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CHAPTER-1

BASIC ASPECTS OF PROTECTION

1.0 Principles of RelaysEvery electrical equipment is designed to work under specified normal

conditions. In case of short circuits, earth faults etc., an excessive current will

flow through the windings of the connected equipment and cause abnormal

temperature rise, which will damage the winding. In a power station, non-

availability of an auxiliary, at times, may cause total shut down of the unit,

which will result in heavy loss of revenue.

So, in modern power system, to minimise damage to equipment two alternatives

are open to the designer, one is to design the system so that the faults cannot

occur and other is to accept the possibility of faults and take steps to guard

against the effect of these faults. Although it is possible to eliminate faults to a

larger degree, by careful system design, careful insulation coordination, efficient

operation and maintenance, it is obviously not possible to ensure cent percent

reliability and therefore possibility of faults must be accepted; and the equipment

are to be protected against the faults. To protect the equipment, it is necessary to

detect the fault condition, so that the equipment can be isolated from the fault

without any damage. This function is performed by a relay. In other words,

protective relays are devices that detect abnormal, conditions in electrical

circuits by constantly measuring the electrical quantities, which are different

under normal and faulty conditions. The basic quantities which may change

during faulty conditions are voltage, current, frequency, phase angle etc. Having

detected the fault relay operates to complete the trip circuit which results in the

opening of the circuit breaker thereby isolating the equipment from the fault.

The basic relay circuit can be seen inMg. No. 1. 1

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Resetting Value : It is the value of the characteristic, quantity at which the relay returns

to its initial position.

Unrestricted Protection : It is a protection system which has no clearly defined zone of

operation and which achieves selective operation only by time grading.Basic Symbols: The equipments they represent are as given below:

Sr. No. Symbol Equipment Function

1 Circuit Breaker Switching during normal and abnormal conditions,interrupt the fault currents.

2 Isolator Disconnecting a part of the system from live partsunder no load conditions.

3 Earth switch Discharging the voltage on the lines to the earthafter disconnection.

4 LightingArrestor

Diverting the high voltage surges to earth.

5 CurrentTransformer Stepping down the current for measurement,protection, and control.

6 VoltageTransformer

Stopping down the voltage for the purpose ofprotection, measurement and control.

1.2 Functions of protective Relaying

To sound an alarm, so that the operator may take some corrective action and/or

to close the trip circuit of circuit breaker so as to disconnect a component during

an abnormal fault condition such as overload, under voltage, temperature rise

etc.

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To disconnect the faulty parts as quickly as possible so as to minimize the

damage to the faulty part. Ex: If a generator is disconnected immediately after a



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winding fault only a few coils need replacement. If the fault is sustained, it may

be beyond repairable condition.

To localise the effect of fault by disconnecting the faulty part from the healthy

part, causing least disturbance to the healthy system. To disconnect the faulty part as quickly as possible to improve the system

stability and service continuity.

1.3 The requirements of protective relaying

* Speed: Protective relaying should disconnect a faulty element as quickly as

possible, in order to improve power system stability, decrease the amount of

damage and to increase the possibility of development of one type of fault

into other type. Modern high speed protective relaying has an operating

time of about 1 cycle.

* Selectivity : It is the ability of the protective system to determine the point

at which the fault occurred and select the nearest of the circuit breakers,

tripping of which leads to clearing of fault with minimum or no damage to

the system.

* Sensitivity : It is capability of the relaying to operate reliably under the

actual minimum fault condition. It is desirable to have the protection as

sensitive as possible in order that it shall operate for low value of actuating

quantity.

* Reliability : Protective relaying should function correctly at all times under

any kind of fault and abnormal conditions of the power system for which it

has been designed. It should also not operate on healthy conditions of

system.

* Simplicity : The relay should be as simple in construction as possible. As a

rule, the simple the protective scheme, less the

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number of relays, and contacts it contains, the greater will be the reliability.

* Economy : Cost of the protective system will increase directly with the

degree

of protection required. Depending on the situation a designer should strike

a



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balance between with the degree of protection required and economy.

1.4 Classification of Relays

1.4.1 Depending upontheir principle ofoperation theyare classifted as:

Electromagnetic attraction type relays: These relays operate by the virtue of aplunger being drawn into a solenoid or an armature being attracted towards the

poles of an electromagnet.

Induction type Relays: In this type of relay, a metal disc or cup is allowed to

rotate or move between two electro-magnets. The fields produced by the two

magnets are displaced in space and phase. The torque is developed by

interaction of the flux of one of the magnets and the eddy current induced into

the disc/cup by the other.

Thermal Relays : They operate due to the action of heat generated by the

passage of current through the relay element. The strip consists of two metals

having different coefficients of expansions and firmly fixed together throughout

the length so that different rates of thermal expansion of two layers of metal

cause the strip to bend when current is passed through it.

Static Relays : It employs discrete electronic components like diodes,

transistors, zenners, resistors/capacitors or Integrated circuits and use electronic

measuring circuits like level detectors, comparators, integrators etc. to obtain

the required operating characteristics.

Moving Coil Relays : In this relay a coil is free to rotate in magnetic field of a

permanent magnet. The actuating current flows through the

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coil. The torque is produced by the interaction between the field of the

permanent magnet and the field of the coil.

1.4.2 Relays canbe classified depending upon their application also.

Over voltage, over current and over power relays, in which operation takes place

when the voltage, current or power rises above a specified value.



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Under voltage, under current under frequencies low power relays, in which

operation takes place when the voltage, current frequency or power fall below a

specified value.

Directional or reverse power relays: In which operation occurs when thedirection of the applied power changes.

Distance Relays : In this type, the relay operates when the ratio of the voltage

and current change beyond a specified limit.

Differential Relays : Operation takes place at some specific phase or magnitude

difference between two or more electrical quantities.

1.4.3 Relays can also be classifiedaccording totheir time ofoperation.

Instantaneous Relay : In which operation takes place after negligibly small

interval of time from the incidence of the current or other quantity causing

operation.

Derinite time lag Relay: This operates after a set time lag, during which the

threshold quantity of the parameter is maintained.

Inverse time lag Relay: This operates after a set time Lab, during which the

operating quantity of the parameter is maintained above its operating threshold.

1.5 Operating Principles of different types of Relays

1. 5. 1 Induction over current and earth fault relays

These are quite commonly used relays. Schematic diagram of induction disc

type relay is shown inMg. No. 1.2.

The output of the current transformer is fed to a winding (1) on the centre limb

of the E-shaped core, the second winding (2) on the limb

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is connected to two windings on the poles of the E and U shaped cores. The

magnetic flux across the air gap induce currents in the disc, which in conjunction

with the flux produced by the lower magnet, produces'a rotational torque. A

magnet (3), is used to control the speed of the disc. The time of operation of therelay v aries inversely with the current fed into it by the current transformer of

the protected circuit. The permanent magnet used for breaking has a tendency to

attract iron filings, which can prevent operation. So care has to be taken while

manufacturing this type of relays. Time-current characteristics induction type

relays has been given in Fig. 1.3.

