Restricted Earth Fault Protection of Transformer

An external fault in the star side will result in current flowing in the line current transformer of the affected phase and at the same time a balancing current flows in the neutral current transformer, hence the resultant current in the relay is therefore zero. So this REF relay will not be actuated for external earth fault. But during internal fault the neutral current transformer only carries the unbalance fault current and operation ofRestricted Earth Fault Relay takes place. This scheme of restricted earth fault protectionis very sensitive for internal earth fault of electrical power transformer. The protection scheme is comparatively cheaper than differential protection schemeRestricted earth fault protection is provided in electrical power transformer for sensing internal earth fault of the transformer. In this scheme the CT secondary of each phase ofelectrical power transformer are connected together as shown in the figure. Then common terminals are connected to the secondary of a Neutral Current Transformer or NCT. The CT or Current Transformer connected to the neutral of power transformer is called Neutral Current Transformer or Neutral CT or simply NCT. Whenever there is an unbalancing in between three phases of the power transformer, a resultant unbalancecurrent flow through the close path connected to the common terminals of the CT secondaries. An unbalance current will also flow through the neutral of power transformer and hence there will be a secondary current in Neutral CT because of this unbalance neutral current. In Restricted Earth Fault scheme the common terminals of phase CTs are connected to the secondary of Neutral CT in such a manner that secondary unbalance current of phase CTs and the secondary current of Neutral CT will oppose each other. If these both currents are equal in amplitude there will not be any resultant currentcirculate through the said close path. The Restricted Earth Fault Relay is connected in this close path. Hence the relay will not response even there is an unbalancing in phasecurrent of the power transformer.

Maintenance of Current Transformer

A Current Transformer or CT is very essential equipment installed in an electrical substation for electrical

measurement and protection purpose. If a current transformer does not perform properly, there may be huge

disturbance in the system due to malfunctioning of protection relays. So far accurate measurement and smooth

operation of electrical power system, CTs must be properly maintained. A schedule of such maintenance of Current

Transformer is preferred below for ready reference. Let us first discuss about the maintenance of CT which to be

performed in one year interval.

1) Insulation resistance of the CT must be checked in yearly basis.

During insulation resistance measurement, it must be remembered that, in currenttransformer there are two level of

insulation. The insulation level of primary of CT is quite high as it has to withstand full system voltage. But the

secondary of the CT has low insulation level generally 1.1 KV. So primary to secondary and primary to earth of

a currenttransformer are measured with 2.5 or 5 KV megger. But this high voltage megger can not be used for

secondary measurement, as here insulation level is quite low in the view of economy of the design. So secondary

insulation is measured with 500 V megger.

Hence, primary terminals to earth, primary terminals to secondary measuring core, primary terminals to secondary

protection cores are measured by 2.5 or 5 KV megger. In between secondary cores and secondary to earth

resistances are measured by 500V megger. 

2) Thermo vision scanning of primary terminals and top dome of a live CT should be performed at least once in a

year.

This scanning can be done with help of infra-red Thermo-vision Camera.

3) All the CT secondary connections in CT secondary box and CT junction box must be checked, cleaned and tighten

every year to ensure maximum possible low resistance path for CT secondary currents. It should also be ensured

that CT junction box is properly cleaned.

There are some other maintenance of Current Transformer which to be performed in half yearly basis, such as,

1) The porcelain housing of CTs should be checked for hire crack if any crack is observed on insulator, necessary

advice to be obtained from manufacturer.

2) The porcelain insulator housing of current transformer, to be cleaned properly by cotton clothes.

Now we will discuss about monthly basis maintenance of current transformer.

1) Oil leakage from any joint should be visually inspected if leakage found, it must be plugged by taking shutdown.

2) The secondary terminals are also checked for oil leakage, if leakage found, immediate action to be taken to plug

the leakage.

In addition to these, tans or loss factor measurement to be performed on a currenttransformer, preferably above 66

KV class, once in two years.

Dissolve Gas Analysis of oil also to be done preferably once in 4 years. If the results are found unsatisfactory as per

standard, the insulating oil must be replaced.

Maintenance of Voltage Transformer and Capacitor Voltage Transformer

Construction wise a voltage transformer and a capacitor voltage transformer are same. Hence basis scheme of

maintenance of both voltage transformer and capacitor voltagetransformer are more or less same. As

heavy current does not flow through PT and CVT, the defect and fault generally very low.

That is why monthly maintenance of voltage transformer and maintenance of capacitor voltage  transformer may not

be required. Moreover very frequent maintenance of bus PT or CVT may not also be possible as far taking shutdown

of such PT or CVT total bus section would be out of protection and metering.

Only yearly maintenance of such equipments are sufficient.

Yearly Maintenance of Voltage Transformer or Capacitor Voltage Transformer 

1) The porcelain housing must be cleaned with cotton clothes.

2) The spark gap assembly to be checked on yearly basis. Remove the moveable part of spark gap as assembly,

clean the braes electrode with emery paper and fix it back in position.

3) The high frequency earthing point should be visually checked yearly in the case, the point is not used for PLCC.

4) Thermo vision camera to be used for checking any hot spots in the capacitor stacks to ensure pro action of

rectification.

5) The terminal connections PT junction box including earth connections to be checked for tightness once in a year.

In addition to that, the PT junction box also to be cleaned properly once in a year.

6) The health of all gasket joint also to be visually checked and replaced if any damaged gasket found.

N. B. : In addition all yearly basis maintenance of potential transformer or Capacitor Voltage Transformer,

must also be checked for tan δ once in 3 years. An increase in value of tan δ indicates deterioration of

insulation whereas both increase in tan δ andcapacitance indicates entry of moisture in insulation.

Daily Basis Maintenance and Checking

There are three main things which to be checked on a power transformer in daily basis and they are :

1. Reading of MOG (Magnetic Oil Gage) of main tank and conservator tank.

2. Color of silica gel in breather.

3. Leakage of oil from any point of a transformer.

In case of unsatisfactory oil level in the MOG, oil to be filled in transformer and also the transformer tank to be

checked for oil leakage. If oil leakage is found take required action to plug the leakage. If silica gel becomes pinkish, it

should be replaced.

