Report On Industrial Tour At Khulshi Grid Sub-Station

Introduction: The course Power System Protection covers almost everything related to protection system in power system including standard lead and device numbers, mode of connections at terminal strips, color codes in multi-core cables, Dos and Don’ts in execution. It also covers principles of various power system protection relays and schemes including special power system protection schemes like differential relays, restricted earth fault protection, directional relays and distance relays etc. The details of transformer protection, generator protection, transmission line protection & protection of capacitor banks are also given. It covers almost everything about protection of power system.

The switchgear testing, instrument transformers like current transformer testing voltage or potential transformer testing and associated protection relay are explained in detail. The close and trip, indication and alarm circuits different of circuit breakers are also included and explain.

Objective of Power System Protection:The objective of power system protection is to isolate a faulty section of electrical power system from rest of the live system so that the rest portion can function satisfactorily without any severer damage due to fault current.

Actually circuit breaker isolates the faulty system from rest of the healthy system and this circuit breakers automatically open during fault condition due to its trip signal comes from protection relay. The main philosophy about protection is that no protection of power system can prevent the flow of fault current through the system, it only can prevent the continuation of flowing of fault current by quickly disconnect the short circuit path from the system.

History: Power Grid Company of Bangladesh Ltd. (PGCB) was formed under the restructuring process of Power Sector in Bangladesh with the objective of bringing about commercial environment including increase in efficiency, establishment of accountability and dynamism in accomplishing its objectives. PGCB was incorporated in November 1996 with an authorized capital of Tk.10 billion. It was entrusted with the responsibility to own the national power grid to operate and expand the same with efficiency. Pursuant to Government decision to transfer transmission assets to PGCB from Bangladesh Power Development Board (BPDB) and Dhaka Electric Supply Authority (DESA), PGCB completed taking over of all the transmission assets on 31.12.2002. PGCB expanded its network and capacity manifold and operating those efficiently and effectively.A public limited company. Incorporated through sponsorship of chairman, BPDB and its six members.

76.25% ownership with BPDB & 23.75% with general public.

Its Head Office is at Institution of Engineers of Bangladesh Bhaban (New), the 3rd and 4th floor, 8/A

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Ramna,Dhaka-1000,Bangladesh.To plan, promote, develop, operate and maintain an integrated and efficient power transmission system network in all its aspects including planning, investigation, research, design and engineering, preparation of preliminary feasibility and detailed project reports, construction operation and maintenance of transmission lines, substations, load dispatch centers and communication facilities and appurtenant works, co-ordination of integrated operation of regional, national and international grid systems, providing consultancy services in power systems field, execution of turnkey jobs for other utilities / organization, wheeling of power, purchase and sale of power.

Brief on Substation:

A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels.

Substations may be owned and operated by an electrical utility, or may be owned by a large industrial or commercial customer. Generally substations are unattended, relying on SCADA for remote supervision and control.

A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages. The word substation comes from the days before the distribution system became a grid. As central generation stations became larger, smaller generating plants were converted to distribution stations, receiving their energy supply from a larger plant instead of using their own generators. The first substations were connected to only one power station, where the generators were housed, and were subsidiaries of that power station.

Types of Substation:According to service requirements substation may be classified into-

1. Transformer Substation2. Switching Substation3. Power factor correction Substation4. Frequency changer Substation5. Converting Substation6. Industrial Substation

According to constructional features the substations are classified as-

1. Indoor Substation2. Outdoor Substation3. Underground Substation4. Pole mounted Substation

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KHULSI GRID SUB-STATION EQUIPMENTS & ITS FUNCTIONS

Lightening Arrester:Lightening arrestors are the instrument that are used in the incoming feeders so that to

prevent the high voltage entering the main station. This high voltage is very dangerous to the

instruments used in the substation. Even the instruments are very costly, so to prevent any damage

lightening arrestors are used. The lightening arrestors do not let the lightening to fall on the station. If

some lightening occurs the arrestors pull the lightening and ground it to the earth. In any substation

the main important is of protection which is firstly done by these lightening arrestors. The lightening

arrestors are grounded to the earth so that it can pull the lightening to the ground. The lightening

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arrestor works with an angle of 30° to 45° making a cone.

