10

EE2402 PROTECTION AND SWITCHGEAR

UNIT-I

INTRODUCTION

Importance of protective schemes for electrical apparatus and power system

1. What is meant by switchgear? (M/J-06)

Switchgear is a general term covering switching devices and their combinations with

associated control, measuring and protective equipments.

2. What are the consequences of a short circuit?

The consequences of a short circuit are

1. A reduction in the line voltage over a major part of the power system.

2. Damage to the equipments due to overheating.

3. A single line to ground fault in an ungrounded system gives rise to arching

grounds and also the voltages of the other lines are increased by 3 times the

original value which may stress the insulation of the equipments.

4. The stability of the power system is affected.

3. Define – Protective Zone

Protective zone is defined as a part of the system protected by a certain protective scheme.

The entire power system is covered by several protective zones and no part of the system is

left unprotected. Neighbouring zones are overlapped so that no dead spots are left in the

protected system. The boundary of the protective zone is determined by the location of current

transformers which detects and isolates the system by the circuit breakers during abnormal

operation.

4. What are unit system and non-unit system?

A unit protective system is one in which the protection scheme responds to faults in the

protected zone alone whereas non-unit system does not have exact zone boundaries. Each zone

has certain protective scheme and each protective scheme has several protective systems.

5. What is primary protection?

Primary protection is the main protection provided for protecting the equipments. The

primary protection is the first to act and if the primary protection fails, then the backup

protection comes into action and removes the faulty part from the healthy system.

6. What is backup protection?

Secondary protection is the second line of defence which operates if the primary

protection fails to activate within a definite time period. The methods of backup are classified

as relay backup, breaker backup and remote backup.

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7. Distinguish between primary protection scheme and secondary protection scheme.

Main or primary protection can fail due to non-functioning of one of its components in the

protective scheme such as relay, CT, PT, trip circuit or circuit breaker. If the primary protection

fails, then there must be an additional protection, otherwise the fault may remain uncleared

which may result into a disaster. The backup protection is provided either by time grading

principle or by duplication principle to protect the power system.

Relay terminology - Definitions

8. Define – Energizing Quantity

Energizing quantity is defined as the current, voltage or frequency which is used to operate

the relay under abnormal condition.

9. Define – Operating Time of a relay

Operating Time of a relay is defined as the time interval between the occurrence of

fault and closure of relay contact .

10. Define – Resetting Time of a relay

Resetting Time of a relay is defined as the time which elapses between the instant

when the actuating quantity becomes less than the reset value to the instant when

the relay contact returns to its normal position.

11. What is meant by time setting multiplier in protective relaying?

The operating time of the relay depends upon the distance between the moving contact and

the fixed contact of the relay. The distance between the contacts is adjusted by the movement

of the disc back stop which is controlled by rotating a knurled moulded wheel at the base of the

graduated time multiplier scale. This is known as time multiplier setting.

Essential qualities of protection.

12. What are the various essential qualities of protective relaying?

The various essential qualities of protective relaying are

1. Speed

2. Stability

3. Selectivity

4. Sensitivity

5. Simplicity

6. Reliability

7. Economy

13. What are the functions of protective relaying? (A/M-07)

A fault in the equipment in the supply system leads to disconnection of supply to a large portion

of the system. If the faulty part is quickly disconnected, the damage caused by the fault is minimum

and the faulty part can be repaired quickly and the service can be restored without further delay.

Better service continuity has its own merits. Thus the protective relaying helps in improving service

continuity.

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Protection against over voltages due to lightning and switching

14. What are the causes of over voltage?

The causes of over voltage are

1. Lightning

2. Switching

3. Power frequency

15. What are the protective measures that taken against lightning over voltage?

The protective measures taken that against lightning over voltage are,

1. Use of overhead ground wires

2. Low tower footing resistance

3. Use of lightning arresters

16. State the difference between the nominal system voltage and highest system voltage.

The sinusoidal rms voltage is represented for single phase voltage supply as

V = Vmax sin

where V = Nominal system voltage

Vmax = Highest system voltage

Ground wires

17. What is shielding angle of an overhead ground wire? What are the values as per

American and European practices?

The shielding angle of the ground wire is defined as the angle between the vertical line

passing through the ground wire and the line passing through the outermost power conductor.

The shielding angle should be 30º on plain areas whereas the angle decreases on hilly areas

according to the slope of the hill.