Yearly Basis Transformer Maintenance Schedule

1. The auto, remote, manual function of cooling system that means, oil pumps, air fans, and other items engaged

in cooling system of transformer, along with their control circuit to be checked in the interval of one year. In the case

of trouble, investigate control circuit and physical condition of pumps and fans.

2. All the bushings of the transformer to be cleaned by soft cotton cloths yearly. During cleaning the bushing should

be checked for cracking.

3. Oil condition of OLTC to be examined in every year. For that, oil sample to be taken from drain valve of divertor

tank, and this collected oil sample to be tested for dielectric strength (BDV) and moisture content (PPM). If BDV is low

and PPM for moisture is found high compared to recommended values, the oil inside the OLTC to be replaced or

filtered.

4. Mechanical inspection of Buchholz relays to be carried out on yearly basis.

5. All marshalling boxes to be cleaned from inside at least once in a year. All illumination, space heaters, to be

checked whether they are functioning properly or not. If not, required maintenance action to be taken. All the terminal

connections of control and relay wiring to be checked an tighten at least once in a year.

6. All the relays, alarms and control switches along with their circuit, in R&C panel (Relay and Control Panel) and

RTCC (Remote Tap Changer Control Panel) to be cleaned by appropriate cleaning agent.

7. The pockets for OTI, WTI (Oil Temperature Indicator & Winding Temperature Indicator) on the transformer top

cover to be checked and if required oil to be replenished.

8. The proper function of Pressure Release Device and Buchholz relay must be checked annually. For that, trip

contacts and alarm contacts of the said devices are shorted by a small piece of wire, and observe whether the

concerned relays in remote panel are properly working or not.

9. Insulation resistance and polarization index of transformer must be checked withbattery operated megger of 5 KV

range.

10. Resistive value of earth connection and rizer must be measured annually with clamp on earth resistance meter.

11. DGA or Dissolve Gas Analysis of transformer Oil should be performed, annually for 132 KV transformer, once in 2

years for the transformer below 132 KV transformer and in 2 years interval for the transformer above 132 KV

transformer.

The Action to be taken once in 2 years :

1.The calibration of OTI and WTI must be carried once in two years.

2.Tan & delta; measurement of bushings of transformer also to be done once in two years.

Maintenance of Transformer on Half Yearly Basis

The transformer oil must be checked half yearly basis that means once in 6 months, for dielectric strength, water

content, acidity, sludge content, flash point, DDA, IFT, resistivity for transformer oil.

In case of distribution transformer, as they are operating light load condition all the time of day remaining peak hours ,

so there are no maintenance required.

DIFFERENTIAL PROTECTION:--

Generally Differential protection is provided in the electrical power transformer rated more than 5MVA.

The Differential Protection of Transformer has many advantages over other schemes of protection.

1) The faults occur in the transformer inside the insulating oil can be detected by Buchholz relay. But if any fault

occurs in the transformer but not in oil then it can not be detected by Buchholz relay. Any flash over at the bushings

are not adequately covered by Buchholz relay. Differential relays can detect such type of faults. Moreover Buchholz

relay is provided in transformer for detecting any internal fault in the transformer but Differential Protection scheme

detects the same in more faster way.

2) The differential relays normally response to those faults which occur in side the differential protection zone of

transformer.

Differential Protection Scheme in a Power Transformer

Principle of Differential Protection

Principle of Differential Protection scheme is one simple conceptual technique. The differential relay actually

compares between primary current and secondary current ofpower transformer, if any unbalance found in between

primary and secondary currents the relay will actuate and inter trip both the primary and secondary circuit breaker of

the transformer.

Suppose you have one transformer which has primary rated current Ip and secondarycurrent Is. If you install CT of

ratio Ip/1A at primary side and similarly, CT of ratio Is/1A at secondary side of the transformer. The secondaries of

these both CTs are connected together in such a manner that secondary currents of both CTs will oppose each other.

In other words, the secondaries of both CTs should be connected to same current coil of differential relay in such a

opposite manner that there will be no resultant current in that coil in normal working condition of the transformer. But

if any major fault occurs inside the transformer due to which the normal ratio of the transformer disturbed then the

secondary current of both transformer will not remain the same and one resultant currentwill flow through

the current coil of the differential relay, which will actuate the relay and inter trip both the primary and secondary

circuit breakers. To correct phase shift ofcurrent because of star – delta connection of transformer winding in case of

three phase transformer, the current transformer secondaries should be connected in delta and star as shown here.

At maximum through fault current, the spill output produced by the small percentage unbalance may be substantial.

Therefore, differential protection of transformer should be provided with a proportional bias of an amount which

exceeds in effect the maximum ratio deviation.

Stability on External Earth Fault(E/F) on Delta Side of Star-Delta Power Transformer

If the earthing transformer on the Delta Side is outsides the Zone of protection the Earth Fault(E/F)in the delta

system outside Current Transformer(CT) locations would producecurrent distributions as shown which circulate within

the differential CT secondaries and is kept out of operating coils.

Zig-Zag or inter connected star grounding transformer has normal magnetising impedance of high value but for

E/F, currents flow in windings of the same – core in such a manner that the ampere turn cancel and hence offer lower

impedance.

In cases where the neutral point of three phase system is not accessible like the system connected to the delta

connected side of a electrical power transformer, an artificial neutral point may be created with help of a zigzag

connected earthing transformer.

This is a core type transformer with three limbs. Every phase winding in zigzag connection is divided into two equal

halves. One half of which is wound on one limb and other half is wound on another limb of the core of transformer.

1st half of Red phase winding is wound on the 1st limb of the core and 2nd half of same Red phase is wound on 3rd

limb.

1st half of Yellow phase winding is wound on the 2nd limb of the core and 2nd half of same Yellow phase is wound

on 1st limb.

1st half of Blue phase winding is wound on the 3rd limb of the core and 2nd half of same Blue phase is wound on 2nd

limb.

