INTRODUCTION

TO

SUB STATION

INTRODUCTION:

Substation is the most important part of the power

system. The Electrical Energy which is generated at the various power

stations (viz. Hydro Electric power station, Steam power station, Nuclear

power station etc.) is supplied to the consumers for their household/ other

purposes through these substations.

The Electric Power generated at the power station and

conveyed by the transmission lines is handled by the substations before it is

delivered to the consumers. Therefore, by definition “A substation is

essentially an assembly of apparatus which is installed to control

transmission or distribution of Electric Power.”

The Electric power at the power station is generated by

different means such as at Hydro Electric Station the electricity is generated

with help of water, at Steam power station the Electricity is generated with

help of steam produced through the water, at nuclear power station the

Electricity is produced by means of chemical action of Uranium.

The Electric power generated by means of 3 phase 3 wire

system. The generated voltage is 3.3Kv, 11Kv or 33Kv. However, the most

usual value adopted in India is 11Kv.

Sub station is the most important part of the Electric

system. The Electric power generated at the power stations is conveyed by

the transmission lines to the substations and then it is step down to the lower

value and is distributed through the distribution lines to the consumers of the

Electric energy.

I have under gone my training at 132 KV substation

P.S.E.B. Verka, (Amritsar).

At this substation there are 5 incoming lines of 132 KV

which makes a ring main system in this Northern area of Punjab. These 5

incoming lines are as follows:

1. 132 KV Mall Mandi

2. 132 KV Jandiala.

3. 132 KV Jaintipur.

4. 132 KV Kathunangal.

5. 132 KV Power Colony.

These 132 KV lines are firstly taken on a common bus bar

and then is Stepped down by one transformer to 66 KV. There are two 66KV

outgoing feeders which supply the whole city of Amritsar. These feeders are

as follows:

1. 66 KV Dental College.

2. 66 KV Moodhal.

Other than these 66 KV outgoing feeders, the 132 KV line

is also stepped down through two other transformers (T2 & T3) to get 11KV

output. There are 16 outgoing 11 KV feeders which are given as follows:

1. 11 KV Railway Workshop.

2. 11 KV Shiv Nagar.

3. 11 KV Nehru Colony.

4. 11 KV Kashmir Avenue.

5. 11 KV Mall Mandi.

6. 11 KV Pawan Nagar.

7. 11 KV Verka Colony.

8. 11 KV Khanna Nagar.

9. 11 KV Civil Lines.

10. 11 KV Milk Plant Verka.

11. 11 KV Batala Road.

12. 11 KV TRW No. 1

13. 11 KV Majitha Road.

14. 11 KV MES.

15 & 16 are the 11 KV spare feeders.

VARIOUS

SECTIONS AT

SUB STATION

AN INTRODUCTION TO THE VARIOUS SECTIONS AT

SUB STATION:

Sub station is divided into different sections to work properly and

efficiently. Following are the different sections at substation:

Open yard

Control and relay panel room

Battery room

OPEN YARD:

All the 5 incoming lines coming from different places in

the form of 5 different 132 Kv transmission feeders are received in this open

yard. All these feeders are firstly received at the transmission tower located

in the open yard of the sub station and then it is sent to the circuit breaker

through the isolating switches. An earth switch is also provided at the

incoming end of the transmission line. A wave trap is provided at the

incoming end of the line in order to trap the high frequency signals. After

these signals are trapped, these are sent to the PLC (Power Line

Communication) section.

All the feeders of high voltage are used as the

communication line for the PLC section. At PLC section, the received

frequencies are higher than the 50 Hz because the communication is possible

only at the high frequencies. The wave trap provided only detects between

these frequencies and send them to their appropriate section i.e. 50 Hz

supply frequency is send to the sub station side and the above frequencies

are sent to the PLC section.

After the wave trap, the line is send to the COMMON

BUS BAR of the line voltage at which all the received feeders are connected

in parallel to make the bus bar voltage equal to the voltage of each feeder.

Before this bus bar, a line CT is connected through an isolating switch.

The line is connected to the incoming of the POWER

TRANSFORMER through the isolating switch, current transformer, circuit

breaker and lightening arrester.

The output of the transformer is send to the low voltage

bus bar through the current transformer, circuit breaker and isolating

switches for voltages more than 11Kv. For 11Kv, outputs the output is send

to the 11Kv bus bar formed in between the control and relay panels from

which the output feeder is send to the different consumers as required by

them.

CONTROL AND RELAY PANEL ROOM:

In the control and relay panels section, all the incoming

and outgoing feeders are controlled with the help of operating relays, which

trips the respective circuit breaker when any fault occurs in the feeder. In

these panels, the energy meters are also placed to record the energy

consumption of the feeder or the load on the respective feeder at any

instance of time. The relays, which are secondary feeded equipments, are

placed on the control panels and these relays are used to protect the

equipments and the feeder from any mishap.

BATTERY SECTION:

Battery section is the section, which gives the standby

supply to the substation. The batteries are placed in a well-ventilated room.

The proper checking of the batteries is very much necessary because these

come in to play during any emergency case. The batteries are required to trip

the circuit breakers when the whole of the ring main system is out of power

supply, thus making sure that all the equipments used at the substation

remains safe.

There are 110 cells are provided at 132 KV substation VERKA.

Each cell has an output voltage of 2.1 volts and total capacity of 200

Ampere-hours.

VARIOUS

EQUIPMENTS

USED AT

SUBSTATION

ISOLATING SWITCHES OR ISOLATORS:

Isolator is defined as the device used to open or close a

circuit either when negligible current is interrupted or established or when

no significant change in the voltage across the terminals of each pole of the

isolator.

Isolator is a switch, which isolates one part of the circuit from

the other.

The isolating switches can be divided into following three

categories depending upon their functions performed:

1. Bus Isolator Switch

2. Line Isolator cum Earthing switch

3. Transformer Isolating Switch

These categories are shown in the following diagram of

double bus bar arrangement:

The isolators marked 1 are bus isolators.

The isolators marked 2 are Line isolators

The isolators marked 3 are Transformer isolators

BUS ISOLATORS: These are the isolating switches, which isolates the one

bus bar from the other one. It is only provided in the case where two or more

feeders form the incoming ring of the sub station and all are connected to the

common bus bar. These isolators are provided between the two bus bars in

order to isolate them from one another.

LINE ISOLATORS:

These are the isolators, which are provided in between the

incoming or outgoing line and circuit breaker. These isolators are provided

in order to work properly on the circuit breaker during its maintenance. An

earthing switch is also provided with such type of isolators.

TRANSFORMER ISOALTORS:

These are the isolators, which are used to isolate the

transformer from the supply circuit. This isolating switch is opened before

there is any need of maintenance to the power transformer at the substation.

On the basis of switching, the isolators can be

divided into two parts:

1. OFF LOAD ISOLATORS

2. ON LOAD ISOLATORS

OFF LOAD ISOLATORS:

This is an isolator which is operated in a circuit either

when the isolator is already disconnected from all sources of supply or when

the isolator is disconnected from the supply and the current may be due to

the capacitance current of the bushings, bus bar, connections and very short

length of cable.

ON LOAD ISOLATORS:

It is an isolator, which is operated in a circuit where there

is parallel path of low impedance so that no significant change in the voltage

across the terminals of each pole occurs when it is operated.

The instances of the use of ON LOAD isolators come

across in the double bus bar arrangements. It is not possible, under normal

operation to close any isolator of the second bus bar or transfer the load from

live bus to the dead bus, the bus coupler breaker with associated isolators

has first to be closed. This creates a path of low impedances across bus

isolators of other circuits enabling the simultaneous operation of the bus

isolators of both the bus bars of any circuit.

From constructional point of view, the isolators are

grouped as follows:

1. Three Post, Centre Post Rotating, Double Break Type

2. Two Post Single Break Type

Three Post, Centre Post Rotating, Double Break Type isolators:

The double break, rotating centre post isolating switches

has three posts per phase mounted on a base of fabricated construction.

The centre post carries the moving contact arm, tubular or

flat, with the contacts assembled at the extremities. The moving contacts

engage the fixed contacts on the outer fixed insulator posts. One of the

contacts is male contact while the other is female contact. The female

contacts are generally in the form of spring loaded finger contacts.

The rotating centre posts of the three phases are

interconnected by the operating rods so that simultaneous movement of each

contact arm is ensured. Rotation of the centre posts is effected through a

driver lever fitted at the base of each post, connected by operating rods and

driven from one post by the operating mechanism through the adjustable

lever. Hand operation is done by a lever from a box attached to the

supporting structure.

Two Post Single Break Type isolators:

In this, the contact arm is divided into two parts, one

carrying male contacts and the other carrying female contacts. For

closing or opening both the isolator posts, rotate causing movement of

the contact arm.

OPERATION OF AN ISOLATOR:

The operation of an isolator may be manual i.e. by

hand without using any of supply or storage of energy. Power operated

isolators utilize energy, which is not supplied by the operator, during the

course of the operation. The energy may be electrical, pneumatic or the

energy previously stored in spring or counter weight.

AUXILLIARY SWITCH:

This is an important accessory and is defined as a

switching device working in conjunction with an isolator for controlling a

circuit for auxiliary device, such as trip coils, indicating lamps etc. The

number of normally closed and normally open contacts should be

specifically worked out particularly if electrical interlocking between

circuit breakers and isolators is chosen.

ARCING HORNS:

These are provided on each stack of post insulators for

the purpose of insulation co ordination. This is necessary for safety and

security. Any traveling wave meeting an isolator in the closed or open

position should cause a flash over to earth rather than between phases or

between terminals of the same pole. Therefore, it is necessary to use

arcing horns on the insulator stacks so that the flash over values to the

earth can be brought down.

MAKE BEFORE AND BREAK AFTER CONTACTS:

These are provided in series with the main contacts so

that in case of ON load isolators, the arcing is taken and when ever

necessary only the arcing contacts is repeated.

INTERLOCKING OF ISOLATERS:

The incorrect operation of an isolating switching may

have exceedingly harmful effect and may cause destruction of parts of

the plant as well as costly service interruption. For preventing such

incorrect operation interlocks are used i.e. the isolating switch is blocked

against the circuit breaker, earthing switch or other isolating switches.

The requirements of interlocking are as follows:

1. The isolator cannot be operated unless the associated breaker is

locked in the open position.

2. The earthing switch shall close only when the line isolator is open

and locked and is not in its stroke.

3. The line isolator shall close only when the corresponding circuit

breaker and the earthing switch of corresponding line are open.

4. The circuit breaker shall close only after all isolators associated

with it have been locked either in closed or open position.

Following are the requirements of interlocking for

double bus bar arrangement:

1. The isolator cannot be operated unless the associated breaker is

locked in the open position.

2. The earthing switch shall close only when the line isolator is

open and locked and is not in its stroke.

3. The line isolator shall close only when the corresponding circuit

breaker and the earthing switch of corresponding line are open.

4. The circuit breaker shall close only after all isolators associated

with it have been locked either in closed or open position.

5. One bus selector isolator of any bay excepting the bus coupler

shall close only when:

The circuit breaker of the corresponding bay is open and locked

The other bus isolator of that bay is open

6. The bus isolator of the bus coupler bay shall operate only when

the bus coupler circuit breaker is open.