American standard - 30º

European standard - 45º

Surge absorber and diverters

18. What is surge absorber? How do they differ from surge diverter? (N/D-11)

The surge absorber acts like an air cored transformer which has a primary low value

inductor and the dissipater acts as a single turn short circuit secondary. Whenever the

travelling wave is incident on the surge absorber, a part of the energy contained in the wave is

dissipated as heat due to transformer action and eddy current flows. Because of this series

inductance, the steepness of the wave gets reduced. It is claimed that the stress in the end

turns is reduced by 15% with the help of the surge absorber.

Lightning arrester or surge diverter limits the duration and amplitude of the flow current

while a surge absorber reduces the steepness of the wave front for a particular surge

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19. What is the significance of the coefficient of earthing in the selection of voltage rating of a

surge arrester?

The coefficient of earthing is given as

The rated voltage of the surge arrester should be more than the phase to ground voltage on

unfaulted phase during a single line to ground fault on any other phase. During a single phase

to earth fault on one phase, the phase to ground voltage of other two healthy phases rises to

(Vm * Ce). The rated voltage of the surge arrester should be more than (Vm * Ce) kV rms

continuous across the terminals.

Va > Vm * Ce kV

where Va = Rated voltage of the surge arrester

Vm = Highest phase to phase voltage

Ce = Coefficient of earthing

Power system earthing

20. Define – Earthing

Earthing is defined as the non current carrying parts of equipment connected to earth.

21. Define – Coefficient of Earthing

Coefficient of Earthing is defined as the ratio of healthy phase to ground voltage to the phase

to phase voltage measured during a single phase to ground fault.

Neutral Earthing

22. What is the difference between equipment earthing and neutral earthing?

Equipment earthing refers to the grounding of non-current carrying metal parts to earth. It is

used for safety of personnel.

Neutral earthing refers to the grounding of current carrying conductor to the earth. It is used

for the safety of equipment and to eliminate arching grounds.

23. List out the advantages of neutral grounding of an electrical system.

The advantages of neutral grounding of an electrical power system are

1. Arching grounds are reduced or eliminated. The system is not subjected to overvoltage

surge due to arching grounds.

2. The voltage of healthy phase lines with respect to earth remains at normal value. They

do not increase to 3 time normal value as in the case of ungrounded system.

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3. The life of insulation is long due to prevention of voltage surges caused by arching

grounds. Thereby it has reduced maintenance, repairs, and breakdowns. It improves

continuity.

4. Stable neutral point.

5. The earth fault relaying is relatively simple. Sufficient quantity of earth fault current is

available to operate the earth fault relay.

6. The over voltages due to lightning are discharged to earth.

7. By employing resistance or reactance in earth connection, the earth fault current can be

controlled.

8. Improved service reliability due to limitation of arching grounds and prevention of

unnecessary tripping of the circuit breaker.

9. Greater safety to personnel and equipment due to operation of fuses or relays on earth

fault and limitation of voltages.

10. Life of equipments, machines, installation is improved due to limitation of voltage.

Hence overall economy increases.

Basic ideas of insulation coordination.

24. Define – Insulation Coordination (N/D-11)

Insulation Coordination is defined as the coordination between the withstand levels of

equipment, protective levels of protective devices with adequate protective margin so that the

overall economy is obtained and least damage is caused to the electrical installation during

overvoltage surges. The insulation level of an apparatus is defined as the combination of

voltage value (both power frequency and impulse) which characterised its insulation with

regard to its capability of withstanding the dielectric stresses.

25. Define – Rated Voltage of Surge Arrester

Rated voltage of surge arrester is defined as the maximum permissible rms voltage between

the line terminal and the earth terminal of the arrester.

UNIT-II

OPERATING PRINCIPLES AND RELAY CHARACTERISTICS

Electromagnetic relays

1. State any two applications of electromagnetic relay. (M/J-08)

The applications of electromagnetic relay are

1. Over/ Under current relays

2. Over/ Under voltage relays

3. Earth fault protection relays

4. Differential protection

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2. What are the fundamental requirements of a relay?

The fundamental requirements of a relay are

1. Speed

2. Stability

3. Selectivity

4. Sensitivity

5. Simplicity

6. Reliability

3. What is the need for relay coordination?

The operation of a relay should be fast and selective. It should isolate the fault in the

shortest possible time causing minimum disturbance to the system. Also, if a relay fails to

operate, there should be sufficiently quick backup protection so that the rest of the system is

protected. By relay coordination, faults can always be isolated quickly without serious

disturbance to the power system.

4. Explain the following:

1. Sensitivity of a relay

Sensitivity of a relay refers to the smallest value of actuating quantity at which the

relay starts operating in relation with the minimum value of fault current in the protected

zone.

2. Stability of a protective system

Stability is defined as the quality of protective system by the virtue of which, the

protective system remains inoperative and stable under certain specified conditions such as

system disturbance through faults and transients.