End point of all three winding ultimately connected together and forms a common neutral point. Now if any fault

occurs at any of the phases in delta connected system, the zero sequence fault current has close path of circulating

through earth as shown in the figure.

In normal condition of the system, the voltage across the winding of the earthing transformer is 1/√3 times of rated

per phase voltage of the system. But when single line to ground fault occurs on any phase of the system, as shown in

the figure, zero sequence component of the earth fault current flows in the earth and returns to the electrical

powersystem by way of earth star point of the earthing transformer. It gets divided equally in all the three phases.

Hence, as shown in the figure, the currents in the two different halves of two windings in the same limb of the core

flow in opposite directions.

And therefore the magnetic flux set up by these two currents will oppose and neutralize each other. As there is no

increase in flux due to fault current, there is no change of dφ/dt means no choking effect occurs to impede the flow of

fault current. So it can be concluded like that, the zigzag type earthing or grounding transformer maintains the rated

supplyvoltage at normal current as well as when a solid single line to ground fault current flows through it.

The rated voltage of an earthing or grounding transformer is the line to line voltage on which it is intended to be used.

Current rating of this transformer is the maximum neutralcurrent in Amperes that the transformer is designed to carry

in fault condition for a specific time. Generally the time interval, for which transformer designed to carry the maximum

fault current through it safely, is taken as 30 second.

Over Fluxing in TransformerAs per present day transformer design practice, the peak rated value of the flux density is kept about 1.7 to 1.8 Tesla,

while the saturation flux density of CRGD steel sheet of core of transformer is of the order of 1.9 to 2 Tesla which

corresponds to about 1.1 times the rated value. If during operation, an electrical power transformer is subjected to

carry rather swallow more than above mentioned flux density as per its design limitations, the transformer is said to

have faced over fluxing problem and consequent bad effects towards its operation and life.

Depending upon the design and saturation flux densities and the thermal time constants of the heated component

parts, a transformer has some over excitation capacity. I.S. specification for electrical power transformer does not

stipulate the short time permissible over excitation, though in a round about way it does indicate that the

maximum over fluxing in transformer shall not exceed 110%.

The flux density in a transformer can be expressed by

B = C V/f, 

where, C = A constant, 

V = Induced voltage, 

f = Frequency.

The magnetic flux density is, therefore, proportional to the quotient of voltage and frequency (V/f). Over fluxing can,

therefore, occur either due to increase in voltage or decrease in-frequency of both.

The probability of over fluxing is relatively high in step-up transformers in Power stations compared to step – down

transformers in Sub-Stations, where voltage and frequency usually remain constant. However, under very abnormal

system condition, over-fluxing trouble can arise in step-down Sub-Station transformers as well.

Effect of Over Fluxing in Transformers

The flux in a transformer, under normal conditions is confined to the core of transformerbecause of its high

permeability compared to the surrounding volume. When the flux density in the increases beyond saturation point, a

substantial amount of flux is diverted to steel structural parts and into the air. At saturation flux density the core steel

will over heat.

Structural steel parts which are nu-laminated and are not designed to carry magnetic fluxwill heat rapidly. Flux flowing

in unplanned air paths may link conducing loops in the windings, loads, tank base at the bottom of the core and

structural parts and the resulting circulating currents in these loops can cause dangerous temperature increase.

Under conditions of excessive over fluxing the heating of the inner portion of the windings may be sufficiently extreme

as the exciting current is rich in harmonies. It is obvious that the levels of loss which occur in the winding at high

excitation cannot be tolerated for long if the damage is to be a voided.

Physical evidences of damage due to over fluxing will very with the degree of over excitation, the time applied and the

particular design of transformer. The Table given below summarizes such physical damage and probable

consequences.

SLComponent

involvedPhysical evidences Consequences

1

Metallic support

and surfaces

structure for core

and coils

Discoloration or metallic parts and

adjacent insulation.Possible

carbonized material in oil.

Evolution of combustible gas.

Contamination of a oil and

surfaces of insulation.

Mechanical weakening of

insulation Loosing of structure.

Mechanical structure

2 WindingsDiscoloration winding insulation

evolution of gas.

Electrical and mechanical

weakling of winding insulation

3 Lead conductors.

Discoloration of conductor

insulation or support, evolution of

gas.

Electrical and mechanical

weakening of insulation,

Mechanical Weakening of

support.

4 Core lamination. Discoloration of insulating material

in contact with core. Discoloration

Electrical weakening of major

insulation (winding to core)

and carbonization of

organic/lamination insulation

Evaluation of gas.

increased interlaminar eddy

loss.

5 Tank Blistering of paintsContamination of oil if paint

inside tank is blistered.

It may be seen that metallic support structures for core and coil, windings, lead conductors, core lamination, tank etc.

may attain sufficient temperature with the evolution of combustible gas in each case due to over fluxing of transformer

and the same gas may be collected in Buchholz Relay with consequent Alarm/Trip depending upon the quantity of

gas collected which again depends upon the duration of time the transformer is subjected to over fluxing. 

Due to over fluxing in transformer its core becomes saturated as such induced voltage in the primary circuit

becomes more or less constant. If the supply voltage to the primary is increased to abnormal high value, there must

be high magnetising current in the primary circuit. Under such magnetic state of condition of transformer core linear

relations between primary and secondary quantities (viz. for voltage and currents) are lost. So there may not be

sufficient and appropriate reflection of this high primary magnetising currentto secondary circuit as such mismatching

of primary currents and secondary currents is likely to occur, causing differential relay to operate as we do not have

overfluxing protection for sub-stn. transformers. 

Stipulated Withstand-Duration of Over Fluxing in Transformers

Over fluxing in transformer has sufficient harmful effect towards its life which has been explained. As overfluxing

protection is not generally provided in step-down transformers of Sub-Station, there must be a stipulated time which

can be allowed matching with the transformer design to withstand such overfluxing without causing appreciable

damage to the transformer and other protections shall be sensitive enough to trip the transformer well within such

stipulated time, if cause of overfluxing is not removed by this time.