7. The bypass isolator( if provided) of the feeder shall also close

when the feeder circuit breaker and its adjoining isolators are

closed.

RATING OF ISOLATORS:

According to the Indian standards IS 1818-1961

the required ratings and test requirements of isolators are as

follows:

1. Voltage rating

2. Insulation level

3. Frequency

4. Normal current

5. Max. duration of short circuit( or short time current)

6. Pressure of a compressed air supply

7. Supply voltage of an operating device or an auxiliary circuit

8. Supply frequency of an AC operating device or an auxiliary

circuit.

TESTS PERFORMED ON ISOLATORS:

Following are the tests performed on the isolators:

1. TYPE TESTS

2. ROUTINE TESTS

TYPE TESTS:

Following are tests, which are performed under the

heading of the type tests:

a) Impulse voltage test

b) Power frequency voltage test

c) Test on auxiliary equipment

d) Temperature rise test

e) Measurement of resistance of main circuit

f) Short circuit current carrying capability

g) Making test for earthing switches

h) Operation test

i) Mechanical endurance test

ROUTINE TEST:

Following are tests, which are performed under the

heading of the routine tests:

a) Power frequency voltage test

b) Test on auxiliary equipment

c) Operation test

d) Measurement of resistance of main circuit

VOLTAGE AND CURRENT TRANSFORMERS

POTENTIAL OR VOLTAGE TRANSFORMER:

Voltage transformers are the transformers, which step

down the system voltage to sufficiently low values. These transformers are

necessary on every system for the following purposes:

Indication of the voltage conditions

Metering of the supply of the energy

Relaying

Synchronizing

The indicating instruments, meters and relays are designed

for the voltages obtainable from the secondary sides of the voltage

transformers. The calibration of the indicating instruments and the meters is,

however done according to the primary voltages of the voltage transformers.

CLASSIFICATION OF THE VOLTAGE TRANSFORMERS:

Voltage transformers are classified into the following

categories:

a) Magnetic Type

b) Capacitive Type or Capacitive Voltage Transformers (C.V.T.)

MAGNETIC TYPE VOLTAGE TRANSFORMER:

These transformers work on the same principle as that of

the power transformers. The design of this transformer is different because

of the different requirements. The load on the voltage transformers is

generally limited to a few Hundred VA. The main object in the design of a

voltage transformer is to minimize ratio and phase angle errors. These errors

are caused due to the following reasons:

a) Voltage drop in the primary winding caused by exciting

current

b) Voltage drop in both the windings caused by the load

current.

The former accounts for errors at Zero Burden and the

latter is for the slope of the ratio and the phase angle curves. Since the load

current is fixed for a given burden, the drop, which it causes can be reduced

only by reducing the resistance and reactance of the transformer. This is

done by using relatively few turns and a large cross section of both Iron and

Copper.

In the lower voltage group of voltage transformers the

active part is contained in the steel housing and the primary terminals are

brought out through bushing. On higher system voltages [above 66 KV], the

active part is contained in porcelain housing, whether the voltage

transformer is contained in steel tank or in porcelain housing. The secondary

terminals are marked and brought out in steel housing.

X V

v, x are secondary terminal x

Schematic diagram of a magnetic type voltage transformer

CAPACITIVE VOLTAGE TRANSFORMER:

These type of transformers are used more and more for

voltage measurement in high voltage transmission networks {particularly for

132 KV and above} where it become increasingly more economical.

It enables measurement of the line to earth voltage to be

made simultaneous provision for carrier frequency coupling, which has

reduced wide application in telemetering, and telephone communication

purposes.

These capacitive type voltage transformers are of two

types:

1. Coupling Capacitor Type

2. Bushing Type

V Phase conductor

C1

C2

X

T

x R

v Z D

Tr F

V primary terminal C1 primary capacitance

C2 secondary cap. D compensating indicating coil

Tr intermediate T/F Z damping impedance

F spark gap

R resistor

X high freq. coupling ter.

v, x sec. terminal

T earthing terminal

Schematic diagram of coupling capacitor voltage transformer

COUPLING CAPACITOR VOLTAGE TRANSFORMER:

The capacitors C1 and C2 are made of oil impregnated

paper and aluminum foil. Each capacitor is assembled in such a way that the

capacitor inductance remains low. A tap is taken in between to connect the

magnetic voltage transformer (Tr) across the capacitor and earth.

Special auxiliary circuit elements are:

1. Compensating Inductance Coil (D)

2. Damping Impedance (Z)

3. Resistor ( R)

4. Spark Gap (F)

The compensating inductance coil in series with the

primary of the intermediate transformer compensates the voltage increases in

the capacitive voltage divider. The damping impedance in the secondary

circuit avoids the Ferro resonance. The resistance and the spark gap provides

necessary protection against over voltages.

BUSHING TYPE CAPACITIVE VOLTAGE TRANSFORMER:

The condenser type bushings are primarily rolls of

vanished, impregnated type paper cut, and lay in between the paper layers.

The sheath may consist of Aluminum foil or a coating of Graphite. The

voltage distribution between the various layers is properly designed and

predetermined.

The low capacitance imposes severe restrictions on the

output power of such C.V.T’s and therefore limits the application to

synchronizing and voltage indication.

Table of max. output for various system voltages that are obtainable

with typical bushings

L-L KV OUTPUT

66

110

132

220

5

12

15

25

RESIDUAL VOLTAGE TRANSFORMER ( RVT):

According to the Indian standards the rated secondary

voltages are specified as 110 V or 110/√3 ( 63.5) volts.

The values of the rated residual secondary voltage of

residual voltage transformer (RVT) shall be 110/√3 ( 63.5) and 110√3 (190)

volts.

The rated residual voltages imply nominal voltages of the

individual winding constituting the broken delta connection of 110/√3( 63.5)

volts each.

The residual voltage transformer with broken delta

connection is required to produce a residual voltage, which maintains the

same phase relationship with the associated spill current for an earth fault on

any phase. This is required to operate the voltage polarized directional earth

fault relays. The broken delta may also be used to actuate an indicator for

showing the presence of an earth fault on an unearthed system.

Under balanced three phase conditions, the voltage across

open delta is negligibly small. In the event of a ground fault on the system

the voltage across open delta appears maximum between 110/√3 ( 63.5) and

110√3 (190) volts depending on the system neutral earthing and voltage

transformer primary neutral earthing, former when both are solidly earthed

and latter when both are isolated.

Limits of voltage error and phase displacement( for measurement VT)

Class Ratio (voltage) error Phase displacement

0.1

0.2

0.5

1.0

3.0

± 0.1

± 0.2

± 0.5

± 1.0

± 3.0

± 5

± 10

± 20

± 40

__

For Protective VT

Class Ratio (voltage) error Phase displacement

3.0

5.0

± 3.0

± 5.0

± 120

± 300

For Residual VT

Class Ratio (voltage) error Phase displacement

5.0

10.0

± 5.0

± 10.0

± 200

± 600

R Y B

AUXILLIARY VT

VT RELAY

Use of interposing VT to obtain Residual Voltage from Open delta

connection

Use of Magnetic Type VT and Capacitive VT:

In case we need voltage supply for Voltmeters,

synchronizing, energy meters, distances relays (without carrier), magnetic

type voltage transformer can serve alone this purpose. If however, it is

required to adopt carrier protection, it is necessary that coupling capacitors

are used on each phase along with the Voltage Transformers. In such a case,

we can use either capacitive voltage transformer alone or Magnetic Voltage

transformer with coupling capacitors. If only carrier communication is

required, the purpose can be served with only one coupling capacitor per

circuit and magnetic voltage transformers or capacitive voltage transformers

only.

On 132 KV lines, the desirability of providing carrier

protection has to be checked. The requirement of one coupling capacitor or

three with the magnetic voltage transformers can influence the comparison

of price with capacitive voltage transformers.

Where the outgoing/ incoming feeders are more than two then one of the

following arrangements can be applied:

1 Phase VT

BUS 1

BUS 2

1Phase VT

CVT CVT CVT

R Y B

Figure shows three CVT’s on each feeder and one CVT or single phase

VT on each bus

In figure, a set of capacitive voltage transformers

has been provided on each incoming/ outgoing line. Each set will feed its

own protection independently. For synchronizing single capacitive voltage

transformer or a single magnetic voltage transformer can be provided on

each bus. In such an arrangement, a capacitive voltage transformer (CVT) on

bus bars also may be preferable because of the same equipment being used

on each line.

BUS1

BUS2

1 PHASE BUS VT

R Y B

In this figure, only coupling capacitors have been shown

on feeders. The number of coupling capacitors on each circuit will depend

upon the purpose. For metering requirements of the voltage supply for

various purposes e.g. voltmeter, synchronizing, metering, relaying only one

set of magnetic type voltage transformer has been purposed on each bus. The

potential transformer would need to be designed for higher rated burdens so

that each set can meet requirements of all the feeders.

TESTS PERFORMED ON VOLTAGE TRANSFORMERS

Following are the tests, which are performed on voltage

transformers:

Type test

Routine test

TYPE TEST:

Type tests for all the Electromagnetic voltage transformers

specified in IS 3156- 1965are as follows

1. Verification of terminal markings and polarity

2. Power frequency tests on primary windings

3. Power frequency tests on secondary windings

4. Determination of errors according to the requirements of

appropriate accuracy.

5. Temperature rise test

6. Impulse voltage test on voltage transformers for service in

electrically exposed installations.

The following are the additional type tests for CVT’s

as laid in IS 3156 (part 4) - 1967:

7. Applied high voltage power frequency with stand test on the

primary voltage capacitors.

8. Applied high voltage power frequency with stand test on the

intermediate voltage capacitors

9. High voltage power frequency withstand test on intermediate

voltage circuit (of electromagnetic unit)

10.Accuracy tests at the limits of frequency range.

11.Settling of the protective gap.

12.Ferro resonance test.

13.Primary short circuit test.

ROUTINE TEST:

1. Verification of terminal markings and polarity

2. Power frequency tests on primary windings

3. Power frequency tests on secondary windings

4. Determination of errors according to the requirements of

appropriate accuracy.

Following are the additional routine tests on capacitive

voltage transformers:

5. Applied high voltage power frequency with stand test on the

primary voltage capacitors.

6. Applied high voltage power frequency with stand test on the

intermediate voltage capacitors

7. High voltage power frequency withstand test on intermediate

voltage circuit (of electromagnetic unit)

8. Accuracy tests at the limits of frequency range

9. Settling of the protective gap

CURRENT TRANSFORMER

The current transformer is similar in construction to the

single-phase power transformer and obeys the same functional laws.

However, the primary current of the current transformer is not controlled

by the power demand in the secondary circuit but imposed on the

transformer by the primary supply system. The primary ampere-turns

produce a magnetic flux in the iron core, which in turns induces an EMF

in the secondary circuit; this causes the current to flow through the

burden connected to the secondary terminals. At the same time, the

ampere-turns produced by the secondary current oppose the primary

ampere-turns, thus balancing the two-ampere turns system.

The following equation applies to an ideal current

transformer:

I1 N1 = I2 N2

I 1 = N 2 I2 N1

N1 = Turns in primary winding

N2 = Turns in secondary winding

I1 = primary current

I2 = secondary current

The secondary terminal voltage V2 is controlled by the

burden of the transformer i.e. by the impedance Z2 of the secondary load

circuit.

