Once there is a ground fault in the load side, current from phase will directly pass through earth and it will not

return through neutral through ELCB. That means once side current is going and not returning and hence

because of this difference in current ELCB will trip and it will safe guard the other circuits from faulty loads. If the

neutral is not grounded fault current will definitely high and that full fault current will come back through ELCB,

and there will be no difference in current.

2) What is the difference between MCB & MCCB, Where it can be used?

MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current

rating circuit.

Normally it is used where normal current is less than 100A.

MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for

instant trip in short circuit condition. Under voltage and under frequency may be inbuilt.

Normally it is used where normal current is more than 100A.

3) Why in a three pin plug the earth pin is thicker and longer than the other pins?

It depends upon R=ρL/A where area (A) is inversely proportional to resistance (R), so if area (A) increases, R

decreases & if R is less the leakage current will take low resistance path so the earth pin should be thicker. It is

longer because the The First to make the connection and last to disconnect should be earth Pin. This assures

Safety for the person who uses the electrical instrument.

4) Why Delta Star Transformers are used for Lighting Loads?

For lighting loads, neutral conductor is must and hence the secondary must be star winding and this lighting load

is always unbalanced in all three phases.

To minimize the current unbalance in the primary we use delta winding in the primary So delta / star transformer

is used for lighting loads.

5) What are the advantages of star-delta starter with induction motor?

The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting

current is reduced to 3-4 times of current of Direct online starting Hence the starting current is reduced , the

voltage drops during the starting of motor in systems are reduced.

6) What is meant by regenerative braking?

When the supply is cut off for a running motor, it still continue running due to inertia. In order to stop it quickly we

place a load (resistor) across the armature winding and the motor should have maintained continuous field supply

so that back e.m.f voltage is made to apply across the resistor and due to load the motor stops quickly. This type

of breaking is called as ―Regenerative Breaking‖.

7) When voltage increases then current also increases then why we need of over voltage relay and over

current relay? Can we measure over voltage and over current by measuring current only?

No. We cannot sense the over voltage by just measuring the current only because the current increases not only

for over voltages but also for under voltage (As most of the loads are non-linear in nature).So, the over voltage

protection & over current protection are completely different.

Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing.

Over current relay meant for sensing any internal short circuit, over load condition, earth fault thereby reducing

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the system failure & risk of fire. So, for a better protection of the system. It should have both over voltage & over

current relay.

8) If one lamp connects between two phases it will glow or not?

If the voltage between the two phases is equal to the lamp voltage then the lamp will glow.

When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow

depending on the type of lamp.

9) What are HRC fuses and where it is used?

HRC stand for ―high rupturing capacity‖ fuse and it is used in distribution system for electrical transformers

10) Mention the methods for starting an induction motor?

The different methods of starting an induction motor

DOL:direct online starter

Star delta starter

Auto transformer starter

Resistance starter

Series reactor starter

11) What is the difference between earth resistance and earth electrode resistance?

Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should

recourse to its definition:

Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and

another point of the earth, which is far away.

The resistance of the electrode has the following components:

(A) the resistance of the metal and that of the connection to it.

(B) The contact resistance of the surrounding earth to the electrode.

12) Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?

4-20 mA is a standard range used to indicate measured values for any process. The reason that 4ma is chosen

instead of 0 mA is for fail safe operation.

For example: A pressure instrument gives output 4mA to indicate 0 psi up to 20 mA to indicate 100 psi or full

scale. Due to any problem in instrument (i.e) broken wire, its output reduces to 0 mA. So if range is 0-20 mA then

we can differentiate whether it is due to broken wire or due to 0 psi.

13) Two bulbs of 100w and 40w respectively connected in series across a 230v supply which bulb will glow

bright and why?

Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant

across the Bulb (P=V^2/R).So the resistance of 40W bulb is greater and voltage across 40W is more (V=IR) so

40W bulb will glow brighter.

14) What happen if we give 220 volts dc supply to bulb or tube light?

Bulbs or devices for AC are designed to operate such that it offers high impedance to AC supply. Normally they

have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so

high that it may damage the bulb element

15) What is meant by knee point voltage?

Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It

is the voltage at which a CT gets saturated.

16) What is reverse power relay?

Reverse Power flow relay are used in generating stations‘ protection.

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A generating station is supposed to feed power to the grid and in case generating units are off, there is no

generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we

use reverse power relay.

17) What will happen if DC supply is given on the primary of a transformer?

Mainly transformer has high inductance and low resistance. In case of DC supply there is no inductance, only

resistance will act in the electrical circuit. So high electrical current will flow through primary side of the

transformer. So for this reason coil and insulation will burn out

When AC current flow to primary winding it induced alternating flux which also link to secondary winding so

secondary current flow in secondary winding according to primary current.Secondary current also induced emf

(Back emf) in secondary winding which oppose induced emf of primary winding and thus control primary current

also.

If DC current apply to Primary winding than alternating flux is not produced so no secondary emf induced in

secondary winding so primary current may goes high and burn transformer winding.

18) Different between megger and contact resistance meter?

Megger used to measure cable resistance, conductor continuity, phase identification where as contact resistance

meter used to measure low resistance like relays, contactors.

19) When we connect the capacitor bank in series?

We connect capacitor bank in series to improve the voltage profile at the load end in transmission line there is

considerable voltage drop along the transmission line due to impedance of the line. so in order to bring the

voltage at the load terminals within its limits i.e (+ or – %6 )of the rated terminal voltage the capacitor bank is used

in series

20) What is Diversity factor in electrical installations?

Diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a

system, or part of a system, to the maximum demand of the whole system, or part of the system, under

consideration. Diversity factor is usually more than one.

21) Why humming sound occurred in HT transmission line?

This sound is coming due to ionization (breakdown of air into charged particles) of air around transmission

conductor. This effect is called as Corona effect, and it is considered as power loss.

22) Why frequency is 50 hz only & why should we maintain the frequency constant?

We can have the frequency at any frequency we like, but then we must also make our own motors, transformers

or any other equipment we want to use.

We maintain the frequency at 50 Hz or 60hz because the world maintains a standard at 50 /60hz and the

equipments are made to operate at these frequency.

23) If we give 2334 A, 540V on Primary side of 1.125 MVA step up transformer, then what will be the

Secondary Current, If Secondary Voltage=11 KV?

As we know the Voltage & current relation for transformer-V1/V2 = I2/I1

We Know, VI= 540 V; V2=11KV or 11000 V; I1= 2334 Amps.

By putting these value on Relation-

540/11000= I2/2334

So,I2 = 114.5 Amps

24) What are the points to be considered for MCB (miniature circuit breaker selection)?

I(L)x1.25=I(MAX) maximum current. Mcb specification is done on maximum current flow in circuit.

25) How can we start-up the 40w tube light with 230v AC/DC without using any choke/Coil?

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It is possible by means of Electronic choke. Otherwise it‘s not possible to ionize the particles in tube. Light, with

normal voltage.

26) What is “pu” in electrical engineering?

Pu stands for per unit and this will be used in power system single line diagram there it is like a huge electrical

circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all

the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the

component is considered as base values, then all other component ratings will get back into this basis. Those

values are called as pu values. (p.u=actual value/base value).

27) Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?

Link is provided at a Neutral common point in the circuit from which various connections are taken for the

individual control circuit and so it is given in a link form to withstand high Amps.

But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the

particular circuit (i.e load) only. So if any malfunction happens the fuse connected in the particular control circuit

alone will blow off.

If Fuse is provided in Neutral and if it is blowout and at the same time Supply is on than due to open or break

Neutral Voltage is increase and equipment may be damage.

28) If 200w, 100 w and 60 w lamps connected in series with 230V AC , which lamp glow brighter? Each lamp

voltage rating is 230V.

Each bulb when independently working will have currents (W/V= I)

For 200 Watt Bulb current (I200) =200/230=0.8696 A

For 100 Watt Bulb current (I100) =100/230=0.4348 A

For 60 Watt Bulb current (I60) =60/230=0.2609 A

Resistance of each bulb filament is (V/I = R)

For 200 Watt Bulb R200= 230/0.8696= 264.5 ohms

For 100 Watt Bulb R100= 230/0.4348 = 528.98 ohms and

For 60 Watt Bulb R60= 230/0.2609=881.6 ohms respectively

Now, when in series, current flowing in all bulbs will be same. The energy released will be I2R

Thus, light output will be highest where resistance is highest. Thus, 60 watt bulb will be brightest.

The 60W lamp as it has highest resistance & minimum current requirement.

Highest voltage drop across it X I [which is common for all lamps] =s highest power.

Note to remember:

Lowest power-lamp has highest element resistance.

And highest resistance will drop highest voltage drop across it in a Series circuit

And highest resistance in a parallel circuit will pass minimum current through it. So minimum power dissipated

across it as min current X equal Voltage across =s min power dissipation

29) How to check Capacitor with use of Multi meter.

Most troubles with Capacitors either open or short.

An ohmmeter (multi meter) is good enough. A shorted Capacitor will clearly show very low resistance. A open

Capacitor will not show any movement on ohmmeter.

A good capacitor will show low resistance initially, and resistance gradually increases. This shows that Capacitor

is not bad. By shorting the two ends of Capacitor (charged by ohmmeter) momentarily can give a weak spark. To

know the value and other parameters, you need better instruments

30) What is the difference between Electronic regulator and ordinary rheostat regulator for fans?

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The difference between the electronic and ordinary regulator is that in electronic regulator power losses are less

because as we decrease the speed the electronic regulator give the power needed for that particular speed .But

in case of ordinary rheostat type regulator the power wastage is same for every speed and no power is saved. In

electronic regulator triac is employed for speed control. by varying the firing angle speed is controlled but in

rheostat control resistance is decreased by steps to achieve speed control.

31) What will happen when power factor is leading in distribution of power?

If there is high power factor, i.e if the power factor is close to one:

Losses in form of heat will be reduced,

Cable becomes less bulky and easy to carry, and very cheap to afford.

It also reduces over heating of transformers.

32) What the main difference between UPS & inverter?

Uninterrupted power supply is mainly use for short time. Means according to ups VA it gives backup. Ups is also

two types: on line and offline. Online ups having high volt and amp for long time backup with high dc voltage. But

ups start with 12v dc with 7 amps. but inverter is start with 12v,24,dc to 36v dc and 120amp to 180amp battery

with long time backup

33) Which type of A.C motor is used in the fan?

It is Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.

34) What is the difference between synchronous generator and asynchronous generator?

In simple, synchronous generator supplies‘ both active and reactive power but asynchronous generator (induction

generator) supply‘s only active power and observe reactive power for magnetizing. This type of generators is

used in windmills.

35) What is the Polarization index value?

Its ratio between insulation resistance (IR)i.e meager value for 10min to insulation resistance for 1 min. It ranges

from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5 .

36) What is Automatic Voltage regulator (AVR)?

AVR is an abbreviation for Automatic Voltage Regulator.

It is important part in Synchronous Generators; it controls the output voltage of the generator by controlling its

excitation current. Thus it can control the output Reactive Power of the Generator.

37) Difference between a four point starter and three point starters?

The shunt connection in four point starter is provided separately from the line where as in three point starter it is

connected with line which is the drawback in three point starter

38) What happens if we connect a capacitor to a generator load?

Connecting a capacitor across a generator always improves power factor, but it will help depends up on the

engine capacity of the alternator, otherwise the alternator will be over loaded due to the extra watts consumed

due to the improvement on pf.

Don‘t connect a capacitor across an alternator while it is picking up or without any other load

39) Why the capacitors work on ac only?

Generally capacitor gives infinite resistance to dc components (i.e., block the dc components). It allows the ac

components to pass through.

40) Why the up to dia 70mm² live conductor, the earth cable must be same size but above dia 70mm² live

conductor the earth conductor need to be only dia 70mm²?

The current carrying capacity of a cable refers to it carrying a continuous load.

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An earth cable normally carries no load, and under fault conditions will carry a significant instantaneouscurrent

but only for a short time most Regulations define 0.1 to 5 sec before the fuse or breaker trips. Its size therefore

is defined by different calculating parameters.

The magnitude of earth fault current depends on:

(a) the external earth loop impedance of the installation (i.e. beyond the supply terminals)

(b) the impedance of the active conductor in fault

(c) the impedance of the earth cable.

i.e. Fault current = voltage / a + b + c

Now when the active conductor (b) is small, its impedance is much more than (a), so the earth (c) cable is sized

to match. As the active conductor gets bigger, its impedance drops significantly below that of the external earth

loop impedance (a); when It is quite large its impedance can be ignored. At this point there is no merit in

increasing the earth cable size

i.e. Fault current = voltage / a + c

(c) is also very small so the fault current peaks out.

The neutral conductor is a separate issue. It is defined as an active conductor and therefore must be sized for

continuous full load. In a 3-phase system,

If balanced, no neutral current flows. It used to be common practice to install reduced neutral supplies, and cables

are available with say half-size neutrals (remember a neutral is always necessary to provide single phase

voltages). However the increasing use of non-linear loads which produce harmonics has made this practice

dangerous, so for example the current in some standard require full size neutrals. Indeed, in big UPS installations

I install double neutrals and earths for this reason.

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Electrical Q&A Part-2

1) Why We use of Stones/Gravel in electrical Switch Yard

Reducing Step and Touch potentials during Short Circuit Faults

Eliminates the growth of weeds and small plants in the yard

Improves yard working condition

Protects from fire which cause due to oil spillage from transformer and also protects from wild habitat.

2) What is service factor?

Service factor is the load that may be applied to a motor without exceeding allowed ratings.

For example, if a 10-hp motor has a 1.25 service factor, it will successfully deliver 12.5 hp (10 x 1.25) without

exceeding specified temperature rise. Note that when being driven above its rated load in this manner, the motor

must be supplied with rated voltage and frequency.

However a 10-hp motor with a 1.25 service factor is not a 12.5-hp motor. If the 10-hp motor is operated

continuously at 12.5 hp, its insulation life could be decreased by as much as two-thirds of normal. If you need a

12.5-hp motor, buy one; service factor should only be used for short-term overload conditions

3) Why transmission line 11KV OR 33KV, 66KV not in 10KV 20KV?

The miss concept is Line voltage is in multiple of 11 due to Form Factor. The form factor of an alternating current

waveform (signal) is the ratio of the RMS (Root Mean Square) value to the average value (mathematical mean of

absolute values of all points on the waveform). In case of a sinusoidal wave, the form factor is 1.11.

The Main reason is something historical. In olden days when the electricity becomes popular, the people had a

misconception that in the transmission line there would be a voltage loss of around 10%. So in order to get 100 at

the load point they started sending 110 from supply side. This is the reason. It has nothing to do with form factor

(1.11).

Nowadays that thought has changed and we are using 400 V instead of 440 V, or 230 V instead of 220 V.

Also alternators are now available with terminal voltages from 10.5 kV to 15.5 kV so generation in multiples of 11

does not arise. Now a days when, we have voltage correction systems, power factor improving capacitors, which

can boost/correct voltage to desired level, we are using the exact voltages like 400KV in spite of 444KV

4) What is electrical corona?

Corona is the ionization of the nitrogen in the air, caused by an intense electrical field.

Electrical corona can be distinguished from arcing in that corona starts and stops at essentially the same voltage

and is invisible during the day and requires darkness to see at night.

Arcing starts at a voltage and stops at a voltage about 50% lower and is visible to the naked eye day or night if

the gap is large enough (about 5/8″ at 3500 volts).

5) What are the indications of electrical corona?

A sizzling audible sound, ozone, nitric acid (in the presence of moisture in the air) that accumulates as a white or

dirty powder, light (strongest emission in ultraviolet and weaker into visible and near infrared) that can be seen

with the naked eye in darkness, ultraviolet cameras, and daylight corona cameras using the solar-blind

wavelengths on earth created by the shielding ozone layer surrounding the earth.

6) What damage does corona do?

The accumulation of the nitric acid and micro-arcing within it create carbon tracks across insulating materials.

Corona can also contribute to the chemical soup destruction of insulating cements on insulators resulting in

internal flash-over.

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The corona is the only indication. Defects in insulating materials that create an intense electrical field can over

time result in corona that creates punctures, carbon tracks and obvious discoloration of NCI insulators.

7) How long does corona require creating visible damage?

In a specific substation the corona ring was mistakenly installed backwards on a temporary 500kV NCI insulator,

at the end of two years the NCI insulator was replaced because 1/3 of the insulator was white and the remaining

2/3 was grey.

8) What voltage are corona rings typically installed at?

It varies depending upon the configuration of the insulators and the type of insulator, NCI normally start at 160kV,

pin and cap can vary starting at 220kV or 345kV depending upon your engineering tolerances and insulators in

the strings.

9) How do we select transformers?

Determine primary voltage and frequency.

Determine secondary voltage required.

Determine the capacity required in volt-amperes. This is done by multiplying the load current (amperes) by the

load voltage (volts) for single phase.

For example: if the load is 40 amperes, such as a motor, and the secondary voltage is 240 volts, then 240 x 40

equals 9600 VA. A 10 KVA (10,000 volt-amperes) transformer is required.