5. List out the applications of electromagnetic relay.

The applications of electromagnetic relay are

1. Protection of various components

2. It can be used for differential protection

3. Used as auxiliary relays in protective relaying schemes

Over current, directional and non-directional, distance, negative sequence, differential and

under frequency relays

6. What is the purpose of distance relay? (A/M-07)

Distance relays are double actuating quantity relays with one coil energized by voltage

and the other energized by current. The torque is produced in such a way that when V/I

reduce below a set value, the relay operates. Distance relays are widely used protective

devices for the protection of high and extra high voltage transmission line. They can be used

for primary as well as backup protection. Distance relays can be used in carrier aided distance

schemes and also in auto-reclosing schemes.

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7. A relay is connected to 400/5 ratio current transformer with a current setting of 150%.

Calculate the plug setting multiplier when the circuit carries a fault current of 4000 A.

(N/D-11)

Given:

Current transformer turns ratio = 400/5

Current setting = 150%

Primary fault current = 4000 A

To find:

Plug setting multiplier (PSM)

Formula:

Solution:

PSM = 4000/(1.5*(400/5)) = 33.33.

8. Define – Pickup (M/J-06)

Pickup is defined as the operation of a relay. The pickup value is the minimum value of

operating quantity at which the relay operates and closes its contacts.

9. Define – Plug Setting Multiplier (M/J-06)

The plug setting multiplier is given as

10. Define – Over Current Relay

Over current relay is defined as the relay that operates when the current in a line exceeds

its predetermined value. e.g. Induction type non-directional/directional over-current relay,

differential over-current relay.

11. Define – Undercurrent Relay

Undercurrent relay is defined as the relay which operates whenever the current in a circuit

or transmission line drops below its predetermined value. e.g. differential over-voltage relay.

12. What are the different types of over current relays?

The different types of over current relays are

1. Instantaneous Over-current Relay

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2. Time Over-current Relay

3. Inverse Time Over-current Relay

4. Directional Over-current Relay

13. What is directional relay?

The relay which responds to a fault current in a particular direction is called the

directional relay.

Three conditions must be satisfied for its operation. They are current magnitude, time

delay and directionality. The directionality of the current flow can be identified using the

voltage as a reference vector.

14. What are the features of a directional relay?

The features of a directional relay are

1. High speed operation

2. High sensitivity

3. Ability to operate at low voltages

4. Adequate short-time thermal ratio

5. Burden must not be excessive.

15. What is the use of distance relay?

Distance relays respond to the voltage and current, i.e., the impedance, at the relay

location. These are widely used protective devices for the protection of high and extra high

voltage transmission line. It can be used for primary as well as for backup protection.

Distance relays can be used in carrier aided distance schemes and also in auto-reclosing

schemes.

16. Explain the significance of the following terms in the distance protection:

Reach of a distance relay

The limiting distance covered by the relay. The faults beyond this distance are not

covered within the reach of the protection and it will be covered by other protective relay.

Under reach of a distance relay

Under reach of a distance relay is the failure of distance relay to operate within the set

protected transmission line distance.

17. Distinguish between circulating current differential protection and balanced voltage

differential protection with reference to the behaviour of CT.

In circulating current differential protection the principle of circulating current is used

where the balanced voltage differential protection works on the principle of balanced

opposing voltages.

18. List out any two applications of differential relay.

The applications of differential relay are

1. Protection of generator & transformer unit

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2. Protection of large motors

3. Bus bars.

19. What is biased differential bus zone protection?

In biased differential protection, the relay element has a restraining coil in addition to

the operating coil. The circulating current flows through the restraining coil and the spill

current flows through the operating coil. For external faults, the restraining current is more

and the relay does not operate.

20. What is an under frequency relay?

An under frequency relay is one which operates when the frequency of the system

(usually an alternator or transformer) falls below 5 to 10% of its original value.

Introduction to static relays.

21. What are the advantages of static relay? (M/J-06)

The advantages of static relay are

1. No moving contacts (no erosion on contacts)

2. Consumes less power

3. Burdens on CTs and PTs are low

22. State any two advantages of static relay over electromagnetic relay.

The advantages of static relay over electromagnetic relay are

1. In static relays, there are no moving contacts and hence associated problems of

arising, erosion of contacts, replacement of contacts, as in the case of

electromagnetic relays, do not exist.

2. Static relays consume lesser power than the electromagnetic relays. i.e., static

relays have fewer burdens on CT’s and PT’s compared to electromagnetic relays.

Static relays power consumption is one mill watt whereas in electromagnetic

relay, the power consumption is 2 watts.