Wave Trap:Wave trap is an instrument using for tripping of the wave. The function of this trap is that it

traps the unwanted waves. Its function is of trapping wave. Its shape is like a drum. It is connected to

the main incoming feeder so that it can trap the waves which may be dangerous to the instruments

here in the substation.

C.V.T:A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down

extra high voltage signals and provide low voltage signals either for measurement or to operate a

protective relay. In its most basic form the device consists of three parts: two capacitors across which

the voltage signal is split, an inductive element used to tune the device to the supply frequency and a

transformer used to isolate and further step-down the voltage for the instrumentation or protective

relay. The device has at least four terminals, a high-voltage terminal for connection to the high

voltage signal, a ground terminal and at least one set of secondary terminals for connection to the

instrumentation or protective relay. CVTs are typically single-phase devices used for measuring

voltages in excess of one hundred kilovolts where the use of voltage transformers would be

uneconomical. In practice the first capacitor, C1, is often replaced by a stack of capacitors connected

in series. This results in a large voltage drop across the stack of capacitors that replaced the first

capacitor and a comparatively small voltage drop across the second capacitor, C2, and hence the

secondary terminals.

Instrument Transformer:Instrument transformers are used to step-down the current or voltage to measurable values.

They provide standardized, useable levels of current or voltage in a variety of power monitoring and

measurement applications. Both current and voltage instrument transformers are designed to have

predictable characteristics on overloads. Proper operation of over-current protection relays requires

that current transformers provide a predictable transformation ratio even during a short circuit.

These are further classified into two types which are discussed below.

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a. Current Transformers

b. Potential Transformers

Current Transformer:Current transformers are basically used to take the readings of the currents entering the

substation. This transformer steps down the current from 800 amps to 1 amp. This is done because

we have no instrument for measuring of such a large current. The main use of this transformer is

a. Distance Protection

b. Backup Protection

c. Measurement

A current transformer is defined as an instrument transformer in which the secondary current

is substantially proportional to the primary current (under normal conditions of operation) and differs

in phase from it by an angle which is approximately zero for an appropriate direction of the

connections. This highlights the accuracy requirement of the current transformer but also important

is the isolating function, which means no matter what the system voltage the secondary circuit need

to be insulated only for a low voltage.

The current transformer works on the principle of variable flux. In the ideal current

transformer, secondary current would be exactly equal (when multiplied by the turns ratio) and

opposite to the primary current. But, as in the voltage transformer, some of the primary current or the

primary ampere-turns are utilized for magnetizing the core, thus leaving less than the actual primary

ampere turns to be transformed into the secondary ampere-turns. This naturally introduces an error in

the transformation. The error is classified into current ratio error and the phase error.

Potential Transformer:There are two potential transformers used in the bus connected both side of the bus. The

potential transformer uses a bus isolator to protect itself. The main use of this transformer is to

measure the voltage through the bus. This is done so as to get the detail information of the voltage

passing through the bus to the instrument. There are two main parts in it;

a. Measurement

b. Protection

The standards define a voltage transformer as one in which the secondary voltage is

substantially proportional to the primary voltage and differs in phase from it by an angle which is

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approximately equal to zero for an appropriate direction of the connections. This in essence means

that the voltage transformer has to be as close as possible to the ideal transformer.

In an ideal transformer, the secondary voltage vector is exactly opposite and equal to the

primary voltage vector when multiplied by the turn’s ratio.

In a practical transformer, errors are introduced because some current is drawn for the

magnetization of the core and because of drops in the primary and secondary windings due to

leakage reactance and winding resistance. One can thus talk of a voltage error which is the amount

by which the voltage is less than the applied primary voltage and the phase error which is the phase

angle by which the reversed secondary voltage vector is displaced from the primary voltage vector.

Bus Bar:The bus is a line in which the incoming feeders come into and get into the instruments for

further step up or step down. The first bus is used for putting the incoming feeders in la single line.