It is already mentioned that the flux density ‘B’ in transformer core is proportional to v/f ratio. Power transformers are

designed to withstand (Vn/fn x 1.1) continuously, where Vn is the normal highest r.m.s. voltage and fn is the standard

frequency. Core design is such that higher v/f causes higher core loss and core heating. The capability of a

transformer to withstand higher v/f values i.e. overfluxing effect, is limited to a few minutes as furnished below in the

Table

F = (V/f)/(Vn/fn) 1.1 1.2 1.25 1.3 1.4

Duration of with stand limit (minutes) continuous 2 1 0.5 0

From the table above it may be seen that when over fluxing due to system hazards reaches such that the factor F

attains a values 1.4, the transformer shall be tripped out of service instantaneously otherwise there may be a

permanent damage.

Protection Against Over fluxing (v/f – Protection) in Transformer

The condition arising out of over-fluxing does not call for high speed tripping. Instantaneous operation is undesirable

as this would cause tripping on momentary system disturbances which can be borne safely but the normal condition

must be restored or the transformer must be isolated within one or two minutes at the most.

Flux density is proportional to V/f and it is necessary to detect a ratio of V/f exceeding unity, V and f being expressed

in per unit value of rated quantities. In a typical scheme designed for over fluxing protection, the system voltage as

measured by the voltages transformer is applied to a resistance to product a proportionate current; this current on

being passed through a capacitor, produces a voltage drop which is proportional to the functioning in question i.e. V/f

and hence to flux in the power transformer. This is accompanied with a fixed reference D.C. voltage obtained across

a Zener diode . When the peak A.C. signal exceeds the D.C. reference it triggers a transistor circuit which operates

two electromechanical auxiliary elements. One is initiated after a fixed time delay, the other after an additional time

delay which is adjustable. The over fluxing protection operates when the ratio of the terminal voltage to frequency

exceeds a predetermined setting and resets when the ratio falls below 95 to 98% of the operating ratio. By

adjustment of a potentiometer , the setting is calibrated from 1 to 1.25 times the ratio of rated volts to rated frequency.

The output from the first auxiliary element, which operates after fixed time delay available between 20 to 120 secs.

second output relay operates and performs the tripping function.

It is already pointed out that high V/f occur in Generator Transformers and Unit-Auxiliary Transformers if full exaltation

is applied to generator before full synchronous speed is reached. V/f relay is provided in the

automatic voltage regulator of generator. This relay blocks and prevents increasing excitation current before full

frequency is reached.

When applying V/f relay to step down transformer it is preferable to connect it to the secondary (L.V. said of the

transformer so that change in tap position on the H.V. is automatically taken care of Further the relay should initiate

an Alarm and the corrective operation be done / got done by the operator. On extreme eventuality the transformer

controlling breaker may be allowed to trip.

Tan Delta Test |Loss Angle Test | Dissipation Factor Test

Principle of Tan Delta Test

A pure insulator when is connected across line and earth, it behaves as a capacitor. In an ideal insulator, as the

insulating material which acts as dielectric too, is 100 % pure, theelectric current passing through the insulator, only

have capacitive component. There is no resistive component of the current, flowing from line to earth through

insulator as in ideal insulating material, there is zero percent impurity.

In pure capacitor, the capacitive electric current leads the applied voltage by 90°.

In practice, the insulator cannot be made 100% pure. Also due to ageing of insulator the impurities like, dirt and

moisture enter into it. These impurities provide conductive path to the current. Consequently, leakage electric

current flowing from line earth through insulator has also resistive component.

Hence, it is needless to say that, for good insulator, this resistive component of leakageelectric current is quite low. In

other way the healthiness of an electrical insulator can be determined by ratio of resistive component to capacitive

component. For good insulator this ratio would be quite low. This ratio is commonly known as tanδ or tan delta.

Sometimes it is also referred as dissipation factor.

In the vector diagram above, the system voltage is drawn along x-axis. Conductive electric current i.e. resistive

component of leakage current, IR will also be along x-axis.

As the capacitive component of leakage electric current IC leads system voltage by 90°, it will be drawn along y-axis.

Now, total leakage electric current IL(Ic + IR) makes an angle δ (say) with y-axis.

Now, from the diagram above, it is cleared, the ratio, IR to IC is nothing but tanδ or tan delta.

NB: This δ angle is known as loss angle.

Method of Tan Delta Testing

The cable, winding, current transformer, potential transformer, transformer bushing, on which tan delta

test or dissipation factor test to be conducted, is first isolated from the system. A very low frequency test voltage is

applied across the equipment whose insulation to be tested. First the normal voltage is applied. If the value of tan

delta appears good enough, the applied voltage is raised to 1.5 to 2 times of normal voltage, of the equipment. The

tan delta controller unit takes measurement of tan delta values. A loss angle analyser is connected with tan delta

measuring unit to compare the tan delta values at normal voltage and higher voltages, and analyse the results.

During test it is essential to apply test voltage at very low frequency.

Reason of applying Very Low Frequency

If frequency of applied voltage is high, then capacitive reactance of the insulator becomes low, hence capacitive

component of electric current is high. The resistive component is nearly fixed, it depends upon applied voltage and

conductivity of the insulator. At high frequency as capacitive current, is large, hence, the amplitude of vector sum of

capacitive and resistive components of electric current becomes large too.

Therefore, required apparent power for tan delta test would become high enough which is not practical. So to keep

the power requirement for this dissipation factor test, very low frequency test voltage is required. The frequency

range for tan delta test is generally from 0.1 to 0.01 Hz depending upon size and nature of insulation.

There is another reason for which it is essential to keep the input frequency of the test as low as possible.

As we know,

That means, dissipation factor tanδ ∝ 1 / f.

Hence, at low frequency, the tan delta number is high, the measurement becomes easier.

How to predict the Result of Tan Delta Testing

There are two ways to predict the condition of an insulation system during tan delta or dissipation factor test.

First one is, comparing the results of previous tests to determine, the deterioration of the condition of insulation due

ageing affect.