23. Draw the block diagram of a static relay indicating its basic elements.

Block diagram of a Static relay

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24. Distinguish between static relay and electromagnetic relay.

For simple protective functions and for protection of simple low power equipment,

electromechanical relays are preferred.

For complex protective functions requiring accurate characteristics for various protective

functions and for protection of costly and large equipments, static relays are preferred. These

may be hard wired or programmable.

25. What is meant by static relay?

A static relay is an electric relay in which the response is developed by

electronic/magnetic/optical or other components, without mechanical motion of its

components. This relay is designed to give an output signal when a threshold condition is

achieved to operate a tripping device.

26. What are the merits and the demerits of a static relay over electromagnetic relay?

Merits and demerits of a static relay over electromagnetic relay are

1. Low power consumption as low as 1mW

2. No moving contacts; hence associated problems of arcing, contact bounce,

erosion, replacement of contacts are avoided.

3. No gravity effect on operation of static relays. Hence it can be used in ships,

aircrafts etc.

4. A single relay can perform several functions like over current, under voltage,

single phasing protection by incorporating respective functional blocks. This is

not possible in electromagnetic relays.

5. Static relay is compact.

6. Superior operating characteristics and its accuracy is more.

7. Programmable operation is possible with static relay.

8. Effect of vibration is negligible; hence it can be used in earthquake-prone areas.

9. Simplified testing and servicing. It can convert even non-electrical quantities to

electrical in conjunction with transducers.

27. List out the applications of static relay.

The applications of static relay are

1. Bus bar protection

2. Transformer protection

3. Transmission line protection

4. Sequential operation tap changing and voltage control

5. Load shedding system restoration etc.,

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UNIT – III

APPARATUS PROTECTION

PART – A

Main considerations in apparatus protection

1. What is meant by time-graded system of protection? (N/D-11)

Time-graded system of protection is a non-unit type of protection which is used when

a time-lag can be permitted and instantaneous operation is not necessary. This is also called a

backup protection to the primary main protection.

2. What are the main safety devices used for a transformer protection?

The main safety devices used for a transformer protection are

1. Buchholz relay

2. Pressure relief value

3. HRC fuses

4. Horn gaps

5. R-C surge suppressors

6. Surge arresters

7. Percentage differential protection

8. Over fluxing and over voltage protection

9. Over current protection

10. Earth fault protection

11. Thermal overload relay

Transformer, generator and motor protection

3. What are the various types of transformer faults? (M/J-08)

The various types of transformer faults are

1. Incipient fault

2. Internal fault

3. Phase-to-phase and phase-to-ground fault

4. Saturation of magnetic circuit

5. Earth fault

6. Through fault

7. Overloading

8. High voltage surges due to lightning

4. Why is the protection of a generator complex? (MJ-08)

The protection of a generator is complex because of the following reasons

1. Generator is connected to the transmission grid through bus bars, transformers and

circuit breakers. The protection requires to ascertain the overload, over/under

voltage, over/under frequency at each instance during its operation. In addition,

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the stator/rotor faults along with the operating range within the specified

temperature rise of the generator need to be accessed as per standard.

2. The excitation system, prime mover status, automatic voltage regulator status and

its cooling system is not single equipment but comprises of several equipments.

Therefore the protection of generator should be co-ordinated with these

equipments.

3. The continuity of power is the basic requirement since the sudden shut down of

the generator would result in power shortage and would affect the stability of the

power system.

5. What are the limitations of Buchholz relay?

The limitations of Buchholz relay are

1. Incipient faults below oil levels are detected.

2. Setting the mercury switch cannot be too accurate as there can be a false operation

due to its sensitivity during vibrations, earthquakes, mechanical shocks to the pipe

etc.

3. The minimum operating time is 0.1 second and average time is 0.2 second. Such a

slow relay is unsatisfactory for detecting faults in the transformer quickly.

4. These are not provided for transformers below 500 kVA, since it is costly.

5. This protection needs conservator, otherwise the relay cannot be installed.

6. What are the problems that arise in differential protection of a power transformer and

how are they overcome?

The problems that arise in differential protection of a power transformer are

1. Difference in lengths of pilot wires on either side of relays. This difficulty is

overcome by connecting adjustable resistors in the pilot wires.

2. Difference in CT ratios due to ratio and phase angle errors causes the relay

operation during through faults. This difficulty can be overcome by using biased

differential relay or percentage differential relay.

3. Tap changing alters the ratio of voltage between H.V. side and L.V. side in a

transformer. Differential protection should be provided with bias which prevents

the effect of variation in secondary current due to tap changing.

4. Magnetizing inrush current takes place during switching on the transformer. To

avoid this effect, the relay will be provided with a time lag of 0.2 second.