1.5. 2 Balanced Beam Relays

It,consists of a horizontal beam pivoted centrally, with one armature attached to

either side. There are two coils one on each side. The current in one coil gives

operating torque. The beam is given a slight mechanical bias by means of a

spring so that under normal conditions trip contacts will not make and the beam

remains in horizontal position. When the operating torque increases then the

beam tilts and closes the trip contacts. In current balance system both coils are

energised by current derived from CT's. In impedance relays, one coil is

energised by current and other by voltage. In these relays the force is

proportional to the square of the current, so it is very difficult to design the relay.

This type of relay is fast and instantaneous. In modern relays electromagnets are

used in place of coils (See Mg.. 1.4.).

1.5.3 Permanent - MagnetMoving - Coil Relgys:

There are two general types of moving coil relays. One type is similar to that of

a moving coil indicating instrument, employing a coil rotating.between the poles

of a permanent magnet. The other is, employing a coil moving at right angles to

the plane of the poles of a permanent magnet. Only direct current measurement

is possible with both the types.

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The action of a rotating coil type is shown in the Fig. 1.5. A light rectangular coil

is pivoted so that its sides lie in the gap between the two poles of a permanent

magnet and a soft iron core. The passage of current through the coil produces a

deflecting torque by the reaction between the permanent magnetic field and thefield of the coil

(See Fig. 1. 5)

The moving contact is carried on an arm which is attached to the moving coil

assembly. A phosper bronze spiral spring provides the resetting torque.

Increasing the contact gap and thus increasing the tension of the spring permits

variation in the quantity required to close the contacts.

Time current characteristic of a typical moving coil permanent magnetic relays is

shown in Fig. 1. 6.

1.5. 4 Attracted armaturerelays :

It is required to clear the faults in power system as early as possible. Hence, high-

speed relay operation is essential. Attracted armature relays have a coil or an

electromagnet energised by a coil. The,coil is energised by the operating quantity

which may be proportional to circuit current or voltage. A plunger or a rotating

vane is subjected to the action of magnetic field produced by the operating

quantity. It is basically single actuating quantity relay.

Attracted armature relays respond to both AC and DC quantities. They are very

fast in operation. Their operating time will not vary much with the amount of

current. Operating time of the relay is as low as 10-15 m seconds and resetting

time is as low as 30 m sec can be obtained in these relays. These relays are non-

directional and are simple type of relays. Examples of attracted armature type

relays are given in Fig.1.7

1.5.5 Time Lag Relays:

These are commonly used in protection schemes as a means of time

discrimination. They are also frequently used in control, delayed

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auto-reclosing and alarm scheme to allow time for the required sequence of

operations to take place, and to measure the duration of the initial condition to

ensure that it is not merely transient.



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Various methods are used to obtain a time lag between the initiation of the relay

and the operation of its contact mechanism. These includes gearing, permanent

magnet damping, friction, thermal means or R.C. circuits. In some cases the time

lag in operation of the contacts is achieved by a separate mechanism released by avoltage operated elements. The release mechanism may be an attracted armature

or solenoid and plunger. The operating time of such relay is independent of the

voltage applied to the relay coil. One of the simplest forms of time lag relay is

provided by a 'Mercury switch in which the flow of mercury is impeded by a

constriction in the mercury bulb. The switch is tilted by a simple attracted

armature mechan,iwn. The time setting of such a relay is fixed by the design of

the tube. Another method of obtaining short time delays is to delay operation of a

normally instantaneous relay by means of a device which delays the build up or

decay of the flux in the operating magnet. The device consists of a copper ring

(slug) around the magnet and can produce delay on pickup as well as delay on

reset.

1.6 Testing and Maintenance of Protective Relays :

Unlike other equipment, the protective relays remain without any operation until a

fault develops. However, for a reliable service and to ensure that the relay is

always vigilant, proper maintenance is a must. Lack of proper maintenance may

lead to failure to operate.

It is possible for dirt and dust created by operating conditions in the plant to get

accumulated around the moving parts of the relay and prevent it from operating.

To avoid this, relays are to be cleaned periodically.

In general, overload relays sense over load by means of thermal element. Loose

electrical connections can cause extra heat and may

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result in false operation of the relay. To avoid this, all the relay connections are to

be tightened every now and then.

To confirm that the relay operation at the particular setting under particular

conditions for which the relay is meant for operating, we should perform number

of tests on the relays. Quality control is given foremost consideration in

manufacturing of relay. Tests can be grouped into following five classes:



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* Measuring insulation resistance of circuits.

* Checking C.T. Ratios

* Checking P.T. ratio, polarity and phasing

* Conducting secondary injection test on relays.* Conducting primary injection test

* Checking tripping and alarm circuits.

* Stability check for balanced protections like differential/REF.

1.6.3Maintenance Tests

Maintenance tests are done in field periodically. The performance of a relay is

ensured by better maintenance. Basic requirements of sensitivity, selectivity,

reliability and stability can be satisfied only if the maintenance is proper.

The relay does not deteriorate by. normal use; but other adverse conditions cause

the deterioration. Continuous vibrations can damage the pivots or bearings.

Insulation strength is reduced because of absorption of moisture; polluted

atmosphere affects the relay contacts, rotating systems etc., Relays room,

therefore, be maintained dust proof. Insects may cause mal-operation of the

relay. Relay maintenance generally consists of

a) Inspection of contacts

b) Foreign matter removal

c) Checking adjustments

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d) Checking of breaker operations by manual contact closing of relays.

e) Cleaning of covers etc.

1.6.4 Maintenance Schedule:

1. Continuous Supervision: Trip circuit supervision, pilot supervision, relay,

auxiliary voltage supervision, Battery supervision, CT circuit supervision.

2. Relay flags are to be checked and resetted in every shift.

3. Carrier current protection testing is to be carried out once in a week.

4. Six monthly inspections : Tripping tests, insulation resistance tests, etc.

Secondary injection tests are to be carried out at least once in a year.

The following tests are to be performed during routine maintenance:



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Inspection : Before the relay cover is removed, a visual check of the cover is

necessary. Excessive dust, dirt, metallic material deposited on the cover should

be removed. Removing such material will prevent it from entering the relay

when the cover is removed. Fogging of the cover glass should be noted andremoved when the cover has been removed. Such fogging is due to volatile

material being driven out of coils and insulating materials. However, if the

fogging is excessive, cause is to be investigated. Since most of the relays are

designed to operate at 400C, a check of the ambient temperature is advisable.

The voltage and current carried by the relay are to be checked with that of the

name plate details.

1.6. 5 Mechanical, adjustments and Inspection:

The relay connections are to be tight, otherwise it may. cause overheating at the

connections. It will cause relay vibrations also. All gaskets should be free from

foreign matter. If any foreign matter is found gaskets are to'be checked and

replaced if required.,

Contact gaps and pressure are to be measured and compared with the previous

readings. Large variation in these measurements will

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indicate excessive wear, in which case worn contacts are to be replaced.

Contacts alignment is to be checked for proper operation.

1.6.6 Electrical Tests and Adjustments

Contact function : Manually close or open the contacts and observe that they

perform their required function.

Pick up: Gradually apply actuating quantity (current or voltage) to see that

pickup is within limits.

Drop out or reset: Reduce the actuating quantity (current or voltage) until the

relay drops out or fully resets. This test will indicate excess friction.

Repair tests involve recalibration, and are performed after major repairs.

Manufacturers tests include development tests, type and routine tests.