Second one is, determining the condition of insulation from the value of tanδ, directly. No requirement of comparing

previous results of tan delta test.

If the insulation is perfect, the loss factor will be approximately same for all range of test voltages. But if the insulation

is not good enough, the value of tan delta increases in higher range of test voltage.

From the graph it is clear that, the tan&delta number non linearly increases with increasing test very low frequency

voltage. The increasing tan&delta, means, high resistiveelectric current component, in the insulation. These results

can be compared with the results of previously tested insulators, to take proper decision whether the equipment

would be replaced or not.

External Faults in Power Transformer

External Short – Circuit of Power Transformer

The short – circuit may occurs in two or three phases of electrical power system. The level of fault current is always

high enough. It depends upon the voltage which has been short – circuited and upon the impedance of the circuit up

to the fault point. The copper loss of the fault feeding transformer is abruptly increased. This increasing copper loss

causes internal heating in the transformer. Large fault current also produces severe mechanical stresses in the

transformer. The maximum mechanical stresses occurs during first cycle of symmetrical fault current.

High Voltage Disturbance in Power Transformer

High Voltage Disturbance in Power Transformer are of two kinds,

(1) Transient Surge Voltage

(2) Power Frequency Over Voltage

Transient Surge Voltage

High voltage and high frequency surge may arise in the power system due to any of the following causes,

(a) Arcing ground if neutral point is isolated.

(b) Switching operation of different electrical equipment.

(c) Atmospheric Lightening Impulse.

Whatever may be the causes of surge voltage, it is after all a traveling wave having high and steep wave form and

also having high frequency. This wave travels in the electrical powersystem network, upon reaching in the power

transformer, it causes breakdown the insulation between turns adjacent to line terminal, which may create short

circuit between turns.

Power Frequency Over Voltage

There may be always a chance of system over voltage due to sudden disconnection of large load. Although the

amplitude of this voltage is higher than its normal level but frequency is same as it was in normal condition.

Over voltage in the system causes an increase in stress on the insulation of transformer. As we know that, voltage V

= 4.44Φ.f.T ⇒ V ∝ Φ, increased voltage causes proportionate increase in the working flux. This therefore causes,

increased in iron loss and dis – proportionately large increase in magnetizing current. The increase flux is diverted

from the transformer core to other steel structural parts of the transformer. Core bolts which normally carry little flux,

may be subjected to a large component of flux diverted from saturated region of the core alongside. Under such

condition, the bolt may be rapidly heated up and destroys their own insulation as well as winding insulation.

Under Frequency Effect in Power Transformer

As, voltage V = 4.44Φ.f.T ⇒ V ∝ Φ.f as the number of turns in the winding is fixed.

Therefore, Φ ∝ V/f

From, this equation it is clear that if frequency reduces in a system, the flux in the core increases, the effect are more

or less similar to that of the over voltage.

Internal Faults in Power Transformer

The principle faults which occurs inside a power transformer are categorized as,

(1) Insulation breakdown between winding and earth

(2) Insulation breakdown in between different phases

(3) Insulation breakdown in between adjacent turns i.e. inter – turn fault

(4) Transformer core fault

Internal Earth Faults in Power Transformer

Internal Earth Faults in a Star Connected Winding with Neutral Point Earthed through an Impedance

In this case the fault current is dependent on the value of earthing impedance and is also proportional to the distance

of the fault point from neutral point as the voltage at the point depends upon, the number of winding turns come under

across neutral and fault point. If the distance between fault point and neutral point is more, the number of turns come

under this distance is also more, hence voltage across the neutral point and fault point is high which causes higher

fault current. So, in few words it can be said that, the value of fault current depends on the value of earthing

impedance as well as the distance between the faulty point and neutral point. The fault current also depends up

on leakage reactance of the portion of the winding across the fault point and neutral. But compared to the earthing

impedance,it is very low and it is obviously ignored as it comes in series with comparatively much higher earthing

impedance.

Internal Earth Faults in a Star Connected Winding with Neutral Point Solidly Earthed

In this case, earthing impedance is ideally zero. The fault current is dependent up onleakage reactance of the portion

of winding comes across faulty point and neutral point of transformer. The fault current is also dependent on the

distance between neutral point and fault point in the transformer. As said in previous case the voltage across these

two points depends upon the number of winding turn comes across faulty point and neutral point. So in star

connected winding with neutral point solidly earthed, the fault currentdepends upon two main factors, first the leakage

reactance of the winding comes across faulty point and neutral point and secondly the distance between faulty point

and neutral point. But the leakage reactance of the winding varies in complex manner with position of the fault in the

winding. It is seen that the reactance decreases very rapidly for fault point approaching the neutral and hence the

fault current is highest for the fault near the neutral end. So at this point, the voltage available for fault current is low

and at the same time the reactance opposes the fault current is also low, hence the value of fault current is high

enough. Again at fault point away from the neutral point, the voltage available for fault current is high but at the same

time reactance offered by the winding portion between fault point and neutral point is high. It can be noticed that the

fault current stays a very high level throughout the winding. In other word, the fault current maintain a very high

magnitude irrelevant to the position of the fault on winding.

Internal Phase to Phase Faults in Power Transformer

Phase to phase fault in the transformer are rare. If such a fault does occur, it will give rise to substantial current to

operate instantaneous over current relay on the primary side as well as the differential relay.

Inter Turns Fault in Power Transformer

Power Transformer connected with electrical extra high voltage transmission system, is very likely to be subjected to

high magnitude, steep fronted and high frequency impulsevoltage due to lightening surge on the transmission line.

The voltage stresses between winding turns become so large, it can not sustain the stress and causing insulation

failure between inter – turns in some points. Also LV winding is stressed because of the transferred surge voltage.

Very large number of Power Transformer failure arise from fault between turns. Inter turn fault may also be occurred

due to mechanical forces between turns originated by external short circuit.