7. What is an over fluxing protection in a transformer?

Power transformers are designed to withstand the ratio of its applied voltage with

respect to frequency (V/f) continuously, where V is the normal highest rms voltage applied

and fn is its frequency. High V/f can occur in a transformer if full excitation is applied to

generator before reaching its full synchronous speed. Over fluxing protection in a transformer

blocks increasing excitation current of a generator before it reaches its full speed.

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8. State the short comings of Merz-Price protection scheme applied to a power

transformer.

The short comings of Merz-price protection scheme applied to a power transformer are

1. Magnetizing current inrush in a transformer during switching on.

2. Fixed current transformer ratios on a variable ratio power transformers due to its

tap changers.

3. Unavoidably different characteristics of CTs on the high and low voltages sides of

the transformer during through faults.

9. What are the uses of Buchholz relay?

The uses of Buchholz relay are

1. To protect the transformer from incipient faults below the oil level, which results

in the decomposition of oil

2. To give advance warning and prevent the transformer from short circuit condition

and subsequent damage

10. What are the various faults to which a turbo alternator is likely to be subjected?

The various faults to which a turbo alternator is likely to be subjected are

1. Thermal overloading

2. Unbalanced loading

3. Stator winding faults

4. Field winding faults

5. Over voltages

6. Loss of synchronism

7. Over speeding

8. Vibration

9. Excessive bearing temperature

10. Wrong synchronization

11. What are the causes of over speed in an alternator and how is it prevented?

The causes of over speed in an alternator are

1. Sudden loss of electrical load

2. Tripping of circuit breaker before disconnection of prime mover

3. This can be prevented by over speed protection provided along with governing

mechanism.

12. What are the main types of stator winding faults?

The main types of stator winding faults are

1. Phase-to-phase faults

2. Phase-to-earth faults

3. Stator inter-turn faults

4. Three phase fault.

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13. What is the purpose of adding a neutral resistor between neutral and earth of an

alternator?

The capacitance ground current may not be large enough to demand reactance

grounding and also the ground fault current for solid grounding becomes excessive in 3.3 kV

and 33 kV transmission lines,. Hence it is needed to connect the neutral point through

resistance to ground to reduce its fault current.

14. Why is the backup protection used for an alternator?

If the primary protection fails in an alternator due to failure in CT, PT, trip circuit,

circuit breaker, etc., the backup protection comes into action and removes the faulty part from

the healthy system. When the main protection is made inoperative for the purpose of

maintenance, testing, etc., backup protection acts as the main protection. As a measure of

economy, backup protection is provided with primary protection against short circuits.

Protection of busbars

15. What is the importance of bus bar protection? (M/J-06)

The busbar protection is important because of the following reasons

1. Fault level at busbar is very high

2. The stability of the system is affected by fault in bus zone.

3. The fault in a busbar causes discontinuation of power to a large portion of the

system.

16. What are the types of protections used for bus-bars? (A/M-07)

The types of protections used for bus-bars are

1. Over current protection

2. Differential protection

3. Earth fault protection

4. Over voltage protection

5. Surge voltage protection

Transmission line protection

17. What are the common methods used for line protection? (M/J-06)

The common methods used for line protection are

1. Over current protection

a) Time graded

b) Current graded

2. Distance protection

3. Earth fault protection

4. Differential protection

5. Carrier current protection

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18. Define – Power Line Protection

Power line protection is the protection of transmission lines under fault conditions

using power line as a medium. The use of channels to compare conditions at the terminals of

a power line provides selective means of high speed clearing of end zone faults. This type of

protection includes carrier current protection.

19. What is meant by auto reclosing?

The process of automatic closing of circuit breaker after its opening during temporary

outage of a transmission line is called auto reclosing.

20. List out any two disadvantages of carrier current protection scheme for transmission

line.

The disadvantages of carrier current protection scheme for transmission line are

1. In order to avoid operation due to spurious signals, the carrier and receiver signals

should be set at a higher power level.

2. Amplifier and oscillator are continuously energized and the stability and response

time of these units are constraints.

Zones of protection

21. What are the causes of bus zone faults?

The causes of bus zone faults are

1. Failure of support insulator resulting in earth faults.

2. Flashover across support insulator during over voltages.

3. Heavily polluted insulator causes flashover.

4. Earthquake

5. Mechanical damage etc.

22. What are the problems in bus zone differential protection?

The problems in bus zone differential protection are

1. Large number of circuits.

2. Saturation of CT core during short circuit which introduces ratio error.

3. Sectionalising of bus makes the circuit complicated.

4. Setting of relays needs to be changed with large changes in loads.

CTs and PTs and their applications in protection schemes

23. What is meant by burden on C.T.? (A-07)

The circuit connected to the secondary winding of CT is termed as burden of the

current transformer. Burden is expressed preferably in terms of impedance of the circuit

connected to the secondary. It is specified in volt-ampere at rated secondary current at

specified power factor.