1.7 Test Equipment

1.7. 1 Primary current injection test sets:



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Generally protective gear is fed from a current transformer on the equipments to

be protected and primary current injection testing checks all parts of the

protection system by injecting the test current through the primary circuit. The

primary injection tests can be carried out by means of primary injection test sets.The sets are comprising current supply unit, control unit and other accessories.

The test set can give variable output current. The output current can be varied

by means of built-in auto transformer. The primary injection test set is

connected to AC single phase supply. The output is connected to primary circuit

of CT. The primary current of CT can be varied by means of the test set. By

using this test one can find at what value of current the relay is picking up and

dropping out.

1.7.2 Secondary current injectiontest set:

It checks the operation of the protective gear but does not check the overall

system including the current transformer. Since it is a rare

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PAGE 19occasion for a fault to occur in CT, the secondary test is sufficient for most

routine maintenance. The primary test is essential when commissioning a new

installation, as it checks the entire system. A simple test circuit is given inMg.

1. 8.

1.7.3 Test Benches

Test benches comprise calibrated variable current and voltage supplies and timing

devices. These benches can be conveniently used for testing relays and obtaining

their characteristics.



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1.8 Static Relaying Concepts

1.8. 1 Introduction

Static Relay is a relay in which the comparison or measurement of electrical

quantities is done by stationary network which gives a tripping signal when thethreshold value is crossed. In simple language static relay is one in which there

are no moving parts except in the output device. The static relay includes

electronic devices, the output circuits of which may be electric, semiconductor or

even electromagnetic. But the output device does not perform relay

measurement, it is essentially a tripping device. Static relay employs electronic

circuits for the purpose of relaying. The entity voltage, current etc. is rectified

and measured. When the output device is triggered, the current flows in the trip

circuit of the circuit breaker.

With the inventions of semiconductors devices like diodes, transistors, thyristors,

zener diodes etc., there has been a tremendous leap in the field of static relays.

The development of integrated circuits has made an impact in static relays. The

static relays and static protection has grown into a special branch.

1.8.2 Advantages of Static Relays:

The static relays compared to the electromagnetic relays have many advantages

bind a few limitations.

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1.8. 3 Low Power Consumption

Static relays provide less burden on CTs and PTs as compared to conventional

relays. In other words, the power consumption in the measuring circuits of static

relays is generally much lower than that for the electromechanical versions. The

consumption of one milli-VA is quite common in static over current relay

whereas as equivalent electromechanical relay can have consumption of about 2-

3 VA. Reduced consumption has the following merits.

a) CTs and PTs of less ratings are sufficient

b) The accuracy of CTs and F'Ts are increased

c) Air gaped CTs can be used

d) Problems arising out of CT saturation are avoided

e) Overall reduction in cost.



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1.8.4 Operating time

The static relays do not have moving parts in their measuring circuits, hence

relay times of low values can be achieved. Such low relay times are impossible

with conventional electromagnetic relays.By using special circuits the resetting, times and the overshoot time can be

improved and also high value of drop off to pick up ratio can also be achieved.

1.8.5 Compact

Static relays are compact. The use of integrated circuit have further reduced

their size. Complex protection schemes may be obtained by using logic circuits

or matrix. Static relays can be designed with good repeat accuracies. Number of

characteristics can be obtained in a single execution, unlike in case of their

Electro-mechanical counter parts.

Most of the components in static relays including the auxiliary relays in the

output stage are relatively immune to vibrations and shocks. The risk of

unwanted tripping is therefore less with static relays as

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compared to electromagnetic relays. So, these can be applied in earthquake prone

areas, ships, vehicles, aeroplanes etc.

1.8. 6 Transducers

Several non-electrical quantities can be converted into electrical quantities and

then fed to static relays. Amplifiers are used wherever necessary.

1.8.7 Limitations

Auxiliary voltage requirement : This disadvantage is not of any importance as

auxiliary voltage can be obtained from station battery supply and conveniently

stepped down to suit load requirements.

Static relays are sensitive to voltage spikes or voltage transients. Special

measures are taken to overcome this difficulty. These include use of surge

supressors and filter circuits in relays, use of screened cables in input circuits,

use of galvanically isolated auxiliary power supplies like d.c./d.c. convertors, use

of isolating transformers with grounded screens for C.T./P.T. input circuits etc.

1.8.8 Temperature Dependence of Static Relays



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The characteristic of semiconductors are influenced by ambient temperatures.

For example, the amplification factor of a transistor, the forward voltage drop of

a diode etc., changes with. Temperature variation. This was a serious limitation

of static relays in the beginning. Accurate measurement of relay should not beaffected by temperature variation. Relay should be accurate over a wide range

of temperature. (-200C to +50

OC) this difficulty is overcome by

a) Individual components in circuits are used in such a way that change in

characteristic of component does not affect the characteristic of the

complete relay.

b) Temperature compensation is provided by thermistor circuits.

Extra precaution for quality control of the components has to be taken. As the

failure rate is highest in early period of components life, artificial ageing of the

components is normally done by heat soaking.

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1.8. 9 Level Detectors

A relay operates when the measured quantity changes, either from its normal

value or in relation to another quantity. The operating quantity in most

protective relays is the current entering the protected circuit. The relay may

operate on current level against a standard bias or restrain, or it may compare the



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current with another quantity of the circuit such as the bus voltage or the current

leaving the protected circuit (Fig. 1. 9).

In a simple electromagnetic relay-used as level detector, gravity or a spring can

provide the fixed bias or reference quantity, opposing the force produced by theoperating current in electromagnet. In static relays the equivalent is a D.C.

voltage bias.

E.g. In the semiconductor circuit (See Fig.1.10) the transistor is reverse biased in

normal conditions. No current flows through the relay coil. Under fault

conditions capacitor will be charged to +ve potential at the base side. If this

potential exceeds that of the emitter,, the B-E junction will be forward biased

and transistor will conduct there by tripping the relay. Thus the comparison is

made against the D.C. fixed bias.

1.8.10 Comparators

In order to detect a fault or abnormal conditions of the power system, electrical

quantities or a group of electric quantities are compared in magnitude or phase

angle and the relay operates in response to an abnormal relation of these

quantities. The quantities to be compared are fed into a comparators as two or

more inputs; in complex relays each input is the vectorial sum or difference of

two currents or voltages of the protected circuit, which may be shifted in phase -

or, changed in magnitude before being compared.

1.8.11 Types of comparators :

Basically there are two types of comparators, viz.

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a) Those whose output is a D.C. voltage proportional to the vector product

of the two A.C. input quantities;

b) Those which give an output whose polarity depends upon the phase

relation of the inputs. The later are sometimes called coincidence type and canbe direct acting or integrating.

1.8.13 Operating Principles of Static Time Current Relays:

Fig. 1. 1 7 shows the block diagram of a static time current relay. The auxiliary

C.T. has taps on the primary for selecting the desire pickup and current range.

Its rectified output is supplied to, a fault detector and an RC timing circuit.

When the voltage of the timing capacitor has reached the value for triggering the

level detector, tripping occurs.

Operation ofa typical static time current relay : The current from the main 0,.T.

is first rectified and smoothed by the capacitor 'Cs' and then passed through the

tapped resistor 'Rs' so that the voltage across it is proportional to the secondary

current. The spike filter RC protects the rectifier bridge against transient over

voltages in the incoming current signal, Fig. 1. 18.