Always select Transformer Larger than Actual Load. This is done for safety purposes and allows for expansion, in

case more loads is added at a later date. For 3 phase KVA, multiply rated volts x load amps x 1.73 (square root of

3) then divide by 1000.

Determine whether taps are required. Taps are usually specified on larger transformers.

10) Why Small Distribution Transformers not used for Industrial Applications?

Industrial control equipment demands a momentary overload capacity of three to eight times‘ normal capacity.

This is most prevalent in solenoid or magnetic contactor applications where inrush currents can be three to eight

times as high as normal sealed or holding currents but still maintain normal voltage at this momentary overloaded

condition.

Distribution transformers are designed for good regulation up to 100 percent loading, but their output voltage will

drop rapidly on momentary overloads of this type making them unsuitable for high inrush applications.

Industrial control transformers are designed especially for maintaining a high degree of regulation even at eight

time‘s normal load. This results in a larger and generally more expensive transformer.

11) Can 60 Hz transformers be used at higher frequencies?

Transformers can be used at frequencies above 60 Hz up through 400 Hz with no limitations provided nameplate

voltages are not exceeded.

However, 60 Hz transformers will have less voltage regulation at 400 Hz than 60 Hz.

12) What is meant by regulation in a transformer?

Voltage regulation in transformers is the difference between the no load voltage and the full load voltage. This is

usually expressed in terms of percentage.

For example: A transformer delivers 100 volts at no load and the voltage drops to 95 volts at full load, the

regulation would be 5%. Distribution transformers generally have regulation from 2% to 4%, depending on the

size and the application for which they are used.

13) Why is impedance important?

It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a

transformer.

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Example: Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 KVA single phase

transformer with 4% impedance, to be operated from a 480 volt 60 Hz source.

Calculate:

Normal Full Load Current = Nameplate Volt Amps / Line Volts = 10,000 VA / 480 V = 20.8 Amperes

Maximum Short Circuit Amps = Full Load Amps / 4% =20.8 Amps / 4%= 520 Amp

The breaker or fuse would have a minimum interrupting rating of 520 amps at 480 volts.

Example: Determine the interrupting capacity, in amperes, of a circuit breaker or fuse required for a 75 KVA,

three phase transformer, with a primary of 480 volts delta and secondary of 208Y/120 volts. The transformer

impedance (Z) = 5%. If the secondary is short circuited (faulted), the following capacities are required:

Normal Full Load Current =Volt Amps / √ 3 x Line Volts= 75,000 VA / √ 3 x Line Volts √ 3 x 480 V =90 Amps

Maximum Short Circuit Line Current = Full Load Amps / 5%= 90 Amps / 5% =1,800 Amps

The breaker or fuse would have a minimum interrupting rating of 1,800 amps at 480 volts.

Note: The secondary voltage is not used in the calculation. The reason is the primary circuit of the transformer is

the only winding being interrupted.

14) What causes flash-over?

Flash-over causes are not always easily explained, can be cumulative or stepping stone like, and usually result in

an outage and destruction. The first flash-over components are available voltage and the configuration of the

energized parts, corona may be present in many areas where the flash-over occurs, and flash-over can be

excited by stepping stone defects in the insulating path.

15) What are taps and when are they used?

Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions,

and still deliver full rated output voltages at the secondary terminals. Taps are generally set at two and a half and

five percent above and below the rated primary voltage.

16) Can Transformers be reverse connected?

Dry type distribution transformers can be reverse connected without a loss of KVA rating, but there are certain

limitations. Transformers rated 1 KVA and larger single phase, 3 KVA and larger three phases can be reverse

connected without any adverse effects or loss in KVA capacity.

The reason for this limitation in KVA size is, the turns ratio is the same as the voltage ratio.

Example: A transformer with a 480 volt input, 240 volt output— can have the output connected to a 240 volt

source and thereby become the primary or input to the transformer, then the original 480 volt primary winding will

become the output or 480 volt secondary.

On transformers rated below 1 KVA single phase, there is a turn‘s ratio compensation on the low voltage winding.

This means the low voltage winding has a greater voltage than the nameplate voltage indicates at no load.

For example, a small single phase transformer having a nameplate voltage of 480 volts primary and 240 volts

secondary, would actually have a no load voltage of approximately 250 volts, and a full load voltage of 240 volts.

If the 240 volt winding were connected to a 240 volt source, then the output voltage would consequently be

approximately 460 volts at no load and approximately 442 volts at full load. As the KVA becomes smaller, the

compensation is greater—resulting in lower output voltages.

When one attempts to use these transformers in reverse, the transformer will not be harmed; however, the output

voltage will be lower than is indicated by the nameplate.

17) What is the difference between “Insulating”, “Isolating”, and “Shielded Winding” transformers?

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Insulating and isolating transformers are identical. These terms are used to describe the separation of the primary

and secondary windings. A shielded transformer includes a metallic shield between the primary and secondary

windings to attenuate (lessen) transient noise.

18) How many BTU’s of heat does a transformer generate?

The heat a transformer generates is dependent upon the transformer losses. To determine air conditioning

requirements multiply the sum of the full load losses (obtained from factory or test report) of all transformers in the

room by 3.41 to obtain the BTUs/hour.

For example: A transformer with losses of 2000 watts will generate 6820 BTUs/hour.

19) What is a transformer and how does it work?

A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. It can

be designed to ―step up‖ or ―step down‖ voltages and works on the magnetic induction principle.

A transformer has no moving parts and is a completely static solid state device, which insures, under normal

operating conditions, a long and trouble-free life. It consists, in its simplest form, of two or more coils of insulated

wire wound on a laminated steel core.

When voltage is introduced to one coil, called the primary, it magnetizes the iron core. A voltage is then induced

in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio) between the primary

and secondary depends on the turns ratio of the two coils.

20) Factors Affecting Corona Discharge Effect:

Corona Discharge Effect occurs because of ionization if the atmospheric air surrounding the voltage conductors,

so Corona Discharge Effect is affected by the physical state of the atmosphere as well as by the condition of the

lines.

(1) Conductor: Corona Discharge Effect is considerably affected by the shape, size and surface conditions of the

conductor .Corona Discharge Effect decreases with increases in the size (diameter) of the conductor, this effect is

less for the conductors having round conductors compared to flat conductors and Corona Discharge Effect is

concentrated on that places more where the conductor surface is not smooth.

(2) Line Voltage: Corona Discharge effect is not present when the applied line voltages are less. When the

Voltage of the system increases (In EHV system) corona Effect will be more.

(3) Atmosphere: Breakdown voltage directly proportional to the density of the atmosphere present in between

the power conductors. In a stormy weather the ions present around the conductor is higher than normal weather

condition So Corona Breakdown voltage occurs at low voltages in the stormy weather condition compared to

normal conditions

(4)Spacing between the Conductors: Electro static stresses are reduced with increase in the spacing between

the conductors. Corona Discharge Effect takes place at much higher voltage when the distance between the

power conductors increases.

21) Will a transformer change Three Phases to Single Phase?

A transformer will not act as a phase changing device when attempting to change three phase to single phase.

There is no way that a transformer will take three phase in and deliver single phase out while at the same time

presenting a balanced load to the three phase supply system.

There are, however, circuits available to change three phase to two phase or vice versa using standard dual

wound transformers. Please contact the factory for two phase applications.

22) Can 60 Hz transformers be operated at 50 Hz?

Transformers rated below 1 KVA can be used on 50 Hz service.

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Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service, due to the higher losses

and resultant heat rise. Special designs are required for this service. However, any 50 Hz transformer will operate

on a 60 Hz service.

23) Can transformers be used in parallel?

Single phase transformers can be used in parallel only when their impedances and voltages are equal. If unequal

voltages are used, a circulating current exists in the closed network between the two transformers, which will

cause excess heating and result in a shorter life of the transformer. In addition, impedance values of each

transformer must be within 7.5% of each other.

For example: Transformer A has an impedance of 4%, transformer B which is to be parallel to A must have

impedance between the limits of 3.7% and 4.3%. When paralleling three phase transformers, the same

precautions must be observed as listed above, plus the angular displacement and phasing between the two

transformers must be identical.

24) What are causes of insulator failure?

Electrical field intensity producing corona on contaminated areas, water droplets, icicles, corona rings, … This

corona activity then contributes nitric acid to form a chemical soup to change the bonding cements and to create

carbon tracks, along with ozone and ultraviolet light to change the properties of NCI insulator coverings. Other

detrimental effects include water on the surface or sub-surface freezing and expanding when thawing, as a liquid

penetrating into a material and then a sudden temperature change causes change of state to a gas and rapid

expansion causing fracture or rupture of the material.

25) Causes of Corona

Corona is causes by the following reasons:

The natural electric field caused by the flow of electrons in the conductor. Interaction with surrounding air.

Poor or no insulation is not a major cause but increases corona.

The use of D.C (Direct Current) for transmission.(Reason why most transmission is done in form of AC)

26) Effects of Corona

1) Line Loss – Loss of energy because some energy is used up to cause vibration of the air particles.

2) Long term exposure to these radiations may not be good to health (yet to be proven).

3) Audible Noise

4) Electromagnetic Interference to telecommunication systems

5) Ozone Gas production

6) Damage to insulation of conductor.

27) What is polarity, when associated with a transformer?

Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding.

Transformers 600 volts and below are normally connected in additive polarity — that is, when tested the terminals

of the high voltage and low voltage windings on the left hand side are connected together, refer to diagram below.

This leaves one high voltage and one low voltage terminal unconnected.

When the transformer is excited, the resultant voltage appearing across a voltmeter will be the sum of the high

and low voltage windings.

This is useful when connecting single phase transformers in parallel for three phase operations. Polarity is a term

used only with single phase transformers.

28) What is exciting current?

Exciting current, when used in connection with transformers, is the current or amperes required for excitation. The

exciting current on most lighting and power transformers varies from approximately 10% on small sizes of about 1

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KVA and smaller to approximately .5% to 4% on larger sizes of 750 KVA. The exciting current is made up of two

components, one of which is a real component and is in the form of losses or referred to as no load watts; the

other is in the form of reactive power and is referred to as KVAR.

29) What is Boucholz relay and the significance of it in to the transformer?

Boucholz relay is a device which is used for the protection of transformer from its internal faults,

it is a gas based relay. whenever any internal fault occurs in a transformer, the boucholz relay at once gives a

horn for some time, if the transformer is isolated from the circuit then it stop its sound itself otherwise it trips the

circuit by its own tripping mechanism.

30) Why we do two types of earthing on transformer (Body earthing & neutral earthing)

The two types of earthing are Familiar as Equipment earthing and system earthing.

In Equipment earthing: body (non conducting part) of the equipment should be earthed to safeguard the human

beings.

The System Earthing : In this neutral of the supply source ( Transformer or Generator) should be grounded. With

this, in case of unbalanced loading neutral will not be shifted. So that unbalanced voltages will not arise. We can

protect the equipment also. With size of the equipment ( transformer or alternator)and selection of relying system

earthing will be further classified into directly earthed, Impedance earthing, resistive (NGRs) earthing.

31) Conductor corona is caused by?

Corona on a conductor can be due to conductor configuration (design) such as diameter too small for the applied

voltage will have corona year-around and extreme losses during wet weather, the opposite occurs during dry

weather as the corona produces nitric acid which accumulates and destroys the steel reinforcing cable (ACSR)

resulting in the line dropping. Road salts and contaminants can also contribute to starting this deterioration.

32) What is flash-over and arcing?

Flash-over is an instantaneous event where the voltage exceeds the breakdown potential of the air but does not

have the current available to sustain an arc, an arc can have the grid fault current behind it and sustain until the

voltage decreases below 50% or until a protective device opens.

Flash-over can also occur due to induced voltages in unbounded (loose bolts, washers, etc) power pole or

substation hardware, this can create RFI/TVI or radio/TV interference. Arcing can begin at 5 volts on a printed

circuit board or as the insulation increases it may require 80kVAC to create flash-over on a good cap and pin

insulator.

33) How to Minimizing Corona Effects

Installing corona rings at the end of transmission lines.

A corona ring, also called anti-corona ring, is a toroid of (typically) conductive material located in the vicinity of a

terminal of a high voltage device. It is electrically insulated.

Stacks of more spaced rings are often used. The role of the corona ring is to distribute the electric field gradient

and lower its maximum values below the corona threshold, preventing the corona discharge.

34) What is BIL and how does it apply to transformers?

BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that consist of the application of a

high frequency steep wave front voltage between windings, and between windings and ground. The Basic

Impulse Level of a transformer is a method of expressing the voltage surge (lightning, switching surges, etc.) that

a transformer will tolerate without breakdown.

All transformers manufactured in this catalog, 600 volts and below, will withstand the NEMA standard BIL rating,

which is 10 KV.

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This assures the user that he will not experience breakdowns when his system is properly protected with lightning

arrestors or similar surge protection devices.

35) The difference between Ground and Neutral?

NEUTRAL is the origin of all current flow. In a poly-phase system, as its phase relationship with all the three

phases is the same, (i.e.) as it is not biased towards any one phase, thus remaining neutral, that‘s why it is called

neutral.

Whereas, GROUND is the EARTH on which we stand. It was perceived to utilize this vast, omnipresent conductor

of electricity, in case of fault, so that the fault current returns to the source neutral through this conductor given by

nature which is available free of cost. If earth is not used for this purpose, then one has to lay a long. long metallic

conductor for the purpose, thus increasing the cost.

Ground should never be used as neutral. The protection devices (eg ELCB, RCD etc) work basically on principle

that the phase currects are balanced with neutral current. In case you use ground wire as the neutral, these are

bound to trip if they are there – and they must be there. at least at substations. And these are kept very sensitive

i.e. even minute currents are supposed to trip these.

One aspect is safety – when someone touches a neutral, you don‘t want him to be electrocuted – do you? Usually

if you see the switches at home are on the phase and not neutral (except at the MCB stage). Any one assumes

the once the switch is off, it is safe (the safety is taken care of in 3 wire system, but again most of the fixtures are

on 2 wire) – he will be shocked at the accidental touching of wire in case the floating neutral is floating too much.

36) What is impedance of a transformer?

If you mean the percentage impedance of the transformed it means the ratio of the voltage( that if you applied it to

one side of the transformer while the other side of the transformer is short cuitcuted, a full load current shall flow

in the short circuits side), to the full load current.

More the %Z of transformer, more Copper used for winding, increasing cost of the unit. But short circuit levels will

reduce, mechanical damages to windings during short circuit shall also reduce. However, cost increases

significantly with increase in %Z.

Lower %Z means economical designs. But short circuit fault levels shall increase tremendously, damaging the

winding & core.

The high value of %Z helps to reduce short circuit current but it causes more voltage dip for motor starting and

more voltage regulation (% change of voltage variation) from no load to full load.

37) How are transformers sized to operate Three Phase induction type squirrel cage motors?

The minimum transformer KVA rating required to operate a motor is calculated as follows:

Minimum Transformer KVA =Running Load Amperes x 1.73x Motor Operating Voltage / 1000

NOTE: If motor is to be started more than once per hour add 20% additional KVA. Care should be exercised in

sizing a transformer for an induction type squirrel cage motor as when it is started, the lock rotor amperage is

approximately 5 to 7 times the running load amperage. This severe starting overload will result in a drop of the

transformer output voltage.

When the voltage is low the torque and the horsepower of the motor will drop proportionately to the square of the

voltage.

For example: If the voltage were to drop to 70% of nominal, then motor horsepower and torque would drop to 70

% squared or 49% of the motor nameplate rating.

If the motor is used for starting a high torque load, the motor may stay at approximately 50% of normal running

speed The underlying problem is low voltage at the motor terminals. If the ampere rating of the motor and

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transformer over current device falls within the motor‘s 50% RPM draw requirements, a problem is likely to

develop. The over current device may not open under intermediate motor ampere loading conditions.

Overheating of the motor and/or transformer would occur, possibly causing failure of either component.

This condition is more pronounced when one transformer is used to power one motor and the running amperes of

the motor is in the vicinity of the full load ampere rating of the transformer. The following precautions should be

followed:

(1)When one transformer is used to operate one motor, the running amperes of the motor should not exceed 65%

of the transformer‘s full load ampere rating.

(2) If several motors are being operated from one transformer, avoid having all motors start at the same time. If

this is impractical, then size the transformer so that the total running current does not exceed 65% of the

transformer‘s full load ampere rating.

38) Which Point need to be consider while Neutral Earthing of Transformer?

The following points need to check before going for Neutral Grounding Resistance.

Fault current passing through ground, step and touch potential.

Capacity of transformer to sustain ground fault current, w.r.t winding, core burning.

Relay co-ordination and fault clearing time.

Standard practice of limiting earth fault current. In case no data or calculation is possible, go for limiting E/F

current to 300A or 500A, depending on sensivity of relay.

39) Why a neutral grounding contactor is needed in diesel generator?