There may be double line in the bus so that if any fault occurs in the one the other can still have the

current and the supply will not stop. The two lines in the bus are separated by a little distance by a

conductor having a connector between them. This is so that one can work at a time and the other

works only if the first is having any fault.

A bus bar in electrical power distribution refers to thick strips of copper or aluminum that

conduct electricity within a switchboard, distribution board, substation, or other electrical apparatus.

The size of the bus bar is important in determining the maximum amount of current that can be

safely carried. Bus bars are typically either flat strips or hollow tubes as these shapes allow heat to

dissipate more efficiently due to their high surface area to cross sectional area ratio. The skin effect

makes 50-60 Hz AC bus bars more than about 8 mm (1/3 in) thick inefficient, so hollow or flat

shapes are prevalent in higher current applications. A hollow section has higher stiffness than a solid

rod of equivalent current carrying capacity, which allows a greater span between bus bar supports in

outdoor switchyards. A bus bar may either be supported on insulators or else insulation may

completely surround it. Bus bars are protected from accidental contact either by a metal enclosure or

by elevation out of normal reach.

Neutral bus bars may also be insulated. Earth bus bars are typically bolted directly onto any

metal chassis of their enclosure. Bus bars may be enclosed in a metal housing, in the form of bus

duct or bus way, segregated-phase bus, or isolated-phase bus.

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Circuit Breaker:The circuit breakers are used to break the circuit if any fault occurs in any of the instrument.

These circuit breaker breaks for a fault which can damage other instrument in the station. For any

unwanted fault over the station we need to break the line current. This is only done automatically by

the circuit breaker. There are mainly two types of circuit breakers used for any substations. They are

a. SF6 circuit breakers

b. Spring circuit breakers.

c. Vacuum circuit breakers.

The use of SF6 circuit breaker is mainly in the substations which are having high input kv

input, say above 220kv and more. The gas is put inside the circuit breaker by force i.e. under high

pressure. When if the gas gets decreases there is a motor connected to the circuit breaker. The motor

starts operating if the gas went lower than 20.8 bar. There is a meter connected to the breaker so that

it can be manually seen if the gas goes low. The circuit breaker uses the SF6 gas to reduce the torque

produce in it due to any fault in the line. The circuit breaker has a direct link with the instruments in

the station, when any fault occur alarm bell rings.

The spring type of circuit breakers is used for small KV stations. The spring here reduces the

torque produced so that the breaker can function again. The spring type is used for step down side of

132kv to 33kv also in 33kv to 11kv and so on. They are only used in low distribution side.

VCB, vacuum is used as the arc quenching medium. Since vacuum offers the highest

insulating strength, it has far superior arc quenching properties than any other medium. VCB are

being employed for outdoor application ranging from 22kv to 66kv.

Transformer:There are three transformers in the incoming feeders so that the three lines are step down at

the same time. In case of a 220KV or more KV line station auto transformers are used. While in case

of lower KV line such as less than 132KV line double winding transformers are used.

The transformer is transported on trailor to substation site and as far as possible directly

unloaded on the plinth. Transformer tanks up to 25 MVA capacity are generally oil filled, and those

of higher capacity are transported with N2 gas filled in them +ve pressure of N2 is maintained in

transformer tank to avoid the ingress of moisture. This pressure should be maintained during storage,

if necessary by filling N2 Bushings - generally transported in wooden cases in horizontal position

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and should be stored in that position. There being more of fragile material, care should be taken

while handling them. Radiators – These should be stored with ends duly blanked with gaskets and

end plates to avoid in gross of moisture, dust, and any foreign materials inside. The care should be

taken to protect the fins of radiators while unloading and storage to avoid further oil leakages. The

radiators should be stored on raised ground keeping the fins intact.