1.8.14 Timing Circuit

The rectified voltage across the 'Rs' charges the capacitor 'Ct' through resistor

'Rt'. When the capacitor voltage exceeds the base emitter voltage 'Vt' the

transistor 'T2' in the Fig. 1,20 becomes conductive, triggering transistor 'T3' and

operating the tripping relay.

Resetting circuit : In order that the relay sha.11 have an instantaneous reset, the

capacitor 'Ct' must be discharged as quickly as possible. This is achieved by the

detector as follows (Fig. 1. 1 9).

The base of the transistor 'Tl' is normally kept sufficiently positive relative to

emitter to keep it conductive and hence short circuiting the timing capacitor 'Ct'

at YY in Fig. 1.20. When a fault occurs the over current through the resistor 'Rs'

makes the base of 'Tl' negative and cuts it off leaving 'Ct' free to be charged.

When the fault is cleared the

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current falls to zero and the negative bias on 'Tl' disappears so that 'Ct'is again

short circuited and discharged immediately.

A weakness of very fast instantaneous units is the tendency to over sensitivity on

off-set current waves. The instantaneous unit can be made insensitive to theD.C. off set component by making the auxiliary C.T. saturate just above the

pickup current value and connecting the capacitor and a resistor across the

rectified input to the level detector. This prevents tripping until both halves of

the current wave are above pickup valve. That is, until the off set has gone, the

short delay thus entailed is acceptable with time current relaying.

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CHAPTER-2

INDUCTION DISC TYPE IDMT

OVER CURRENT RELAYS

2.1 Introduction

Induction types relays are most widely used for protective relaying purposes

involving A.C. quantities. Torque is produced in these relays when alternating

flux reacts with eddy currents induced in a disc by another alternating flux of the

same frequency but displaced in time and space. These relays are used as over

current or earth fault relay. In its simplest form, such a relay consists of a

metallic disc which is free to rotate between the poles of two electromagnets

(Fig. 2. 1).

The spindle of this disc carries a moving contact which bridges two fixed

contacts when the disc rotates through an angle which is adjustable. By

adjusting this angle the travel of the moving contact can be adjusted so that the

relay can be given any desired time setting which is indicated by a pointer on a

time setting dial. The dial is calibrated from 0 to 1. These figures do not

represent the actual .operating times but are multipliers to be used to convert the

time known from the relay name plate curve into the actual operating time.

The upper electromagnet has a primary and a secondary winding. The primary

is connected to the secondary of a C.T. in the line to be protected and is provided

with tappings. These tappings are connected to a plug setting bridge which isusually arranged to giv@, seven selections of tapping, the over current range



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desired tapping and, therefore, current setting can be selected by inserting a pin

between the spring loaded jaws of the bridge type socket at the appropriate tap

value. When the pin is withdrawn for the purpose of changing the setting while

the relay is in service, the relay automatically adopts a high setting, thus ensuring

that the C.T. secondary is not open circuited and that the relay remains operative

for faults during the process of changing the settings. The secondary winding

surrounds the limbs of the lower electromagnet as well. The torque exerted on



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the disc is due to the interaction, of eddy currents produced therein by means of

the leakage flux from the upper electromagnet and the flux from the lower

electromagnet: these two fluxes having a phase displacement between them.

2.2 Characteristic CurveA set of typical time current characteristic curves of this type of relay is shown

in Fig. 2.2. The curve shows the relation between the operating current in terms

of current setting multiplier along the x-axis and operating time in seconds along

the y-axis. A current setting multiplier indicates the number of times the relay

cut-rent is in excess of the current setting. The current setting multiplier is also

referred to as plug setting multiplier (P. S. M.). Thus

P.S.M. = Primary CurrentPrimary Setting Current

= Primary Current

Relay Current Setting XC.T.Ratio

where, as is usually the case, the rated current of the relay is equal to the rated

secondary current of C.T. From the figure the operating time, when current

setting multiplier is 10 and the time multiplier is sea at 1, is 3 seconds. This is

sometimes called the basic 3/ 10 curve.

It is evident that at the same current setting but the time multiplier set at 0.8, the

time of operation is 2.4 seconds. Thus to get the actual time of operation a ainst

any particular time multiplier setting, 9

multiply the time of operation of the basic curve by the multiplier

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setting. Thus in this example the time of operation is 3 x 0.8 2.4 secs.

The time current characteristics ofFYg. 2.2 are the inverse definite minimum

time (I.D.M.T.) type since the time of operation is approximately inversely

proportional to smaller values of current and tends to a definite minimum time asthe current increases above 10 times the setting current.

The D.M.T. characteristic is obtained by saturating the iron in the upper magnet

so that there is practically no increase in flux after current has reached a certain

value. This results in the flattening out of the current time curve.

Example : An I.D.M.T. over current relay has a current setting of 150% and has

a time multiplier setting of 0.5. The relay is connected in the circuit through a

C.T. having ratio 500:5 amps. Calculate the time of operation of the relay if the

circuit carries a fault current of 6000 A. The relay characteristic is shown is Fig.

2.3.

Solution: Sec fault current 6000 X 5 60A

500

Plug Setting multiplier (P. S. M.) = Actual Current in Relay = 60 = 8

Setting Current 5 x 1.5

Time from graph against this multiplier of 8 = 3 15 sec.

Operating time = 3. 1 5 x 0. 5 = 1. 575 sec.

PAGE 37

CHAPTER - 3

MOTOR PROTECTION

Electrical Motor is an important component of an industry. Squirrel cage

induction motor is most widely used in power stations and industries. To protectthe motor from different faults condition various protection are provided, which

are as listed below.

3.1 Overload Protection

A motor may get overloaded during its operation because of excessive

mechanical load; (b) Single phase; (c) Bearing fault. An overloaded' motor

draws overcurrent resulting in overheating of the winding insulation. A

reasonable degree of overload protection can be provided by Bi-metallic thermal



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starting of all motors together, when the supply is subsequently restored. Thus,

prevents stressing of the supply source.

Composite Motors Protection Relays (Conventional analog types) provide

following protection functions.a) Thermal overload (Alarm/Trip) - ITH

b) Short circuit (Isc)

c) Single Phasing (I2)

d) Earth Fault (I0)

e) Stalling (lIt)

Numerical versions are now available which offer following additional

protection functions, besides those given above.

f) Prolonged starting time

g) Too many start

h) Loss of load

The Numerical versions have data acquisition capabilities and provide useful

service Data (such as load currents, I2/I. content in load current, thermal status

etc.), historic data fault data on operation. These relays have programmable

settings, programmable output relays and continuous self monitoring against any

internal failures.

PAGE 40

CHAPTER - 4

TRANSFORMER PROTECTIONS

4.1 Transformer protections are provided

a) against effects of faults in the system to which the transformer isconnected.

b) against effects of faults arising in the transformer itself.

4.1.2 Protections against faultsin the System

a) Short Circuits

b) High Voltage, high frequency disturbance

c) Flure Earth Faults.

4.1.3 System Short Circuits



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A short circuit may occur across any two phases (phase to phase) or between any

one line and earth neutral (phase to earth). The effect is excessive over current

and electromagnetic stresses proportional to square of short circuit current. For

these type of faults additional reactance and additional bracing of the transformerwinding and end leads is resorted to. This reactance may be incorporated in the

design itself or separate series reactance with primary of transformer is provided.

