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INTRODUCTION

A substation is a part of an electrical generation,

transmission and distribution system. Substation transforms

voltage from high to low or reverse or any of other important

functions. Electric power may flow through several substations

between generating plant and consumer, and its voltage may

change in several steps.

A substation has a step up transformer increases the

voltage while decreasing the current, while a step down

transformer decreases the voltage while increases the current

for domestic and commercial distribution. The word substation

comes from the day before the distribution system became a

grid. The first substations where connected to only one power

station, where the generations were house and were

substations of the power station.

In ‘SEALDAH POWER HOUSE’, the control is divided into

two sections. They are –

The Low Transmissions Sections

The High Transmissions Sections

The main components of Power House are-

Transformers (300KVA & 500KVA)

Circuit Breaker (ACB, VCB & OCB) o ACB- Air Circuit Breaker (Used in low Transmission Section

o VCB- Vacuum Circuit Breaker (Used in High Transmission

Section)

o OCB- Oil Circuit Breaker

Relays (EFR & OCR) o EFR- Earth Fault Relay

o OCR- Over Current Relay

Bus Coupler

Hooter / Alarm

Isolators (P.F= 1 Substation)

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The components are describe in brief from the Power House

Below-

TRANSFORMER-

A transformer is a device that transfers electrical energy from

one circuit to another circuit through inductively coupled

conductors. The transformer coli is varying current in the first

or primary winding creates a varying magnetic flux in the

transformer core and thus a varying magnetic field through the

secondary winding. This varying magnetic field induces a

varying electromotive force (EMF) or voltage in the secondary

winding. This effect is called Mutual Induction. If a load is

connected to the secondary, an electric current will flow in the

secondary winding and electrical energy will be transferred

from the primary circuit through the transformer to the load. In

an ideal transformer, the induced voltage (VS) in the

secondary winding is proportional to the primary voltage (VP)

and is given by the ratio of the number of turns in the

secondary (NS) to the number of turns in primary (NP) as

follows-

VS/VP=NS/NP

BY appropriate selection

of the ratio of turns, a

transformer thus allows

an alternating current

voltage to be “Stepped

Up” by making NS

greater than NP or

“Stepped Down” by

making NS less than NP.

In the vast Majority of

transformers, the

windings are coils would

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around a ferromagnetic core, air-core transformers being a

notable exception.

In SEALDAH POWER HOUSE two transformers are used, one

300KVA and another one is 500KVA transformer is kept for

backup.

THE 500 KVA TRANSFORMER SPECIFICATION-

1 C Serial No. 1581/D7-D8

2 Makers Name Automatic Electro Group

3 KVA 500 KVA

4 Voltage at No Load H.V- 600 V & L.V- 115 V

5 Amperes H.V- 18.11 A & L.V- 695.62 A

6 Phases H.V-3 & L.V- 3

7 Type of Cooling Oil Natural Cooling

8 Frequency 50 C/S

9 Impedance Voltage 4.37%

10 Vector group REF DY11

11 Core & Wedges 1140 KG

12 Weight of Oil 451 KG

13 Total Weight 2015 Kg

14 Oil Amount 530 liters

15 Maximum Temperature of Oil 450

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CIRCUIT BREAKER-

A circuit breaker is an automatically operated electrical switch

designed to protect an electrical circuit from damage caused

by overload short circuit. Its basic function is to detect a fault

condition and interrupt current flow. Unlike a fuse, which

operates once and then must be replaced, a circuit breaker

can be reset (either manually or automatically) to resume

normal operation. Circuit breakers are made in varying sizes,

from small devices that protect an individual household

appliance up to large switchgear designed to protect high

voltage circuits feeding an entire city or Substation or

Powerhouse.

CLASSIFICATION OF CIRCUIT BREAKERS-

There are quite a few ways to classify the circuit breakers.

However, the most general ay of classification is based on

medium used for the arc extinction. Other bases for the

classification of circuit breakers are summarized in the

flow-

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1. BASED ON VOLTAGE-

Based on the voltage levels for which they are used,

the circuit breakers are classified as listed in table

(with corresponding voltage ranges of use)

Category Range of Voltage

Low Voltage Less than 1KV

Medium Voltage 1KV to 52 KV

High/ Extra High Voltage 66 KV to 765 KV

Ultra High Voltage Above 765 KV

2. BASED ON LOCATION -

Circuit breakers are based upon where they are

located, classified as, indoor and outdoor types.