There would not be any current flow in neutral if DG is loaded equally in 3 phases , if there any fault(earth fault or

over load) in any one of the phase ,then there will be un balanced load in DG . at that time heavy current flow

through the neutral ,it is sensed by CT and trips the DG. so neutral in grounded to give low resistance path to fault

current.

An electrical system consisting of more than two low voltage Diesel Generator sets intended for parallel operation

shall meet the following conditions:

(i) Neutral of only one generator needs to be earthed to avoid the flow of zero sequence current.

(ii) During independent operation, neutrals of both generators are required in low voltage switchboard to obtain

three phases, 4 wire system including phase to neutral voltage.

(iii) required to achieve restricted earth fault protection (REF) for both the generators whilst in operation.

Solution:

Considering the requirement of earthing neutral of only one generator, a contactor of suitable rating shall be

provided in neutral to earth circuit of each generator. This contactor can be termed as ―neutral contactor‖.

Neutral contactors shall be interlocked in such a way that only one contactor shall remain closed during parallel

operation of generators. During independent operation of any generator its neutral contactor shall be closed.

Operation of neutral contactors shall be preferably made automatic using breaker auxiliary contacts.

40) Neutral grounded system vs solidly grounded system

In India, at low voltage level (433V) we MUST do only Solid Earthing of the system neutral.

This is by IE Rules 1956, Rule No. 61 (1) (a). Because, if we option for impedance earthing, during an earth fault,

there will be appreciable voltage present between the faulted body & the neutral, the magnitude of this voltage

being determined by the fault current magnitude and the impedance value.

This voltage might circulate enough current in a person accidentally coming in contact with the faulted equipment,

as to harm his even causing death. Note that, LV systems can be handled by non-technical persons too. In solid

earthing, you do not have this problem, as at the instant of an earth fault, the faulted phase goes to neutral

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potential and the high fault current would invariably cause the Over current or short circuit protection device to

operate in sufficiently quick time before any harm could be done

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Electrical Q&A Part-3

1) What is the reason of grounding or earthing of equipment?

with a ground path, in case of short circuit the short circuit current goes to the body of the equipment & then to the

ground through the ground wire. Hence if at the moment of fault if a person touches the equipment body he will

not get a shock cause his body resistance (in thousands of ohms) will offer a high resistance path in comparison

to the ground wire. Hence the fault current will flow thru the ground wire & not thru human body.

Providing a ground path helps in clearing the fault. A CT in the ground connection detects the high value fault

current hence the relay connected to the CT gives breaker a trip command.

Grounding helps in avoiding arcing faults. IF there would have been no ground then a fault with the outer body

can cause a arcing to the ground by breaking the air. This is dangerous both for the equipment & the human

beings.

2) A type-C MCB has thermo magnetic capability 5In to 10In that means a short circuit current will be

interrupted as the value will reach between 5In to 10In but the MCB breaking capacity is (for example) define

as 10kA.

5In to 10In is the pickup threshold for the magnetic trip element. The MCB will trip instantaneously when the

current is between these limits. 10kA is the short circuit withstands capacity of the MCB.

Under normal condition, a current limiting type MCB will trip on short circuit (magnetic trip) and the current during

circuit interruption will be much less than the prospective current. However, the MCBs have to have a short circuit

capacity more than or equal to the fault level at the location where it is installed.

3) What is Ferrari Effect?

Ferranti Effect is due to the rise in voltage at the receiving end than that of the sending end. This occurs when the

load on the system reduces suddenly.

Transmission line usually consists of line inductance, line to earth capacitance and resistance. Resistance can be

neglected with respect to the line inductance .When the load on the system falls the energy stored in the

capacitance gets discharged. The charging current causes inductive reactance voltage drop. This gets added

vector ally to the sending end voltage and hence causes the voltage at the receiving end to raise

A Long transmission line draws significant amount of charging current. If such line is open circuited or very lightly

loaded at the receiving end, the voltage at the receiving end may become greater than sending end voltage. This

effect is known Ferranti effect and is due to the voltage drop across the line inductance (due to charging current)

being in phase with the sending end voltages. Therefore both capacitance and inductance is responsible to

produce this phenomenon.

The capacitance (charging current) is negligible in short lines, but significant in medium and long transmission

line. Hence, this phenomenon is applicable for medium and long transmission line.

The main impact of this phenomenon is on over voltage protection system, surge protection system, insulation

level etc.

4) Can single phase transformers be used for three phase applications?

Yes. Three phase transformers are sometimes not readily available whereas single phase transformers can

generally be found in stock. Three single phase transformers can be used in delta connected primary and wye or

delta connected secondary. They should never be connected wye primary to wye secondary, since this will result

in unstable secondary voltage. The equivalent three phase capacity when properly connected of three single

phase transformers is three times the nameplate rating of each single phase transformer.

5) What is BIL and how does it apply to transformers?

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BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that consist of the application of a

high frequency steep wave front voltage between windings, and between windings and ground. The BIL of a

transformer is a method of expressing the voltage surge that a transformer will tolerate without breakdown.

6) Where Auto-recloser is used?

For Generator protection / Transformer Protection / Transmission Line / Bus bar protection.

Many faults on overhead transmission lines are transient in nature 90% of faults are used by birds, tree branches.

These condition results in arching faults and the arc in the fault can be extinguished by de-energizing the lines by

opening of CB on the both ends of the lines.

Open-0.3 second-Close-3minute-Close this is the sequence of AR. i.e.-OPEN,C-CLOSED

whenever faults occurs CB opens, then after 0.3 sec it closes automatically, if faults persists then it will open after

3 min it closes and if still fault persists. It remains in open condition.

Auto reclosure is generally used for Transmission lines where the general types of faults are transient in nature.

It can be three phase auto-reclosure or single pole auto-reclosure.

The single pole auto reclosures are generally for 400kV line below this three pole auto- reclosures are used.

The reason for a line the single pole reclosures provides a better stability of the system since some part of power

is still transferred through the healthy phases.

Also 400kV breaker till date has a independent drive/ trip/ close coils for the three poles, below that all breakers

have common drive/ trip / closing coils for the three poles.

7) What is difference between power transformers & distribution transformers?

Distribution Transformers are designed for a maximum efficiency at 50% of load. Whereas power transformers

are designed to deliver max efficiency ay 90% and above loads.

The distributions transformers have low impedance so as to have a better regulation power transformers have

higher so as to limit the SC current.

Power transformers are used to step up voltages from 11 KV which is the generating voltage to 132 or whatever

will be the transmission voltage levels. Power transformers are having Star-Delta connection. It will be located at

power generating stations.

Distribution transformers are used to step down voltages from transformer levels to 11 KV/415 V. Will be having

Delta-Star. It will be located in substations near load centers.

The main basic difference lies in the Design stage itself as power transformer are to operate at near full load so

there sensing is such that they achieve equal. of copper losses & iron losses at full loads whereas this is achieved

in the design itself at about 50% loading in dist transformer but friends there is a dilemma as our dist. transformer

are almost fully loaded & beyond so they never go operate at their full eff. & also poor voltage regulation.

The difference between power and distribution transformers refers to size & input voltage. Distribution

transformers vary between 25 kVA and 10 MVA, with input voltage between 1 and 36 kV. Power transformers are

typically units from 5 to 500 MVA, with input voltage above 36 kV. Distribution transformer design to have a max

efficiency at a load lower than full load. Power transformer design to have a max efficiency at full load

8) What will be happen if the neutral isolator will be open or close during the running condition of power?

During normal condition the neutral isolating switch should be kept close. In case it is kept open, under balanced

load conditions the current through neutral will not flow & nothing harmful will take place but in case an earth fault

takes place then there will be no earth fault current flowing through the system & the generator will run as a

ungrounded generator. Thus the earth fault will not be cleared.

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If more number of generators are connected parallel. We will have a close loop and hence negative sequence

current will flow. This will increase the rotor temperature. Hence if more number of generators are connected then

only one is earthed and others are open.

In case of Two or more generators connected to a common bus without a transformer in between, basically in

hydro stations, one of the Neutral Isolation Switch(NIS) is kept closed & rest are opened to prevent circulating

currents to flow between generators. Hence the above explanation will not be valid for such systems.

Sometime we may want to test generator and may want to isolate the neutral from ground. like for example

meggaring etc. In such case we would like to open ground connection cable in case we want to remove the NIS?

we will certainly not like to open all the bolted connections for just a small test like checking insulation with a

meggar etc. for such things we need a NIS.

Neutral isolator is required if we have delta transmission system and at the time to connection with the Grid

Neutral isolation is required.

If we ungrounded the neutral then the generator is connected to the ground via Phase to earth capacitances.

Hence during faults arcing grounds can take place. Which are dangerous both to human & equipment.

When we provide earthed neutral, for a fault, earth fault current will start flowing through the neutral, which we

can sense thru a CT & relay & hence can immediately identify & clear the fault in about 100 ms by opening the

associated breaker/prime mover/excitation. Quicker the fault clearance less is the damage.

9) Why shorting type terminal required for CT?

During maintenance or secondary injection you will need to bypass the CT & for the same you need shorting link.

During sec. injection you will short circuit the main CT & bypass it. Open circuiting the CT will saturate it &

damage it.

10) Why fuse is given for only PT and not CT?

Fuse if given for CT blows off due to a fault then rather than protecting the CT it will make it open circuited hence

it will be saturated & damaged. For PT it gives overload & SC protection.

11) Why is insulating base required for LA?

The LA is provided with a dedicated Prper earthing which may be in the form of a buried treated electrode next

toit.LA connection is securely made with the electrode via a surge counter. If we directly earth the LA through

structure then the surge counter will not be able to measure the no of surges. For lesser rating the counter is not

provided, hence we can bypass the insulated base. But then proper earthing has to be assured.

12) Can 60 Hz transformers be operated at 50 Hz?

Transformers 1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service due to higher losses and

resultant heat rise. However, any 50 Hz transformer will operate on 60 Hz service.

13) Can transformers be used in parallel?

Single phase transformers can be used in parallel only when their voltages are equal. If unequal voltages are

used, a circulating current exists in the closed network between the two transformers which will cause excess

heating and result in a shorter life of the transformer. In addition impedance values of each transformer must be

within 7.5% of each other.

14) Can Transformers be reverse connected?

Dry type distribution transformers can be reverse connected without a loss of KVA rating, but there are certain

limitations. Transformers rated 1 KVA and larger single phase, 3 KVA and larger three phases can be reverse

connected without any adverse effects or loss in KVA capacity.

15) Why short circuit do not take place when electrode is touched to ground.

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Basically during welding we force a short-circuit at the electrode tip. The fault condition produces large magnitude

currents. Greater the Current value have greater I2R heat produced. The arcing energy elevates the temperature

& hence melts the electrode material over the joint.

The transformer is designed to withstand such high currents. But welding is a very complex & detailed

phenomenon. Besides there are many principles on which welding operates. Some may be a welding, dc welding,

arc, constant voltage, constant current etc

16) What’s the difference between generator breaker and simple breaker?

Breaker is one which disconnects the circuit in fault condition and It is similar for all equipment. Based on the

equipment voltage and maximum short circuit current the ratings will be decided. For better understanding we call

generator or transformer or line etc breakers.

17) What is accuracy Class of the instrument?

Generally the class indicates the accuracy with which the meter will indicate or equipment will measure with

respect to its input.

The accuracy of different equipment will depend on number of factors.

For example for a PT accuracy will depend on its leakage reactance & winding resistance. For a PT accuracy

gives the voltage & phase error & it varies with the VA burden of secondary. Also better core material will give

better heat dissipation & reduce error. class of accuracy will give the voltage error for a PT

different type of PTs available are:0.1, 0.2, 0.5, 1, 5 & error values will be: class% voltage error(+/_) phase

displacement

Similarly indicating instruments shall have accuracies & accordingly application as depicted below for testing the

following values are generally used:

for routine tests : accuracy class 1

for type tests : accuracy class 0.5 or better.

indicating meters generally will have accuracy of 1.

18) First pole to clear factor-Circuit breakers

The first pole to clear factor (kpp) is depending on the earthing system of the network. The first pole to clear factor

is used to calculating the transient recovery voltage for three phase faults. In general following cases apply:-

1. kpp = 1.3 corresponds to three phase faults in system with an earthed neutral.

2. kpp = 1.5 corresponds to three phase faults in isolated or resonant earthed system.

3. kpp = 1.0 corresponds to special cases e.g. railway systems.

A special case is when there is a three phase fault without involving earth in a system with earthed neutral. This

case responds to kpp = 1.5 . This special case is however not normally considered in the standards.

19) Why we use a resistance to ground the neutral when we need always low resistivity for the grounding?

If we ground the generator directly then whenever a fault will take place at any phase with ground the fault current

flowing throw the faulted phase-to ground-to neutral will be very high cause there will be no resistance to limit the

value of fault current. Hence we insert a resistance in the neutral circuit to limit this fault current. Also we need to

reduce the fault current to such a value that the protection CTs are able to identify the fault current without

saturating the CTs. Communicate it to the protection relays & hence the relays can then isolate the system from

the fault; so that the system is isolated from the fault before the harm is done by the fault current. That is the

reason that all the equipment will be designed for fault KA values for 1 sec so that the total operation(CT sensing-

relay functioning-circuit breaker operation ) time will be less than 1 sec. hence the Breakers will isolate the fault

before 1 sec i.e. within the time period the equipment are designed to carry the fault current. Thus all your

objectives of:

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preventing the arcing.

limiting the fault current.

isolating the faulted system are achieved

20) Why are NGR’s rated for 10sec?

NGR are placed in the neutral circuit & hence will be energized only in the fault conditions thus their continuous

loading is not expected. Hence they are selected for intermittent rating. Similarly when we place a transformer in

the neutral grounding circuit the KVA rating obtained after the calculation is multiplied by a diversity factor to

obtain smaller rating cause the therefore It will not be continuously rated.

NIS is also provided to cut the circulating negative sequence current in 2 more generators connected in parallel.

in some grid conditions they ask to keep neutral isolated after being connected to grid.

21) How to calculate knee point voltage and significance of knee point voltage?

Knee point voltage: That point on the magnetizing curve (BH curve) where an increase of 10% in the flux density

(voltage) causes an increase of 50% in the magnetizing force (current). Its significance lies mainly in PS class

core of CTs used for diff protection

22) Design method for neutral grounding resistor?

NGR design basics:

Capacitive coupling of generator, equipment and the ground

-Generator to ground capacitance.

-Generator cable to ground capacitance (or bus duct as the case may be)

-Low voltage winding of trafo & ground capacitance.

-Surge arrestor capacitance.

The total capacitance is then obtained from the above values & then we calculate from that the capacitive

reactance. The capacitive current then produced is calculated from the generator voltage & the capacitive

reactance obtained above. Once the current is obtained we can then calculate the electrostatic KVA from the

current multiplied with voltage.

23) Criterion is there for selection of Insulation Disc in Transmission and Distribution Line.

11kV is the phase to earth voltage for 220kV =220/ (sqrt(3)*11)=12 No‘s of disc are suitable.The number can be

increased to increase the creep age distance.

While selecting the disc insulators one has to keep in mind the following things:

1. EM-strength of the string. All the forces coming on to the string & the ability of the string to withstand them.

2. Sufficient Cree page distance so as not to cause a flashover .

3. Interface with the type of conductor used (moose, tarantula, zebra etc)

So we will get the value of no of discs by dividing the phase to earth voltage with 1.732. Once that is done then

we need to see its suitability with respect to EM strength.

After this we need to consider the force that the stack has to bear. If we have a strain type of fitting i.e. the stack

has to bear horizontal conductor tension, weight load of the conductor, wind load, ice load etc then the number of

insulator discs required may be more.

But for a suspension type system which has to bear only the weight then number of discs required may be less

than what we get by dividing by 11. That is the reason we have seen only 23/24 discs in 400 kv line cause in that

case the creep age obtained must have been enough & also the strain requirement.

33kv insulators are generally used in a vertical installation & are not stacked together because that will make the

suspension very rigid

24) Do taps work the same when a transformer is reverse fed?

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Taps are normally in the primary winding to adjust for varying incoming voltage. If the transformer is reverse fed,

the taps are on the output side and can be used to adjust the output voltage.

25) Why may I get the wrong output voltage when installing a step up transformer?

Transformer terminals are marked according to high and low voltage connections. An H terminal signifies a high

voltage connection while an X terminal signifies a lower voltage connection. A common misconception is that H

terminals are primary and X terminals secondary. This is true for step down transformers, but in a step up

transformer the connections should be reversed. Low voltage primary would connect to X terminals while high

voltage secondary would connect on the H terminals.

26) Can a single phase transformer be used on a three phase source?

Yes. Any single phase transformer can be used on a three phase source by connecting the primary leads to any

two wires of a three phase system, regardless of whether the source is three phase 3-wire or three phase 4-wire.

The transformer output will be single phase.