V2 = I2 Z2

Resistance and leakage reactance of the secondary

winding can be combined in the internal impedance Zi of the transformer.

The voltage E2 is induced by the magnetic flux and therefore

E2 – V2 = E2 – I2 Z2 = I2 Zi

Therefore E2 = I2 ( Z2 + Zi )

The magnetic flux thus depends on the secondary

voltage E2. In actual practice the condition I1 N1 = I2 N2 is not fully

realized. As in the case of power transformer there is always another

current I0 called no load current, which disturbs the ideal balance. In case

of power transformer the no load current I0 must flow in the primary

winding in order to produce the required magnetic flux and to provide for

the iron losses. Thus, no load current is independent of the secondary

current.

The primary draws the additional current from the

supply system so that the primary ampere turns balance the secondary

ampere turns. In case of the current transformer, the current of the supply

system is flowing in full through the primary winding. A part of this

current, called the no load current is consumed to produce the required

flux.

Line I1 Line

N1

N2 , Zi

I2 V2

Schematic connections of current transformer

N1I0

-I2N2

N1I0

Ф

I2N2

E2

Vector diagram

Current I 1 circulates in the primary winding, which is

connected in series with the lines, the secondary delivers a current I2 to the

impedance of the winding and that of the instruments and relays. The flux Ф

generates the EMF E2 in the secondary sufficient to cause the flow of current

I2 through the impedance Z2 + Zi. This flux is produced by a component of

the no load current I0, the vertical component of which accounts for the

losses of the transformers. A part of the primary ampere-turns i.e. I0 N1 are

therefore, consumed in the excitation of the core, to induce sufficient voltage

in the secondary winding so that a rated secondary current I2 can be forced

through the total impedance Z2 + Zi.

Current I0 is responsible for the difference in actual ratio

I1/ I2 and the displacement in the vector N1 I1 and –N2 I2. These are called

the Current (or Ratio) Error and Phase Error.

Therefore

(I1 – I0 ) N1 = I2 N2

Limits of error for measuring Current Transformer:

Accuracy Percentage current error at

percentage of rated current

Phase displacement in minutes

at percentage of rated current

10 20 100 120 10 20 100 120

0.1

0.2

0.5

1.0

±0.25

±0.5

±1.0

±2.0

±0.2

±0.35

±0.75

±1.5

±0.1

±0.2

±0.5

±1.0

±0.1

±0.2

±0.5

±1.0

±10

±20

±60

±120

±8

±15

±45

±90

±5

±10

±30

±60

±5

±10

±30

±60

Accuracy Percentage current error at percentage of rated current

3.0

5.0

± 3.0

± 5.0

± 3.0

± 5.0

Limits of error for protective current transformer

Accuracy

class

Current error at

rated primary

current(percent)

Phase displacement

at rated primary

current (minutes)

Composite error at

rated accuracy limit

primary current (%)

5P

10P

15P

± 1

± 3

± 5

± 60

__

__

5

10

15

Difference between Instrument Transformer and

Protective Transformer

Instrument Transformer:

Precision measurement and metering has assumed

increasing importance because of the growth of supply systems particularly

where energy interchange between different power systems is concerned.

With the large quantities of the energy transferred, the financial effects of

the measuring errors assume considerable importance. It is also required that

small fractions of the rated primary current should be measured with

adequate accuracy.

Some extension beyond rated current is also necessary to

take consideration of the normal system overloads. The instrument

transformer is therefore required to maintain the accuracy class within 10%

to 125 % of the rated current and small measuring errors within the range.

An ideal transformer should have the following characteristics:

I2

IN2

IN1 I1

I1 = Primary Current

I2 = Secondary Current

IN1 = Rated Primary Current

IN2 = Rated Secondary Current

PROTECTIVE TRANSFORMER:

Unlike instrument transformers, it is not in the function of

the protective transformer to maintain great precision over the normal

operating range. Rather it should maintain the current error and phase

difference with in reasonable limits in the fault current range. It is necessary

that the protective transformer does not separate upto the current necessary

to operate relays.

I2

IN2

IN1 I1

CONSTRUCTION OF CURRENT TRANSFORMER:

According to the constructional point of view, the current

transformer can be divided into following two groups:

BAR TYPE CURRENT TRANSFORMER

WOUND TYPE CURRENT TRANSFORMER

BAR TYPE CURRENT TRANSFORMER:

For large primary currents the bar type construction is

ideal because it can meet with the burden and accuracy requirements and at

the same time can have high thermal and dynamic short time factors. This

bar primary transformer may be further sub divided into following:

1. Separately Mounted Type

2. Bushing Type

The bushing type is mainly employed on the bushing of

the transformers or bulk oil circuit breakers. This type of CT has serious

limitations with regard to the burden and accuracy. The rated primary

ampere turns is equal to the primary current and only being limited, there is

an upper limit to the rated output to be expected from the Current

transformer, generally 15 VA in Class 0.5 or 10P15 or 15P20 for currents of

400 Amperes. On lower currents the rated output will be less unless either

the accuracy or accuracy limit factor are reduced.

In case of separately mounted bar type transformers the

difficulty to keep the measuring errors within the limits is over come either

with the use of Nickel Iron alloy core or adopting a special long cylindrical

design of core. There is no difficulty in having high saturation factor at high

burdens because the core section can be increased to the desired extent. The

space is not restricted as in the case of the bushing type transformers.

WOUND TYPE CURRENT TRANSFORMERS:

Where the primary currents are low or the burden and

accuracy requirements are high, it is generally necessary to build wound

type transformers. The primary winding consists of a number of turns, which

depends on the primary current. The greater the number of turns, lesser the

thermal and dynamic short time current factors.

MULTI RATIO TRANSFORMERS

In the case of bar type transformers the taps, for obtaining

different ratios can be fixed only on the secondary side. Since the secondary

output depends on the square of the ampere turns the output at half the ratio

will be 1/4th only.

In case of wound type transformers, it is possible to

provide necessary tapings on the primary side. The arrangement is of the

advantage where no special fault current capacity is required. Any desirable

ratio can be obtained without sacrificing either burden or accuracy. Short

circuit proof wound transformers are not provided with primary tapings. The

primary windings are split into two or four sections insulated from each

other.

100 Amps. Series connection

100 Amps. Series connection

200 amp series- parallel connection

200 Amps. Parallel connection

400 Amps. Parallel connection

MULTI CORE TRANSFORMERS

In the separately mounted current transformers, it would

be very much uneconomical to build separate instrument and protective

transformers. The current transformers can therefore be provided with two or

more cores. Each core carries its own secondary winding. The primary

winding is common to all of them. A current transformer with two cores

operates in exactly the same way as two independent transformers with the

same primary current. If the arrangement of the primary winding is changed

then the ratio of the both transformers is changed. It is of course possible to

employ different numbers of turns for the two secondary windings. A double

core transformer needs not to have the same turn ratio for both the cores.

SELECTION OF A CURRENT TRANSFORMER

Following points need to be considered while selecting a

current transformer:

1. Type

2. Number of Secondaries

3. Accuracy Class of each secondary

4. Rated Burden

5. Accuracy Limit Factor

6. Short time Current Rating

7. Insulation Values e.g. power frequency dry and wet with stand values,

impulse with stand values.

TESTS PERFORMED ON CURRENT TRANSFORMERS

According to the Indian Standards IS 2705, following are

the tests, which are performed on the current transformers:

Type Tests :

1. Verification of the terminals marking and polarity

2. High voltage power frequency test on primary windings

3. High voltage power frequency test on secondary windings

4. Over voltage inter turn test

5. Determination of error according to requirement of appropriate

accuracy classes

6. Short time current test

7. Temperature rise test

8. Impulse voltage test for current transformers for service in

Electrically exposed installation

ROUTINE TEST:

1. Verification of the terminals marking and polarity

2. High voltage power frequency test on primary windings

3. High voltage power frequency test on secondary windings

4. Over voltage inter turn test

5. Determination of error according to requirement of appropriate

accuracy classes

ADDITIONAL TESTS FOR MEASURING CURRENT

TRANSFORMERS:

TYPE TESTS:

1. Accuracy test

2. Instrument security current test

ROUTINE TEST:

1. Accuracy test

ADDITIONAL TESTS FOR PROTECTIVE TRANSFORMERS

A) For Class 5P, 10P and 15P

Type and Routine test:

1) Current Error and phase displacement test

2) Composite error test

B) For Class PS (special purpose) transformers

ROUTINE TESTS:

1) Knee point voltage

2) Exciting Current

3) Secondary Winding Resistance

4) Turns Ratio

CONTROL AND RELAY PANELS

PROTECTIVE RELAYS:

The protective relay may be defined as an Electrical

device interposed between the main circuit and the circuit breaker in such a

manner that any abnormality in the circuits acts on the relays, which in turn,

if the abnormality is of a dangerous character, causes the breaker to open

and so to isolate the faulty element. The relay ensures the safety of the

circuit equipment from any damage which might be other wise caused by the

fault. All the relays have three essential fundamental elements:

To Trip OR Signal Circuit

Sensing Comparison Control Element Element Element

SENSING ELEMENT:

It responds to the change in the actuating quantity, the

current in a protected system in case of over current relay. It is some times

called the measuring element.

COMPARING ELEMENT:

It serves to compare the action of the actuating quantity on

a relay with the pre-selected relay setting.

CONTROL ELEMENT:

It accomplishes a sudden change in the control quantity

such as closing of the operative current circuit.

OPERATING PRINCIPLE:

The operating principles of relays are of the following

types:

Electromagnetic Induction - For AC only

Electromagnetic Attraction – For both AC and DC

TYPES OF ACTUATING STRUCTURES:

1. Shaded Pole Structure

2. Watt Hour Meter Structure

3. Induction Cup and Double Induction Loop Structure

4. Single Induction Loop Structure

The mechanical movement of the operating mechanism is

imparted to a contact structure to close or to open contacts. The relays are

further provided with

1. Operation Indicators :

Operation indicators are actuated either mechanically by

movement of the relays operating mechanism or electrically by the flow of

contact current.

2. Seal in and Holding Coils or Seal In Relay :

The relay contacts are generally inadequate to interrupt the

flow of the current once established in the circuit breaker trip coil. To save

the relay contacts from damage from an inadvertent attempt to interrupt the

flow of the trip coil current in the circuit breaker trip coil. Seal in contacts

are also provided to reinforce the main contacts, as they (main contacts) may

not be able to take care of contact bounce in case of low torque relays. Quite

often, the seal in contacts are also not suitable to open the trip coil circuit.

This is done by the circuit breaker auxiliary contacts.

ELECTROMAGNETIC ATTRACTION RELAYS:

OPERATING PRINCIPLE:

The electromagnetic force exerted on the moving element

is proportional to the Square of the flux in the air gap. Neglecting saturation

effect the total actuating force may be expressed as:

F = K1 I²rms – K2

Where F is net force, K1 is the force conversion constant, Irms is the current

flowing through the actuating coil and K2 is the restraining constant.