Medium and low voltage breakers are categorized as

Indoor breakers, whereas the circuit breakers, which

have, air as external insulating medium are classified

as outdoor circuit breakers.

3. BASED ON EXYERNAL DESIGNED-

Outdoor circuit breakers can be identified as either

dead tank or live tank type circuit breakers, from the

point of view of their physical structural design. In the

dead tank circuit breakers, the switching device is

located with suitable insulator supports, inside a

metallic vessel at ground potential and filled with

insulating medium. In dead tank circuit breakers, the

incoming and outgoing conductors are taken out

through suitable insulator bushing, and low voltage

type current transformers are located at lower end of

both insulator bushing, i.e. at the line side and the load

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side. In live tank circuit breakers, the interrupter is

located in an insulator bushing, at potential above

ground potential. The live tank circuit breakers are

cheaper (with no current transformer), and require less

mounting space.

4. BASED ON INTERRUPTING MEDIA-

The interrupting media has been a vital factor in the

evolution of circuit breakers. It dedicates the overall

design parameters of the breaker. The choice of air

and oil, as the interrupting media, was predominant till

late 70s. However, today, vacuum and SF6 are the

only dominant interrupting technologies, for medium

and high voltage segments of circuit breaker design

respectively. The medium used for the arc extinction

can be-

Oil.

Air.

Vacuum.

Sulphur Hexafluoride (SF6).

Accordingly, the circuit breakers may be classified into

following categories (which will be treated in detail in

the present report):

Oil Circuit Breakers.

Air-Blast Circuit Breakers.

Sulphur Hexafluoride (SF6) Circuit Breakers.

Vacuum Circuit Breakers.

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In this Powerhouse, they are used three types of

Circuit Breakers. They are-

a) ACB- Air Circuit Breaker

b) VCB- Vacuum Circuit Breaker

c) OCB- Oil Circuit Breaker

ACB (AIR CIRCUIT BREAKER)-

These circuit breakers employ a high-pressure air-blast as

an arc-quenching medium. The contacts are opened in a flow

of air-blast established by the opening of the blast valve. The

air-blast sweeps away the arcing products of the

atmosphere. Consequently, the arc is extinguished and flow

of current is interrupted. Whenever current at high voltages

needs to be interrupted, more breaking units are used, in

series. Dry and clean air supply is one of the most essential

requirements forth operation of the Air Circuit Breakers.

In addition, other gases such as nitrogen, carbon dioxide,

and Hydrogen can also be used. However, air is preferred

because of the fact that the carbon dioxide tends to freeze,

and the hydrogen gas is very explosive. This type of circuit

breaker has been used earlier for open terminal 11KV

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applications, for voltages of

245KV and 400Kv up to 765

KV, especially where faster

breaker operation was

required. This type of breaker

has been used for special

applications where several

super thermal power stations

located in close vicinity to

each other have been inter-

connected. These one-cycle

breakers from the inter-

connecting link which, in the

event of a fault level for the feeder breaker.

The interrupting capability of air circuit breaker is increased

by increasing the normal pressure range. Normally, the

pressure level is around 30 to 35 bars. In order to maintain

the insulation level and reliability of operation, it is also

necessary for the condition of the air to be very dry. Currently,

however, SF6 circuit breakers have practically eliminated the

use of this technology.

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ADVANTAGES-

1. The risk of fire is eliminated in these circuit breakers.

2. The arcing products are completely removed by the blast

whereas the oil deteriorates with successive operations.

So the expenditure of oil replacement is avoided in air-

blast circuit breakers.

3. The size of these breakers is reduced, as the dielectric

strength grows so rapidly that final contact gap for the arc

extinction is very small.

4. Due to the rapid growth of the dielectric strength, the

arcing time is also very small. It causes less burning of

oil. The arc energy is also very small fraction of that in oil

circuit breakers.

5. The arc extinction is facilitate by the high-pressure air and

is independent of the fault current to be interrupted.