4.1.4 High Voltage High frequency surges:

These surges may be due to arching grounds, switching operation surges or

atmospheric disturbances. These surges have very high amplitudes, steep wave

front currents and high frequencies. Because of this, the breakdowns of the

transformer turns adjacent to line terminals occurs causing short circuit between

the turns.

To take care of this, the transformer winding is to be designed to withstand the

impulse surge voltages as specified below and then protect it by surge divertors.

PAGE 41

System Voltage KV(RMS) Impulse voltage withstand level(Peak value)

7.2 KV 60 MV

12.5 KV 75 KV

33 KV 170 KV

66 KV 250 KV

145 KV 550 KV

245 KV 900 KV

400 KV 1350 KV

Surge divertors are provided from each line to earth. These consist of several spark

gaps in series with a non-linear resistance. This spark gap breakdown when surge

reaches the divertor and disturbance is discharged to earth through nonlinear resistance

since at high voltage divertors resistance is low. These surge divertors should have

rapid response, non-linear characteristics, high thermal capacity, high system flow

current interrupting capacity and consistent characteristics under all conditions.

4.1.5 System Earth Faults



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a) When neutral of the system is earthed :-It represents short circuit across the

phase. Hence, same protection as for short circuit can be provided.

b) When neutral is not earthed transformer is used. :-Surge divertor gears in

front of transformer is used.4.2 Protection against Internal Faults

a) Electrical faults which cause serious immediate damage but are detectable

by unbalance of current or voltage.

i) Phase to Earth Fault or phase to phase faults on HV and LV external

terminals.

ii) Phase to earth faults or phase to phase faults on HV & LV winding.

iii) Short circuit between turns on HV & LV winding (inter turn faults)

iv) Earth faults on tertiary winding or short circuit between turn of tertiary

winding.

v) Problem in tap changer gear.

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b) Incipient faults : These are initially minor but subsequently develops

itself resulting into damage to the transformer. These may be due. to -

i) Poor electrical connection of conductors due to breakdown of insulation of

laminations, core bolt faults, clampings, rings etc.

ii) Coolant failure

iii) Blocked oil flow causing local hot spot on winding.

iv) Continuous uneven load sharing between transformers in parallel causing

overheating due to circulating current.

4.3 Principles of Protection System Principles used are -

i) Overheating

ii) Over current

iii) Un-restricted earth faults

iv) Restricted earth-faults

v) Percentage bias differential protection

vi) Gas detection due to incipient faults

vii) Over fluxing

viii) Tank earth current detection



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ix) Over voltage

x) Tap changer problems.

4.4 Gas Detection

a) Buchholz relay protectionb) Pressure relief valves/switches (for heavy internal faults)

4.4.1 Buchholz Protection

This is for two types of faults inside the transformer.

a) for incipient faults because of -

i) bolt insulation failure

ii) short circuit in laminations

iii) local over heating because of clogging of oil

iv) Excess ingress of air in oil system

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v) loss of oil due to heavy leakage

vi) Uneven load sharing between two transformers in parallel causing

overheating due to circulating current.

These generate gases causing operation of upper float and energises the alarm

circuits.

b) For serious faults inside the transformer due to -

i) short circuit between phases

ii) winding earth faults

iii) puncture on bushing

iv) tap changer problems

These types of faults are of serious nature and operate both the floats provided in

the buchholz relay and trip out the transformer.

4.4.2 Principles ofBuchholz Relay Operation (Fig. 4. 1)

This relay is provided in the connecting pipe from transformer tank to

conservator. Two floats are provided inside the relay and are connected to

mercury switches. Normally the relay is full of oil and in case of gas collection

the floats due to their buyopancy, rotate on their supports until they engage their

respective stops. Initially fault develops slowly an heat is produced locally

which begins to decompose solid or liquid insulating material and thus produce



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inflammable gases. Gas bubbles are collected in relay causing oil level to lower

down. The upper float rotates as the oil level in the relay goes down and when

sufficient oil is displaced the mercury switch contacts close and initiates alarm.

For serious faults as described above, gas generation is more violent and the oildisplaced by gas bubbles flows through connecting pipe to conservator. This

abnormal flow of oil causes deflection of both float and trip out the transformer.

Recently the dissolved gas analysis technique (gas chromatography) is in use for

pre-detection of type of slowly develo ing faults inside the transformer which

helps to decide whether the transformer maintenance/internal inspection is

required to be

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PAGE 45

carried out or otherwise, and thus helps to predict transformer damage in future.

4.4.3 Dissolved Gas Analysis

The inflammable gases dissolved in the transformer oil are mainly hydrocarbongases (methane CH4, Ethane C2H6, Ethylene C2H4, Acetylene C2H2, Propane,



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hydrogen, carbon monoxide and carbon dioxide). With the help of dissolved gas

analysis equipment the concentration of these gases in PPM can be known and

can be cross checked with the IS standard. Also with the help of Roger's ratio

method, the type of probable incipient fault can be judged and corrective actioncan be taken in advance to prevent failure of the transformer (Ref. Annex. 1 &,

H).

4.5 Over Heating Protection

Protection is mainly required for continuous over load of the transformer.

a) Protection is based on measurement of winding temperature which is

measured by thermal image technique.

b) Thermal sensing element is placed in small pocket located near the top

transformer tank in the hot oil. A small heater fed from a current

transformer (winding temp. C.T.) in the lower voltage terminal of one

phase, is also located in this pocket and produces a local temp. rise,

similar to that of main winding and proportional to copper losses, above

general temp. of oil.

c) Winding temperature high alarm/trip is provided through mercury

switches in the winding temp. indicators.

d) By thermometers, mercury switches heat sensing silicon resistance are

also used for sensing the temp. rise.

e) Thermisters are provided mainly in the dry type transformers for

temperature sensing.

Temperature of 550 above ambient of 500C is generally provided for tripping.

PAGE 46

4.6 Over Current & Earth leakage protection

4.6.1 EarthLeakage Protection

In case of transformer earthed through resistance or earthed through impedance.

Resistance Grounding : The earth fault current in faulty winding in resistance

grounded transformer depends on voltage between neutral and fault point and is

inversely proportional to neutral resistance.

10kv x P

ly = 3. Rn



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Where ly is earth fault current; P = percentage of winding to be protected; KV -

line to line voltage and Rn = Neutral Grounding resistance. Suitable earth fault

relay can be provided across C.T. in the neutral of the transformer depending

upon the minimum earth fault current to be detected.Impedance Grounding Transformer neutral is connected to the primary of

neutral grounding transformer. The suitable resistance is connected in parallel

with the secondary of this neutral grounding transformer. The earth fault relay

(neutral displacement relay) is connected across this resistance. The earth fault

relay can be set at about 2.5 percent of maximum neutral voltage The relay is

time delayed for transient free operation.

4.6.2 Overcurrent protection

i) HRC fuses are provided for small distribution transformer.

ii) Over current relays are used for power transformers, considering

the following:

a) IDMT relays should be chosen

b) Discrimination with circui t protection of secondary side should be

provided;

c) instantaneous trip f@cility for high speed clearance of terminals short

circuit should be provided.

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d) Setting depends on transformer reactance or percentage impedance,

faults MVA, type of relay used.

e) Setting of over current relays can be slightly higher than rated full load

current (say 120 percent of FL) with proper discrimination.