When the relay is on the verge of picking up, the net force

is zero.

i.e. K1 I²rms – K2 = 0

or Irms = √ K2/ K1 = constant

Such relays are normally used for D.C. operating

quantities, but these can also be used for A.C. operation by providing

shading rings on their poles to split the air flux into two out of phase

components. These relays are not suitable for continuous operation on A.C.

in the picked up position because there would be excessive vibration and

noise.

There are two types of relays based on this type of

principle.

1. Attracted Armature type Relay

2. Solenoid type Relay

ATTRACTED ARMATURE TYPE RELAY:

This relay consists of laminated electromagnet and a

pivoted laminated armature. The armature is usually balanced by a counter

weight and carries a pair of spring contact fingers at its free end. When the

fault or abnormal condition take place, the relay armature is attracted, so the

stationary contacts attached to the relay frame are bridged and trip circuit is

completed. The magnet coil is tapped at the intervals and the tapping points

are brought out to a number of terminals or contacts on a plug section

bridge, so the number of turns in use and consequently the setting value at

which the relay operates may varied.

Electromagnetic relays are usually instantaneous type. A

definite time lag can be obtained by using an oil dashpot or an escapement

chamber or a clockwork mechanism. The oil dashpot or escapement

chamber must be widened at one end so that there should be a free

movement over the last part of the stroke to make a good contact.

By placing, a fuse in parallel with an instantaneous or

definite time lag relay can be made as inverse time lag relay.

SOLENOID AND PLUNGER TYPE IMPEDANCE RELAY:

This relay consists of a pivoted beam, with an iron core at

either end, which dips into coils. One of the solenoid coil is energized by the

line voltage while the other by the current of the feeder to be protected. The

voltage coil produces a restraining torque, which tends to prevent the trip

circuit contacts from closing where as the operating torque produced by the

current coils tends to close the contacts.

Voltage Trip Circuit Coil Current Coil

I

V Feeder

Solenoid and Plunger Type Relay

Under healthy conditions, the restraining torque developed

by the voltage coil is much more than operating torque developed by the

current coil carrying normal load current and the beam is in horizontal

position therefore the relay does not operate.

Let the restraining torque developed by the voltage coil, T1 = K1V²

And the operating torque developed by the current coil, T2 = K2 I²

The relay will operate if:

T2 > T1

Or K2 I² > K1V²

Or V = √ K2/ K1

I

Or Z = √ K2/ K1

Thus, the relay operates if the line impedance falls below

a certain value. For nay fault within the length of the line corresponding to

setting, the operating torque will pre dominate the restraining torque and the

relay will operate to close the trip circuit contacts.

ELECTROMAGNETIC INDUCTION RELAYS

INDUCTION TYPE OVER CURRENT RELAY:

It consists essentially of an A.C. Energy meter mechanism

with slight modification to give required characteristics. The relay has two

electromagnets. The upper electromagnet has two windings, one of these

primary and is connected to the secondary of a CT in the line to be protected

and is tapped at intervals. The tapings are connected to a plug setting bridge

by which the number of turns in use can be adjusted, there by giving the

desired current setting. The plug bridge is usually arranged to give seven

sections of tapings to give over current range from 50% to 200% in steps of

25%. If the relay is required to response for earth fault, the steps are

arranged to give a range from 10% to 70%. Adjustment of the current setting

is made by inserting a pin between the spring-loaded jaws of the bridge

socket at the tap value required. When the pin is withdrawn for the purpose

of changing the setting value while the relay in service, the relay

automatically adopts higher setting, thus the CT’s secondary is not open

circuited.

The second winding is energized by induction from the

primary, and is connected in series with the winding on the lower magnet.

By this arrangement, leakage flux of upper and lower electromagnets are

sufficiently displaced in space and phase to set up a rotational torque on the

aluminum disc suspended between the two magnets, as in the shaded pole

induction motor. This torque is controlled by the spiral spring and

sometimes by the permanent magnet.

INDUCTION TYPE REVERSE POWER RELAY:

This arrangement is similar to the Induction type

Energy Meter. The relay has two Electromagnets. The upper

electromagnet has winding on its middle limb, connected to the supply

through the potential transformer. The lower electromagnet has the

separate winding connected to the secondary of the current transformer in

line to be protected. The current winding is provided with tapings, which

are connected to the Plug Bridge. The torque developed on the disc

suspended between the two magnets is proportional to VI. When the

power flows in the normal direction, the torque developed on the disc

assisted by the spring tends to turn away the moving contact from the

fixed trip circuit contacts. A reversal of the current reverses the torque

produced on the disc and when this is large enough to control the spring

torque, the disc rotates in the reverse direction and the moving contacts

closes the trip circuit. The relay can be made sensitive by having a very

light control spring so that a very small reversal of power will cause the

relay to operate.

INDUCTION TYPE DIRECTIONAL OVER CURRENT

RELAY:

This relay is a combination of Non- directional over

current relay and the Reverse power relay, which has a current coil and a

pressure coil and provides directional features to the relay. When the

power flow in the normal direction the disc of the reverse power relay

(lower element) does not move but as soon as there is reversal of current

or power the disc starts rotating and completes the circuit for over current

or earth fault elements, which energize the relay. Due to over current a

torque is set up in the disc and the action closes the trip coil circuit, there

by enabling the circuit breaker to operate. The directional element is

made as sensitive as possible to ensure positive operation – even 2% of

the power in the reverse direction operates it. A plug bridge is provided

to adjust the current setting. The relay operates only when

(i) The direction of current is in reverse direction

(ii) Current is in reverse direction exceeds the pre set value.

(iii) Excessive current (greater than its pre set value) persists for

longer time than its time setting.

Such relays are employed when graded time over load

protection is applied to ring mains and inter connected networks, since fault

current can flow in either direction.

DIFFERENTIAL RELAYS

A differential relay is that relay which operates when the

phase difference of two or more similar Electrical quantities exceeds a pre-

determined amount. Almost any type of relay, when connected in particular

way, can be made to operate as differential relay.

Figure shows an arrangement of an over current relay

connected to operate as differential relay. In this arrangement, a pair of

current transformers is fitted on either ends of the element to be protected

and secondary windings of CT’s are connected in series so that they carry

induced currents in the same direction. Normally when there is no fault or

there is no external fault, the current in two CT’s secondaries are equal and

relay operating coil, therefore does not carry any current. Whenever there is

internal fault, current in two secondaries of CT’s fitted on either end will be

different and the relay operating coil will be energized by the current equal

to their difference and completes the trip circuit to operate the circuit

breaker.

Alternator neutral Alternator winding load

I1 I2

Relay opreating coil

Pilot wires

This system is adopted for the protection of feeders,

alternators and transformers. The CT’s of equal ratio are employed when

used either at the two ends of an alternator winding or at the two ends of the

feeder with no taping. While using this system for protection of transformers

correction must be made for different currents determining approximately by

the transformer turn ratio.

The most extensively used form of differential relay is

percentage differential relay or biased beam relay. This system consists of an

additional restraining coil in which current induced in both CT’s flows.

Relay operating coil is fed from taping on restraining coil.

Alternator winding

Restraining coil trip circuit

Oprerating coil

The torque due to restraining coil prevents the closing of

trip circuit contacts, while the torque due to operating coils tends to close the

trip circuit contacts. The differential current required to operate this relay is

variable quantity, owing to the effect of restraining coil. The differntial

current in the operating coil is I1 – I2 and the equivalent current in the

restraining coil is proportional to the I1 + I2/ 2 since the operating coil is

connected to the mid point of the restraining relay.

VOLTAGE BALANCE DIFFERENTIAL RELAY:

This is an alternative arrangement of obtaining, a

differential protective gear. In this arrangement two current transformers are

fitted at either end of each phase but secondaries of the current transformers

are connected so that E.M.F.’s in both opposes i.e. current only flows

through operating coil when there is any difference of the induced voltages

in the secondaries of two C.T.’s. In normal conditions i.e. when there is no

fault in the system equal currents flows at the two differen ends, so induced

voltages in the secondaries of the C.T.’s are equal, so no current will flow

through the operating coil, but when ever fault occurs, currents will differ at

the two ends, so induced E.M.F.’s in the secondaries of the C.T.’s will differ

and circulating current will flow through the operating coil, which will close

the trip circuit.

Alternator neutral Alternator winding load

E2

E1 Relay operating coil

DISTANCE RELAYING

In distance relaying there are two torques – one is

operating and the other is Restraining. For faults beyond the protective zone,

the restraining torque is more whereas for faults within the protective zone

the operating torque is more. The operating torque is produced generally by

the current or a component of that and the restraining torque is produced by

the voltage or a component of that. At the point of short circuit, the voltage

collapses to zero. For any magnitude of fault current, the voltage seen by the

relay will be multiple of the current and the impedance of the line section

between the points of the faults and the relay installation. In general, there

are three types of distance relays:

1. Simple Impedance

2. Reactance

3. Mho Principles

SIMPLE IMPEDANCE:

This relay consists of a pivoted beam, with an iron core at

either end, which dips into coils. One of the solenoid coil is energized by the

line voltage while the other by the current of the feeder to be protected. The

voltage coil produces a restraining torque, which tends to prevent the trip

circuit contacts from closing where as the operating torque produced by the

current coils tends to close the contacts.

BALANCED BEAM TYPE RELAY

Under healthy conditions, the restraining torque developed

by the voltage coil is much more than operating torque developed by the

current coil carrying normal load current and the beam is in horizontal

position therefore the relay does not operate.

Let the restraining torque developed by the voltage coil, T1 = K1V²

And the operating torque developed by the current coil, T2 = K2 I²

The relay will operate if:

T2 > T1

Or K2 I² > K1V²

Or V = √ K2/ K1

I

Or Z = √ K2/ K1

Thus, the relay operates if the line impedance falls below

a certain value. For nay fault within the length of the line corresponding to

setting, the operating torque will pre dominate the restraining torque and the

relay will operate to close the trip circuit contacts.

REQUIREMENTS OF A PROTECTIVE SYSTEM

Whenever a fault develops in a piece of equipment or

section of a circuit, it is highly desirable to de energize the faulty section

quickly and selectively:

1. To arrest the spread of the damage in the faulty equipment or other

parts of the system.

2. To continue supply to the healthy parts of the system.

These functions are performed by protective relays

individually or in combination and are aided in the task by circuit breakers,

which disconnects the faulty elements under instructions from the protective

relays. The most important requirements for a protective system are as

follows:

1. Sensitivity

2. Selectivity

3. Speed

4. Reliability

Relays are used for the protection of the following

components:

Power Transformers

Transmission Line Feeders

Bus Bars

Shunt Capacitor Banks

Earthing Transformers

Station Transformers

POWER TRANSFORMERS:

A power transformer is subjected to the following faults:

1. Overloads and External Short Circuits

2. Terminal Faults

3. Winding Faults

4. Incipient Faults

All faults conditions produce mechanical and thermal

stress with the transformer windings. Excessive overloading results in

deterioration of the insulation and subsequent failure. It is usual to provide

winding and oil temperature indicators with alarm and trip contacts. As soon

as the temperature of the winding and the oil exceed the predetermined

values, the contacts are bridged, completing the circuit of an alarm bell and

if the temperature reaches the hot spot value, the transformer is tripped.

Terminal faults on the feeding side has no adverse effect

on the transformer but faults on the load side cannot operate the gas and oil

pressure relay (buchholz relay). These do fall in the protective zone of the

restricted earth fault or differential relays.