Core Fault in Power Transformer

In any portion of the core lamination is damaged, or lamination of the core is bridged by any conducting material

causes sufficient eddy current to flow, hence, this part of the core becomes over heated. Some times, insulation of

bolts (Used for tightening the core lamination together) fails which also permits sufficient eddy current to flow through

the bolt and causing over heating. These insulation failure in lamination and core bolts causes severe local heating.

Although these local heating, causes additional core loss but can not create any noticeable change in input and

output current in the transformer, hence these faults can not be detected by normal electrical protection scheme. This

is desirable to detect the local over heating condition of the transformer core before any major fault occurs. Excessive

over heating leads to breakdown of transformer insulating oil with evolution of gases. These gases are accumulated

in Buchholz relay and actuating Buchholz Alarm.

Sweep Frequency Response Analysis Test | SFRA TestThis is very reliable and sensitive method or tool for condition monitoring of the physical condition of transformer

windings. The winding of transformer may be subjected to mechanical stresses during transportation, heavy short

circuit faults, transient switching impulses and lightening impulses etc. These mechanical stresses may cause

displacement of transformer windings from their position and may also cause deformation of these windings.

Windings collapse in extreme cases, such physical defects eventually lead to insulation failure or dielectric faults in

the windings. 

Sweep Frequency Response Analysis Test or in short SFRA Test can detect efficiently, displacement of transformer

core, deformation and displacement of winding, faulty core grounds, collapse of partial winding, broken or loosen

clamp connections, short circuited turns, open winding conditions etc. 

Principle of SFRA Test

The principle of SFRA is quite simple. As all the electrical equipments theoretically have

some resistance, inductor and some capacitance values hence each of them can be considered as a complex RLC

circuit. The term ‘theoretically’ means some equipment may have very low or zero resistance compared to

their inductor and capacitance values again, some equipments may have very low or zero inductor compared to

their resistance andcapacitance and again some equipments may have very low or zero capacitance compared to

their resistance and inductor but theoretically all of them can be considered as RLC circuit although may be R = 0, or

L = 0 or C = 0. But in most cases the resistance, inductorand capacitance of an equipment have non zero values.

Hence most of the electrical equipments can be considered as RLC circuit hence they response to the sweep

frequencies and produce an unique signature. As in a transformer each winding turn is separated from other by paper

insulation which acts as dielectric and windings themselves have inductor and resistance, a transformer can be

considered as a complicated distributed network of resistance, inductance, and capacitance or in other words a

transformer is a complicated RLC circuit.

Because of that each winding of a transformer exhibits a particular frequency response.

In Sweep Frequency Response Analysis a sinusoidal voltage Vi is applied to one end of a winding and

output voltage Vo is measured at the other end of the winding. Other windings are kept open. 

As the winding is itself an distributed RLC circuit it will behave like RLC filter and gives different output voltages at

different frequencies. That means if we go on increasing the frequency of the input signal without changing

its voltage level we will get different output voltages at different frequencies depending upon the RLC nature of the

winding. If we plot these output voltages against the corresponding frequencies we will get a particular patter for a

particular winding. 

But after transportation, heavy short circuit faults, transient switching impulses and lightening impulses etc, if we do

same Sweep Frequency Response Analysis test and superimpose the present signature with the earlier patterns and

observe some deviation between these tow graphs, we can asses that there is mechanical displacement and

deformation occurred in the winding.

In addition to that, SFRA test also helps us to compare between physical condition of the same winding of different

phases at the same tap position.

It also compares different transformers of the same design.

Analysis

Low frequency response

1) Winding behaves as a simple RL circuit formed by series inductor and resistance of the winding (At low

frequencies capacitance cats as almost open circuit)

2) At low frequency winding inductances are determined by the magnetic circuit of the transformer core.

High frequency response

3) At high frequency winding behaves as RLC circuits

4) Winding exhibits many resonant points 

5) Frequency responses are more sensitive to winding movement.

Different Connection During SFRA Test

SIGNAL APPLIED ACROSS TRANSFORMER

TERMINALSCONDITIONS

HV Red phase to Neutral LV Red Yellow Blue phases are open

HV Yellow phase to Neutral LV Red Yellow Blue phases are open

HV Blue phase to Neutral LV Red Yellow Blue phases are open

HV Red phase to Neutral LV Red Yellow Blue phases are shorted

HV Yellow phase to Neutral LV Red Yellow Blue phases are shorted

HV Blue phase to Neutral LV Red Yellow Blue phases are shorted

LV Red to Yellow phaseHV Red Yellow Blue phases and LV Blue

phase are open

LV Yellow to Blue phaseHV Red Yellow Blue phases and LV Red

phase are open

LV Blue to Red phaseHV Red Yellow Blue phases and LV Yellow

phase are open

Example Data Sheet for SFRA Test Result

Transformer Oil and Winding Temperature Rise Test

Temperature Rise Test for Top Oil of Transformer

1. First the LV winding of the transformer is short circuited.

2. Then one thermometer is placed in a pocket in transformer top cover. Other two thermometers are placed at the inlet

and outlet of the cooler bank respectively.

3. The voltage of such value is applied to the HV winding that power input is equal to no load losses plus load losses

corrected to a reference temperature of 75°C.

4. The total losses are measured by three watt-meters method.

5. During the test, hourly readings of top oil temperature are taken from the thermometer already placed in the pocket of

top cover.

6. Hourly readings of the thermometers placed at inlet and outlet of the cooler bank are also noted to calculate the mean

temperature of the oil.

7. Ambient temperature is measured by means of thermometer placed around the transformer at three or four points

situated at a distance of 1 to 2 meter from and half-way up the cooling surface of the transformer.

8. Temperature rise test for top oil of transformer should be continued until the top oil temperature has reached an

approximate steady value that means testing would be continued until the temperature increment of the top oil

becomes less than 3°C in one hour. This steady value of top oil is determined as final temperature rise of transformer

insulating oil.

9. There is another method of determination of oil temperature. Here the test in allowed to be continued until the top oil

temperature rise does not vary more than 1°C per hour for four consecutive hours. The least reading is taken as final

temperature rise of the oil.