27) Why in Double circuit wire are transposed (R – B, Y – Y, B – R)

This is done to avoid

1. Proximity effect

2. Skin effect

3. Radio interference

4. Reduction in noise in communication Signals

28) Selection of LA

The voltage rating of LA is selected as: Line voltage x sqrt(2)/ sqrt(3) so for 11kV line its 9kV

In that case also the values would not differ much if We takes the TOV factor as 1.4. However, we can take the

value of 1.56 as TOV to be more precise.

29) Which is more dangerous AC or DC

Low frequency (50 – 60 Hz) AC currents can be more dangerous than similar levels of DC current since the

alternating fluctuations can cause the heart to lose coordination, inducing ventricular fibrillation, which then rapidly

leads to death.

However any practical distribution system will use voltage levels quite sufficient to ensure a dangerous amount of

current will flow, whether it uses alternating or direct current. Since the precautions against electrocution are

similar, ultimately, the advantages of AC power transmission outweighed this theoretical risk, and it was

eventually adopted as the standard.

30) What all are the applications where high speed grounding switches are used.

Generator neutral is earthed directly or through distribution transformer. This neutral earthing is through done

through a switch. This is general practice for only one generator.

For two generators in parallel to a bus the neutral earthing is different. If both the neutral earthing is closed the

negative sequence current will be flowing though both the generator taking earth as path. This leads to increase

in loss and increase in temperature (This may leads to false tripping also). Hence once the second generator is

synchronized with the bus or grid the neutral is isolated.

Neutral grounding switch we do not need a high speed grounding switch. A normal switch with the correct rating

capacity would also work.

31) What is Skin Effect and how does it happen??

According to faradays law of electromagnetic induction, a conductor placed in a changing magnetic field induces

an emf. The effect of back emf is maximum at the centre because of maximum lines of field there. Hence the

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maximum opposition of current at inner side of conductor and minimum opposition at the surface. Hence the

current tries to follow at the surface. It is due to this reason that we take hollow tube conductors in bus duct.

Taking into account the inductance effect, its simple consider the DC current. Since its constant & not varying

hence no back emf but if we gradually start increasing the frequency then the flux cutting the conductor goes on

increasing, hence greater the frequency greater the alternating flux cutting the conductor & hence greater the

back emf & therefore greater the skin effect.

32) Why we ground the sheath of single core power cables and to avoid grounded at both the ends?

A single core cable with a sheath is nothing but a conductor carrying current surrounded by another conductor

(sheath). Hence the Alternative current in the conductor induces voltages in the sheath or the armour. Hence

grounding these cables at both ends will cause the potential of the armour to be same as ground potential &

hence shall become safe for the personnel.

But grounding the cables at both the end will cause a problem. In that case the circulating currents will start

flowing with the armor, the ground & with the two ends of the grounding completing the circuit. This will also

provide path for the fault currents to flow. Hence this whole thing will cause the cable to produce some I2R

losses, hence heating & hence the current carrying capacity will be de rated. This system of cable earthing is

called both-end bonding. This system is suggested only when one wants to avoid the voltage development

because can either go with the de rated cable or if one updates the cable in advance.

When only one end of the cable sheath is grounded then there is no path for the circulating current to flow. Hence

the current carrying capacity of the cable will be good. But in this case potential will be induced between sheath &

ground. This potential is proportional to the length of the cable & hence this will limit the length of the cable used.

This method is called single point bonding. This is thus used only for short lengths.

There is another system called the cross bonding system in which the sheath are sectionaliosed & cross

connected so that the circulating currents are minimized. Although some potential will also exist between sheath

& ground, the same being maximum at the link boxes where bonding is done. This method provides maximum

possible current carrying capacity with the maximum possible lengths.

33) What is EDO & MDO type breaker?

In the Breakers for the operation spring charging is must.

In EDO breaker the spring charging is done with a motor and draw out manually by hand. so EDO means

Electrically spring charged Draw Out breaker

In MDO breaker the spring charging is also done by hand manually and draw out about also by hand only. so

MDO means Manual spring charge Draw Out breaker

34) Why transformer rating is in KVA or KW?

Because power factor of the load is not defined in case of transformer that‘s why it is not possible to rate

transformer in KW.

The losses (cu loss and iron loss) of the transformer depends on current and voltage purely, not on load i.e,

phase angle between the current and voltage i.e. why transformer rated in kVA

Transformer is not a load and having no effect on P.F (that‘s why no change in its power factor) and it only

transfer the constant power from one voltage level to another voltage level without changing frequency. since

both the losses viz copper loss(depends on current) and iron loss(depends on voltage) are independent of power

factor, that is why a Transformers rating is not on kW, but on KVA

35) Why the secondary of CT never open when burden is connected on the CT.?

secondary of CT is never opened as because CT is always connected to the line so opening the secondary will

mean there will be no counter mmf to balance the primary current as a result of which a very high induced emf

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may appear in the secondary as flux is very high and no counter mmf and this will be dangerous for the personnel

in the secondary side and for pt if it is shorted then with full voltage applied to the primary.

If we short the secondary then much high current will circulate in the secondary due to high induced emf much

higher than the actual full load current as a result of which the transformer‘s secondary winding may burn out.

36) Distance relay setting

Step1:

Get the conductor Details (i.e Positive Sequence Impedance (Z), Zero Sequence Impedance(Z0)) which is in

Primary value. Convert in terms of secondary values.

Step 2 :

Based upon the calculated value divide into various zones

Zone 1 (Forward) means 80% of your protected line length.

Zone 2 (Forward) means 100% of protected line length + 20% Adjacent Shortest line

Zone 3 (Forward) means 100% of protected line length + 50% Adjacent Longest line.

Zone 4 (Reverse) means 10% of protected line.

37) Difference between CT class 0.2 and 0.2S?

0.2S & 0.5S: Special type of measurement CTs they guarantee the declared accuracy, even with

20% loading. And some definite error can be defined even with a load as low as 1%. Thus they are suitable for

industries where loads are commissioned in steps or stages. Also for tariff metering purposes.

0.2S: Special class for metering. It is more accurate than 0.2 classes. Generally if we use 0.2s class CT than VA

burden of core is also come down.

In 0.2 classes CT, ratio & phase angle errors must be within the specified limits at 5%, 20%, 100% & 120% of

rated secondary current. Whereas in 0.2s class CT, ratio & phase angle errors must be within the specified limits

at 1%, 5%, 20%, 100% & 120% of rated secondary current. Also in 0.2s class, Ratio & Phase angle errors limits

are lower than 0.2 classes.

38) Why we use inductors

Inductors have the property to oppose sudden changes in Current. When connected to the primary side of

transformer, if any sudden short circuit (very high) current flows due to some fault in the system, the inductor will

oppose the flow of that current saving the transformer.

Secondly, for the problem of lagging current. Capacitors are connected across the inductor to improve the lagging

current. So Mainly Inductor is used to (i) protected the transformer, (ii) solved the problem of lagging current.

39) Why do we need a bigger breaker when reverse feeding a transformer?

Typically the output winding is wound first and is therefore closest to the core. When used as exciting winding a

higher inrush current results. In most cases the inrush current is 10 to 12 times the full load current for 1/10 of a

second. When the transformer is reverse fed the inrush current can be up to 16 times greater. In this case a

bigger breaker with a higher AIC rating must be used to keep the transformer online.

40) How many types of Neutral grounding system?

There are primarily three types of grounding system which are:

(1)Solid grounding – The neutral point of the system is grounded without any resistance. If the ground fault

occurs, high ground current passes through the fault. Its use is very common in low voltage system, where line to

neutral voltage is used for single phase loads.

(2) Low Resistance grounding (LRG) – This is used for limiting the ground fault current to minimize the impact

of the fault current to the system. In this case, the system trips for the ground fault. In this system, the use of line

to neutral (single phase) is prohibited. The ground fault current is limited to in the rage from 25A to 600A.

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(3) High Resistance Grounding (HRG) – It is used where service continuity is vital, such as process plant

motors. With HRG, the neutral is grounded through a high resistance so that very small current flows to the

ground if ground fault occurs. In the case of ground fault of one phase, the faulty phase goes to the ground

potential but the system doesn‘t trip. This system must have a ground fault monitoring system. The use of line to

neutral (single phase) is prohibited (NEC, 250.36(3)) in HRG system, however, phase to neutral is used with

using the additional transformer having its neutral grounded. When ground fault occurs in HRG system, the

monitoring systems gives alarm and the plant operators start the standby motor and stop the faulty one for the

maintenance. This way, the process plant is not interrupted. The ground fault current is limited to 10A or less.

There are other two types such as Corner Grounding (for Delta system) and ungrounded system but they are not

commonly used.

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Electrical Q&A Part-4

1) What value AC meters show, is it the RMS or peak voltage?

AC voltmeters and ammeters show the RMS value of the voltage or current. DC meters also show the RMS value

when connected to varying DC providing the DC is varying quickly, if the frequency is less than about 10Hz you

will see the meter reading fluctuating instead.

2) Why in the transmission tower construction Middle arm is longer than the upper and lower Arm.

Conductor of Upper Arm and Lower Arm will stay apart.

To prevent big birds (Ostriches etc) from bumping their heads against the conductor above when they sit on the

wire below.

Designed to maintain the mechanical requirement to prevent arching between conductors while maintaining a

tower height that is manageable, and of course preventing head injuries to birds

The arms are of different links to prevent a broken upper line from falling on one or more of the phase lines below.

The clearance from other phase.

Mutual inductance minimization.

Preventing droplet of water/ice to fall on bottom conductor.

3) What is the difference between Surge Arrester & Lightning Arrestor

LA is installed outside and the effect of lightning is grounded, where as surge arrestor installed inside panels

comprising of resistors which consumes the energy and nullify the effect of surge.

Transmission Line Lightning Protection:

The transmission line towers would normally be higher than a substation structure, unless you have a multi-storey

structure at your substation.

Earth Mats are essential in all substation areas, along with driven earth electrodes (unless in a dry sandy desert

site).

It is likewise normal to run catenaries‘ (aerial earth conductors) for at least 1kM out from all substation structures.

Those earth wires to be properly electrically to each supporting transmission tower, and bonded back to the

substation earth system.

It is important to have the catenaries‘ earth conductors above the power conductor lines, at a sufficient distance

and position that a lightning strike will not hit the power conductors.

In some cases it is thus an advantage to have two catenary earth conductors, one each side of the transmission

tower as they protect the power lines below in a better manner.

In lightning-prone areas it is often necessary to have catenary earthing along the full distance of the transmission

line.

Without specifics, (and you could not presently give tower pictures in a Post because of a CR4 Server graphics

upload problem), specifics would include:

Structure Lightning Protection:

At the Substation, it is normal to have vertical electrodes bonded to the structure, and projecting up from the

highest points of the structure, with the location and number of those electrodes to be sufficient that if a lightning

strike arrived, it would always be a vertical earthed electrode which would be struck, rather than any electrical

equipment.

In some older outdoor substation structures, air-break isolator switches are often at a very high point in the

structure, and in those cases small structure extension towers are installed, with electrodes at the tapered peak of

those extension towers.

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The extension towers are normally 600mm square approximately until the extension tower changes shape at the

tapered peak, and in some cases project upwards from the general structure 2 to 6 metres, with the electrode

some 2 to 3 metres projecting upwards from the top of the extension tower.

The substation normally has a Lightning Counter – which registers a strike on the structure or connected to earth

conductors, and the gathering of that information (Lightning Days, number per Day/Month/Year, Amperage of

each strike)

4) How Corona Discharge Effect Occur in Transmission Line?

In a power system transmission lines are used to carry the power. These transmission lines are separated by

certain spacing which is large in comparison to their diameters.

In Extra High Voltage system (EHV system ) when potential difference is applied across the power conductors in

transmission lines then air medium present between the phases of the power conductors acts as insulator

medium however the air surrounding the conductor subjects to electro static stresses. When the potential

increases still further then the atoms present around the conductor starts ionize. Then the ions produced in this

process repel with each other and attracts towards the conductor at high velocity which intern produces other ions

by collision.

The ionized air surrounding the conductor acts as a virtual conductor and increases the effective diameter of the

power conductor. Further increase in the potential difference in the transmission lines then a faint luminous glow

of violet color appears together along with hissing noise. This phenomenon is called virtual corona and followed

by production of ozone gas which can be detected by the odor. Still further increase in the potential between the

power conductors makes the insulating medium present between the power conductors to start conducting and

reaches a voltage (Critical Breakdown Voltage) where the insulating air medium acts as conducting medium

results in breakdown of the insulating medium and flash over is observed. All this above said phenomenon

constitutes CORONA DISCHARGE EFFECT in electrical Transmission lines.

5) Methods to reduce Corona Discharge Effect:

Critical Breakdown voltage can be increased by following factors

By increasing the spacing between the conductors:

Corona Discharge Effect can be reduced by increasing the clearance spacing between the phases of the

transmission lines. However increase in the phases results in heavier metal supports. Cost and Space

requirement increases.

By increasing the diameter of the conductor:

Diameter of the conductor can be increased to reduce the corona discharge effect. By using hollow conductors

corona discharge effect can be improved.

By using Bundled Conductors:

By using Bundled Conductors also corona effect can be reduced this is because bundled conductors will have

much higher effective diameter compared to the normal conductors.

By Using Corona Rings or Grading Rings:

This is of having no greater significance but i presented here to understand the Corona Ring in the Power system.

Corona Rings or Grading Rings are present on the surge arresters to equally distribute the potential along the

Surge Arresters or Lightning Arresters which are present near the Substation and in the Transmission lines.

6) How to test insulators?

Always remember to practice safety procedures for the flash-over voltage distance and use a sturdy enclosure to

contain an insulator that may shatter, due to steam build-up from moisture in a cavity, arcing produces intense

heat, an AM radio is a good RFI/arcing detection device, a bucket truck AC dielectric test set (130KV) is a good

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test set for most pin and cap type insulators. A recent article said the DC voltage required to ―search out defects

can be 1.9 times the AC voltage.

Insulators have a normal operating voltage and a flash-over voltage. Insulators can have internal flash-over that

are/are not present at normal operating voltage. If the RFI is present, de-energize the insulator (line) and if the

RFI goes away, suspect the insulator (line). Then there can be insulators that have arcing start when capacitor or

other transients happen, stop when the line is de-energized or dropped below 50% of arc ignition voltage. Using a

meg-ohm-meter can eliminate defective insulators that will immediately arc-over tripping the test set current

overload.

7) How to identify the starting and ending leads of winding in a motor which is having 6 leads in the

terminal box

If it is a single speed motor then we have to identify 6 leads.

Use IR tester to identify 3 windings and their 6 leads. Then connect any two leads of two winding and apply small

voltage across it and measure the current.

Then again connect alternate windings of same two windings and apply small amount of voltage (same as before)

and measure current.

Check in which mode you get the max current and then mark it as a1-a2 & b1-b2. You get max current when a2-

b1 will be connected and voltage applied between a1-b2.

Follow the same process to identify a1-a2, b1-b2, c1-c2.now we will be able to connect it in delta or star.

8) How to measure Transformer Impedance?

Follow the steps below:

(1) Short the secondary side of the transformer with current measuring devices (Ammeter)

(2) Apply low voltage in primary side and increase the voltage so that the secondary current is the rated

secondary current of the transformer. Measure the primary voltage (V1).

(3) Divide the V1 by the rated primary voltage of the transformer and multiply by 100. This value is the percentage

impedance of the transformer.

When we divide the primary voltage V1 with the full load voltage we will get the short circuit impedance of the

transformer with refereed to primary or Z01. For getting the percentage impedance we need to use the formula =

Z01*Transformer MVA /(Square of Primary line voltage).

9) Why Bus Couplers are normally 4-Pole. Or When Neutral Isolation is required?

Neutral Isolation is mandatory when you have a Mains Supply Source and a Stand-by Power Supply Source. This

is necessary because if you do not have neutral isolation and the neutrals of both the sources are linked, then

when only one source is feeding and the other source is OFF, during an earth fault, the potential of the OFF

Source‘s Neutral with respect to earth will increase, which might harm any maintenance personnel working on the

OFF source. It is for this reason that PCC Incomers & Bus Couplers are normally 4-Pole. (Note that only either

the incomer or the bus coupler needs to be 4-pole and not both).

3pole or 4pole switches are used in changing over two independant sources ,where the neutral of one source and

the neutral of another source should not mix the examples are electricity board power supply and standalone

generator supply etc. the neutral return current from one source should not mix with or return to another source.

as a mandatory point the neutral of any transformer etc are to be earthed, similarly the neutral of a generator also

has to be earthed. While paralling (under uncontrolled condition) the neutral current between the 2 sources will

crises cross and create tripping of anyone source breakers.

also as per IEC standard the neutral of a distribution system shall not be earthed more than once. means earthing

the neutral further downstream is not correct,

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10) Why Three No’s of Current transformer in 3 phase Star point is grounded.

For CT‘s either you use for 3 phase or 2 phase or even if you use only 1 CT‘s for the Over current Protection or

for the Earth Faults Protection, their neutral point is always shorted to earth. This is NOT as what you explain as

above but actually it is for the safety of the CT‘s when the current is passing thru the CT‘s.