4.6.3 Combined overcurrent and unrestricted EIP Protection (Ref. -Kg. 4.2)

a) Typical over current/earth fault protection is shown for a Delta/start

transformer in Fig. 4.2.

b) IDMT O/C elements on delta and star side, primarily serve as back up

protection against downstream short circuits and are time coordinated

with downstream O/C protections.

c) The high set instantaneous O/C elements on Delta side (connected to

source) are provided to detect severe terminal short circuits and quickly



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isolate the transformer. These are set over and above the maximum

short circuit current infeeds of the transformer for star side faults.

d) The start side earth fault protection (IDMT) serves as a backup against

downstream earth faults and is required to be suitably time graded. Thiscan either be residually connected across the phase C.Ts or operated off

a C.T. in the Neutral Earth connection (standby earth fault relay). The

latter is considered to be advantageous since it can detect star winding

earth faults, beside providing backup for downstream earth faults. Since

the neutral C.T. ratio is not tied up with the load current, a lower C.T.

ratio consistent with the maximum E/F current limited by NGR can be

provided. This renders good sensitivity for the standby E/F protection.

e) The E/F protection on delta side is inherently restricted to delta winding

earth faults and does not respond to earth faults on the star side, due to

zero sequence isolation provided by the delta connection. The delta side

E/-F protection, therefore, assumes the form of REF

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protection, enabling sensitive setting and instantaneous operation. The relay is

connected in high impedance mode with a series stabilizing resistor, as shown.

4.6.4 Restricted EarthFault Protection : (Ref. Mg. 4.3)

a) REF protection is used to supplement the differential protection,particularly where star neutral of the transformer is grounded through a

neutral grounding resistor to limit the earth fault current. REF protection

provides increased coverage to the star winding against earth faults.

b) The REF protection operates on the principle of Kirchoff's law and

requires CTs of identical ratio and ratings as the phases and neutral

earth connection. The relay is connected across the parallel combination

of the CTs in High Impedance mode.

c) For external earth fault, the associated CTs have dissimilar polarities

forming a series connection. Thus, the resulting current through the

relay is negligible. For internal fault, however, the CTs have similar

polarities, forming a parallel connection, thus adding up the current in the

relay branch. This ensures positive operation of the relay.

4.7 Percentage Bias Differential Protection

a) In this protection, operating current is a function of differential current.

b) The value of restraining current depends on 2nd and 5th harmonic

component of differential current during magnetic inrush and over excited

operation.

c) Bias current is a function of through current (external fault current) and

stabilizes the relays against heavy external fault.

4.7. 1 Basic Consideration for differentiazprotection

a) Transformer ratio : the current transformers should match to the rated

currents of the primary windings.

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b) Transformer Connection: In delta star connected transformer, the phase

shift of 300C between primary and secondary. side current exist. Also

zero sequence current flowing on the star side will not produce the

reflected current. in the delta on the other side. To eliminate zero

sequence compo nent on star side the current transformer must be



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connected in delta and the current transformer of delta: side must be

connected in start.

c) For star/star transformer CTs on both sides should be connected in . delta.

d) In order that secondary currents from two groups of CTs may have thesame.magnitude (i.e.. primary side CTs and secondary side CTs). The

ratio of star connected CTs if 5 Amp, then those of delta connected group

may be 5/ @3 2.89 Amps.

e) The operating current is a appropriate percentage of reflected through

fault current in the. restraining (bias) coils and the ratio is termed as

percentage slope.

f) Operating coil is provided with vectors sum of the currents in the

transformer windings and the bias coil sees the average scaler sum of the.

reflected through fault current. Spill current required to operate the relay

is expressed as percentage of through current.

g) The relay is also provided with an unrestrained differential high set, to

protect against heavy faults which are en ough to saturate the line current

transformers. The settin of this high set unit is kept above the maximum

in rush current magnitude. This will operate in typically one cycle for

heavy internal faults.

4.7.2 Operating Principlesfor Internalfault & externalfaults

During external fault condition (through fault) (Fig. 4.4):

Current in pilot wires would pass, through whole of bias coils and only .ou@t of

balance current due tb mis-match caused by OLTC and C.T. errors



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PAGE 52

would flow through operating coil. Under this condition biasing effect

predominates and prevents the relay operation.

During Internal faults: (FYg. 4. 5)In this case, the reflected current flows through only one half of bias coil and the

operating coil and back to CT neutral connection. Here the operating quantity

predominates resulting into operation of the relay.

4.8 Over Voltage Protection

a) Two stage protection is provided

b) the delayed trip is set at 1 10 percent of the rated voltage with two second

time delay (typical).

c) Instantaneous setting is kept at 1 15 - 120 percent of the rated voltage

d) During voltage fluctuations the AVR (Automatic Voltage Regulator) will

take care to avoid over voltage condition if fluctuations are within its

operating limits (for Generator step-up transformer).

4.9 C)ver Fluxing Protection

a) This protection is commonly used for Generator Transformers and large

inter connecting transformers in the Grid.

b) This condition arises during abnormal operating conditions i.e. heavy

voltage fluctuations at lower frequency conditions. This condition is

experienced by the transformer during heavy power swings, cascade

tripping of the generator sets and HT line in the Grid, interstate system

separation conditions and due to AVR malfunctioning during start-up or

shutting down in case of Generator Transformers.

c) The power frequency over -voltage cause both stress on insulation and

proportionate increase in the magnetising flux inside the transformer due to

which the iron losses are increased and the core bolts get maximum

component of flux, thereby rapidly heating and damaging its own

insulation and coil insulation. Reduction in frequency during high voltage

fluctuation has the same effect.



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PAGE 54

d) Transformer should be isolated within one or two minutes or as

recommended by the manufacturer.

e) The core flux @ V/f where V- impressed voltage and f- frequency. Theindex of over fluxing is, therefore, V/f. Over fluxing relays having variable

V/f setting and time delays are used for this protection.

4.10 Overall DifferentialProtection

a) This is provided for complete protection of generator, and generator

transformer and as such is a compound overall differential protection.

b) In addition to normal differential protection of generator, overall biased

differential protection relay is connected to protect the unit as shown, in Mg. 4.6.

c) 200/o pickup and 20% bias setting is provided.(The values are typical).

d) This is a supplementary protection for individual differential protection of

the generator.

e) Unit auxiliary transformers . are provided with separate differential

protection.

PAGE 55

ANNEXURE-I

PERMISSIBLE CONCENTRATIONS OF DISSOLVED

GASES IN THE OIL OF HEALTHY TRANSFORMER

(TRANSFORMERS UNIO AG)

GasLess than four

years in service4-10 years in

serviceMore than 10

years in service

Hydrogen 100 / 150 ppm 200 / 300 ppm 200 / 300 ppm

Methane 50 / 70 ppm 100 / 150 ppm 200 / 300 ppm

Acetylene 20 / 30 ppm 30 / 50 ppm 100 / 150 ppm

Ethylene 100 / 150 ppm 150 / 200 ppm 200 / 400 ppm

Ethane 30 / 40 ppm 100 / 150 ppm 800 / 1000 ppm

CarbonMonoxide 200 / 300 ppm 400 / 500 ppm 600 / 700 ppm

Carbondioxide 3000 / 3500 ppm 4000 / 5000 ppm 9000 / 12000 ppm



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CHAPTER - 5

GENERATOR PROTECTIONS

5.1 Introduction

Generators are designed to run at a high load factor for. a large number of yearsand permit certain incidences of abnormal working conditions. The machine and

its auxiliaries are supervised by monitoring devices to keep the incidences of

abnormal working conditions down to a minimum. Despite of this monitoring,

electrical and mechanical faults may occur, and the generators must be provided

with protective relays, which in case' of a fault, quickly initiate a disconnection

of the machine from the system and, if necessary, initiate a complete shut-down

of the machine.