The majority of the internal faults either are earth faults or

inter turn faults. The severity of the fault depends on the location of the

fault, transformer design and the method of the system earthing.

Incipient faults are internal faults, which constitutes no

immediate hazard. The main faults in this group are core faults due to

insulation failure between the core laminations and oil failure due to loss of

oil or obstruction in circulation of oil. The protective systems applied to the

power transformer are:

1. Gas and Oil Pressure Relays (Buchholz Relay)

2. Over Current and Earth Fault Relay (Unrestricted)

3. Restricted Earth Fault

4. Frame Leakage

5. Differential Relay

BUCHHOLZ RELAY:

The device comprises a housing, which contains two

elements. The upper element consists of a float and a mercury switch. The

lower element consists of a baffle plate and a mercury switch.

The use of this device is possible only in conservative type

transformers. The relay is located in the pipe connection between the main

tank and conservator.

Incipient fault conditions such as insulation faults between

turns, breakdown of core insulation, core heating, impulse failure, fall of oil

level produces gas. These gasses rise up and accumulated in the upper part

of the housing. Hence, the oil level falls down and the float sinks thereby

tilting the mercury switch. The contact is closed and the alarm circuit is

energized. After the operation is completed, the gasses are allowed to escape

through the release cock provided at the top of the relay.

More severe type of faults, such as short circuit between

phases or to earth and the faults in the tap changing equipment are

accompanied by the surge of oil which strikes the baffle plate and causes

mercury switch of the lower element to close. This generally energizes the

trip circuit of the breaker.

Buchholz relay protection is always used in conjunction

with some other form of electrically operated protective gear as it can only

operate for internal faults of transformer.

OVER CURRENT AND UNRESTRICTED EARTH FAULT:

Simple over current and earth fault protection is applied

against short circuits and excessive over loads. These over current and earth

fault relays may be of inverse and definite minimum time type or definite

time relays.

Over current relay

Earth fault relay

Star Connected Winding

BALANCED EARTH FAULT PROTECTION SCHEME:

Earth fault relay Earth Fault Relay Delta connected transformer winding

Star connected transformer winding

For the star connected winding three line CT’s are

balanced against a current transformer in the neutral connection. These four

CT’s must have same CT ratio and magnetization characteristics. In delta-

connected windings the three line CT’s are paralleled and connected against

an earth fault relay. Some times the same the line CT’s may be used for the

over current protection as well as restricted earth fault protection.

An external earth fault on star side will result in current

flowing in the line current transformer of the affected phase and a balancing

current in the relay is therefore zero. During an internal fault the neutral CT

only carries current and operation results. This form of protection is very

sensitive to the internal faults.

FRAME LEAKAGE PROTECTION:

This type of protection is simple, convenient and cheaper.

The scheme is generally known as ‘Tank Protection’.

The transformer is lightly insulated from earth by

mounting it on a concrete plinth. The transformer tank is connected to earth

through a CT across which an instantaneous earth fault relay is connected.

Earth fault current due to insulation breakdown in any winding of the

transformer will flow to the earth through the tank and single earth

connection, thus energizing the circuit transformer and operating the relay.

This scheme is extremely sensitive in detecting the earth faults in the

transformer zone. In order to obtain successful results it is essential that the

flow of fault current be confined only to the path of the faulty transformer

earth connection.

DIFFERENTIAL RELAY:

It is common practice to provide differential relays on all

transformers above 5 MVA. The advantages of this protection scheme over

the others are:

1. The Buchholz relay can detect faults caused under oil only. The flash

over at the bushings are not adequate covered by the Buchholz relay.

The differential relay detects such faults and on the leads between the

current transformer and power transformer provided the CT’s are

separately mounted and not in the transformer bushings.

In case of very severe internal faults differential relay

operates faster than the Buchholz relay thus controlling the extent of the

damage.

2. The differential protection responds to phase-to-phase faults with in

the protection zone. This generally comprises all equipment and

connections between the current transformers on all sides of the

transformers.

A transformer differential relay operates on circulating

current principle, by comparing the currents in the various windings of

the transformers. The ratio and connection of the CT’s on various

windings of the power transformers are so chosen that their secondary

currents are equal in magnitude and phase under normal operating

conditions or for faults externals to the protective zone.

Transformer

Relay

The polarity of the current transformers is such that the

relay receives the vector sum of the secondary currents, which should be

zero for the normal conditions and the external fault conditions. If however,

a fault to earth or a fault between phases occurs in the transformer protective

zone the balance between the corresponding current transformers will be

disturbed and current will flow through the relay to cause operation of the

circuit breakers.

The following are the factors, which disturbs the balance

between the current transformers under the normal operation:

1. Current Transformer Characteristics :

The ratios of protective current transformers on different

sides of the power transformer generally vary in the inverse ratio of the

voltages.

2. Unequal length of current transformer secondary leads may well cause

a difference in VA burden between the two sets of CT’s. This

generally tends to give current error between the sets of CT’s.

3. Ratio changes because of change of tapings.

4. Magnetizing inrush current.

FEEDER PROTECTION:

The feeders may be classified as:

a) Cable feeders

b) Over head transmission lines

Recommended scheme for different type of feeders are as

follows:

1) For Radial Feeders :

a) Time graded over current and earth fault at input end

b) Instantaneous pilot wire protection

c) Time graded over current and earth fault as backup

protection

2) For Parallel Feeders:

a) Time graded over current and earth fault

b) Instantaneous directional over current and earth fault

at input end.

c) Instantaneous directional differential and time graded

over current and earth fault at input ends only.

3) For Ring Main Feeders:

a) Time graded over current and earth fault at each feeder except

at power source

b) Time graded over current and earth fault at feeder ends

connected to power supply source only.

SHUNT CAPACITORS or CAPACITOR BANK

In A.C. systems, the reactive power is expressed in KVAR

(or MVAR). It is an essential part of the power system. The demand of

reactive power arises out of electromagnetic circuits of motors, transformers

and inductance of lines, electric furnaces, uncorrected fluorescent lights etc.

The equipments, which are used for producing reactive power, are:

1. Synchronous condensers

2. Static capacitors

The static capacitors are further divided into following:

1. Shunt Capacitors

2. Series Capacitors

R Y B

Shunt cap. Bank Series cap. Bank

Shunt capacitors are very commonly used in all voltages

and in all sizes. Fundamental effects of shunt capacitor may be summed up

as under:

1. Reduction of line current

2. Increased voltage level at the load

3. Reduced system losses

4. Increased power factor of a source current

5. Reduced loading on source generators and circuits

6. Reduced system investment per kilowatt of load

Shunt capacitors draw almost a fixed amount of leading

current, which is super imposed on the load current. This reduces the

reactive component of the load current, there by improving the power factor.

Reduced current and improved power factor reduce voltage drop in the

various components of the system and thus voltage is improved.

Series capacitors have no control over the flow of current.

It is the system load current, which always passes through them. The

capacitor reactance counter acts partly the inductance of the line thus

reducing the effective value of X.

The voltage regulation is thereby automatic with the

decrease or increase in the load current. The improvement in the power

factor is only consequential. During faults conditions the voltage across the

series capacitors exceeds upto 15 to 20 times, therefore series capacitor have

to be protected under system fault conditions. These are used at extra high

voltage and are in large sized banks.

CONSTRUCTION OF SHUNT CAPACITORS:

The active parts of a capacitor are composed of two

Aluminum foils (App.7Microns thick) separated by several layers of

impregnated papers. The paper varies in thickness from 8 to 24 microns

(1micron = 0.001mm) depending on the voltage for which the insulation is

designed. Due to change in layers of paper, the cost per KVAR goes high in

low-tension capacitor units.

The capacitor sections are wound into rolls where after

they are flattened out, compressed into packs, enclosed in multi purpose

layers of heavy paper insulations and inserted into the containers. After the

lid is welded, the whole unit is dried and impregnated in large autoclaves by

combination of heat and vaccum. After the paper is dried out the capacitor

tank is filled with impregnated degassed at the same vaccum.

The mineral oils are poured in the tank. These oils have

low conductivity but high breakdown strength. The limitations of the oil are

as follows:

Dielectric constant is low

Voltage distribution is not uniform

They are subjected to oxidation; the oxidation products are acids,

water and sludge

They are attacked by the gaseous electrical discharge producing

hydrogen and low molecular weight hydrocarbons.

They are inflammable and this requires greater installation cost to

provide safety.

DETERMINATION OF BANK RATING REQUIRED

The size of the capacitor bank required may be determined

by the following formula:

Q = P ( tan θ – tan θc)

Where Q = KVAR required, P = Active power in KW

Cos θ = Power factor before compensation.

Cos θc = Power factor after compensation

BANK PROTECTION:

Failure of the capacitor bank will cause the voltage to rise

on the remaining units in the series group, which contained the failed unit.

An additional failure in that group will cause the voltage to rise still higher

on the remaining units. Since the capacitor, units have the maximum

continuous voltage limitations of 110%, some automatic indication of failed

units is often desired so that the bank can be removed from the service and

failed units replaced before others can be damaged due to the over voltage.

The most common methods of protection are shown in figure:

In figure (a), a voltage transformer is connected in parallel

with each phase. The secondary terminals of these three transformers are

connected in open delta. In case of a failure, the voltage become

unsymmetrical, causing the voltage relay connected across open delta

winding to pick up and give an alarm or a trip signal. It is necessary to check

up the voltage rise on the other units in series in case of failure of one unit

and to see if failure of single unit causes adequate voltage across open delta

to cause the voltage relay to pick up.

In figure (b), the capacitor bank arrangement has two units

in series per phase. Across each unit is connected a discharge coil, which

serves two fold purpose i.e. to detect voltage unbalance of capacitor series

group and to remove dangerous residual charge of capacitors after they are

switched off the service.

In figure (c), the capacitor bank has been shown as star

connected with the neutral point of the bank grounded through potential

transformer, the secondary of which is connected across a voltage relay.

TESTING OF CAPACITORS

Following are the tests, which are performed on the

capacitor banks:

TYPE TESTS :

1. Test for dielectric loss angle

2. Test for capacitor losses

3. Stability test

4. Impulse voltage test between terminals and container

ROUTINE TEST:

1. Test for output and/ or capacitance.

2. Voltage test between terminals (for capacitor units)

As a D.C. test, the test voltage should be 4.3 V

As an A.C. test, the test voltage should be 2.15 V

3. Voltage test between terminals and containers

4. Voltage test between terminals and earth.

5. Insulation resistance test

6. Test for efficiency of discharge device.

7. Test for output and/ or capacitance.

CIRCUIT BREAKES

Circuit breakers are the mechanical devices that are

designed to close or open contact members, thus closing or opening an

electrical circuit under normal and abnormal conditions.

Automatic circuit breakers, which are usually employed

for the protection of electric circuits, are equipped with a trip coil connected

to the relay or other means, designed to open the breaker under abnormal

conditions. The automatic circuit breaker performs the following duties:

1. It carries the full load current continuously without overheating or

damage.

2. It opens and closes the circuit on no load.

3. It makes and breaks the normal operating current.

4. It makes and breaks the short circuit currents of magnitude upto,

which it is designed.