In generally, tripping of Earth faults and Over current Protection has nothing to do with the earthing the neutral of

the CT‘s. Even these CT‘s are not Grounded or Earthed, these Over current and the Earth Faults Protection

Relay still can operated.

Operating of the Over current Protection and the Earth Faults Relays are by the Kirchhoff Law Principle where the

total current flowing into the points is equal to the total of current flowing out from the point.

Therefore, for the earth faults protection relays operating, it is that, if the total current flowing in to the CT‘s is NOT

equal total current flowing back out of the CT‘s then with the differences of the leakage current, the Earth Faults

Relays will operated.

11) What is tertiary winding of Transformer?

Providing a tertiary winding for a transformer may be a costly affair. However, there are certain constraints in a

system which calls for a tertiary transformer winding especially in the case of considerable harmonic levels in the

distribution system. Following is an excerpt from the book ―The J&P Transformer Book‖.

Tertiary winding is may be used for any of the following purposes:

(A)To limit the fault level on the LV system by subdividing the infeed that is, double secondary transformers.

(B)The interconnection of several power systems operating at different supply voltages.

(C) The regulation of system voltage and of reactive power by means of a synchronous capacitor connected to

the terminals of one winding.

It is desirable that a three-phase transformer should have one set of three-phase windings connected in delta

thus providing a low-impedance path for third-harmonic currents. The presence of a delta connected winding also

allows current to circulate around the delta in the event of unbalance in the loading between phases, so that this

unbalance is reduced and not so greatly fed back through the system.

Since the third-order harmonic components in each phase of a three-phase system are in phase, there can be no

third-order harmonic voltages between lines. The third-order harmonic component of the magnetising current

must thus flow through the neutral of a star-connected winding, where the neutral of the supply and the star-

connected winding are both earthed, or around any delta-connected winding. If there is no delta winding on a

star/star transformer, or the neutral of the transformer and the supply are not both connected to earth, then line to

earth capacitance currents in the supply system lines can supply the necessary harmonic component. If the

harmonics cannot flow in any of these paths then the output voltage will contain the harmonic distortion.

Even if the neutral of the supply and the star-connected winding are both earthed, then although the transformer

output waveform will be undistorted, the circulating third-order harmonic currents flowing in the neutral can cause

interference with telecommunications circuits and other electronic equipment as well as unacceptable heating in

any liquid neutral earthing resistors, so this provides an added reason for the use of a delta connected tertiary

winding.

If the neutral of the star-connected winding is unearthed then, without the use of a delta tertiary, this neutral point

can oscillate above and below earth at a voltage equal in magnitude to the third-order harmonic component.

Because the use of a delta tertiary prevents this it is sometimes referred to as a stabilizing winding.

When specifying a transformer which is to have a tertiary the intending purchaser should ideally provide sufficient

information to enable the transformer designer to determine the worst possible external fault currents that may

flow in service. This information (which should include the system characteristics and details of the earthing

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arrangements) together with a knowledge of the impedance values between the various windings, will permit an

accurate assessment to be made of the fault currents and of the magnitude of currents that will flow in the tertiary

winding. This is far preferable to the purchaser arbitrarily specifying a rating of, say, 33.3%, of that of the main

windings.

12) Why do transformers hum?

Transformer noise is caused by a phenomenon which causes a piece of magnetic sheet steel to extend itself

when magnetized. When the magnetization is taken away, it goes back to its original condition. This phenomenon

is scientifically referred to as magnetostriction.

A transformer is magnetically excited by an alternating voltage and current so that it becomes extended and

contracted twice during a full cycle of magnetization. The magnetization of any given point on the sheet varies, so

the extension and contraction is not uniform. A transformer core is made from many sheets of special steel to

reduce losses and moderate the ensuing heating effect.

The extensions and contractions are taking place erratically all over a sheet and each sheet is behaving

erratically with respect to its neighbour, so you can see what a moving, writhing construction it is when excited.

These extensions are miniscule proportionally and therefore not normally visible to the naked eye. However, they

are sufficient to cause a vibration, and consequently noise. Applying voltage to a transformer produces a

magnetic flux, or magnetic lines of force in the core. The degree of flux determines the amount of

magnetostriction and hence, the noise level Why not reduce the noise in the core by reducing the amount of flux?

Transformer voltages are fixed by system requirements. The ratio of these voltages to the number of turns in the

winding determines the amount of magnetization. This ratio of voltage to turns is determined mainly for

economical soundness. Therefore the amount of flux at the normal voltage is fixed. This also fixes the level of

noise and vibration. Also, increasing (or decreasing) magnetization does not affect the magnetostriction

equivalently. In technical terms the relationship is not linear.

13) How can we reduce airborne noise?

Put the transformer in a room in which the walls and floor are massive enough to reduce the noise to a person

listening on the other side. Noise is usually reduced (attenuated) as it tries to pass through a massive wall. Walls

can be of brick, steel, concrete, lead, or most other dense building materials.

Put the object inside an enclosure which uses a limp wall technique. This is a method which uses two thin plates

separated by viscous (rubbery) material. As the noise hits the inner sheet some of its energy is used up inside the

viscous material. The outer sheet should not vibrate.

Build a screen wall around the unit. This is cheaper than a full room. It will reduce the noise to those near the wall,

but the noise will get over the screen and fall elsewhere (at a lower level). Screens have been made from wood,

concrete, brick and with dense bushes (although the latter becomes psychological)

Do not make any reflecting surface coincident with half the wave length of the frequency. What does this mean?

Well, every frequency has a wave length. To find the wave length in air, for instance, you divide the speed of

sound, in air (generally understood as 1130 feet per second) by the frequency. If a noise hits a reflecting surface

at these dimensions it will produce what is called a standing wave. Standing waves will cause reverberations

(echoes) and an increase in the sound level. If you hit these dimensions and get echoes you should apply

absorbent materials to the offending walls (fibreglass, wool, etc.)

14) What is polarity, when associated with a transformer?

Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding.

Transformers 600 volts and below are normally connected in additive polarity. This leaves one high voltage and

one low voltage terminal unconnected. When the transformer is excited, the resultant voltage appearing across a

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voltmeter will be the sum of the high and low voltage windings. This is useful when connecting single phase

transformers in parallel for three phase operations. Polarity is a term used only with single phase transformers.

15) What is exciting current?

Exciting current is the current or amperes required for excitation. The exciting current on most lighting and power

transformers varies from approximately 10% on small sizes of about 1 KVA and less to approximately 2% on

larger sizes of 750 KVA.

16) Can a three phase transformer be loaded as a single phase transformer?

Yes, but the load cannot exceed the rating per phase and the load must be balanced. (KVA/3 per phase)

For example: A 75 kVA 3 phase transformer can be loaded up to 25 kVA on each secondary. If you need a 30

kVA load, 10 kVA of load should be supplied from each secondary.

17) What are taps and when are they used?

Taps are provided on some transformers on the high voltage winding to correct for high or low voltage conditions,

and still deliver full rated output voltages at the secondary terminals.

Standard tap arrangements are at two-and-one-half and five percent of the rated primary voltage for both high

and low voltage conditions.

For example, if the transformer has a 480 volt primary and the available line voltage is running at 504 volts, the

primary should be connected to the 5% tap above normal in order that the secondary voltage be maintained at

the proper rating.

18) What is the difference between “Insulating,” “Isolating,”and“Shielded Winding” transformers?

Insulating and isolating transformers are identical. These terms are used to describe the isolation of the primary

and secondary windings, or insulation between the two.

A shielded transformer is designed with a metallic shield between the primary and secondary windings to

attenuate transient noise.

This is especially important in critical applications such as computers, process controllers and many other

microprocessor controlled devices.

All two, three and four winding transformers are of the insulating or isolating types. Only autotransformers, whose

primary and secondary are connected to each other electrically, are not of the insulating or isolating variety.

19) Can transformers be operated at voltages other than nameplate voltages?

In some cases, transformers can be operated at voltages below the nameplate rated voltage.

In NO case should a transformer be operated at a voltage in excess of its nameplate rating, unless taps are

provided for this purpose. When operating below the rated voltage, the KVA capacity is reduced correspondingly.

For example, if a 480 volt primary transformer with a 240 volt secondary is operated at 240 volts, the secondary

voltage is reduced to 120 volts. If the transformer was originally rated 10 KVA, the reduced rating would be 5

KVA, or in direct proportion to the applied voltage.

20) Can a Single Phase Transformer be used on a Three Phase source?

Yes. Any single phase transformer can be used on a three phase source by connecting the primary leads to any

two wires of a three phase system, regardless of whether the source is three phase 3-wire or three phase 4-wire.

The transformer output will be single phase.

21) Can Transformers develop Three Phase power from a Single Phase source?

No. Phase converters or phase shifting devices such as reactors and capacitors are required to convert single

phase power to three phases.

22) Can Single Phase Transformers be used for Three Phase applications?

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Yes. Three phase transformers are sometimes not readily available whereas single phase transformers can

generally be found in stock.

Three single phase transformers can be used in delta connected primary and wye or delta connected secondary.

They should never be connected wye primary to wye secondary, since this will result in unstable secondary

voltage. The equivalent three phase capacity when properly connected of three single phase transformers is three

times the nameplate rating of each single phase transformer. For example: Three 10 KVA single phase

transformers will accommodate a 30 KVA three phase load

23) Difference between Restricted Earth Fault & Unrestricted Earth Fault protections?

Restricted earth fault is normally given to on star connected end of power equipment like generators, transformers

etc. mostly on low voltage side. For REF protection 4 no‘s CTs are using one each on phase and one in neutral. It

is working on the principle of balanced currents between phases and neutral. Unrestricted E/F protection working

on the principle of comparing the unbalance on the phases only. For REF protection PX class CT are using but

for UREF 5P20 Cts using.

For Differential Protection CTs using on both side HT & LV side each phase, and comparing the unbalance

current for this protection also PX class CTs are using.

24) Can transformers be operated at voltages other than nameplate voltages?

In some cases, transformers can be operated at voltages below the nameplate rated voltage. In NO case should

a transformer be operated in excess of its nameplate rating unless taps are provided for this purpose. When

operating below the rated voltage the KVA capacity is reduced correspondingly.

25) How many types of cooling system it transformers?

ONAN (oil natural,air natural)

ONAF (oil natural,air forced)

OFAF (oil forced,air forced)

ODWF (oil direct,water forced)

OFAN (oil forced,air natural)

26) What is the function of anti-pumping in circuit breaker?

when breaker is close at one time by close push button, the anti pumping contactor prevent re close the breaker

by close push button after if it already close.

27) There are a Transformer and an induction machine. Those two have the same supply. For which device

the load current will be maximum?

The motor has max load current compare to that of transformer because the motor consumes real power.. and

the transformer is only producing the working flux and it‘s not consuming. Hence the load current in the

transformer is because of core loss so it is minimum.

28) Where the lighting arrestor should be placed in distribution lines?

Near distribution transformers and out going feeders of 11kv and incoming feeder of 33kv and near power

transformers in sub-stations.

29) Why Delta Star Transformers are used for Lighting Loads?

For lighting loads, neutral conductor is must and hence the secondary must be star winding. and this lighting load

is always unbalanced in all three phases.

To minimize the current unbalance in the primary we use delta winding in the primary. So delta / star transformer

is used for lighting loads.

30) NGR grounded system vs. solidly grounded system

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In India, at low voltage level (433V) we must do only Solid Earthing of the system neutral. This is by IE Rules

1956, Rule No. 61 (1) (a).Because, if we have opt for impedance earthing, during an earth fault, there will be

appreciable voltage present between the faulted body & the neutral, the magnitude of this voltage being

determined by the fault current magnitude and the impedance value.

This voltage might circulate enough current in a person accidentally coming in contact with the faulted equipment,

as to harm his even causing death. Note that, LV systems can be handled by non-technical persons too.

In solid earthing, you do not have this problem, as at the instant of an earth fault, the faulted phase goes to

neutral potential and the high fault current would invariably cause the Over current or short circuit protection

device to operate in sufficiently quick time before any harm could be done.

31) Why Do not We Break Neutral in AC Circuits?

Neutral is connected to earth at some point, thus it has some value as a return path in the event of say and

equipment earth being faulty. It‘s a bit like asking ‗why don‘t we break the Earth connection‘

It was stupid and dangerous, as it was possible for the neutral fuse to blow; giving the appearance of ‗no power‘

when in fact the equipment was still live.

32) What is Minimum Value of Insulation Resistance / Polarization Index?

Motor Insulation Resistance:

The acceptable meg-ohm value = motor KV rating value + 1 (For LV and MV Motor).

Example, for a 5 KV motor, the minimum phase to ground (motor body) insulation is 5 + 1 = 6 meg-ohm.

Panel Bus Insulation Resistance:

The acceptable meg-ohm value = 2 x KV rating of the panel.

Example, for a 5 KV panel, the minimum insulation is 2 x 5 = 10 meg-ohm

IEEE 43 – INSULATION RESISTANCE AND POLARIZATION INDEX (min IR at 400C in MΩ)

Minimum Insulation

Resistance TEST SPECIMEN

R1 min = kV+1 R1

min = 100

For most windings made before about 1970, all field windings, and others not

described below For most dc armature and ac windings built after about 1970 (form

wound coils)

R1 min = 5 For most machines with random -wound stator coils and form-wound coils rated

below 1kV

33) What is service factor?

Service factor is the load that may be applied to a motor without exceeding allowed ratings. For example, if a 10-

hp motor has a 1.25 service factor; it will successfully deliver 12.5 hp (10 x 1.25) without exceeding specified

temperature rise. Note that when being driven above its rated load in this manner, the motor must be supplied

with rated voltage and frequency.

Keep in mind, however, that a 10-hp motor with a 1.25 service factor is not a 12.5-hp motor. If the 10-hp motor is

operated continuously at 12.5 hp, its insulation life could be decreased by as much as two-thirds of normal. If you

need a 12.5-hp motor, buy one; service factor should only be used for short-term overload conditions.

34) Calculate the size the CT on the neutral point of the secondary side of 11/0.415 kV Transformer

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For high impedance relays (differential or restricted earth fault relays), ‗Class X‘ current transformers are

recommended to be used.

Please note that both CTs (neutral & phase) shall have the same characteristics. The following is an example to

size the CT:

Input data: 11/0.415 kV ,2500 KVA Power transformer ,Transformer impedance is 6% ,Length of cable from

neutral CT to the relay is 200 m ,Cross section of CT cable to be used is 6 mm² -copper and resistance is 0.0032

Ω/m

Step 1: Calculation of CT Rated Primary Current

I = kVA/ (0.415×1.732) = 2500/ (0.415×1.732) = 3478.11 A, CT with primary current of 4000 A to be selected.

Select the secondary current of the CT 1 or 5 A. selecting 1 A secondary current, as the cross section and length

of pilot wires can have a significant effect on the required knee voltage of the CT and therefore the size and cost

of the CT. When the relay is located some distance from the CT, the burden is increased by the resistance of the

pilot wires.

Step 2: Calculation of maximum Fault Current

Ift = kVA/ (0.415×1.732x Z)

Ift = 2500/ (0.415×1.732×0.06) = 57968.59 A (say 58000 A)

Step 3: Calculation of the Knee Voltage of the CT (Vkp)

Vkp = (2x Iftx (Rct+Rw)/CT transformation ratio)

Where: Rct is the CT resistance (to be given by the manufacturer), Here Rct is1.02 Ω.

Rw: total CT cable resistance= 2x cable length (200 m) x wire resistance= 2x200x0.0032= 1.28 Ω

CT transformation ratio = CT Primary Current/CT Secondary Current

CT transformation ratio = 4000/5= 800 A, for CT with 5 A secondary current; or,

CT transformation ratio = 4000/1= 4000 A, for CT with 1 A secondary current. We will use 1 A in this example.

Vkp = (2x58000x (1.02+1.28)/4000)= 66.7 V.

The Vkp of the CT should be higher than the setting of relay stability voltage (Vs), to ensure stability of the

protection during maximum Through fault current.

To calculate the stability voltage,we should follow the related formula given by the relay manufacturer, as each

relay manufacturer has its own formula.

we may calculate the Vkp as above using a CT with secondary current of 5 A, and you will notice the difference in

the Vkp.

35) When should we use Molded Case Circuit Breakers and Mini Circuit Breakers?

MCB is Miniature Circuit Breaker, since it is miniature it has limitation for Short Circuit Current and Amp

RatingMCB:

MCB are available as Singe module and used for :-

Number of Pole :- 1,2,3,4 – 1+ N , & 3 + N

Usually Current range for A.C. 50-60 HZ, is from 0.5 Amp – 63 Amp. Also available 80A, 100A, and 125 Amp.

SC are limited 10 KA

Applications are as: – Industrial, Commercial and Residential application.

Tripping Curve:

(1) B Resistive and lighting load,

(2) C Motor Load,

(3) D Highly inductive load.

MCCB:

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MCCB: – Moulded Case Circuit Breaker.