DISADVANTAGES-

1. These circuit breakers are very sensitive to variations in

the rate of rise of restriction voltage.

2. The compressor plant that needs considerable

maintenance supplies the air-blast.

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THE AIR CIRCUIT BREAKER SPECIFICATION

(Specification based on one sample on Low

Transmission Panel)-

Makers Name Pulsar

Frame LH800 DMIT3P

SL. No. Y606183

IIEC-947.2 IS: 3947 (Part 2)

Utilization Category 8

Rated Characteristics

IN 400A

ITH @ 400 C 800A

ICS & ICU 50k

ICW 50KA, 1 Sec

Power Factor 0.25

UI=1000v UE=4.15v Frequency = 50/60HZ

U/V 40VCA

VCB (VACUUM CIRCUIT BREAKER)-

In such circuit breakers,

the vacuum is used as

the arc-quenching

medium. The vacuum

circuit breaker takes the

advantage of non-

sustainability of electric

arc in vacuum, and

employs the principle of

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contact separation under

vacuum where there is no

ionization due to medium.

The initial arc caused by

field and thermionic

emission during the contacts

ionization because of

vacuum. The degree of

vacuum in these circuit

breakers is in the range from

10-7 to 10-5 torr. Since

vacuum offers the highest

insulating strength, it has far

superior arc –quenching

properties than any other

medium. When the contacts

in the vacuum circuit breakers are opened in vacuum, an arc

is produced between the contacts by the ionization of metal

vapors of contact. However, the arc is quickly extinguished

because the metallic vapors, electrons and ions produced

during arc rapidly condense on the surface of the circuit

breaker contacts.

CONSTRUCTION-

The vacuum circuit breaker

consists of fixed contact, moving

contact and arc shied mounted

inside the vacuum chamber. The

movable member is connected to

the control mechanism by

stainless steel bellows. This

enables the permanent sealing of

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the vacuum chamber to eliminate the possibility of leak. A

glass vessel or ceramic Bessel is used as the outer insulating

body. The arc shield prevents the deterioration of the internal

dielectric strength by preventing metallic vapors falling on the

inside surface of the outer insulating cover.

APPLICATONS-

For a country, where distances are quite long and

accessibility to remote areas is difficult, the installation of

such outdoor, maintenance-free circuit breakers should

prove definite advantage. Vacuum circuit breakers are being

employed for outdoor applications ranging from 22kv to 66kv.

This technology has been found to be most suitable for

medium voltage application though the experimental

interrupters for the 72.5kv and 145kv have been developed,

they were not found to be commercially viable.

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ADVANTAGES-

1. Vacuum circuit breakers are compact in size and have

longer lives.

2. Operating energy requirements are low, because the

mechanism must move only relatively small masses at

moderate speed, over very short distances.

3. Because of the very low voltage across the metal vapor

arc, energy is very low. (arc voltage is between 50v to

100 v)

4. Due to the very low arc energy, the rapid movement of

the arc root over the contact and to the fact that most of

the metal vapor re-condenses on the contact, contact

erosion is extremely small.

5. There is no generation of gases during and after the

circuit breaker operation.

6. The outstanding feature of these breakers is that it can

break any heavy fault current perfectly just before the

contacts reach a definite open position.

7. They can withstand lighting surges.

8. It is now possible to produce cost-effective VI (Vacuum

Interrupter) designs with electrical lives that exceed the

required mechanical life of the circuit breakers, and that

will even be able to satisfy a recent requirement of

extended short-circuit operating life.

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THE VACUUM CIRCUIT BREAKER SPECIFICATION-

TYPE VBBA

SL. NO. VBBA-4742

Year of Manufacture 2006

Specification IS> 1881

Rated Voltage 7.2 KV

I/L 20/60 KV

Rated Current 800 AMPS

Frequency 50 HZ

Making Capacity 62.5 AMPS

STC 25 KA

OCB (OIL CIRCUIT BREAKER)-

In such circuit breakers, some insulating

oil (i.e. transformer oil) is used as an

arc-quenching medium. The contacts

are opened under oil and an arc is

struck between them. The heat of the

arc evaporates the surrounding oil and

dissociates it into a substantial volume

of hydrogen gas at a high pressure. This

large volume of the hydrogen gas

pushes the oil away from the arc. In an

oil circuit breaker, the arc quenching

process is entirely dependent on arc

energy generated. The arc drawn

across the contacts is contained inside

the interrupting pot, and this the

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hydrogen gas formed by the vaporized oil (gas) is also