OPERATING PRINCIPLE:

A circuit breaker consists of fixed and moving contacts,

which are touching each other under normal conditions i.e. when circuit

breaker is closed. Whenever a fault occurs, the trip coil gets energized, the

moving contacts are pulled by some mechanism and therefore the circuit

breaker is opened and the circuit is broken.

The basic construction of a circuit breaker requires the

separation of the contacts in an insulating fluid, which serves two functions:

1. Extinguishes the arc drawn between the contacts when the circuit

breaker is open.

2. Provides insulation between the contacts from each contact to earth.

The fluids commonly used for this purpose are as follows:

1. Air at atmospheric pressure.

2. Compressed air

3. Oil producing hydrogen for arc extinction

4. Ultra high vaccum

5. Sulphur Hexa- Fluoride

The fluids used in the circuit breakers should have the

following properties:

High dielectric strength

Thermal and chemical stability

Non- Inflammability

Arc extinguishing ability

High thermal stability

Commercial availability at moderate cost

INTIATION OF ARC:

When the moving contacts of a circuit breaker begin to

part, the heavy current passes through the gap causing a small voltage drop.

A small separation of contacts does not cause immediate interruption of

current, because as the contacts separate, the resistance between them

increases and a lot of heat is produced due to ohmic losses. This heat is

sufficient to ionize air or vaporize and ionize the oil between the contacts.

The ionized air acts as a conductor and the current remains uninterrupted

across the low resistance arc produced. The potential drop between the

contacts is quite small and is just sufficient to maintain the arc.

There are two methods of extinguishing the arc, which are

as follows:

1. HIGH RESISTANCE METHOD :

By increasing the arc resistance with the time, the

current is reduced to such a value that heat produced by it is not sufficient

to maintain the arc and thus the current is interrupted or the arc is

extinguished. This method is employed in D.C. circuit breakers and low

and medium power industrial type air break circuit breakers.

The resistance of the arc can be increased by following

methods:

Decreasing the concentration of ionized particles

Increasing the length of the arc

Reducing the cross section of the arc

Splitting the arc

2. ZERO POINT or LOW RESISTANCE METHOD :

This method is applicable only to A.C. circuits because

there is natural zero of current 100 times in a second for 50 Hz supply

system. In this method the arc resistance is kept low until the current is zero

where the arc extinguishes naturally and is prevented from restriking.

The phenomenon of arc extinction is explained by two

theories as follows:

1. ENERGY BALANCE THEORY :

This theory states that if the rate of heat dissipation

between the contacts is greater than the rate at which heat is generated, the

arc will be extinguished. Hence, by increasing the rate of heat dissipation

such as by cooling, lengthening and splitting the arc it can be extinguished.

2. VOLTAGE RACE THEORY:

Immediately after the current zero, the space between the

contacts contains ions and electrons, therefore, has a finite resistance, and

can easily be broken down again by the rising contact voltage, known as

restriking voltage. If such a breakdown occurs, the arc will persists for

another half cycle when the process will be repeated. The problem is,

therefore, to remove the ions and electrons either by causing them to

recombine into neutral molecules as soon as the current becomes zero, so

that the rising contact voltage or restriking voltage cannot breakdown the

space between the contacts. This can be achieved by following methods:

High Pressure

Cooling

Blast Effect

MINIMUM OIL CIRCUIT BREAKER

As the system voltages and fault level increased the

bulk, oil breaker required huge quantities of insulating oil and became

unwieldy in size and weight. This added enormously to the cost of a power

system. This led to the development of the minimum oil circuit breakers in

which arc chamber contains a minimum quantity of oil and is mounted on a

porcelain insulator to insulate it from earth. Such circuit breakers are less

suitable where very frequent operation occurs because the degree of

carbonization produced in small volume of oil is far more severe than in the

conventional bulk oil circuit breakers and this leads to deterioration in the

dielectric strength across the contact system in the open position.

The circuit breaker is of the single break type in which a

moving contact tube moves in the vertical line to make or break contacts at

the upper fixed contacts mounted within the controlled device. A low ring of

fixed contacts are in permanent contact with the moving arm to provide the

other terminal of the phase unit. Within the moving contact tube is a fixed

piston, which, as the tube moves downwards on opening, forces the column

of oil inside the tube into the arc control device.

This has two effects, firstly, a partial pressure balance is

ensured, so that the pressure generated inside the are control device has little

effect on the acceleration of the moving contact and secondly, the amount of

cavitations caused by removal of the moving contact is controlled and the

efficiency of arc extinction is increased.

The “turbulator” arc control device is built up of oil

impregnated vulcanized fiber plates held under compression by tension

members the plates being arranged to form a series of vents on one side of

the arc, and a series of oil pockets on the other. The fixed arcing tip is so

arranged that when the circuit breaker opens, the arc is drawn in front of the

vents where it can be more easily extinguished. To ensure that the oil

vaporized by the arc is replaced quickly when the arc is extinguished a

filling valve is fitted in the top casting of the arc control device. This valve,

lightly spring loaded, closes when the pressure is set up with in the device,

but as this pressure dies away when the arc is extinguished, the valve opens

to allow as in rush of oil.

The upper chamber contains a separator, which eliminates,

by centrifugal action, loss of oil when the breaker operates under fault

conditions. It has a spring-loaded vent valve, a breather to prevent moisture

from entering the circuit breaker, and a safety diaphragm, under the domed

cover, designed to lift and protect the circuit breaker from damage.

VACCUM CIRCUIT BREAKER

During the contact separation of the circuit breaker arc is

formed due to ionization of the particles in the medium between the

contacts. Idea behind the vaccum circuit breaker is to eliminate the medium

between the contacts i.e. vacuum. As we know that the vaccum means the

pressure below the atmospheric pressure which is 760 mm of Hg. In this

type of circuit breaker vaccum of the order of 10‾5 to 10‾6 torr (one torr is

equal to1 mm of Hg) is maintained. In such a low-pressure means free path

of the electrons is of the order of few meters and thus when the electrodes

are separated by few mm an electron crosses the gap without any collision.

Because of its dielectric strength of the vaccum is 1000 times more than that

of any other medium.

CONSTRUCTION:

It is very simple in construction as compared to other

circuit breakers. The outer envelope is made of glass, which is joined with

metallic end caps. Glass envelope facilitates the examining of the breaker

from outside after the operation. If it becomes milky white from its original

finish of silver mirror then it is a sign of losing vaccum. After the envelope

there is a sputter shield made of stainless steel to prevent the metal vapour

reaching the envelope. Inside the shield the breaker has one fixed and other

moving contact. The moving contacts moves through the metallic bellows

made of stainless steel.

CURRENT CHOPPING IN VACCUM CIRCUIT BREAKERS:

In case of vaccum circuit breakers current chopping

depends upon the vapour pressure and electron emission properties of the

contact material. The current chopping can be reduced by reducing arc

current to a very low value. In a vaccum circuit breaker it is done by

choosing such contact materials, which give out sufficient metal vapour.

However, this leads to reduction of dielectric strength.

SULPHUR HEXA-FLUORIDE (SF6) CIRCUIT BREAKERS :

Sulphur Hexa- Fluoride gas has the following properties,

which proves it superior to other mediums such as oil or air for use in circuit

breakers.

It has a very high dielectric strength, roughly 24 times of that of air.

The fast recombination property after removal of the source

energizing the spark makes it very effective for extinction of the arc.

It is an inert gas hence non-reactive to the other components of the

circuit breaker.

Its heat transfer property is about 1.6 times of that of air.

The thermal time constant of SF6 is low hence it can be stored at a

relatively smaller pressure than that of air.

CONSTRUCTION:

Such a circuit breaker consists of only two parts, namely

1. The Interrupter Unit

2. The Gas System

The Interrupter Unit:

The unit consists of two contacts, one is fixed and

other is moving contact. The fixed contact comprises a set of fingers,

which make contact round the circumference of the moving contact in

the circuit breaker’s close position. The contacts are surrounded by

the interrupting nozzles and a blast shield to control the arc

displacement. When the moving contact is withdrawn, an arc is drawn

between the nozzles and arcing probe, which is quenched by the gas

flow from the high pressure to the low-pressure systems.

The Gas System:

This system deals with the gas only. It consists of a

compressor to maintain the required pressure (16 atmospheres on

high-pressure side and 3 atmospheres at the low-pressure side) and a

heater backed with a thermostat set at 16°C to prevent the

liquidification of gas.

Advantages of SF6 breakers:

SF6 circuit breakers have the following advantages

over the conventional circuit breakers:

1. Electric clearances are very much reduced because of high

dielectric strength of SF6.

2. Its performance is independent of ambient conditions.

3. It gives noiseless operation.

4. Closed circuit gas cycle reduces the moisture problem.

5. Current chopping is minimum for this type of circuit breaker.

6. Arcing time is small owing to arc quenching properties of SF6;

therefore the contact erosion is less.

7. There is no reduction in dielectric strength of SF6 since no

carbon particle is formed during arcing.

TESTING OF CIRCUIT BREAKERS:

The tests preformed on the circuit breakers are divided

into two types:

TYPE TESTS : These are the tests, which are performed to satisfy

the general design, and the results of such tests, conducted on circuit

breakers, are applicable to all other circuit breakers.

1) Short circuit tests comprising of:

(a) Breaking capacity test for recovery voltage.

(b) Making capacity tests for applied voltage and making current.

(c) Test duty

(d) Short time current test.

2) (a) Temperature rise tests of the main circuit.

(b) Temperature rise tests of the auxiliary circuit.

(c) Measurement of the resistance of the main circuit.

3) Operation tests

4) Mechanical endurance test

5) (a) Impulse voltage dry with stand test.

(b) One minute power frequency voltage dry with stand tests.

(c) One minute power frequency voltage wet with stand tests.

ROUTINE TESTS :

The routine tests are carried out on each individual circuit

breaker, before it is placed in service, to ensure that, though of proven

design, the breaker is not faulty by reason if incorrect assembly or inferior

material.

1. Operation test

2. Measurement of resistance of the main circuits

3. One-minute power frequency voltage dry withstand test.

LIGHTENING ARRESTORS

Lightening arrestor is the most common device used for

protection of the power system against the high voltage surge, which is

connected between the line and earth so divert the incoming high voltage

wave to earth. It is also known as the “Surge Diverter”.

Surge diverters acts similar to a safety valve. When a high

voltage reaches the surge diverter, it sparks over and provides a conducting

path of relatively low impedance between the line and earth so the resulting

current flows to earth. As soon as the voltage again comes to the normal

value, it stops the flow of current.

The properties of good diverters are as follows:

It should not take any current on the working voltage of the system or

its breakdown voltage must be more than system normal working

voltage.

Any abnormal transient voltage above the breakdown value must

cause it to breakdown as quickly as possible, so that it may provide a

conducting path to earth.

When the breakdown have taken place, it should be capable of

carrying the resulting discharge current without getting damaged itself

and without the voltage across it exceeding the breakdown voltage.

The power frequency current following the breakdown must be

interrupted as soon as the transient voltage has fallen below the

breakdown value.

LIGHTENING STROKES AND OVER VOLTAGES:

The overhead transmission lines and the connected

apparatus i.e. power transformers, switch gears etc. are subjected to over

voltages on account of lightening discharge caused by atmospheric

disturbances and or by switching operations. Abnormal voltages are caused

by atmospheric disturbances because of following:

1. Direct Strokes :

Direct stroke on the phase conductor or ground wire or to

supporting structure results into abnormal transient voltages, which gets

super imposed on the power network.