During temperature rise test for top oil of transformer we make the LV winding short circuited and

apply voltageto the HV winding. So for full load rated current flows in the transformer, the

supplyvoltage required will much less than rated transformer voltage. We know that core loss of a

transformer depends upon voltage. So there will not be any considerable core loss occurs in the transformer

during test. But for getting actual temperature rise of the oil in a transformer, we have to compensate the

lack of core losses by additional copper loss in the transformer. For supplying this total losses, transformer

draws current from the source much more than its rated value for transformer.

Temperature rise limits of transformer when it is oil immersed, given in the table below

 

TEMPERATURE RISE

LIMIT

FOR AIR AS 

COOLING MEDIUM

TEMPERATURE RISE

LIMIT

FOR WATER AS

COOLING MEDIUM

CONDITION

WINDING55oC 60oC When oil circulation is

natural

60oC 65oC

When oil circulation is forced

TOP OIL

50oC 55oC

When transformer is sealed

&

equipped with conservator

tank

45oC 50oC

When transformer is neither

sealed 

nor equipped with

conservator tank

NB: These temperature rises limits mentioned in the above table are the temperature rise above the temperature of

cooling medium. That means these are the difference between winding or oil temperature and temperature of cooling

air or water.

Winding Temperature Rise Test on Transformer

1. After completion of temperature rise test for top oil of transformer the current is reduced to its rated value for

transformer and is maintained for one hour.

2. After one hour the supply is switch off and short circuit and supply connection to the HV side and short circuit

connection to the LV side are opened.

3. But, the fans and pumps are kept running (if any).

4. Then resistance of the windings are measured quickly.

5. But there is always a minimum 3 to 4 minutes time gap between first measurement of resistance and the instant of

switching off the transformer, which can not be avoided.

6. Then the resistances are measured at the same 3 to 4 minutes time intervals over a period of 15 minutes.

7. Graph of hot resistance versus time is plotted, from which winding resistance (R2) at the instant of shut down can be

extrapolated.

8. From this value, θ2, the winding temperature at the instant of shut down can be determined by the formula given

below-

Where, R1 is the cold resistance of the winding at temperature t1.

For determining winding temperature rise we have to apply the above discussed indirect method. That

means hot winding resistance is measured and determined first and then from that value we have to

calculate the winding temperature rise, by applying resistance temperature relation formula. This is because

unlike oil the winding of transformer is not accessible for external temperature measurement.

Insulation Dielectric Test of TransformerThe dielectric test of transformer is generally performed in two different steps, likewise, separate

source voltage withstand test and induced voltage withstand test of transformer, which we have discussed one by

one below.

Separate Source Voltage Withstand Test of Transformer

This dielectric test is intended to check the the ability of main insulation to earth and between winding.

Procedure

1. All three line terminals of the winding to be tested are connected together.

2. Other winding terminals which are not under test and also tank of the transformer should be connected to earth.

3. Then a single-phase power frequency voltage of shape approximately sinusoidal is applied for 60 seconds to the

terminals of the winding under test.

4. The test shall be performed on all the windings one by one.

5. The test is successful if no break down in the dielectric of the insulation occurs during test.

In this transformer testing, the peak value of voltage is measured, that is why the capacitor voltage  divider with digital

peak voltmeter is employed as shown in the diagram above. The peal value multiplied by 0.707 (1/√2) is the test

voltage.

The values of test voltage for different fully insulated winding are furnished below in the table.

NOMINAL SYSTEM

VOLTAGE RATING 

FOR EQUIPMENT

HIGHEST SYSTEM

VOLTAGE RATING 

FOR EQUIPMENT

RATED SHORT DURATION 

POWER FREQUENCY

WITHSTAND

VOLTAGE

415V 1.1 KV 3 KV

11 KV 12 KV 28 KV

33 KV 36 KV 70 KV

132 KV 145 KV 230 / 275 KV

220 KV 245 KV 360 / 395 KV

400 KV 420 KV 570 / 630 KV

WINDING WITH GRADED INSULATION, WHICH HAS NEUTRAL INTENDED FOR DIRECT

EARTHING, 

IS TESTED AT 38KV

Induced Voltage Test of Transformer

The induced voltage test of transformer is intended to check the inter turn and line end insulation as well as main

insulation to earth and between windings-

1. Keep the primary winding of transformer open circuited.

2. Apply three phase voltage to the secondary winding. The applied voltage should be twice of rated voltage of

secondary winding in magnitude and frequency.

3. The duration of the test shall be 60 second.

4. The test shall start with a voltage lower than 1/3 the full test voltage, and it shall be quickly increased up to desired

value.

The test is successful if no break down occurs at full test voltage during test.

Vector Group Test of Transformer

The vector group of transformer is an essential property for successful parallel operation of transformers. Hence

every electrical power transformer must undergo through vector group test of transformer at factory site for

ensuring the customer specified vector group of transformer.

Transformer Winding Resistance MeasurementIn the factory, it helps in determining the following :

Calculation of the I2R losses in transformer.

Calculation of winding temperature at the end of temperature rise test of

transformer.

As a benchmark for assessing possible damages in the field.

It is done at site in order to check for abnormalities due to loose connections,

broken strands of conductor, high contact resistance in tap changers,

high voltage leads and bushings.

Procedure of Transformer Winding Resistance Measurement

For star connected winding, the resistance shall be measured between the line

and neutral terminal.

For star connected auto-transformers the resistance of the HV side is measured

between HV terminal and IV terminal, then between IV terminal and the neutral.

For delta connected windings, measurement of winding resistance shall be

done between pairs of line terminals. As in delta connection the resistance of

individual winding can not be measured separately, the resistance per winding

shall be calculated as per the following formula:

Resistance per winding = 1.5 x Measured value

The resistance is measured at ambient temperature and then converted to resistance at 75˚C for all practical purposes of comparison with specified design values, previous results and diagnostics.

Winding Resistance at standard temperature of 75° C

Rt = Winding resistance at temperature t.t = Winding temperature.

