Important Protection & Switchgears Questions For Interview Part – 2

1. What are the functions of protective relays

To detect the fault and initiate the operation of the circuit breaker to isolate the defective element from the rest of the system, thereby protecting the system from damages consequent to the fault.

2. Give the consequences of short circuit.

Whenever a short-circuit occurs, the current flowing through the coil increases to an enormous value. If protective relays are present , a heavy current also flows through the relay coil, causing it to operate by closing its contacts.The trip circuit is then closed , the circuit breaker opens and the fault is isolated from the rest of the system. Also, a low voltage may be created which may damage systems connected to the supply.

3. Define protected zone.

Are those which are directly protected by a protective system such as relays, fuses or switchgears.If a fault occurring in a zone can be immediately detected and or isolated by a protection scheme dedicated to that particular zone.

4. What are unit system and non unit system?

A unit protective system is one in which only faults occurring within its protected zone are isolated.Faults occurring elsewhere in the system have no influence on the operation of a unit system.A non unit system is a protective system which is activated even when the faults are external to its protected zone.

5. What is primary protection?

Is the protection in which the fault occurring in a line will be cleared by its own relay and circuit breaker.It serves as the first line of defence.

6. What is back up protection?

Is the second line of defence , which operates if the primary protection fails to activate within a definite time delay.

7. Name the different kinds of overcurrent relays.

Induction type non-directional overcurrent relay,Induction type directional overcurrent relay & current differential relay.

8. Define energizing quantity.

It refers to the current or voltage which is used to activate the relay into operation.

9. Define operating time of a relay.

It is defined as the time period extending from the occurrence of the fault through the relay detecting the fault to the operation of the relay.

10. Define resetting time of a relay.

It is defined as the time taken by the relay from the instant of isolating the fault to the moment when the fault is removed and the relay can be reset.

11. What are over and under current relays?

Overcurrent relays are those that operate when the current in a line exceeds a predetermined value. (eg: Induction type non-directional/directional overcurrent relay, differential overcurrent relay)whereas undercurrent relays are those which operate whenever the current in a circuit/line drops below a predetermined value.(eg: differential over-voltage relay)

12. Mention any two applications of differential relay.

Protection of generator & generator transformer unit; protection of large motors and busbars.

13. What is biased differential bus zone reduction?

The biased beam relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected zone. It is essentially an over-current balanced beam relay type with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force.

14. What is the need of relay coordination?

The operation of a relay should be fast and selective, ie, 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 coordinating relays, faults can always be isolated quickly without serious disturbance to the rest of the system.

15. 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 a certain value.

16. What are the features of directional relay?

High speed operation; high sensitivity; ability to operate at low voltages; adequate short-time thermal ratio; burden must not be excessive.

17. What are the causes of over speed and how alternators are protected from it?

Sudden loss of all or major part of the load causes over-speeding in alternators. Modern alternators are provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime mover when a dangerous over-speed occurs.

18. Define an under current relay?

Relays which operates whenever the current in a circuit drops below a predetermined value.

19. Differentiate between a fuse and a circuit breaker.

Fuse is a low current interrupting device. It is a copper or an aluminium wire.Circuit breaker is a high current interrupting device and it act as a switch under normal operating conditions. 30. How direct tests are conducted in circuit breakers? Using a short circuit generator as the source. Using the power utility system or network as the source.

20. What is dielectric test of a circuit breaker?

It consists of over voltage withstand test of power frequency lightning and impulse voltages.Tests are done for both internal and external insulation with switch in both open and closed conditions.

21. What is the backup protection available for an alternator?

Overcurrent and earth fault protection is the backup protections.

22. What is REF relay? It is restricted earth fault relay.

When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small,which may not be sufficient to activate the relay, unless the relay is set for a very low current. Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.

23. Why busbar protection is needed?

(a) Fault level at busbar is high (b) The stability of the system is affected by the faults in the bus zone. (c) A fault in the bus bar causes interruption of supply to a large portion of the system network.

24. What are the errors in CT?

(a) Ratio error Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100 The value of transformation ratio is not equal to the turns ratio. (b) Phase angle error: Phase angle =180/ [(ImCos -I1Sin )/nIs]

25. What is circuit breaker?

It is a piece of equipment used to break a circuit automatically under fault conditions. It breaks a circuit either manually or by remote control under normal conditions and under fault conditions.

26. Write the classification of circuit breakers based on the medium used for arc extinction?

Air break circuit breaker Oil circuit breaker Minimum oil circuit breaker Air blast circuit breaker SF6 circuit breaker Vacuum circuit breaker

27. Define a over current relay.

Relay which operates when the current ia a line exceeds a predetermined value.

28. What are the uses of Buchholz’s relay?

Buchholz relay is used to give an alarm in case of incipient( slow-developing) faults in the transformer and to connect the transformer from the supply in the event of severe internal faults. It is usually used in oil immersion transformers with a rating over 750 KVA.