2. Indirect Strokes :

Direct strokes are the vicinity of the equipment or charged

cloud over the power line includes abnormal voltages.

E.H.V. transmission lines and sub stations are designed to

take care of direct strokes by providing:

Higher impulse level

Shielding and lower footing resistance

Lightening arresters for draining undesirable voltage to the ground.

TYPES OF LIGHTENING ARRESTERS

Following are the different types of lightening arresters:

1. ROD GAP or SPHERE TYPE :

It is very simple protective device i.e. gap is provided

across the stack of insulators to permit flash over when undesirable voltages

are impressed on the system. It is not an ideal lightening arrester because

short circuit will be caused on the system every time a surge causes a flash

over. Flash over conditions are also affected by rain, pollution humidity,

temperature and polarity of incident waves. In view of these disadvantages,

it can be only used as back up protection in case main lightening arrester

gets damaged.

2. EXPULSION TYPE LIGHTENING ARRESTER :

It consists of an insulating tube, which has got an

electrode at each end and discharge hole at the lower end. The length of the

tube is such that spark over occurs in the tube between the two electrodes.

The arc produced inside the tube by “Follow up Current” produces gas,

which drives out ionized air through the bottom vent. The “Follow up

current” at its zero finds the arc path deionized and the space between the

electrodes fully insulated to prevent the flow of “Follow up current”. The

rapid expulsion of the gasses in the tube normally interrupt the short circuit

power follow current within the first or second half cycle.

3. VALVE TYPE LIGHTENING ARRESTER :

It consists of number of spark gaps in series with non-

linear resistors, the whole assembly being rigidly housed inside a

hermetically sealed bushing.

Under normal conditions, power frequency system voltage

does not cause breakdown of series spark gaps and thereby insulate the line

from ground for the highest system voltage. When undesirable transient

voltages due to lightening are superimposed over the system, the series gap

assemblies spark over at a pre-determined value. After the breakdown of the

gaps, the non-linear resistors conduct the surge current to the ground

offering very low resistance and limit the power frequency current, to a

value, the gaps can interrupt at the first current zero. During the flow of the

discharge current the non-linear resistor, limit the voltage drop across the

arrester to a value far below the BIL of the equipment.

VOLTAGE RATING OF LIGHTENUNG ARRESTER

The rated voltage in case of lightening arrester is defined

as the maximum permissible R.M.S. value of power frequency voltage,

which it can withstand while still carrying out effectively and without

damage the automatic extinction of the follow up current. According to IS

4004- 1967, while determining the maximum phase to earth frequency

voltage at the arrester location, the highest system voltage shall be

multiplied by the coefficient of earthing at a point of installation of the

arrester on the system.

The coefficient of the earthing is defined as the “rate of

highest R.M.S. voltage to earth of sound phase or phases, the point of

application of an arrester during line to ground fault, to the highest line-to-

line R.M.S. voltage expressed as a percentage of the latter voltage”.

1. Effectively Earthed System :

When the coefficient of the earthing does not exceed 80%,

the system is said to be effectively earthed. This value is generally obtained

when the ratio of zero sequence reactance to positive sequence reactance is

between zero and plus one.

2. Non-Effectively Earthed System :

In this case, the coefficient of earthing system is more than

80%, but does not exceed 100%. This happens only when the resistors or arc

suppression coil have been used while earthing the neutral. In such cases, the

lightening arresters rated at 100% of the system highest voltage are used.

3. Unearthed System :

In case of isolated or unearthed neutral system, line to

earth voltage of healthy phase may exceed 100% line-to-line voltage in the

event of a ground fault on one phase.

LOCATION OF LIGHTENING ARRESTERS

In order to ensure effective protection of the equipment

the lightening arresters should be located at the following places:

Very close to, the equipment to be protected and connected with

shortest leads on both the line and ground side to reduce the inductive

effects of the leads while discharging large surge currents.

To order to ensure the protection of transformer windings, it is

desirable to interconnect the ground lead of the arrester with the tank

and the neutral of secondary. This interconnection reduces the stress

imposed on the transformer winding by the surge currents to the

extent of the drop across the earth resistance and the inductive drop

across the ground lead.

TESTING OF LIGHTENING ARRESTERS

In IS 3070-1965, the tests are classified as:

Type tests

1. Voltages withstand tests of arrester insulation.

2. Power frequency spark over tests.

3. Hundred percent I.2/50 microsecond impulse spark over test

4. Front of wave impulse spark over test

5. Residual voltage test.

Acceptance tests

1. Dry power frequency spark over test.

2. Hundred percent I.2/50 microsecond impulse spark over test

3. Residual voltage test

4. Temperature cycle test on porcelain housing.

Routine test

1. Dry power frequency spark over test

POWER AND DISTRIBUTION TRANSFORMER

The transformer is a most and convenient and

economical apparatus for transfer of power from one voltage to other.

It consists of a core built with laminated high-grade

silicon or other steel plates around which set of windings are wound.

The winding to which supply voltage is connected is known as

primary winding and load side winding is known as secondary

winding. Alternating voltage when applied to the primary winding

builds up alternating flux in the core, which induces voltage in the

secondary winding. When load is connected to the secondary winding

current starts flowing through it.

Iron core

Primary Winding Secondary Winding

To AC To Load Circuit Source

Laminations

Transformers are wound as core type and shell type form.

In the core type transformers windings are wound around the two legs of the

magnetic material, while in case of shell type transformers, the windings are

surrounded by the two magnetic paths. Core type transformers provide long

mean length of magnetic circuit and short mean length of the winding.

FITTINGS OF TRANSFORMER:

Power and distribution transformers are installed with

various fittings and devices, which are necessary for their proper

functioning. Some of the fittings are as follows:

Dial type oil temperature gauge.

Dial type oil winding temperature gauge.

Conservator tank

Silica gel breather

Magnetic type oil level indicator

Pressure relief vent

Buchholz relay

Tap changing gear

Oil filling, drain valves and plug

Oil filter valves

Terminal kiosk

Earthing terminal, name plates, radiators, rollers etc

COOLING ARRANGEMENTS AND TYPE OF TRANSFORMERS:

In order to get rid of the heat generated by energy losses in

the transformer, coolants are employed. According to standards, there are

three types of cooling agents i.e. Air, Mineral oil and Synthetic liquid.

Transformers can be classified according to the method of cooling:

Air Cooled Transformers :

In case of air-cooled transformers, heat generated in the

windings and core is directly dissipated to the atmosphere by natural

convections. In some cases air is blown through the ducts (by means of fans)

provided in the windings.

Oil Cooled Transformers :

Mineral oil is extensively used as coolant and an

insulating medium in case of an power and distribution transformers.

Various types of transformers according to the cooling arrangement are

described as under:

1. Natural- O.N. type

2. Air Blast- O.B. type

3. Oil Immersed Water Cooled- O.W. type

4. Forced Oil Water Cooled- O.F.W. type

Askarel Transformers :

In order to eliminate the risk of fire, which is invariably

connected with oil fitted transformers non- flammable fluid known as

ASKAREL is used as coolant. A typical askarel is Pyroclor; a fire resistant

fluid based on a blend of Monosanto Aroclor 1260 and highly purified

Trichlor Benzene. Pyroclor does not produce explosive gasses. Askarel type

transformers designed to operate at temperatures specified for oil-immersed

transformers, but Pyroclor can be operated safely at high temperatures.

Askarel transformers are costly; as such, they are only used where it is not

possible to go in for oil-immersed transformers on technical grounds.

DISTRIBUTION TRANSFORMERS:

Distribution transformers transform H.V. or E.H.V. to low

voltages, which are used for transmitting energy to ultimate consumer. They

are usually of O.N. type; transformers of A.N. and A.B. are also used.

PROTECTIVE DEVICES & THEIR MAINTENANCE

BUCHHOLZ RELAY :

The device comprises a housing, which contains two

elements. The upper element consists of a float and a mercury switch. The

lower element consists of a baffle plate and a mercury switch.

The use of this device is possible only in conservative type

transformers. The relay is located in the pipe connection between the main

tank and conservator.

Incipient fault conditions such as insulation faults between

turns, breakdown of core insulation, core heating, impulse failure, fall of oil

level produces gas. These gasses rise up and accumulated in the upper part

of the housing. Hence, the oil level falls down and the float sinks thereby

tilting the mercury switch. The contact is closed and the alarm circuit is

energized. After the operation is completed, the gasses are allowed to escape

through the release cock provided at the top of the relay.

More severe type of faults, such as short circuit between

phases or to earth and the faults in the tap changing equipment are

accompanied by the surge of oil which strikes the baffle plate and causes

mercury switch of the lower element to close. This generally energizes the

trip circuit of the breaker.

Buchholz relay protection is always used in conjunction

with some other form of electrically operated protective gear as it can only

operate for internal faults of transformer.

Buchholz relay helps in detecting the under mentioned

faults:

Core laminations short circuit

Over heating of windings

Arcing due to bad contacts

Earth faults

Short circuit of windings

Puncture of bushing insulators inside tank

IDENTIFICATION OF GAS

Color of the gas accumulated in the relay can help in

identifying the material affected. Since the gas collected in the relay losses

its color in very short time, necessary identification should be done without

loss of time.

Color of gas Identification/ Affected Material

Colorless

White

Yellow

Grey

Black

Air

Decomposed paper and cloth insulation

Decomposed wood insulation

Over heated oil

Decomposed oil due to arcing

MAINTENANCE:

Buchholz relay should be critically inspected and

subjected to operation test at least once in year. While performing test, dry

air or nitrogen should be injected through lower pet cock and its behavior

should be recorded. While doing mechanical inspection oil level should be

brought down and buckets should be checked for free movement. Mercury

switches should be checked for their performance.

DE- HYDRATING BREATHERS :

Presence of moisture in oil results into reduction of its di-

electric strength and may cause failure of transformer. As such, it is

necessary to ensure that oil remains clean and dry. Conservator tank is

mounted on the top of the transformer and is connected to the main tank by

means of small diameter pipe. The shape of the conservator is so designed

that minimum surface area of oil comes into contact with the atmosphere.

Interior of the tank is connected to the silica gel breather by a pipe. Silica

gel breather absorbs the moisture when the transformer breathes due to

expansion and contraction of the oil in account of changes in temperature.

While fixing the breather, make sure that all joint are

airtight. Silica gel is BLUE in color when dry. With the absorption of the

moisture, its color changes to VIOLET and then PINK. Pink color indicates

that the silica gel is saturated and is ineffective. It should be either replaced

or reactivated. While charging the breather following procedure must be

followed:

1. Remove the wing nuts supporting the body.

2. Glass container should be squarely fitted on its gasket, then pour

reactivated or fresh silica gel into the container upto a level ¼ inch

from top.

3. Fix the assembly to the top plate with inspection window facing

outward from the transformer and secure it with the wing nuts.

Ensure that the top gasket is in position.

4. Transformer oil should be poured into oil cup until over flows

through the screw hole and fix it to the assembly with the nut.

5. Silica gel is reactivated by applying heat to it or heating it in the oven

until its color is restored to blue. While baking silica gel temperature

should not exceed 150°C.