Oil Piping. The Oil piping should also be blanked at the ends with gasket and blanking plates to

avoid in gross of moisture, dust, and foreign All other accessories like temperature meters, oil flow

indicators, PRVs, buchholz relay; oil surge relays; gasket ‘ O ‘ rings etc. should be properly packed

and stored indoor in store shed. Oil is received in sealed oil barrels. The oil barrels should be stored

in horizontal position with the lids on either side in horizontal position to maintain oil pressure on

them from inside and subsequently avoiding moisture and water ingress into oil. The transformers

are received on site with loose accessories hence the materials should be checked as per bills of

materials.

Tap changing:Off-circuit designs (NLTC or DETC)

In low power, low voltage transformers, the tap point can take the form of a connection terminal, requiring a power lead to be disconnected by hand and connected to the new terminal. Alternatively, the process may be assisted by means of a rotary or slider switch.

Since the different tap points are at different voltages, the two connections cannot be made simultaneously, as this would short-circuit a number of turns in the winding and produce excessive circulating current. Consequently, the power to the device must be interrupted during the switchover event. Off-circuit or de-energized tap changing (DETC) is sometimes employed in high voltage transformer designs, although for regular use, it is only applicable to installations in which the loss of supply can be tolerated. In power distribution networks, transformers commonly include an off-circuit tap changer on the primary winding to accommodate system variations within a narrow band around the nominal rating. The tap changer will often be set just once, at the time of installation, although it may be changed later during a scheduled outage to accommodate a long-term change in the system voltage profile.

On-load designs (OLTC)

Also called on circuit tap changer or On Load Tap Changer (OLTC). For many power transformer applications, a supply interruption during a tap change is unacceptable, and the transformer is often fitted with a more expensive and complex on-load tap-changing (OLTC, sometimes LTC) mechanism. On-load tap changers may be generally classified as either mechanical, electronically assisted, or fully electronic.

Isolator: of 15

The use of this isolator is to protect the transformer and the other instrument in the line. The isolator

isolates the extra voltage to the ground and thus any extra voltage cannot enter the line. Thus an isolator is

used after the bus also for protection.

Control and Relay Panel:The control and relay panel is of cubical construction suitable for floor mounting. All

protective, indicating and control elements are mounted on the front panel for ease of operation and

control. The hinged rear door will provide access to all the internal components to facilitate easy

inspection and maintenance. Provision is made for terminating incoming cables at the bottom of the

panels by providing separate line-up terminal blocks. For cable entry provision is made both from

top and bottom. The control and relay panel accepts CT, PT aux 230 AC and 220V/10V DC

connections at respective designated terminal points. 220V/10V DC supply is used for control supply

of all internal relays and timers and also for energizing closing and tripping coils of the breakers.

230V AC station auxiliary supply is used for internal illumination lamp of the panel and the space

heater. Protective HRC fuse are provided with in the panel for P.T secondary. Aux AC and battery

supplies. Each Capacitor Bank is controlled by breaker and provided with a line ammeter with

selector switch for 3 phase system & over current relay (2 phases and 1 Earth fault for 3 ph system).

Under voltage and over voltage relays. Neutral Current Unbalance Relays are for both Alarm and

Trip facilities breaker control switch with local/remote selector switch, master trip relay and trip

alarms acknowledge and reset facilities.

Protective Relaying:Protective relays are used to detect defective lines or apparatus and to initiate the operation of

circuit interrupting devices to isolate the defective equipment. Relays are also used to detect

abnormal or undesirable operating conditions other than those caused by defective equipment and

either operate an alarm or initiate operation of circuit interrupting devices. Protective relays protect

the electrical system by causing the defective apparatus or lines to be disconnected to minimize

damage and maintain service continuity to the rest of the system. There are different types of relays.

i. Numerical relay

ii. Distance relay

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iii. Differential relay

iv. Directional over current relay

i. Numerical Relay

The over current relay responds to a magnitude of current above a specified value. There are

four basic types of construction: They are plunger, rotating disc, static, and microprocessor type. In

the plunger type, a plunger is moved by magnetic attraction when the current exceeds a specified

value. In the rotating induction-disc type, which is a motor, the disc rotates by electromagnetic

induction when the current exceeds a specified value.