Generally transformer windings are immersed in insulation liquid and covered with

paper insulation, hence it is impossible to measure the actual winding temperature

in a de-energizing transformer at time of transformer

winding resistance measurement. An approximation is developed to calculate

temperature of winding at that condition, as follows

Temperature of winding = Average temperature of insulating oil.

(Average temperature of insulating oil should be taken 3 to 8 hours after de-energizing transformer and when the difference between top & bottom oil temperatures becomes less than 5° C.)

The resistance can be measured by simple voltmeter ammeter method, Kelvin

Bridge meter or automatic winding resistance measurement kit. (ohm meter,

preferably 25 Amps kit)

Caution for voltmeter ammeter method: Current shall not exceed 15% of the

rated currentof the winding. Large values may cause inaccuracy by heating the

winding and thereby changing its temperature and resistance.

NB: – Measurement of winding resistance of transformer shall be carried out

at each tap. 

Current Voltage Method of Measurement of Winding Resistance

The transformer winding resistances can be measured by current voltage method.

In this method of measurement of winding resistance, the test current is injected to

the winding and corresponding voltage drop across the winding is measured.

By applying simple Ohm’s law i.e. Rx = V ⁄ I, one can easily determine the value

of resistance.

Procedure of Current Voltage Method of

Measurement of Winding Resistance

Before measurement the transformer should be kept in OFF condition without

excitation at least for 3 to 4 hours. During this time the winding temperature will

become equal to its oil temperature.

Measurement is done with D.C.

To minimize observation errors, polarity of the core magnetization shall be kept

constant during all resistance readings.

Voltmeter leads shall be independent of the current leads to protect it from high

voltages which may occur during switching on and off the current circuit.

The readings shall be taken after the current and voltage have reached steady

state values. In some cases this may take several minutes depending upon the

winding impedance.

The test current shall not exceed 15% of the rated current of the winding. Large

values may cause inaccuracy by heating the winding and thereby changing

itsresistance.

For expressing resistance, the corresponding temperature of the winding at the

time of measurement must be mentioned along with resistance value. As we said

earlier that after remaining in switch off condition for 3 to 4 hours, the winding

temperature would become equal to oil temperature. The oil temperature at the

time of testing is taken as the average of top oil and bottom oil temperatures of

transformer.

For star connected three phase winding, the resistance per phase would be half of

measured resistance between two line terminals of the transformer.

For delta connected three phase winding, the resistance per phase would be 0.67

times of measured resistance between two line terminals of the transformer.

This current voltage method of measurement of winding resistance of

transformer should be repeated for each pair of line terminals of winding at every

tap position.

 

Bridge Method of Measurement of Winding Resistance

The main principle of bridge method is based on comparing an

unknown resistance with a knownresistance. When currents flowing through the

arms of bridge circuit become balanced, the reading of galvanometer shows zero

deflection that means at balanced condition no current will flow through the

galvanometer. Very small value ofresistance ( in milli-ohms range) can be

accurately measured by Kelvin bridge method whereas for higher value

Wheatstone bridge method of resistance measurement is applied. In bridge

method of measurement of winding resistance, the errors is minimized.

The resistance measured by Kelvin bridge,

All other steps to be taken during transformer winding resistance measurement in these methods are similar to that of current voltage method of measurement of windingresistance of transformer, except the measuring technique of resistance.

The resistance measured by Wheatstone bridge,

The phase sequence or the order in which the phases reach their maximum positive voltages, must be identical for

two paralleled transformers. Otherwise, during the cycle, each pair of phases will be short circuited.

The several secondary connections are available in respect of various primary three phase connection in a the three

phase transformer. So for same primary applied three phasevoltage there may be different three phase secondary

voltages with various magnitudes and phases for different internal connection of the transformer.

Let’s have a discussion in detail by example for better understanding.

We know that, the primary and secondary coils on any one limb have induced emfs that are in time-phase. Let’s

consider two transformers of same number primary turns and the primary windings are connected in star. The

secondary number of turns per phase in both transformers are also same. But the first transformer has star

connected secondary and other transformer has delta connected secondary. If same voltages are applied in primary

of both transformers, the secondary induced emf in each phase will be in same time-phase with that of respective

primary phase, as because the the primary and secondary coils of same phase are wound on the same limb in

the core of transformer. In first transformer, as the secondary is star connected, the secondary line voltage is √3

times of induced voltage per secondary phase coil. But in case of second transformer, where secondary is delta

connected, the line voltage is equal to induced voltage per secondary phase coil. If we go through the vector diagram

of secondary line voltages of both transformer, we will easily find that there will be a clear 30o angular difference

between the line voltages of these transformers. Now, if we try to run these transformers in parallel then there will be

a circulating current flows between the transformers as because there is a phase angle difference between their

secondary line voltages. This phase difference can not be compensated. Thus two sets of connections giving

secondary voltages with a phase displacement can not be intended for parallel operation of transformers.

The following table gives the connections for which from the view point of phase sequence and angular divergences,

transformer can be operated parallel. According to their vector relation, all three phase transformers are divided into

different vector group of transformer. All electrical power transformers of a particular vector group can easily be

operated in parallel if they fulfill other condition for parallel operation of transformers.

GRO

UPCONNECTION CONNECTION

0

(0O)

YY0 DD0

1

( 30O)

YD1 DY1

6

( 18

0O)

YY6 DD6

11

( –

YD11 DY11

30O)

Procedure of Vector Group Test of Transformer

Let’s have a YNd11 transformer.

1. Connect neutral point of star connected winding with earth.

2. Join 1U of HV and 2W of LV together.

3. Apply 415 V, three phase supply to HV terminals.

4. Measure voltages between terminals 2U-1N, 2V-1N, 2W-1N, that means voltages between each LV terminal and HV

neutral.

5. Also measure voltages between terminals 2V-1V, 2W-1W and 2V-1W.

For YNd11 transformer, we will find,

2U-1N > 2V-1N > 2W-1N

2V-1W > 2V-1V or 2W-1W .

The vector group test of transformer for other group can also be done in similar way.

TT