The following are the various types of protections provided for a 200 / 2 1 0 MW

Generator.

1. Stator ground (earth) fault protection

a) 95% stator ground fault protection

b) 100% stator ground fault protection

2. Rotor earth fault protection

a) First rotor earth fault protection

b) Second rotor earth fault protection

3. Generator Interturn fault protection

4. Generator Negative phase sequence protection

5. Generator Loss of excitation protection

6. Generator Minimum Impedance (MHO backup) protection

7. Generator Differential protection

8. Generator Overall differential protection

9. Generator Reverse power protection

10. Generator Over frequency protection

11. Generator Under frequency protection

12. Generator Thermal overload protection

13. Generator Over voltage protection

14. Generator out of step (Pole slipping) Protection



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c) D.C. injection method

5.3.1 Potentiometer Method(Fig. 5.3)

In this scheme, a centre tapped resistor is connected in parallel with the main

field winding as shown in Fig. 5.3. The centre point of the resistor is connectedto earth through a voltage relay. An earth fault on the field winding will produce

a voltage across the relay. The maximum voltage occurring for faults at the ends

of the winding.

A 'blind spot'exists at the centre of the field winding, this point being at a

potential equal to that of the tapping point on the potentiometer. To avoid a fault

at this location remaining undetected, the tapping point on the potentiometer is

varied by a push button or switch. It is essential that station instructions be

issued to make certain that the blind spot is checked at least once per shift. The

scheme is simple in that no auxiliary supply is needed. A relay with a setting 5%

of the exciter voltage is adequate. The potentiometer will dissipate about 60

volts.

5.3.2 A. C. InjectionMethod(Mg. 5. 4)

This scheme is shown in Fig. 5.4. It comprises of an auxiliary supply

transformer, the secondary of which is connected between earth and one side of

the field circuit through an interposed capacitor and a relay coil.

The field circuit is subjected to an alternating potential at the same level through

out, so that an earth fault anywhere in the field system will give rise to a current

which is detected by the relay. The capacitor limits the magnitude of the current

and blocks the normal field voltage, preventing the discharge of a large direct

current through the transformer.



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This scheme has an advantage over the potentiometer method in that there is no

blind spot in the supervision of the field system. It has the disadvantage that

some current will flow to earth continuously through the capacitance of the field

winding. This current may flow through the machine bearings, causing erosionof the bearing surface. It is a common practice to insulate the bearings and to

provide an earthing brush for the shaft. and if this is done the capacitance current

would be harmless.

5.3.3 D. C. InjectionMethod(FYg. 5. 5)

The capacitance current ob ection to the a.c. injection scheme is' overcome by

rectifying the injection voltage as shown in Fig. 5.5. The d.c. out put of a

transformer rectifier power unit is arranged to bias the positive side of the field

circuit to a negative voltage relative to earth. The negative side of the field

system is at a greater negative voltage to earth, so an earth fault at any point in

the field winding will cause current to flow through the power unit. The current

is limited by including a high resistance in the circuit and a sensitive relay is

used to detect the current.

The fault current varies with fault position, but this is not detrimental provided

the relay can detect the minimum fault current and withstand the maximum.

The relay must have enough resistance to limit the fault current to a harmless

value and be sufficiently sensitive to respond to a fault which at the low injection

voltage may have a fairly high resistance. The relay must not be so sensitive as

to operate with the normal insulation leakage current, taking into account of the

high voltage to earth at the negative end of the winding and any over voltage due

to field forcing and so on.

5.3.4 (a) SecondRotor Earth Fault Protection 64R2 (FYg. 5.5 a)

In this test system is replaced by a replica field system in the form of potential

divider, two IK potentiometers in parallel with station D.C. is used as shown in

Figure 5.5 (a) with SWI at lst rotor E/F position. Close switch S 1 check that 1

st rotor E/ F relay VAEM (64R 1) operated.



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5.6.1 Out of Step Protection ofGenerator

A generator may lose synchronism with the power system, without failure of the

excitation system, because of a severe system fault disturbance or operation at a

high load with a leading power factor and hence a relatively weak field. In thiscondition, which is quite different from the failure of field system, the machine

is subject to violent oscillations of torque, with vide variations, in current, power

and power factor. Synchronism can be regained if the load is sufficiently

reduced but if this does not occur within a few seconds it is necessary to isolate

the generator and then resynchronize.

The impedance of the generator measured at the 'stator terminals changes mostly

when synchronism is lost. by the machine. The terminal voltage will begin to

decrease and the current to increase, resulting in a decrease of impedance and

also a change in power factor.

A pole slipping protection comprising of two ohm relays is used to detect out of

step operation. The relay monitors the load impedance at the machine terminals

and operates when the terminal impedance locus sequentially crosses both ohm

relay characteristics which corresponds to one pole slip between the defaulting

machine and the system.

5.7 Generator Minimum Impedance (MHO Back) Protection (2101, G2, G3):

The Generator minimum impedance protection (or Impedance back-up

protection) is primarily provided to protect the Generator against uncleared

external short circuits on the lines emanating from the station bus bars. The

relay has an impedance or offset MHO characteristic and is set to cover the

impedance of the longest line. The Generator transformer being delta/star,

introduces a 300 phase shift on @the HV side. To ensure correct impedance

measurement of the lines, the machine voltage fed to the relay (via Generator

V.TS), is phase corrected by using Interposing voltage transformers (delta/ star)

connected in the same vector group as that of the Gen.Transformer.

PAGE 72

The relay operation is delayed by using external or built-in timer so as to

discriminate with line back-up protections.



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Over current type of back-up protection is also used for Generator. This is

usually of voltage restraint or voltage controlled type where the voltage input

from the Generator V.T. is used to sensitise the over current protection on fault.

This ensures positive operation even though the sustained fault current is lessthan the full load current of the machine due to the effect of armature reaction.

The over current backup is also set with adequate time delay to coordinate with

down stream backup protections.

5.8 Generator Differential Protection (87A,B,C) (Fig. 5. 10)

5.8.1 Principle of Operation

Current transformers at each end of the protected zone are interconnected by an

auxiliary pilot circuit as shown in Fig. 5. 1 0. Current transmitted through the

zone causes secondary current to circulate round the pilot circuit without

producing any current in the relay. A fault within the protected zone will cause

secondary currents of opposite relative phase as compared with the through fault

condition. The summated value of these currents will. flow in the relay, thus

energizes the relay. The relay voltage setting is decided from the secondary load

drop by the following formula.

Vmax = I11(RcT + RL) where

I11

= Secondary subtransient short circuit current.

RL = resistance of pilot wire between current transformer (CT) and relay.

RcT = resistance of the secondar-y winding of the saturated current

transformer.