Static types convert the current to a proportional D.C mill volt signal and apply it to a level

detector with voltage or contact output. Such relays can be designed to have various current-versus-

time operating characteristics. In a special type of rotating induction-disc relay, called the voltage

restrained over current relay. The magnitude of voltage restrains the operation of the disc until the

magnitude of the voltage drops below a threshold value. Static over current relays are equipped with

multiple curve characteristics and can duplicate almost any shape of electromechanical relay curve.

Microprocessor relays convert the current to a digital signal. The digital signal can then be compared

to the setting values input into the relay. With the microprocessor relay, various curves or multiple

time-delay settings can be input to set the relay operation. Some relays allow the user to define the

curve with points or calculations to determine the output characteristics.

ii. Distance Relay

The distance relay responds to a combination of both voltage and current. The voltage

restrains operation, and the fault current causes operation that has the overall effect of measuring

impedance. The relay operates instantaneously (within a few cycles) on a 60-cycle basis for values of

impedance below the set value. When time delay is required, the relays energizes a separate time-

delay relay or function with the contacts or output of this time-delay relay or function performing the

desired output functions. The relay operates on the magnitude of impedance measured by the

combination of restraint voltage and the operating current passing through it according to the settings

applied to the relay. When the impedance is such that the impedance point is within the impedance

characteristic circle, the relay will trip. The relay is inherently directional. The line impedance

typically corresponds to the diameter of the circle with the reach of the relay being the diameter of

the circle.

iii. Differential Relay

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The differential relay is a current-operated relay that responds to the difference between two

or more device currents above a set value. The relay works on the basis of the differential principle

that what goes into the device has to come out .If the current does not add to zero, the error current

flows to cause the relay to operate and trip the circuit.

The differential relay is used to provide internal fault protection to equipment such as

transformers, generators, and buses. Relays are designed to permit differences in the input currents as

a result of current transformer mismatch and applications where the input currents come from

different system voltages, such as transformers. A current differential relay provides restraint coils

on the incoming current circuits. The restraint coils in combination with the operating coil provide an

operation curve, above which the relay will operate. Differential relays are often used with a lockout

relay to trip all power sources to the device and prevent the device from being automatically or

remotely reenergized. These relays are very sensitive. The operation of the device usually means

major problems with the protected equipment and the likely failure in re-energizing the equipment.

iv. Directional Over current Relay

A directional over current relay operates only for excessive current flow in a given direction.

Directional over current relays are available in electromechanical, static, and microprocessor

constructions. An electromechanical overcorrect relay is made directional by adding a directional

unit that prevents the over current relay from operating until the directional unit has operated. The

directional unit responds to the product of the magnitude of current, voltage, and the phase angle

between them or to the product of two currents and the phase angle between them. The value of this

product necessary to provide operation of the directional unit is small, so that it will not limit the

sensitivity of the relay (such as an over current relay that it controls). In most cases, the directional

element is mounted inside the same case as the relay it controls. For example, an over current relay

and a directional element are mounted in the same case, and the combination is called a directional

over current relay. Microprocessor relays often provide a choice as to the polarizing method that can

be used in providing the direction of fault, such as applying residual current or voltage or negative

sequence current or voltage polarizing functions to the relay.

DC Power Supply:I. DC Battery and Charger:

All but the smallest substations include auxiliary power supplies. AC power is required for

substation building small power, lighting, heating and ventilation, some communications equipment,

switchgear operating mechanisms, anti-condensation heaters and motors. DC power is used to feed

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essential services such as circuit breaker trip coils and associated relays, supervisory control and data

acquisition (SCADA) and communications equipment. This describes how these auxiliary supplies

are derived and explains how to specify such equipment. It has Single 100% battery and 100%

charger, Low capital cost, No standby DC System outage for maintenance. Need to isolate

battery/charger combination from load under boost charge conditions in order to prevent high boost

voltages.

II. Battery and Charger configurations:

Capital cost and reliability objectives must first be considered before defining the battery and

battery charger combination to be used for a specific installation.