MCCB are available as Singe module and used for:

Number of Pole :- 3 pole , & 4 Pole

Current range for A.C:

For 3.2 /6.3/12.5/25/50/100/125/160 Amp and Short Circuit Capacity 25/35/65 KA.

For 200 250 Amp and Short Circuit Capacity 25/35/65 KA

For 400 630/800 Amp and Short Circuit Capacity 50 KA

Protection release :

Static Trip :- Continuous adjustable overload protection range 50 to 100 % of the rated current Earth fault

protection can be add on with adjustable earth fault pick up setting 15 to 80 % of the current.

Micro processor Based release:

Over load rated current 0.4 to1.0 in steps of o.1 of in trip time at 600 % Ir (sec) 0.2.0.5,1, 1.5 , 2 ,3

Short Circuit :-2 to10 in steps of 1 lr , short time delay (sec) 0.02.0.05,0.1, 0.2 ,0.3

Instantaneous pick up :2 to10 in steps of 1 in Ground fault pick up Disable: 0.2 to 0.8 in steps of 0.1 of in Ground

fault delay (sec): 0.1 to 0.4 in steps of 0.1

MCB (Miniature Circuit Breaker) Trip characteristics normally not adjustable, factory set but in case of MCCB

(Moulded Case Circuit Breaker) Trip current field adjustable.

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Transformer

APRIL 7, 2011 22 COMMENTS

Standard Transformer Accessories & Fittings:

Standard Transformer Fittings:

1) Standard Fittings

Rating and terminal marking plate.

Tap Changing arrangement

Off – circuit tap changing switch

Off – circuit tap changing link

On Load tap changer

Two earthing terminals

Lifting Lugs

Drain – cum filter valve

Pressure Relief Device

Silica gel dehydrating breather.

Oil Level Indicator.

Thermometer Pocket.

Conservator with drain plug and filling hole.

Air Release plug.

Jacking lugs (above 1600 KVA)

Filter valve (top tank)

Under base unidirectional flat rollers.

2) Terminal Arrangement:

Bare Bushings or Cable box.

Compound filled for PVC cables (up to 33000 Volts) or Air filled for PVC cable s (Up to

11000 Volts) or

Bus Duct (Bare bushing enclosed in housing up to 600 Volts)

Disconnection chamber between cable box and transformer tank.

Additional bare neutral terminal.

3) Optional Fittings:

These are optional fittings provided at an extra cost, if customer specifically orders them.

Winding temperature indicator

Oil temperature indicator

Gas and oil actuated (Buchholz) relay

Conservator drain valve

Shut off valve between conservator and tank.

Magnetic oil level gauge

Explosion vent

Filter valve (Bottom of tank)

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Skid under base with haulage holes

Junction box.

Standard Transformer Accessories:

1) Thermometer Pockets:

This pocket is provided to measure temperature of the top oil in tank with a mercury in glass

type thermometer. It is essential to fill the pocket with transformer oil before inserting the

thermometer, to have uniform and correct reading. One additional pocket is provided for dial

type thermometer (OTI) with contacts

2) Air release plug:

Air release plug is normally provided on the tank cover for transformer with conservator.

Space is provided in the plug which allows air to be escaped without removing the plug fully

from the seat. Plug should be unscrewed till air comes out from cross hole and as soon as

oil flows out it should be closed. Air release plugs are also provided on radiator headers and

outdoor bushings.

3) Winding temperature Indicator

The windings temperature indicator indicates ‗‘ Hot spot‘‘ temperature of the winding. This is

a ‗‘Thermal Image type‘‘ indicator. This is basically an oil temperature indicator with a heater

responsible to raise the temperature equal to the ‗‘Hot spot‘‘ gradient between winding and

oil over the oil temperature. Thus, this instrument indicates the ‗‘Hot Spot‘‘ temperature of the

windings. Heater coil is fed with a current proportional to the windings current through a

current transformer mounted on the winding under measurement. Heater coil is either placed

on the heater bulb enveloping the sensing element of the winding temperature indicator

immersed in oil or in the instrument. The value of the current fed to the heater is such that it

raises the temperature by an amount equal to the hot spot gradient of the winding, as

described above. Thus temperature of winding is simulated on the dial of the instrument.

Pointer is connected thought a mechanism to indicate the hot spot temperature on dial. WTI

is provided with a temperature recording dial main pointer. Maximum pointer and re setting

device and two sets of contacts for alarm and trip.

4) Oil Temperature Indicator

Oil temperature indicator provides local temperature of top oil. Instruments are provided with

temperature sensing bulb, temperature recording dial with the pointer and maximum reading

pointer and resetting device. Electrical contacts are provided to give alarm or trip at a

required setting (on capillary tube type thermometer).

5) Conservator Tank:

It is an Expansion Vessel

It maintains oil in the Transformer above a Minimum Level

It has a Magnetic Oil Level Gage.

It can give an alarm if the oil level falls below the limit

A portion of the Tank is separated for use with OLTC.

This usually has oil level indicators

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Main Conservator Tank can have a Bellow

It has an oil filling provision

It has an oil drain valve

Provision is there for connecting a Breather

6) Silica Gel Breather:

Prevents Moisture Ingress.

Connected to Conservator Tank

Silica Gel is Blue when Dry; Pink when moist

Oil Seal provides a Trap for Moisture before passing thro Silica Gel

7) Cooling:

ONAN .. Oil Natural Air Natural

ONAF .. Oil Natural Air Forced

OFWF .. Oil Forced Water Forced

ODWF .. Oil directed Water Forced.

By Forced Cooling, the Transformer capacity can be increased by more than 50%

8) Bushing:

Insulators and Bushings are built with the best quality Porcelain shells manufactured by wet

process.

For manufacture of electro porcelain, high quality indigenous raw materials viz, China Clay,

Ball Clay, Quartz and Feldspar is used Quartz and feldspar are ground to required

finesses and then intimately mixed with ball and china clay in high speed blungers. They are

then passed through electromagnetic separators, which remove iron and other magnetic

impurities. The slip produced is passed to a filter press where extra water is removed

under pressure and the resulting clay cakes are aged over a period. The aged cakes are

extruded to required form viz., cylinders, on high vacuum de-airing pug mill. The

extruded blanks or cylinders are given shapes of Insulators / Bushings which are

conditioned and are shaped on copying lathes as the case may be.

Testing, Assembly & packing:

All insulators & bushings undergo routine electrical and mechanical tests. The tests before

and after assembly are carried out according to IS Specifications, to ensure their

suitability for actual conditions of use. Porosity tests are also carried out regularly on

samples from every batch, to ensure that the insulators are completely vitrified. These

insulators are then visually checked and sorted, before they are packed in sea worthy

packing, to withstand transit conditions.

Types of Insulators & Bushings:

Bushing Insulators: Hollow Porcelain Bushings up to 33 KV

Application : Transformers, Capacitors, Circuit Breakers

Solid Core Insulators:

Line Post

Long Rod

Support

Special Type Insulators

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C.T. up to 66 KV

P.T. up to 33 KV

Weather Casing

L.T. Insulators

Shackel Type

Spool Type

Pin Type

Guy strain

H.V. Bushings (IS:3347)

Pin Insulators: Up to 33 KV

Post type Insulators: Post type insulators, complete with metal fittings, generally IS

Specifications and other International Standards up to 33 KV

12 to17.5 KV / 250 amps 24 KV / 1000 amps

12 to 17.5 KV / 630 amps 24 KV / 2000 to 3150 amps

12 to 17.5 KV / 1000 amps 36 KV / 250 amps

12 to 17.5 KV / 2000 to 3150 amps 36 KV / 630 amps

24 KV / 250 amps 36 KV / 1000 amps

24 KV / 630 amps 36 KV / 2000 to 3150 amps

L.V. Bushings (IS:3347)

11 KV / 250 amps 1 KV / 2000 amps

1 KV / 630 amps 1 KV / 3150 amps

1 KV / 1000 amps

H.V. Bushings (IS:8603)

12 KV / 250 amps 36 KV / 250 amps

12 / 630 amps 36 KV / 630 amps

12 KV / 1000 amps 6 KV / 1000 amps

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12 KV / 2000 to 3150 amps 36 KV 3150 amps

C.T. Bushings (IS:5612)

11 KV 1 KV / 2000 amps

1 KV / 630 amps 1 KV / 3150 amps

1 KV / 1000 amps

Epoxy Bushing:

All Epoxy Resin Cast Components are made from hot setting reins cured with

anhydrides; hence these provide class-F Insulation to the system. In an oxidizing

atmosphere, certain amine cured Epoxy Resins can start to degrade at 150ºC whereas the

anhydride cured systems are stable at 200ºC therefore our epoxy components are cured with

anhydrides which gives them a longer life.

9) Buchholz Relay:

The purpose of such devices is to disconnect faulty apparatus before large scale damage

caused by a fault to the apparatus or to other connected apparatus. Such devices

generally respond to a change in the current or pressure arising from the faults and are used

for either signaling or tripping the circuits.

Considering liquid immersed transformer, a near ideal protective device is available in

the form of gas and oil operated relay described here. The relay operates on the well

known fact that almost every type of electric fault in a liquid immersed transformer gives rise

to a gas. This gas is collected in the body of the relay and is used in some way or the other

to cause the alarm or the tripping circuit to operate.

In the event of fault in an oil filled transformer gas is generated, due to which buchholz relay

gives warning of developing fault. Buchholz relay is provided with two elements one for minor

faults (gives alarm) and other for major faults (tripping). The alarm elements operate after a

specific volume gets accumulated in the relay. Examples of incipient faults which will

generate gas in oil are:- Buchholz Relay

i) Failure of core bolt insulation.

ii) Shorting of lamination and core clamp.

iii) Bad Electrical contact or connections.

iv) Excessive hot spots in winding.

The alarm element will also operated in the event of oil leakage. The trip element operates

due to sudden oil surge in the event of more serious fault such as: -

i) Earth fault due to insulation failure from winding to earth.

ii) Winding short circuit inter turn, interlayer, inter coil etc.

iii) Short circuit between phases.

iv) Puncture of bushing.

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The trip element will also operate if rapid loss of oil occurs. During the operation of

transformer, if there is an alarm transformer should be isolated from lines and possible

reasons, listed above for the operation of relay should be checked starting with simple

reason such as loss of oil due to leaks, air accumulation in relay chamber which may be the

absorbed air released by oil due to change in temperature etc. Rating of contacts: – 0.5

Amps. At 230 Volts AC or 220 Volts. DC.

Pre commissioning Inspection of Transformer:

Sample of oil taken from the transformer and subjected to electric test (break down value) of

50KV (RMS) as specified in IS : 335.

Release trapped air through air release plugs and valve fitted for the purpose on various

fittings like radiators, bushing caps, tank cover, Bushing turrets etc.

The float lever of the magnetic oil level indicator (if provided) should be moved up and down

between the end position to check that the mechanism does not stick at any point. If the

indicator has signaling contact they should be checked at the same time for correct

operation. Checking the gauge by draining oil is a more positive test.

Check whether gas operated really (if provided) is mounted at angle by placing a spirit level

on the top of the relay. See that the conservator is filled upto the filling oil level marked on

plain oil gauge side and corresponding to the pointer reading in MOG side. Check the

operation of the alarm and trip contacts of the relay independently by injecting air through

the top cocks using a dry air bottle. The air should be released after the tests. Make sure

that transformer oil runs through pert cock of Buchholz relay.

Check alarm and trip contacts of WTIs, Dial type thermometer, magnetic oil gauge etc. (if

provided).

Ensure that off circuit switch handle is locked at the desired tap position with padlock.

Make sure that all valves except drain, filter and sampling valves are opened (such as

radiator valves, valves on the buchholz relay pipe line if Provided).

Check the condition of silicagel in the breather to ensure that silicagel in the breather is

active and colour is blue. Also check that the transformer oil is filled in the silicagel breather

upto the level indicated.

Check tightness of external electrical connections to bushings.

Give a physical check on all bushing for any crack or any breakage of porcelain. Bushing

with cracks or any other defects should be immediately replaced.

Check the neutral earthing if specified.

Make sure that neutrals of HV / LV are effectively earthed.

Tank should be effectively earthed at two points.

Check that the thermometer pockets on tank cover are filled with oil.

If the oil temperature indicator is not working satisfactorily, loosen and remove the

thermometer bulb from the pocket on the cover and place it with a standard thermometer in a

suitable vessel filled with transformer oil. Warm the oil slowly while string it and take reading

of the thermometers if an adjustment of the transformer thermometer is necessary the

same many be done. Also check signaling contacts and set for the desired temperature.

CT secondary terminals must be shorted and earthed if not in use.

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Check relief vent diaphragm for breakage. See that the Bakelite diaphragm at bottom and

glass diaphragm at top are not ruptured.

Check all the gasket joints to ensure that there is no leakage of transformer oil at any point.

Clear off extraneous material like tools earthling rods, pieces of clothes, waste etc.

Lock the rollers for accidental movement on rails.

Touching of paint may be done after erection.

Parts of Transformer:

1) Transformer Oil

Oil is used as coolant and dielectric in the transformer and keeping it in good condition will

assist in preventing deterioration of the insulation, which is immersed in oil. Transformer oil is

always exposed to the air to some extent therefore in the course of time it may oxidize and

form sludge if the breather is defective, oil may also absorb moisture from air thus reducing

dielectric strength.

2) Transformer Winding:

The primary and secondary windings in a core type transformer are of the concentric type

only, while in case of shell type transformer these could be of sand-witched type as well. The

concentric windings are normally constructed in any of the following types depending on the

size and application of the transformer.

(1)Cross over Type.

(2) Helical Type.

(3) Continuous Disc Type.

Distributed.

Spiral.

Interleaved Disc.

Shielded Layer

a) Distributed Winding :

Used for HV windings of small Distribution Transformers where the current does

not exceed 20 amps using circular cross section conductor .

b) Spiral:

Used up to 33 kv for low currents using strip conductor. Wound closely on Bakelite or

press board cylinders generally without cooling ducts. However, multi layer windings are

provided with cooling ducts between layers. No Transposition is necessary.

c) Interleaved Disc:

Used for voltages above 145 kv . Interleaving enables the winding withstand higher impulse

voltages.

d) Shielded Layer :

Used up to 132 kv in star connected windings with graded insulation. Comprises of a number

of concentric spiral coils arranged in layers grading the layers.

The longest at the Neutral and the shortest at the Line Terminal. The layers are

separated by cooling ducts. This type of construction ensures uniform distributed voltages.

e) Cross-over type winding:

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It is normally employed where rated currents are up-to about 20 Amperes or so.

In this type of winding, each coil consists of number of layers having number of turns per

layer. The conductor being a round wire or strip insulated with a paper covering.

It is normal practice to provide one or two extra lavers of paper insulation between lavers.

Further, the insulation between lavers is wrapped round the end turns of the lavers there by

assisting to keep the whole coil compact.

The inside end of a coil is connected to the outside end of adjacent coil. Insulation blocks are

provided between adjacent coils to ensure free circulation of oil.

f) Helical winding:

Used for Low Voltage and high currents .The turns comprising of a number of

conductors are wound axially. Could be single, double or multi layer winding. Since

each conductor is not of the same length, does not embrace the same flux and

of different impedances, and hence circulating currents, the winding is Transposed.

The coil consists of a number of rectangular strips wound in parallel racially such that each

separate turn occupies the total radial depth of the winding.

Each turn is wound on a number of key spacers which form the vertical oil duct and each

turn or group of turns is spaced by radial keys sectors.

This ensures free circulation of oil in horizontal and vertical direction.

This type of coil construction is normally adopted for low voltage windings where the

magnitude of current is comparatively large.

Helical Disc winding:

This type of winding is also termed ―interleaved disk winding.‖

Since conductors 1 – 4 and conductors 9 – 12 assume a shape similar to a wound capacitor,

it is known that these conductors have very large capacitance. This capacitance acts as

series capacitance of the winding to highly improve the voltage distribution for surge.

Unlike cylindrical windings, Helical disk winding requires no shield on the winding outermost

side, resulting in smaller coil outside diameter and thus reducing Transformer dimension.

Comparatively small in winding width and large in space between windings, the construction

of this type of winding is appropriate for the winding, which faces to an inner winding of

relatively high voltage.

Thus, general EHV or UHV substation Transformers employ Helical disk winding to utilize its

features mentioned above.

g) Continuous disc type of windings:

Used for 33kv and 132 kv for medium currents. The coil comprises of a number of

sections axially. Cooling ducts are provided between each section.

IT is consists of number of Discs wound from a single wire or number of strips in parallel.

Each disc consists of number of turns, wound radically, over one another.

Arrangement of layers

The conductor passing uninterruptedly from one disc to another. With ultiple-strip conductor.

Transpositions are made at regular intervals to ensure uniform resistance and length of

conductor. The discs are wound on an insulating cylinder spaced from it by strips running the

whole length of the cylinder and separated from one another by hard pressboard sectors

keyed to the vertical strips.

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This ensures free circulation of oil in horizontal and vertical direction and provides efficient

heat dissipation from windings to the oil.