The relay operating voltage is set higher than Vmax. The minimum operating

current depends mainly on the current setting of the relay, the magnetizing

characteristics current of the associated CTs and CT Ratio.

For internal faults, the fault current equal to or above the minimum operating

value of the relay, the voltage across the relay goes upto the



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Inrush restraint is also required when the unit transformer energized from the HV

bus.

The over excitation restraint is important since there is a possibility of overvoltage when load is suddenly disconnected in which, the differential relay may

trip the generator and the voltage remains high until the automatic voltage

regulator (AVR) brought it back to the normal value.

The relay has an unrestrained differential high set unit. The unrestrained

operation must be set higher than the maximum inrush current of the transformer.

It gives fast tripping (10-20m sec.) The CT and relay connections are shown in

Fig. 5.1 1.

5.10 Generator Reverse Power Protection (32) (Flg. 5.12)

This is basically/the protection provided for the prime mover i.e. turbine. If the

driving torque becomes less such as closure of main steam valves in case of

steam turbo generator, the generator starts to work as a synchronous

compensator, taking the necessary active power from the network. The

reduction of steam flow reduces the cooling effect on the turbine blades and

overheating may occur. The work done by the entrapped steam in the turbine is

then zero. As generator is not designed to run as a motor it should be

immediately tripped when the steam flow to the turbine is stopped and to avoid

damage to the turbine blades.

The generator currents remain balanced when the machine is working as a

motor. For large turbo-generator, where the reverse power may be substantially

less than 1%, reverse power protection is obtained by a minimum power relay,

which normally is set to trip the machine when the active power out put is less

than 1% of rated value.

The relay contains directional current relay which measures the product IX coso,

where 0 is the angle between the polarizing voltage and the, current to the relay.

The scale range used is 5-2OmA for 1A and 30-120 mA for 5A rated CT

secondary currents. Time delay of 2 seconds is provided. The detail

connections of CT and relay are shown in Fig. 5.12.

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5.14 Generator Over voltage Protection (I/II(59A/59B) (Fig. 5.14)

During the starting up of a generator, prior to synchronization, the Generator

terminal voltage is obtained by the proper operation of the automatic voltage

regulator (AVR). After synchronization, the terminal voltage of the machinewill be dictated by its own AVR and also by the voltage level of the system and

the AVRS of nearby machines. It is not possible for one machine to cause any

appreciable rise in the terminal voltage as long as it is connected to the system.

Increasing the field excitation, owing to a fault in the AVR, merely increases the

reactive MVAR output, which may ultimately lead to tripping of the impedance

relay or the V/Hz. Relay. Maximum excitation limit prevents the rotor field

current and the reactive output power from exceeding the design limits.

This protection is used for the insulation level of the generator stator windings.

Severe over voltage will occur, if the generator circuit 'breaker is tripped while

the machine is running at full load and rated power factor, the subsequent

increase in terminal voltage will normally be limited by a quick acting AVR.

However, if the AVR faulty or at this particular time switched over to manual

control, over voltage will occur. This voltage rise will be further increased if

simultaneous over speeding should occur, owing to a slow acting turbine

governor.

Modern unit transformers with high magnetic qualities have a relatively sharp

and well defined saturation level, with a knee point voltage between 1.2 and 1.25

times the rated voltage (Un). A suitable setting of the over voltage relay is,

therefore, between 1. 15 and 1.2 times Un and with a definite delay of 1 to 3 sec.

An instantaneous high set voltage relay can be included to trip the generator

quickly in case of excessive over voltage following a sudden loss of load and

generator over speeding.



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6.2.5 Sensitivity

The protection should be adequately sensitive to clear low in feed ,faults,particularly during minimum generating conditions.

6.2.5 Suitable for use with moderate C. T. ratings

This is necessary since the CTs have to handle high fault currents, worst case

being faults approaching switchgear breaking capacity.

6.2.6 Conftgurable to different Busbar arrangements

The busbar arrangement may undergo changes such as sectionalisation and

additional circuits may be connected in future. The protection should be

extendable to such configuration changes.

6.3 Types of Bus Bar Protection

The most commonly used bus bar protection system are :

1) System Protection covering Busbar

2) Differential protection

6.3.1 System Protections Covering Busbar

These are primarily local or remote backup protection such as over current/earth

fault relay on feeders/transformers or distance protection provided on lines.

The distance protection for example, provides backup protection to remote

busbars in time delayed zone 2 or backup to local busbars in time delayed zone 3

with a small reverse reach. The IDMT overcurrent. Earth fault relays also

provide similar backup protection to the connected circuits against bus faults.

However, these cannot be considered as primary protection for busbar, being

time delayed and non-selective.

6.3.2 Differential Protection

The differential protection is the primary protection for bus bar against both

phase and earth faults. Practical bus differential schemes have all the ingredient

as spelled out under 6.2 above.

6.3.2.1 Operating Principleof DifferentialProtection

The protection uses a circulating current arrangement, with CTs of identical ratio

and ratings on all incoming and outgoing circuits having heir secondaries



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2. This type of protection requires special class PS CTs (with low turns ratio

errors) of identical ratio and ratings on all circuits. Exclusive CT cores

are required for high impedance schemes which cannot share common CT

cores with other protections.3. High impedance schemes are primarily fundamental frequency turned,

over current or over voltage relays and hence simple in design and

execution.

6.3.2.3 Supervision

The differential protection has a fail safe design. Consequently, the relay

becomes potentially unstable for any open circuit or cross connection in the CT

secondary of the associated feeders. The maloperation of the Busbar protection

can be prevented on load under the above condition by setting the pick up

threshold of the differential element over and above the maximum loaded circuit

current. However, the relay may still maloperate on a through fault, if the CT

secondary open circuit goes undetected. A maloperation of busbar protection

could be catastrophic, particularly in interconnected system and hence

continuous supervision of CT secondary is required as an additional safeguard.

The supervision relay is an AC voltage relay, connected across the differential

relay branch, having a sensitive setting range (usually 2 -14 volts) and a fixed

time delay to prevent transient operation on internal faults. The relay is

connected to sound an alarm and short CT secondary Bus wires, on operation.

Typical circuit arrangement for CT supervision relay is shown in Fig. 6.3.2.3.

6.3.2.4 Check Feature

Since stability is a very critical parameter of busbar protection, additional check

feature is usually provided in high impedance schemes to enhance security

against possible maloperation.

The check feature is o erated off a separate CT core on all incoming and

outgoing circuits connected to the bus and is a virtual duplication of the main

differential system. The contacts of the main and check



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Besides, the control breakers are provided with duplicated trip coils. All these

measures, undoubtedly improve the reliability of fault detection and isolation.

However, the possibility of mechanical failures of the switchgear or

interrupter flash overs can not be covered by these means for obvious reasons.A failure of the breaker may therefore, result inspite of correct operation of

the protection and energisation of trip coils. This situation can be corrected

by providing local breaker backup (LBB) or breaker fail protection.

6.5.2 Operating Principle

LBB protection comes into operation, only if, the breaker fails to trip, following

energisation of its trip coil, through the circuit trip relays. The main ingredient

of LBB protection, is a. current check relay initiated by the circuit trip relays and

a follower timer. The current check relay, on initiation, check the presence of

Ele_Pro_Sys

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