III. 400V DC Battery:

Make: Exide

Capacity: 300 AH at 27°

No. of Cells: 110 No.

Date of installation: 06/2001

Make: Universal,

Sr. No. : BC 1020/82

Date of manufacturing: 4/2000

Input Rating: Voltage: 415 V + 10 %

Output Rating: Float: 220 V, (5-10) Amp

Boost: 180 V, 30Amp

Capacitor Bank:The demand of active power is expressing Kilo watt (kW) or megawatt (mw). This power should be

supplied from electrical generating station. All the arrangements in electrical pomes system are done

to meet up this basic requirement. Although in alternating power system, reactive power always

comes in to picture. This reactive power is expressed in Kilo VAR or Mega VAR. The demand of

this reactive power is mainly originated from inductive load connected to the system. These

inductive loads are generally electromagnetic circuit of electric motors, electrical transformers,

inductance of transmission and distribution networks, induction furnaces, fluorescent lightings etc.

This reactive power should be properly compensated otherwise, the ratio of actual power consumed

by the load, to the total power i.e. vector sum of active and reactive power, of the system becomes

quite less. This ratio is alternatively known as electrical power factor, and fewer ratios indicates poor

power factor of the system. If the power factor of the system is poor, the ampere burden of the

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transmission, distribution network, transformers, alternators and other equipment’s connected to the

system, becomes high for required active power. And hence reactive power compensation becomes

so important. This is commonly done by capacitor bank.

Functions of Associated System in Khulshi SubstationFunctions of Associated System in Khulshi Substation is as shown below in table-1

Table-1 Functions of Associated System in Substation

Sr

.

System Function

1. Substation Earthing system

- Earth mat

- Earthing spikes

- Earthing risers

To provide an earth mat for connecting neutral points, equipment

body, support structures to earth. For safety of personnel and for

enabling earth fault protection. To provide the path for discharging the

earth currents from neutrals, faults, Surge Arresters, overheads

shielding wires etc. with safe step-potential and touch potential.

2. Overhead earth wire

shielding or Lightning

masts.

To protect the outdoor substation equipment from lightning strokes.

3. Illumination system

(lighting)

- for switchyard

- buildings

- roads etc.

To provide proper illumination to substation yard.

4. Protection system

- protection relay panels

- control cables

- circuit breakers

- CTs, VTs etc.

To provide alarm or automatic tripping of faulty part from healthy

part and also to minimize damage to faulty equipment and associated

system.

5. Control cable For Protective circuits, control circuits, metering circuits,

communication circuits

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6. Power cable To provide supply path to various auxiliary equipment and machines.

7. PLCC system

power line carrier

communication system

For communication, telemetry, tele-control, power line carrier

protection etc.

8. Telephone, telex,

microwave, OPF

For internal and external communication

9. Auxiliary standby power

system

For supplying starting power, standby power for auxiliaries.

10

.

Fire Fighting system

- Sensors, detection system

- water spray system

- fire port, panels, alarm

System.

- water tank and spray

system

To sense the occurrence of fire by sensors and to initiate water spray,

to disconnect power supply to affected region to pinpoint location of

fire by indication in control room.

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CnclusionThe Electrical and Electronic Engineering Department, organized an industrial visit to PGCB sub-station Khulshi branch (132/33/11 KV) was very informative. From this visit, we got the information and practical knowledge about Power Distribution and Transmission. This visit was extremely beneficial in familiarizing the different equipments and working of a substation. We got the idea how to read the single line diagram of power substation using different symbols used in diagram. We cleared out practical knowledge of transformer as how it step down voltage 132 KV to 33 KV. We understand the reason behind using stones/gravel in the substation which was to reduce the step potential and touch potential when operators work on switch yard and eliminate the growth of small weeds and plants inside the switch yard. We hope that this visit will help us in our future practical life and bring a positive change in our thinking and practical behavior regarding Education and specially Engineering.

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Report On Industrial Tour At Khulshi Grid Sub-Station

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Transcript of Report On Industrial Tour At Khulshi Grid Sub-Station