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24. Why should not the secondary of a current transformer be open? (N/D-11)

If the secondary of a current transformer is opened, then the voltage across the

secondary rises to a high value since there is no back emf. This may cause danger to working

personnel. Also the flux increases and results in the saturation of the transformer core.

Therefore, the secondary of the current transformer should never be opened.

25. What are the merits of carrier current protection?

The merits of carrier current protection are

1. It protects the transmission line if the length of the transmission line is long and

also when the simultaneous opening of circuit breakers at both ends is desirable.

2. Fast clearing prevents shocks to systems .

3. For simultaneous faults, it provides easy discrimination.

4. Tripping due to synchronizing power surges does not occur and it increases

stability of transmission system.

26. What are the various errors of CT?

The various errors of CT are

1. Current error or ratio error

2. Phase angle error

3. Composite error

27. What is meant by a power swing?

During switching of lines, larger loads or generators, surges of real power and

reactive power flow through the transmission lines causing oscillations in the voltage and

current waves. These oscillations are called power swings.

28. What is a programmable relay?

A relay in which the characteristics and behaviour are programmed is called

programmable relay. It has a microprocessor in its circuit. The programme can take care of on

line computation for relay operation and protection purpose.

29. What is a REF relay?

In the absence of earth fault, the vector sum of the line currents Ias + Ibs + Ics is zero.

During earth fault the vector sum of these currents will flow which is called residual current.

The earth fault relay is connected in such a way that the residual current flows through it. If

earth fault occurs, residual current flows through the earth fault relay. If the residual current

is set above pick up value, the earth fault relay operates. In the absence of earth fault, the

residual current is zero and the residually connected earth fault relay does not operate

30. What is meant by field suppression?

When a fault develops in an alternator winding even though the generator circuit

breaker is tripped, the fault continues to be fed because of emf is induced in the generator

itself. Hence the field circuit breaker must be opened and the stored energy in the field

winding is discharged through a resistor. This method is known as field suppression.

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UNIT – IV

THEORY OF CIRCUIT INTERRUPTION

Physics of arc phenomena and arc interruption

1. What are the factors on which the arc resistance depends? (M/J-06)

The factors on which the arc resistance depends are

1. Length of the arc

2. Voltage across the arc

2. What are the two theories used to describe the arc extinction phenomena?

(M/J-08)

The two theories used to describe the arc extinction phenomenon are

1. Slepian’s theory

2. Energy balance theory

3. Write the different methods of high resistance arc interruption.(MAY/JUNE-2008)

The different methods of high resistance arc interruption are

1. Lengthening the arc by means of arc runners

2. Splitting of arc

3. Cooling of arc

4. What is an arc?

An electric arc is a self-sustained discharge of electricity between electrodes in

a gas or a vapour.

5. What is an arcing ground?

A temporary fault caused by falling of a line creates an arc between phase conductor

and ground. The arc extinguishes and restrikes in a repeated and regular manner. This

phenomenon is called an arching ground which is commonly experienced with ungrounded

systems due to charging and discharging of capacitance between line and earth.

6. What is an arc suppression coil?

Arc suppression coil is provided with tappings. This permits selection of reaction of

the coil depending upon the length of the transmission line and the capacitance of line to

ground to be neutralized. The arc suppression coil is connected between neutral and ground.

7. List out any two methods used for arc interruption.

The methods used for arc interruption are

1. High resistance interruption

2. Low resistance or zero point interruption

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Restriking voltage, Recovery voltage, Rate of rise of recovery voltage, Resistance

switching, Current chopping, Interruption of capacitive current

8. Define – Rate of Rise of Restriking Voltage (A/M-07)

The restriking voltage is the recovery voltage having significant transients which

appears across the terminals of a pole of a circuit breaker after the breaking of current. This

may last for hundreds of microseconds. It may be oscillatory or non oscillatory or

combination of both depending upon the characteristic of the circuit and the type of breaker.

Rate of rise of restriking voltage is expressed in volts per micro-second.

9.-Distinguish between recovery voltage and restriking voltage. (N/D-11)

Sl. No. Recovery voltage Restriking voltage

1

It is the normal frequency r.m.s. voltage

which appears across the terminals of a

circuit breaker after the final current zero.