contained inside the chamber. As the contacts continue to

move and the moving contact rod separates itself from the

orifice of the chamber, an exit similar to a nozzle allows

escape of the hydrogen gas trapped inside the interrupting

chamber. The escaping high-pressure hydrogen gas, having

a high thermal conductivity, takes away the heat, thus making

the contact gap cool and free from ionization, immediately

after current zero. However, the oil breakers have prolonged

arcing times, due to insufficient vapor pressure generated at

lower interrupting currents. Periodic monitoring of the oil

dielectric conduction and its maintenance is important for the

effective operation of medium oil circuit breaker. However,

maintenance required for oil circuit breakers is of low

technology.

Mainly two processes facilitate the arc extinction:

1. Firstly, the hydrogen gas has high heat conductivity and

cools the arc, thus aiding the de-ionization of the medium

between the contacts.

2. Secondly, the gas sets up turbulence in the oil and forces

it into the space between contacts, thus eliminating the

arcing products from the arc path. This results in

extinguishing the arc and as a result, the circuit current is

interrupted.

ADVANTAGES-

As an arc-extinguishing medium oil has the following

advantages-

1. Oil absorbs the arc energy to produces hydrogen gas

during arcing. The hydrogen has excellent cooling

properties and helps extinguish the arc. (In addition to

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hydrogen gas, a small proportion of methane, ethylene,

and acetylene arc also generated in oil decomposition.)

2. The oil provides insulation for the live exposed contacts

from the earthed portions of the container.

3. Oil provides insulation between the contacts after the arc

have been extinguished.

4. The oil close to the arc region provides cooling surface.

DIS ADVANTAGE-

1. Oil is inflammable and may cause fire hazards. When a

defective circuit breaker fails under pressure, it may

cause an explosion.

2. The hydrogen generated during arcing, when combined

with air, may form an explosive mixture. During arcing, oil

decomposes and becomes polluted by carbon particles,

which reduces strength. Hence, it requires periodic

maintenance and replacement.

TYPES OF OIL CIRCUIT BREAKERS:

The oil circuit breakers find extensive use in the power

system. They can be classified with the reverence to the

quantity of oil used. The types are:

a. Bulk Oil Circuit Breakers

b. Minimum Oil Circuit Breakers

a. BULK OIL CIRCUIT BREAKERS-

These circuit breakers use a large quantity of oil. The oil has

to serve two purposes:

1. It extinguishes the arc during opening of contacts.

2. It insulates the current conducting parts from one another

and from the earthed tank.

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In the bulk oil circuit breakers, the interrupting unit is placed in

a tank of oil at earth potential and the incoming and outgoing

conductors are connected through insulator bushings.

b. MINIMUM OIL CIRCUIT BREAKERS-

These circuit breakers use a small quantity of oil. In such circuit

breakers, oil is used only for arc extinction; the current

conducting parts insulated by air or porcelain or organic

insulating material. In these circuit breakers, the oil requirement

can be minimized by placing the interrupting units, in insulating

chambers at live potential, on an insulator column.

THE OIL CIRCUIT BREAKER SPECIFICATION

(BASED ON ONE SAMPLE ON L.T. PANEL ON 1. NO.PF

SUBSTATION)-

SL. NO. 2K2132

Type HN2T

Normal Current 400 A

Service Voltage 415 V

Design Frequency 50 HZ

Breaking Capacity 25 MVA

Trip Coil Rating 5 A

Calibration 100/200 %

C.T. Ratio 800/5

RELAY-

A relay is an electromagnetic switch. In other words, it is

activated when a current is applied to it. Normally a relay is

used in a circuit as a type of switch. There are different types

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of relays and they operate at different voltages. When we build

our circuit we must be need to consider the voltage that will

trigger it.