The whole coil structure is mechanically sound and capable of resisting the most enormous

short circuit forces.

This is the most general type applicable to windings of a wide range of voltage and current

Rectangular wire is used where current is relatively small, while transposed cable Fig. (12) is

applied to large current. When voltage is relatively low, a Transformer of 100MVA or more

capacity handles a large current exceeding 1000A. In this case, the advantage of transposed

cable may be fully utilized

Since the number of turns is reduced, even conventional continuous disk construction is

satisfactory in voltage distribution, thereby ensuring adequate dielectric characteristics. Also,

whenever necessary, potential distribution is improved by inserting a shield between turns.

According to the number of layers used the paper is applied as follows.

Two layers: =Where there are two layers both of them are wound in opposite directions.

More than two layers: =Where there are more than two layers all the layers are applied in

the same direction, all, except the outermost layer is butt wound, and the outermost

layer is overlap wound. Within each group of papers the position of the butt joints of any

layer relative to the layer below is progressively displaced by approximately 30 percent of the

paper width.

Note: Overlapping can also be done as per customer requirements.

Grade of paper

The paper, before application, is ensured to be free from metallic and other injurious

inclusions and have no deleterious effect on insulating oil.

The thickness of paper used is between 0.025 mm to 0.075 mm.

Enameled Conductor

Apart from paper covered conductors, we have all the facilities of producing enameled

conductors as per customer specified requirements.

Copper - Usually in 8 – 16mm rods is drawn to the required sizes and then insulated with

paper etc..

Annealing is done for softening and stress relieving in electrically heated annealing plant

under vacuum upto 400-500ºC. After 48hrs when the temperature reaches ambient, the

vacuum is slowly released and the material is transferred to Insulation section.

Conductors are one of the principal materials used in manufacturing of transformers.

Best quality of copper rods are procured from indigenous as well as foreign sources.

Normally 8 mm & 11 mm rods are procured. For each supply of input, test certificate from

suppliers is obtained and at times.

After the wires & strips are drawn as per clients requirements they are moved on to

paper covering process.

To prevent the inclusion of copper dust or other extraneous matter under paper covering

the conductor is fully cleaned by felt pads or other suitable means before entering the

paper covering machine. As per the customers requirements DPC, TPC & MPC

conductors are produced. It is ensured that each layer of paper is continuous, firmly applied

and substantially free from creases.

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No bonding or adhesive material is used except to anchor the ends of paper. Any such

bonding materials used to anchor the ends do not have deleterious effect on

transformer oil, insulating paper or the electric strength of the covering. It is ensured

that the overlapping percentage is not less than 25% of the paper width.

The rectangular paper-covered copper conductor is the most commonly used conductor for

the windings of medium and large power transformers.

These conductors can be individual strip conductors, bunched conductors or continuously

transposed cable (CTC) conductors.

In low voltage side of a distribution transformer, where much fewer turns are involved, the

use of copper or aluminum foils may find preference.

To enhance the short circuit withstand capability, the work hardened copper is commonly

used instead of soft annealed copper, particularly for higher rating transformers

In the case of a generator transformer having high current rating, the CTC conductor is

mostly used which gives better space factor and reduced eddy losses in windings. When the

CTC conductor is used in transformers, it is usually of epoxy bonded type to enhance its

short circuit strength.

3) Transformer Core:

Purpose of the core:

To reduce the magnetizing current. (For topologies such as Forward, Bridge etc we need the

magnetizing current to be as small as possible. For fly-back topology, though the

magnetizing current is used to transfer energy, the size of the transformer will be very large

to get the required inductance if a core is not used.)

To improve the linkage of the flux within windings if the windings are separated spatially.

To contain the magnetic flux within a given volume

In magnetic amplifier applications a saturable core is used as a switch.

Core Material:

Different types of material used for cores

Iron-Silicon Steel- Nickel-Iron-Iron-Cobalt-Ferrite-Molybdenum-Met-glass

Salient characteristics of a core material:

Permeability, Saturation flux density, Coercive force, Remnant flux, Losses due to

Hysteresis & Eddy Current.

The power loss is a function of frequency and the ac flux swing and is given by the equation

P = K1 * (frequency)K2 * (Flux Density)K3

Every transformer has a core, which is surrounded by windings. The core is made out of

special cold rolled grain oriented silicon sheet steel laminations. The special silicon steel

ensures low hysteretisis losses. The silicon steel laminations also ensure high resistively of

core material which result in low eddy currents. In order to reduce eddy current losses, the

laminations are kept as thin as possible. The thickness of the laminations is usually around

0.27 to 0.35 mm.

Transformer cores construction is of two types, viz, core type and shell type. In core type

transformers, the windings are wound around the core, while in shell type transformers, the

core is constructed around the windings. The shell type transformers provide a low reactance

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path for the magnetic flux, while the core type transformer has a high leakage flux and hence

higher reactance.

The limb laminations in small transformers are held together by stout webbing tape or by

suitably spaced glass fiber bends. The use of insulated bolts passing through the limb

laminations has been discontinued due to number of instances of core bolt failures. The top

and bottom mitered yokes are interleaved with the limbs and are clamped by steel sections

held together by insulated yoke bolts. The steel frames clamping the top and bottom yokes

are held together by vertical tie bolts.

Grain Oriented steel sheets namely ORIENTCORE, ORIENTCORE H1-B &

ORIENTCORE HI-B.LS are some of the finest quality of core.

ORENTCORE.HI-B is a breakthrough in that it offers higher magnetic flux density,

lower core loss and lower magnetostriction than any conventional grain-oriented

electrical steel sheet.

ORIENT.HI-B.LS is a novel type with marked lower core losses, produced by laser

irradiation of the surface of ORIENTCORE.HI-B sheets.

Annealing of stacked electrical sheets

Annealing is to be done at 760 to 845ºC to

Reduce mechanical stress

Prevent contamination

Enhance insulation of lamination coating

Though ORIENTCORE and ORIENTCORE.HI-B are grain orient steel sheets with

excellent magnetic properties, mechanical stress during such operations as cutting,

punching and bending affect their magnetic properties adversely. When these stress are

excessive, stress relief annealing is necessary.

Following method is observed for stress relief annealing

Available Grades:

1. Stacked electrical steel sheets are heated thoroughly in the edge-to-edge direction

rather than in the face-to-face direction, because heat transfer is far faster in side

heating.

2. A cover is put over sheets stacked on a flat plate. Because ORIENTCORE

and ORIENTCORE.HI-B have extremely low carbon content and very easily

decarburized at annealing temperatures, the base, cover and other accessories used are of

very low carbon content .

3. To prevent oxidation so as to protect the coating on the sheets, a no oxidizing atmosphere

free from carbon sources is used having less than 2%hydrogen or high-purity nitrogen gas.

Due point of the atmosphere is maintained at 0ºC or less.

4. Care is taken to the flatness of annealing base, because an uneven base distorts

cores, leading to possible distortion during assembly.

5. Annealing temperature ranging from 780ºC to 820ºC is maintained for more than 2

hours or more. Cooling is done upto 350ºC in about 15 hours or more.

ORIENTCORE :M1, M2, M3, M4, M5 & M6

ORIENTCORE.HI-B :23ZH90, 23ZH95, 27ZH95, 27ZH100, 30ZH100,M-0H, M-1H, M-2H,

M-3H

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ORIENTCORE.HI-B.LS: 23ZDKH90, 27ZDKH95

Non-oriented silicon steel, hot rolled grain oriented silicon steel,cold rolled grain oriented

(CRGO) silicon steel, Hi-B, laser scribed and mechanically scribed. The last three materials

are improved versions of CRGO.

Saturation flux density has remained more or less constant around 2.0 Tesla for CRGO; but

there is a continuous improvement in watts/kg and volt-amperes/kg characteristics in the

rolling direction.

The core building technology has improved from the non-mitred to mitred and then to the

step-lap construction

The better grades of core steel not only reduce the core loss but they also help in reducing

the noise level by few decibels

Use of amorphous steel for transformer cores results in substantial core loss reduction (loss

is about one-third that of CRGO silicon steel). Since the manufacturing technology of

handling this brittle material is difficult, its use in transformers is not widespread

In the early days of transformer manufacturing, inferior grades of laminated steel (as per

today‘s standards) were used with inherent high losses and magnetizing volt-amperes. Later

on it was found that the addition of silicon content of about 4 to 5% improves the

performance characteristics significantly, due to a marked reduction in eddy losses (on

account of the increase in material resistivity) and increase in permeability. Hysteresis loss is

also lower due to a narrower hysteresis loop. The addition of silicon also helps to reduce the

aging effects.

Although silicon makes the material brittle, it is well within limits and does not pose problems

during the process of core building.

The cold rolled manufacturing technology in which the grains are oriented in the direc tion of

rolling gave a new direction to material development for many decades, and even today

newer materials are centered around the basic grain orientation process.

Important stages of core material development are: non-oriented, hot rolled grain oriented

(HRGO), cold rolled grain oriented (CRGO), high permeability cold rolled grain oriented (Hi-

B), laser scribed and mechanically scribed.

Laminations with lower thickness are manufactured and used to take advantage of lower

eddy losses. Currently the lowest thickness available is 0.23 mm, and the popular thickness

range is 0.23 mm to 0.35 mm for power transformers.

Maximum thickness of lamination used in small transformers can be as high as 0.50 mm.

Inorganic coating (generally glass film and phosphate layer) having thickness of 0.002 to

0.003 mm is provided on both the surfaces of laminations, which is sufficient to withstand

eddy voltages (of the order of a few volts).

Since the core is in the vicinity of high voltage windings, it is grounded to drain out the

statically induced voltages. While designing the grounding system, due care must be taken

to avoid multiple grounding, which otherwise results into circulating currents and subsequent

failure of transformers.

4) Transformer Core:

a) Core Type Construction: (Mostly Used):

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Generally in India, Core type of construction with Two/Three/Five limbed cores are used.

Generally five limbed cores are used where the dimensions of the Transformer is to be

limited due to Transportation difficulties. In three limbed core the cross section of the Limb

and the Yoke are the same where as in five Limbed core, the cross section of the Yoke and

the Flux return path Limbs are ver y less (58% and 45% of the principal Limb).

Limb:which is surrounded by windings, is called a limb or leg?

York: Remaining part of the core, which is not surrounded by windings, but is essential for

completing the path of flux, is called as yoke.

Advantage:

Construction is simpler, cooling is better and repair is easy.

The yoke and end limb area should be only 50% of the main limb area for the same

operating flux density.

Zero-sequence impedance is equal to positive-sequence impedance for this construction (in

a bank of single-phase transformers).

Sometimes in a single-phase transformer windings are split into two parts and placed around

two limbs as shown in figure (b). This construction is sometimes adopted for very large

ratings. Magnitude of short-circuit forces are lower because of the fact that ampere-

turns/height are reduced. The area of limbs and yokes is the same. Similar to the single-

phase three-limb transformer.

The most commonly used construction, for small and medium rating transformers, is three-

phase three-limb construction as shown in figure (d).For each phase, the limb flux returns

through yokes and other two limbs (the same amount of peak flux flows in limbs and yokes).

limbs and yokes usually have the same area. Sometimes the yokes are provided with a 5%

additional area as compared to the limbs for reducing no-load losses.

It is to be noted that the increase in yoke area of 5% reduces flux density in the yoke by 5%,

reduces watts/kg by more than 5% (due to non-linear characteristics) but the yoke weight

increases by 5%. Also, there may be additional loss due to cross-fluxing since there may not

be perfect matching between lamination steps of limb and yoke at the joint. Hence, the

reduction in losses may not be very significant.

In large power transformers, in order to reduce the height for transportability, three-phase

five-limb construction depicted in figure (e) is used. The magnetic length represented by the

end yoke and end limb has a higher reluctance as compared to that represented by the main

yoke. Hence, as the flux starts rising, it first takes the path of low reluctance of the main

yoke. Since the main yoke is not large enough to carry all the flux from the limb, it saturates

and forces the remaining flux into the end limb. Since the spilling over of flux to the end limb

occurs near the flux peak and also due to the fact that the ratio of reluctances of these two

paths varies due to non-linear properties of the core.

Fluxes in both main yoke and end yoke/end limb paths are non-sinusoidal even though the

main limb flux is varying sinusoidal [2,4]. Extra losses occur in the yokes and end limbs due

to the flux harmonics. In order to compensate these extra losses, it is a normal practice to

keep the main yoke area 60% and end yoke/end limb area 50% of the main limb area.

The zero-sequence impedance is much higher for the three-phase five-limb core than the

three-limb core due to low reluctance path (of yokes and end limbs) available to the in-phase

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zero-sequence fluxes, and its value is close to but less than the positive-sequence

impedance value.

b) Shell-type construction:

Cross section of windings in the plane of core is surrounded by limbs and yokes, is also

used.

Shell type of construction of the core is widely used in USA.

Advantage:

One can use sandwich construction of LV and HV windings to get very low impedance, if

desired, which is not easily possible in the core-type construction.

Analysis of overlapping joints and building factor:

While building a core, the laminations are placed in such a way that the gaps between the laminations at the joint

of

limb and yoke are overlapped by the laminations in the next layer.

This is done so that there is no continuous gap at the joint when the laminations are stacked one above the other

(figure). The overlap distance is kept around 15 to 20 mm.

There are two types of joints most widely used in transformers: non-mitred and mitred joints.

Non-mitered joints:

In which the overlap angle is 90°, are quite simple from the manufacturing point of view, but the loss in the corner

joints is more since the flux in the joint region is not along the direction of grain orientation. Hence, the on-mitred

joints are used for smaller rating transformers. These joints were commonly adopted in earlier days when non-

oriented material was used

Non-mitered joints:

In which the overlap angle is 90°, are quite simple from the manufacturing point of view, but

the loss in the corner joints is more since the flux in the joint region is not along the direction

of grain orientation. Hence, the on-mitred joints are used for smaller rating transformers.

These joints were commonly adopted in earlier days when non-oriented material was used

Mitered joints:

The joint where these laminations meet could be Butt or Mitred. In CRGO, the Mitred Joint

is preferred as it reduces the Reluctance of the Flux path and reduces the No Load

Losses and the No Load current (by about 12% & 25% respectively).

The Limb and the Yoke are made of a number of Laminations in Steps. Each step

comprises of some number of laminations of equal width. The width of the central

strip is Maximum and that at the circumference is Minimum. The cross section of the

Yoke and the Limb are nearly Circular. Mitred joint could be at 35/45/55 degrees but

the 45 one reduces wastage.

The angle of overlap (a) is of the order of 30° to 60°, the most commonly used angle is 45°.

The flux crosses from limb to yoke along the grain orientation in mitred joints minimizing

losses in them. For airgaps of equal length, the excitation requirement of cores with mitred

joints is sin a times that with non-mitred joints.

Better grades of core material (Hi-B, scribed, etc.) having specific loss (watts/kg) 15 to 20%

lower than conventional CRGO material (termed hereafter as CGO grade, e.g., M4) are

regularly used. However, it has been observed that the use of these better materials may not

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give the expected loss reduction if a proper value of building factor is not used in loss

calculations

The building factor generally increases as grade of the material improves from CGO to Hi-B

to scribed (domain refined). This is a logical fact because at the corner joints the flux is not

along the grain orientation, and the increase in watts/kg due to deviation from direction of

grain orientation is higher for a better grade material.

The factor is also a function of operating flux density; it deteriorates more for better grade

materials with the increase in operating flux density. Hence, cores built with better grade

material may not give the expected benefit in line with Epstein measurements done on

individual lamination. Therefore, appropriate building factors should be taken for better grade

materials using experimental/test data.

Also the loss contribution due to the corner weight is higher in case of 90° joints as

compared to 45° joints since there is over-crowding of flux at the inner edge and flux is not

along the grain orientation while passing from limb to yoke in the former case. Smaller the

overlapping length better is the core performance; but the improvement may not be

noticeable.

The gap at the core joint has significant impact on the no-load loss and current. As

compared to 0 mm gap, the increase in loss is 1 to 2% for 1.5 mm gap, 3 to 4% for 2.0 mm

gap and 8 to 12% for 3 mm gap. These figures highlight the need for maintaining minimum

gap at the core joints.

Lesser the laminations per lay, lower is the core loss. The experience shows that from 4

laminations per lay to 2 laminations per lay, there is an advantage in loss of about 3 to 4%.

There is further advantage of 2 to 3% in 1 lamination per lay. As the number of laminations

per lay reduces, the manufacturing time for core building increases and hence most of the

manufacturers have standardized the core building with 2 laminations per lay.

Joints of limbs and yokes contribute significantly to the core loss due to cross-fluxing and

crowding of flux lines in them. Hence, the higher the corner area and weight, the higher is the

core loss.

The corner area in single-phase three-limb cores, single-phase four-limb cores and three-

phase five-limb cores is less due to smaller core diameter at the corners, reducing the loss

contribution due to the corners. However, this reduction is more than compensated by

increase in loss because of higher overall weight (due to additional end limbs and yokes).