The restriking voltage is the

voltage having significant

transients which appears

across the breaker poles of a

circuit breaker after the

breaking of current.

2

The instantaneous voltage of the recovery

voltage at the instant of arc extinction is

called active recovery voltage.

The transient voltage that

appears across the contacts at

the instant of arc extinction is

called restriking voltage.

3 It does not restrike after the arc extinction

It appears and immediately

restrikes after the arc

extinction.

10. Which are the factors affecting the transient recovery voltage. (N/D-11)

The factors affecting the transient recovery voltage are

1. Effects of natural frequency of transient recovery voltage

2. Effect of Power factor on transient recovery voltage

3. Effect of Reactance drop on power frequency recovery voltage

4. Effect of Armature reaction on recovery voltage

5. Effect of circuit condition

11. What is restriking voltage?

Restriking voltage is the transient voltage across the breaker poles immediately after

the arc extinction. It lasts for hundreds of micro-second. It may be oscillatory or non-

oscillatory in nature depending upon the characteristics of the circuit and the type of circuit

breaker.

Restrike is defined as the reappearance of an arc after one-fourth cycle from the arc

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extinction at current zero. In capacitor current breaking, a single restrike gives an overvoltage

of about 4 times and a second restrike gives an overvoltage of about 6 times of the normal

power frequency voltage resulting in internal and external flashovers between phase to phase

as well as phase to ground.

12. Explain, “successive restrikes” and current chopping as applied to the interruption

of capacitive and low inductive currents.

The energy stores in inductance during current interruption are diverted to the

capacitance at the moment of current interruption.

Also capacitor banks are connected in the network to provide reactive power at

leading power factor. The voltage across a capacitor cannot change instantaneously. The

currents supplied to the capacitor are generally of small order and the circuit breaker can

interrupt such currents invariably at the first current zero. Due to the 90º phase difference, the

voltage across the capacitor is at maximum value (ec) at this instant (t1) and the capacitor

remains charged at this voltage (ec). After half cycle (t2) the recovery voltage of approximate

magnitude of (ermax) appears across the circuit breakers and the total voltage across the circuit

breaker is the sum of the two voltages.

eTmax = ermax + ec

where eTmax = maximum voltage across breaker

ermax = maximum value of power frequency recovery voltage

ec = voltage across the capacitor

Thus the recovery voltage of the order of 2Emax, appears across the circuit breaker

pole at the instant t2, after ½ cycle of current zero. Therefore, a restrike is possible.

If a restrike occurs, the LC circuit will oscillate at a frequency given by fn = ½ (LC).

This current tries to maintain the arc. The voltage across the interrupter rises up to 4 times the

normal voltage due to one restrike and up to 6 times with second restrike. The energy to be

dissipated during such arcs is quite large and the interrupters may get damaged in the process

after a restrike.

13. How can restriking transients be damped by connecting a resistance across the

contacts of a circuit breaker?

In resistance switching, a resistance is connected in parallel with the contacts of the

circuit breaker. With the arc so shunted by the resistance a part of an arc current flows

through the resistance. This results in a decrease in arc current and an increase in the rate of

deionization of the arc path and the resistance of the arc. This leads to a further increase in

the current through the shunt resistances. This built up process continues until the current

path through the arc is substituted by that through the resistance either wholly or in greater

part. In the later case, the small value of the current remaining in the arc path becomes so

instable that it gets easily extinguished.

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14. Explain the following terms:

a. Symmetrical breaking capacity

b. Asymmetrical breaking capacity

c. Making capacity

Symmetrical breaking capacity

Symmetrical breaking capacity is the product of symmetric breaking current and

recovery voltage.

Asymmetrical breaking capacity

The Asymmetrical breaking capacity is the product of an asymmetrical breaking

current and the recovery voltage.

Making capacity

Making capacity is the peak value of the maximum current loop including DC

component in a phase during the first cycle of current when the circuit breaker is closed. The

making capacity is specified by the product of making current it can make and carry

instantaneously at the rated service voltage without increasing the temperature rise of its

current carrying parts.

15. An OCB is rated for 1000 MVA, 2 kA, 66 kV, 3 phase, 3 second. Determine the

following:

a. Rated operating voltage

b. Rated operating current

c. Rated symmetrical breaking current

Given data:

Rating of OCB = 1000 MVA

Rated current = 2 kA

Rated voltage = 66 kV

No. of phase = 3

Time period = 3 second

To find:

a) Rated operating voltage

b) Rated operating current

c) Rated symmetrical breaking current

Formula required:

Rated symmetrical breaking current = Rating of the circuit breaker / ( 3 x

Rated voltage)

Solution:

Rated operating voltage = Rated voltage = 66 kV

Rated operating current = Rated current = 2 kA

Rated symmetrical breaking current = (1000 x 106) / ( 3 x 66 x 10

3)

= 8750 A

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UNIT-V CIRCUIT BREAKERS

Types of circuit breaker

1. What is a circuit breaker?

A circuit breaker is an automatic switch which can interrupt fault currents.