In SEALDAH POWER HOUSE, they are used two types of

relay. They are:

a) Over Current Relay.

b) Earth Fault Relay.

a) OVER CURRENT RELAY-

The name ‘over current relay’ implies that this is a special type

of protection, which is used to protect the costly apparatus from

the effect of huge current flow. Over current relays are those

relays, which operate during the excess current flow through

the network, and trips the circuit of circuit breaker, which

isolates the faulty part of the network from the healthy part.

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Now depending upon the time of operating time, over current

relays are divided in-

1. INSTANTANEOUS OVER CURRENT RELAY-

As the name suggests this relay takes no intentional time to act

or close the contacts of circuit breaker. The operating time of

such relay is approximately 0.1 sec.

2. INVERSE TIME CURRENT RELAY-

This type of relay operating time reduces as the actuating

quantity i.e. the amount of current flow increases. It means

that, for severe fault the operating time of the relay will be less

and for minor fault the operating time will be more.

3. INVERSE DEFINITE MINIMUM TIME OVER CURRENT

RELAY (IDMT)-

In this type of relay, the operation and operating time is similar

to inverse time over current relay near the pickup value but it

becomes almost constantly increases.

4. VERY INVERSE RELAYS-

As the name suggests, in this type of relay, the saturation of

the core occurs at a later stage and it has inverse

characteristics.

5. EXTREMELY INVERSE RELAY-

The slope of time current graph is more than in very inverse

over current relay and for this, the saturation of the core occurs

at later stage.

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OVER CURRENT RELAY SPECIFICATION -

Model No. CDG31EG001SBCH

Serial No. 130954160621011

C.T. Sec 5amps; 2.5-10 Amps

Frequency 50 HZ

b) EARTH FAULT RELAY-

Backup protection of electrical transformer is simple Over

Current and Earth Fault

protection applied against

external short circuit and

excessive over loads. These

over current and earth Fault

relays may be of Inverse

Definite Minimum Time (IDMT)

or Definite Time type relays.

Generally, IDMT relays are

connected to the in-feed side of the transformer.

The over current relays cannot distinguish between external

short circuit, over load and internal faults of the transformer. For

any of the above fault, backup protection i.e. over current and

earth fault protection connected to in-feed side of the

transformer will operate. Backup protection is although

generally installed at in feed side of the transformer, but it

should trip both the primary and secondary circuit breakers of

the transformer.

Over Current and Earth Fault protection relays may be provided

in load side of the transformer too, but it should not inter trip the

primary side Circuit Breaker like the case of backup protection

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at in feed side. Primarily current and time setting and the

characteristic curve of the relay govern the operation. To permit

use of over load capacity of the transformer and co-ordination

with other similar relays at about 125 to 150% of full load current

of the transformer but below the minimum short circuit current.

Backup protection of transformer has four elements; three over

current relays connected each in each phase and one earth

fault relay connected to the common point of three over current

relays as shown in the figure. The normal ranges of current

settings available on IDMT over current relays are 50% to 200%

and on earth fault relay 20 to 80%.

Another range of setting on earth fault relay is also available

and may be selected where the earth fault current is restricted

due to insertion of impedance in the neutral grounding. In the

case of transformer, winding with neutral earthed, unrestricted

earth fault protection is obtained by connecting an ordinary

earth fault relay across a neutral current transformer.

The unrestricted over current and earth fault relays should have

proper time lag to co-ordinate with the protective relays of other

circuit to avoid indiscriminate tripping.

EARTH FAULT RELAY SPECIFICATION-

Model No. CDG31EG001SBCH

Serial No. 130954160621011

C.T. Sec 5 Amp ; 1-4 Amps

Frequency 50 Hz

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BUS COUPLER-

Bus coupler is a device, which is used to couple one bus to

the other without any interruption in power supply and without

creating hazardous arcs. Bus coupler is a breaker used to

couple two bus bar in order to perform maintenance on other

circuit breaker associated with those bus bars.

It is achieved with the help of circuit breaker and isolators.

HOOTER/ALARM-

The alarm is employed to the system to indicate the faulty

condition. If any fault occurs in the system components, the

alarm sounds to notify.

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ISOLATOR-

An isolator is a two-port device that

transmits microwave or radio frequency

power in one direction only. It is used to

shield equipment on its input side, from the

effects of conditions on its output side. For

example, to prevent a microwave source

being detuned by a mismatched load.