Building factor is usually in the range of 1.1 to 1.25 for three-phase three-limb cores with

mitred joints. Higher the ratio of window height to window width, lower is the contribution of

corners to the loss and hence the building factor is lower.

Step-lap joint :

It is used by many manufacturers due to its excellent performance figures. It consists of a

group of laminations (commonly 5 to 7) stacked with a staggered joint as shown in figure.

Its superior performance as compared to the conventional mitred construction.

It is shown that, for a operating flux density of 1.7 T, the flux density in the mitred joint in the

core sheet area shunting the air gap rises to 2.7 T (heavy saturation), while in the gap the

flux density is about 0.7 T. Contrary to this, in the step-lap joint of 6 steps, the flux totally

avoids the gap with flux density of just 0.04 T, and gets redistributed almost equally in

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laminations of other five steps with a flux density close to 2.0 T. This explains why the no-

load performance figures (current, loss and noise) show a marked improvement for the step-

lap joints.

The assembled core has to be clamped tightly not only to provide a rigid

mechanical structure but also required magnetic characteristic. Top and Bottom

Yokes are clamped by steel sections using Yoke Studs. These studs do not pass through

the core but held between steel sections. Of late Fiber Glass Band tapes are wound round

the Limbs tightly upto the desired tension and heat treated. These laminations , due to

elongation and contraction lead to magnetostriction, generally called Humming which can be

reduced by using higher silicon content in steel but this makes the laminations

become very brittle.

The choice of operating flux density of a core has a very significant impact on the overall

size, material cost and performance of a transformer.

For the currently available various grades of CRGO material, although losses and

magnetizing volt-amperes are lower for better grades, viz. Hi-B material (M0H, M1H, M2H),

laser scribed, mechanical scribed, etc., as compared to CGO material (M2, M3, M4,M5, M6,

etc.), the saturation flux density has remained same (about 2.0 T).

The peak operating flux density (Bmp ) gets limited by the over-excitation conditions

specified by users.

The slope of B-H curve of CRGO material significantly worsens after about 1.9 T (for a small

increase in flux density, relatively much higher magnetizing current is drawn). Hence, the

point corresponding to 1.9 T can be termed as knee-point of the B-H curve.

It has been seen in example 1.1 that the simultaneous over-voltage and under-frequency

conditions increase the flux density in the core. Hence, for an over-excitation condition (over-

voltage and under-frequency).

When a transformer is subjected to an over-excitation, core contains an amount of flux

sufficient to saturate it. The remaining flux spills out of the core. The over-excitation must be

extreme and of a long duration to produce damaging effect in the core laminations

The laminations can easily withstand temperatures in the region of 800°C (they are annealed

at this temperature during their manufacture), but insulation in the vicinity of core

laminations, viz. press-board insulation (class A: 105°C) and core bolt insulation (class B:

130°C) may get damaged. Since the flux flows in air (outside core) only during the part of a

cycle when core gets saturated, the air flux and exciting current are in the form of pulses

having high harmonic content which increases the eddy losses and temperature rise in

windings and structural parts.

Winding Insulation in Transformer:

Requirement of Insulating Oil:

1.0 lit / kva for Trs from 400 – 1600 Kva

0.6 lit / kva for Trs from 1600 – 80,000 kva

0.5 lit / Kva for Trs above 80,000 Kva.

In Transformers, the insulating oil provides an insulation medium as well as a heat

transferring medium that carries away heat produced in the windings and iron core. Since the

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electric strength and the life of a Transformer depend chiefly upon the quality of the

insulating oil, it is very important to use a high quality insulating oil

Provide a high electric strength.

Permit good transfer of heat.

Have low specific gravity-In oil of low specific gravity particles which have become

suspended in the oil will settle down on the bottom of the tank more readily and at a faster

rate, a property aiding the oil in retaining its homogeneity.

Have a low viscosity- Oil with low viscosity, i.e., having greater fluidity, will cool

Transformers at a much better rate.

Have low pour point- Oil with low pour point will cease to flow only at low temperatures.

Have a high flash point. The flash point characterizes its tendency to evaporate. The lower

the flash point the greater the oil will tend to vaporize When oil vaporizes, it loses in volume,

its viscosity rises, and an explosive mixture may be formed with the air above the oil

The Core Insulation is:

SRBP- Synthetic Resin Bonded Paper

OIP – Oil Impregnated Paper

RIP – Resin Impregnated Pape

Resin Coated Paper/ Kraft Paper/ Crepe Kraft Paper are used for making core for the above

It is Hermetically Sealed.

Pre-compressed pressboard is used in windings as opposed to the softer materials used in

earlier days. The major insulation (between windings, between winding and yoke, etc.)

Mineral oil has traditionally been the most commonly used electrical insulating medium and

coolant in transformers. Studies have proved that oil-barrier insulation system can be used at

the rated voltages greater than 1000 Kv.

A high dielectric strength of oil-impregnated paper and pressboard is the main reason for

using oil as the most important constituent of the transformer insulation system.

Manufacturers have used silicon-based liquid for insulation and cooling. Due to non-toxic

dielectric and self-extinguishing properties, it is selected as a replacement of Askarel. High

cost of silicon is an inhibiting factor for its widespread use.

Super-biodegradable vegetable seed based oils are also available for use in environmentally

sensitive locations.

SF6 gas has excellent dielectric strength and is non-flammable. Hence, SF6 transformers

find their application in the areas where fire-hazard prevention is of paramount importance.

Due to lower specific gravity of SF6 gas, the gas insulated transformer is usually lighter than

the oil insulated transformer. The dielectric strength of SF6 gas is a function of the operating

pressure; the higher the pressure, the higher the dielectric strength.

However, the heat capacity and thermal time constant of SF6 gas are smaller than that of oil,

resulting in reduced overload capacity of SF6 transformers as compared to oil-immersed

transformers. Environmental concerns, sealing problems, lower cooling capability and

present high cost of manufacture are the challenges.

Dry-type resin cast and resin impregnated transformers use class F or C insulation. High

cost of resins and lower heat dissipation capability limit the use of these transformers to

small ratings.

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The dry-type transformers are primarily used for the indoor application in order to minimize

fire hazards. Nomex paper insulation, which has temperature withstand capacity of 220°C, is

widely used for dry-type transformers. The initial cost of a dry-type transformer may be 60 to

70% higher than that of an oil-cooled transformer at current prices, but its overall cost at the

present level of energy rate can be very much comparable to that of the oil-cooled

transformer.

Transformer Noise:

Transformers located near a residential area should have sound level as low as possible.

Levels specified are 10 to 15 dB lower than the prevailing levels mentioned in the

international standards.

Core, windings and cooling equipment are the three main sources of noise.

The core is the most important and significant source of the transformer noise.

The core vibrates due to magnetic and magnetostrictive forces. Magnetic forces appear due

to non-magnetic gaps at the corner joints of limbs and yokes

These magnetic forces depend upon the kind of interlacing between the limb and yoke;

these are highest when there is no overlapping (continuous air gap).

The magnetic forces are smaller for 90° overlapping, which further reduce for

45°overlapping. These are the least for the step-lap joint due to reduction in the value of flux

density in the overlapping region at the joint.

The forces produced by the magnetostriction phenomenon are much higher than the

magnetic forces in transformers.

Magnetostriction is a change in configuration of magnetizable material in a magnetic field,

which leads to periodic changes in the length of material. An alternating field sets the core in

vibration.

This vibration is transmitted, after some attenuation, through the oil and tank structure to the

surrounding air. This finally results in a characteristic hum.

The magnetostriction force varies with time and contains even harmonics of the power

frequency (120, 240, 360, —Hz for 60 Hz power frequency). Therefore, the noise also

contains all harmonics of 120 Hz.

The amplitude of core vibration and noise increase manifold if the fundamental mechanical

natural frequency of the core is close to 120 Hz.

The value of the magnetostriction can be positive or negative, depending on the type of the

magnetic material, and the mechanical and thermal treatments.

Magnetostriction is generally positive (increase in length by a few microns with increase in

flux density) for CRGO material at annealing temperatures below 800°C, and as the

annealing temperature is increased (=800°C), it can be displaced to

negative values.The mechanical stressing may change it to positive values

Magnetostriction is minimum along the rolling direction and maximum along the 90° direction.

Most of the noise transmitted from a core comes principally from the yoke region because

the noise from the limb is effectively damped by windings (copper and insulation material)

around the limb.

The quality of yoke clamping has a significant influence on the noise level.

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Apart from the yoke flux density, other factors which decide the noise level are: limb flux

density, type of core material, leg center (distance between the centers of two adjacent

phases), core weight, frequency, etc.

The higher the flux density, leg centers, core weight and frequency of operation, the higher is

the noise level.

The noise level is closely related to the operating peak flux density and core weight.

If core weight is assumed to change with flux density approximately in inverse proportion, for

a flux density change from 1.6 T to 1.7 T, the increase in noise level is 1.7 dB

Hence, one of the ways of reducing noise is by designing transformer at lower operating flux

density. For a flux density reduction of 0.1 T, the noise level reduction of about 2 dB is

obtained. This method results into an increase of material content and it may be justified

economically if the user has specified a lower no-load loss, in which case the natural choice

is to use a lower flux density.

Use of step-lap joint gives much better noise reduction (4 to 5 dB).

Some manufacturers also use yoke reinforcement (leading to reduction in yoke flux density);

the method has the advantage that copper content does not go up since the winding mean

diameters do not increase. Bonding of laminations by adhesives and placing of anti-

vibration/damping elements between the core and tank can give further reduction in the

noise level.

The use of Hi-B/scribed material can also give a reduction of 2 to 3 dB. When a noise level

reduction of the order of 15 to 20 dB is required, some of these methods are necessary but

not sufficient.

Transformer Protection:

Internal Protection:

(1) Bucholtz Relay:

This Gas operated relay is a protection for minor and major faults that may develop inside

a Transformer and produce Gases.

This relay is located in between the conservator tank and the Main

Transformer tank in the pie line which is mounted at an inclination of 3 to 7 degrees.

A shut off valve is located in between the Bucholtz relay and the Conservator.

The relay comprises of a cast housing which contains two pivoted Buckets counter

balanced weights.

The relay also contains two mercury y switches which will send alarm or trip

signal to the breakers controlling the Transformer. In healthy condition, this relay will be

full of oil and the buckets will also be full of oil and is counter balanced by the

weights.

In the event of a fault inside the transformer, the gases flow up to the conservator via the

relay and pushes the oil in the relay down. Once the oil level falls below the bottom level of

the buckets, the bucket due to the weight of oil inside tilts and closes the mercury switch and

causes the Alarm or trip to be actuated and isolate the transformer from the system.

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(2) Oil Surge / Bucholtz Relay for OLTC:

This relay operating on gas produced slowly or in a surge due to faults inside the

Diverter Switch of OLTC protects the Transformer and isolates it from the system.

(3) Pressure Relief Valve for Large Transformers:

In case of a serious fault inside the Transformer, Gas is rapidly produced.

This gaseous pressure must be relieved immediately otherwise it will damage the Tank

and cause damage to neighboring equipment.

This relay is mounted on the top cover or on the side walls of the Transformer. The

valve has a corresponding port which will be sealed by a stain less steel diaphragm .

The diaphragm rests on a O ring and is kept pressed by two heavy springs. If a

high pressure is developed inside, this diaphragm lifts up and releases the excessive

gas.

The movement of the diaphragm lifts the spring and causes a micro switch to

close its contacts to give a trip signal to the HV and LV circuit breakers and isolate

the transformer.

A visual indication can also be seen on the top of the relay. For smaller

capacity transformer, an Explosion vent is used to relieve the excess pressure

but it cannot isolate the Transformer.

(4) Explosion Vent Low & Medium Transformers :

For smaller capacity Transformers, the excessive pressures inside a Transformer due to

major faults inside the transformer can be relieved by Explosion vents. But this cannot

isolate the Transformer.

(5) Winding /Oil Temperature Protection :

These precision instruments operate on the principle of liquid expansion.

These record the hour to hour temperatures and also record the Maximum temperature

over a period of time by a resettable pointer.

These in conjunction with mercury switches provide signals for excessive temperature

alarm annunciation and also isolate the Transformer for very excessive

temperatures.

These also switch on the cooler fans and cooler pumps if the temperature exceeds the set

values. Normally two separate instruments are used for oil and winding temperatures.

In some cases additional instruments are provided separately for HV,LV and

Tertiary winding temperatures.

The indicator is provided with a sensing bulb placed in an oil pocket located on the

top cover of the Transformer tank. The Bulb is connected to the instrument housing by

means of flexible connecting tubes consisting of two capillary tubes.

One capillary tube is connected to an operating Bellow in the instrument. The

other is connected to a compensating Bellow .

The tube follows the same path as the one with the Bulb but the other end, it does not

end in a Bulb and left sealed. This compensates for variations in Ambient

Temperatures.

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As the temperature varies, the volume of the liquid in the operating system also varies

and operates the operating Bellows transmitting its movements to the pointer and also the

switching disc. This disc is mounted with mercury float switches which when made

provides signals to alarm/trip/cooler controls.

Oil and winding temperature indicators work on the same principles except that the

WTI is provided with an additional bellows heating element. This heating

element is fed by a current transformer with a current proportional to the load in the

winding whose temperature is to be measured/monitored. The tem premature

increase of the heating element is proportional to the temperature rise of winding over top

oil temperature.

The operating bellow gets an additional movement simulating the increase of winding

temperature over top oil temperature and represents the Winding Hot Spot. This is called

Thermal Imaging process.

(6) Conservator Magnetic Oil Level Protection :

Inside the conservator tank, a float is used to sense the levels of oil and move. This is

transmitted to a switch mechanism by means of magnetic coupling. The Float and the

Magnetic mechanism are totally sealed. The pointer connected to the magnetic

mechanism moves indicating the correct oil level and also provision is m ade for

Low oil level alarm by switch.

(7) Silica gel Breather:

This is a means to preserve the dielectric strength of insulating oil and prevent absorption of

moisture, dust etc. The breather is connected to the Main conservator tank. It is provided

with an Oil seal. The breathed in air is passed through the oil seal to retain moisture before

the air passes through the silica gel cr ystals which absorbs moisture before breathing

into the conservator tank. In latest large transformers, Rubber Diaphragm or Air cells

are used to reduce contamination of oil.

(8) Transformer Earthing :

For Distribution Transformers, normally Dy11 vector Group, the LT Neutral is Earthed by a

separate Conductor section of at least half the section of the conductor used for phase

wire and connected to a Separate Earth whose Earth Resistance must be less than 1 ohm.

The Body of the Tank has two different earth connections, which should be connected to

two distinct earth electrodes by GI flat of suitable section.

For Large Power Transformers, Neutral and Body Connections are made

separately but all the Earth Pits are connected in parallel so that the combined Earth

Resistance is always maintained below 0.1 ohm.

The individual and combined earth resistance is measured periodically and the

Earth Pits maintained regularly and electrodes replaced if required.

External Protection:

Lightning Arrestors on HV & LV for Surge Protection

HV / LV Over Current Protection(Instantaneous /IDMT- Back up)

Earth Fault Protection ( Y connected side)

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REF (HV & LV) ( For internal fault protection)

Differential Protection (for internal fault protection)

Over Fluxing Protection (against system Kv & HZ variations)

HG Fuse Protection for Small Capacity Transformers.

Normally Each Power Transformers will have a LV Circuit Breaker. For a Group of

Transformers up to 5 MVA in a substation, a Group control Circuit Breaker is

provided. Each Transformer of 8 MVA and above will have a Circuit Breaker on

the HV side.

Transformer Cooling:

The Heat in a transformer is produced due to I square R in the windings and in the core due

to Eddy Current and Hysteresis Loss.

In Dry type Transformer the Heat is directly dissipated into the atmosphere but in Oil filled

Transformer, the Heat is dissipated by Thermosyphon and transmitted to the

top and dissipated into the atmosphere through Radiators naturally or by forced

cooling fans or by Oil pumps or through Water Coolers.

The following Standard symbols are adopted to denote the Type of Cooling:

A =Air Cooling

N =Natural Cooling by Convection

B= Cooling by Air Blast Fans

O=Oil (mineral) immersed cooling

W= Water Cooled

F =Forced Oil Circulation by Oil Pumps

S=Synthetic Liquid used instead of Oil

G =Gas Cooled (SF6 or N2)

D=Forced (Oil directed)

ONAF=Oil immersed Transformer with natural oil circulation and forced air external cooling

is designated.

ONAN= Oil Immersed Natural cooled

ONAF= Oil Immersed Air Blast

ONWN=Oil Immersed Water Cooled

OFAF=Forced Oil Air Blast Cooled

OFAN=Forced Oil Natural Air Cooled

OFWF=Forced Oil Water Cooled

ODAF=Forced Directed Oil and Forced Air Cooling.

Cooling e.g., ONAN/ONAF or ONAN/OFAF or sometimes three systems e.g., ONAN/ONAF/

OFAF