2. List out the classification of circuit breakers based on the medium used for arc

extinction.

The classifications of circuit breakers based on the medium used for arc extinction are

1. Air break circuit breaker or Miniature circuit breaker

2. Oil circuit breaker

3. Minimum oil circuit breaker

4. Air blast circuit breaker

5. Sulphur hexafluoride circuit breaker

6. Vacuum circuit breaker

3. List out any two applications of a differential relay.

The applications of a differential relay are

1. Protection of generator

2. Protection of generator-transformer unit

3. Protection of transformer

4. Protection of transmission line

5. Protection of bus zone

4. List out all the three symmetrical components.

The three symmetrical components are

1. Positive sequence component

2. Negative sequence component

3. Zero sequence component

5. Define – Positive Sequence Component

Positive sequence component is defined as a component which has three vectors of

equal magnitude but displaced in phase from each other by 120º and has the same phase

sequence as the original vectors.

6. Define – Zero Sequence Component

Zero sequence component is defined as a component which has three vectors of equal

magnitude and also are in phase with each other.

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7. Define – Negative Sequence Component

Negative sequence component is defined as a component which has three vectors of

equal magnitude but displaced in phase from each other by 120º and has the phase sequence

opposite to the original vectors.

8. What is resistance switching?

Resistance switching is the deliberate connection of a resistance in parallel with the

contact space (arc). It is used in circuit breakers having high post zero resistance of contact

space (air blast circuit breaker).

9. What is meant by current chopping? (M/J-06)

When interrupting low inductive currents such as magnetizing currents of transformer

or shunt reactor, the rapid deionization of contact space due to air blast may cause the current

to be interrupted before its natural zero. This phenomenon of the interruption of current

before its natural zero is called current chopping.

10. Which is the method used for capacitive switching?

The problems faced in transmission line during following events are

1. Switching of capacitor banks

2. Switching of unloaded transmission lines

3. Switching of unloaded cables

4. Opening of long transmission line on no load

5. Disconnection of capacitor banks

This can be avoided by resistance switching whereby a resistance is connected in

parallel with the contacts of the circuit breaker. With the arc so shunted by the resistance a

part of an arc current flows through the resistance. This results in a decrease in arc current

and an increase in the rate of deionization of the arc path and the resistance of the arc. This

leads to a further increase in the current through the shunt resistances. This built up process

continues until the current path through the arc is substituted by that through the resistance

either wholly or in greater part. In the later case, the small value of the current remaining in

the arc path becomes so unstable that it gets easily extinguished.

11. A circuit breaker is rated at 1500 amps, 2000 MVA, 33 kV, 3 sec, 3-phase, oil circuit

breaker. Determine the rated normal current, breaking current, making current and

short time rating (current).

Given data:

Rated current of circuit breaker = 1500 A

Rating of the circuit breaker = 2000 MVA

Rated voltage = 33 kV

No. of phase = 3

Time = 3 second

To find:

Rated normal current

32

Breaking current

Making current

Short time rating

Formula required:

Rated normal current = Rated current of the circuit breaker

Breaking current = Rating of the circuit breaker / ( 3 x Rated voltage)

Making current = 2.55 x Breaking current

Short time rating = Breaking current

Solution:

The rated normal current is 1500 amps.

Breaking current =2000/ ( 3 33) =34.99kA

Making current =2.55 34.99 =89.22kA

Short time rating =34.99 kA for 3 sec.

12. A circuit breaker is rated at 1200 amps, 1500 MVA, 33 kV, 3 sec, 3-phase, oil circuit

breaker. Determine the rated normal current, breaking current, making current and

short time rating (current).

Given data:

Rated current of circuit breaker = 1200 A

Rating of the circuit breaker = 1500 MVA

Rated voltage = 33 kV

No. of phase = 3

Time = 3 second

To find:

Rated normal current

Breaking current

Making current

Short time rating

Formula required:

Rated normal current = Rated current of the circuit breaker

Breaking current = Rating of the circuit breaker / ( 3 x Rated voltage)

Making current = 2.55 x Breaking current

Short time rating = Breaking current

Solution:

The rated normal current is 1200 amps.

Breaking current =1500/ ( 3 33) = 26.25 kA

Making current =2.55 26.25 = 67 kA

Short time rating = 26.25 kA for 3 sec