GASKETS AND GASKETTED JOINTS

Gaskets sometimes shrink during prolonged services, it is

therefore necessary to check the tightness of all bolts, fastening gasketed

joints. In order to avoid un even pressure bolts should be tightened evenly.

When gaskets are not available in one piece, they are built

in strips. While jointing the piece special care is taken to ensure that, the

strips fit closely. Following are some standard joints:

Following procedure should be adopted while fixing a new gasket:

1. The metal surface should be made clean paint should be removed.

Surface should be cleaned from oil with non-fluffy rag dipped in

petrol or benzene.

2. Heldite compound or Neoprene solution gasket compound be used for

fixing the gasket in position.

3. While tightening the bolts work alternately from opposite edges and

tighten until gasket is compressed to 75% of its original thickness.

MAGNETIC OIL GAUGE

The magnetic oil level gauge supervises the level of oil in

the conservator tank. The oil level gauge provided on the transformer is of

the dial type with minimum and maximum level markings and pointer,

which indicates the level of oil in the conservator.

The movement of the float arm is transmitted to the switch

mechanism by means of magnetic coupling, thereby sealing of the switch

compartment and tank is assured. The pointer is also magnetically operated.

In order to have shortest float arm length, oil level gauge is usually mounted

midway between high and low level to be indicated. If the oil level falls

below the minimum admissible level, a set of contacts are operated to give a

signal.

PRESSURE RELIEF VALVE/ EXPLOSION VENT

Relief valves are placed at the top of the small

transformers for the release of the gas, which may be generated on account

of over heating, arcing or short circuit under the oil. Transformers of

medium and large capacity are fitted with explosion vent. Explosion vent is

fitted with a diaphragm of mica, bakelite or glass. When heavy fault occurs

gasses are generated by the transformer in larger volume. In case, diaphragm

bursts with the force of pressure generated by these gasses, the transformer

should be isolated immediately. After the removal of defect, the diaphragm

should be replaced by new one and transformer is energized.

STATIONARY STORAGE BETTERY

Great care is required to be exercised while selecting the

right size of battery, as an under capacity battery may fail at the critical time

and non-operation of protective devices may damage important electrical

apparatus. Selection of an over capacity battery will result into investment of

additional money which could have been used anywhere else. The basic

factors, which influence selection of a storage battery, are as under:

Voltage of the battery :

As per recommendations of British Specifications 24,

30, 50, 110, 240 volts batteries may selected for the supply of direct current

to the auxiliary circuits required for actuating protective devices, signaling

equipment and emergency lighting. Batteries with rated voltage less than

110 volts are not recommended. The voltage drop and number of cells are

determined by following formulae:

Vd = 2*R*I

Vr = 100/95 Vd

N = Vr/ Vm

Vd = voltage drop in volts; R = resistance in ohms of the load

I = Max. anticipated current in amperes; Vm = min cell voltage

Capacity of the battery :

The capacity of the batteries is expressed in terms of Watt-

Hours or Ampere- Hours. The Watt-hour capacity is defined as the ability of

battery to do work and is determined by multiplying the Ampere-hour

capacity by the average value of the voltage during discharge. While

determining the size of the battery, current requirements of various services

or appliances are worked out and since switching mechanism requires large

quantity of current, although for short time, current requirements for

simultaneous switching operations are determined and the number of

positive plates to supply the required current at one minute rate may be

estimated. The size of battery depends upon the sum of these estimates.

Some of the factors, which influence the capacity of cell, are as follows:

Thickness

Rate of discharge

Temperature

Electrolyte

Condition of plates

The size of the battery is so determined that it should be

able to deliver the required current without the help of trickle charger for

specified number of hours.

1. Capacity in Ampere-Hours = Continuous load requirement in Watts* time in hours with charger out of circuit divided by rated voltage in volts

2. Maximum discharge current = Continuous load requirement in Watts+ Short time load in Watts Divided by rated voltage in volts

LEAD ACID STORAGE BATTERY

Lead acid storage cell consists of electrodes i.e. Anode

and Cathode in the form of plates immersed in diluted Sulphuric acid placed

in acid resistant container. These containers are usually made of Vulcanized

rubber, glass, plastic, ceramic etc. Glass and plastic containers are normally

used in case of stationary storage batteries. The container is provided with

vent to facilitate the escape of gasses as well as it provides opening for the

addition of distilled water or electrolyte.

Plates are used in shape of grids made of an alloy of lead

and antimony. The use of plates in the shape of the grid is helpful in

providing support to the active material conduction of electric current and

maintenance of uniform of the current through out the mass of the active

material. Lead mon-oxide is normally used as an active material.

Polystyrene Non-Splash Vent Plug Terminal Post Hard Rubber Lid Low Resistance Pillar Plastic Separator Guard Separator Retainer Mitex Micro porous Rubber Separator Semi Perforated End Tube Perforated P.V.C. Outer Sleeve Braided Glass Fiber Inner Sleeve Positive Active Material Corrosion Resistant lead Alloy Plastic Bottom Sealing Negative Plate Hard Rubber Container

THE ELECTROLYTE:

Sulphuric acid of very high purity diluted with distilled

water is used as an electrolyte in case of lead acid batteries. Since specific

heat of the water is higher than Sulphuric acid as such when it is mixed with

water abnormal increase in temperature takes place, it is therefore,

necessary to get the solution cooled down before it is poured into the

battery, to avoid damage to the plates.

The viscosity increase with decrease in temperature,

thereby affecting the capacity of the battery at low temperatures. The value

of the specific gravity is the indicator regarding the condition of the battery.

It is defined as the ratio between the equal volumes of the

liquid and the water at the specific temperature and is measured by means

of hydrometer.

Specific Gravities of the Electrolytes of Various Batteries

Types of battery Specific gravity

Stationary batteries

Truck and tractor batteries

Starting and lighting batteries

Aviation batteries

1.200 to 1.225

1.260 to 1.280

1.200 to 1.233

1.260 to 1.285

ALKALINE BATTERIES

Two types of alkaline storage batteries i.e. Nickle-

Cadmium and Nickle-Iron type. The constructional features of both the

batteries are same except that finely divided metallic Cadmium and finely

divided Iron is used as the negative active material in the respective cases.

Nickle hydro oxide is used as a active material in both the cases. Potassium

hydroxide of required specific gravity is used as an electrolyte.

Since the plates are assembled in steel containers and

electrolyte acts as a conductor, the cells are always alive and as such, coils

have to be insulated from one another to avoid short circuits. The cells are

arranged in hard wood crates.

CHARGING AND DISCHARGING OF THE STATIONARY

BATTERIES:

The storage battery does not generate or stores electrical

energy. Electrical energy in the form of chemical energy due to chemical

actions, which takes place at its electrodes, is stored by it, when it is

charged. When the cell is discharged, chemical energy is reconverted into

electrical energy and supplied to the load. The reversible chemical action in

lead acid cell with diluted Sulphuric acid as electrolyte and in case of

Nickle –Iron and Nickle-Cadmium cells with KOH as electrolyte are as

under:

1. Pb + PbO2 + 2H2SO4 2PbSO4 + 2H2O

2. 2Ni(OH)3 + Fe 2Ni(OH)2 + Fe(OH)2

3. Cd + Ni2O3 CdO + 2NiO

4. Cd + NiO2 CdO + NiO

VARIUOS TYPES OF CHARGING SYSTEMS

a) LEAD ACID BATTERIES:

1. Constant Current Charging System :

In this case, the current is kept constant by means of

rheostat connected in series, by increasing or decreasing the resistance as

the charge progresses.

2. Constant Voltage Charging System:

In this system, the voltage is kept at affixed value per cell.

The maximum voltage should not exceed 2.35 volts per cell. Since the

variation in case of line voltages will influence the charging currents and

excessive charging currents may damage the battery, so the voltage should

be maintained constant.

3. Trickle Charge System:

The battery is continuously charged at a low rate, which

may be sufficient to keep it in fully charged condition. The charging rate is

determined by:

a) Constant small load requirement

b) Charging current required to compensate the loss due to local action

c) For restoration of intermittent discharges of small amount

4. Floating Battery System:

In this case, the load is supplied by the charging

equipment i.e. rectifier, which is connected to alternating current mains.

The battery is also connected to the direct current mains in such a fashion

that the voltage of direct current mains is slightly higher than the open

circuit voltage of the battery. In case of failure of an A.C. system, the load

is supplied by the battery.

5. Boosting Charge:

Charging of a battery at a higher rate of current given to it

is known as Boosting charge, due to this charging the temperature of the

electrolyte rises and this temperature should not exceed 125°F.

6. Equalizing Charge:

Equalizing charge is given to the battery or restoration of

all the cells to a fully charged condition i.e. to correct any inequalities

among the cells of the battery that may get developed during service.

b) ALKALINE BATTERIES:

1. Routine Charging:

Charging equipment should be capable of supplying

charging current at 1.8 and 1.9 volts per cell in case of Ni-Cd and Ni-Fe

batteries, as the charging voltage required are 1.35 to 1.65 and 1.5 to 1.8

volts per cell.

2. Trickle & Floating Charge:

There is no loss of capacity in the absence of self-

discharge in the cell, however the battery can be kept on trickle or floating

charge with advantage for taking care of fluctuating loads.

Safety Precautions:

Protective equipments and recommended safety

precautions are listed below:

a) Protective Equipments:

Goggles

Acid proof gloves and aprons

Water

Bicarbonate of soda

b) Precautions:

1. While preparing electrolyte for lead acid batteries never pour water

into acid but add acid into water.

2. Handles of the tools required for tightening the bolts should be

insulated.

3. Smoking, presence of naked flame should be prohibited in the battery

room

4. Battery room should be well ventilated and provide with exhaust

fans.

5. Acid should be stored in separate rooms

6. Cells should be installed on wooden racks painted with acid resistant

paint.

7. Cells should be insulated from the racks by placing insulators in

between

8. Cells should be leveled during installation by using lead shims

9. Sulphuric acid containers are normally enclosed in wooden crates

10.Never use metallic vessels while handling acid or distilled water.

GENERAL CARE OF BATTERIES

a) LEAD ACID BATTERIES:

1. Electrolyte level must be maintained 10 to 15 mm above the plates

2. Terminal voltage of the cell must not be allowed to fall below 1.85V

3. Batteries should be charged to its rated capacity

4. Battery should not allowed to remain in semi-charged condition

5. Commercial Sulphuric acid should not be used

6. Only distilled water should be used for topping of the battery

7. Excessive charging of the batteries should be avoided.

8. Bare and insulated leads should be painted with recommended paints

9. Batteries should be kept clean and dry ,the battery room should be

well ventilated

10.Terminal posts and connectors should be clean and free from

moisture

11.Nuts and Bolts of cell connectors should be kept tightened and

smeared with Vaseline.

b. ALKALINE BATTERIES:

1. The battery should be kept neat and clean

2. Dirt and foreign matter should not be allowed to accumulated

below the cells, as its accumulation may result into heavy

discharge on account of short circuit

3. Connectors should be coated with petroleum jelly

4. Alkaline batteries should not be installed in the room along

with lead acid batteries

5. Topping should be done with the distilled water

6. Terminal voltage should not be allowed to fall below the

minimum voltage specified