Switchgear and Protection Interview Questions and Answers:

1. What is meant by short circuit ?

Ans. When conductors of different phases come into contact with each other in a power line, power transformer, or any other circuit element, part of the impedance of the system is shunted out of the circuit, a large current flows in one or more phases, a short circuit is said to have occurred.

2. What is difference between a short circuit and an overload ?

Ans. On occurrence of a short circuit, the voltage at the fault point is reduced to zero and current of abnormally high magnitude flows through the network to the fault point. On the other hand, an overload means that load greater than the designed value have been imposed on the system. In case of an overload, the voltage at the overload point may become low, but cannot be zero. The under voltage conditions may extend to some distance beyond the overload point in the remainder of the system. In case of an overload, the currents are high but are substantially lower than in the case of a short circuit.

3. The probability of the failure or occurrence of abnormal conditions is more on the power line. Why ?

Ans. The probability of the failure or occurrence of abnormal conditions is more on the power line. This can be explained by the fact that the power lines are widely branched, have greater length, operate under variable weather conditions and are subject to the action of atmospheric disturbances of electrical nature.

4. Name the different faults likely to occur in an overhead transmission line.

Ans. The different faults likely to occur in an overhead transmission line are (1) Single phase to ground, (2) Phase to phase, (3) Two phases to ground, (4) Phase to phase and third phase to ground, (5) All the three phases shorted and (6) All the three phases to ground.

5. Which type of fault occurs more frequently in overhead lines ?

Ans. Single phase to ground (L-G) fault occurs more frequently in overhead tines.

6. Which type of faults are taken into account while determining the rating of the circuit breaker ?

Ans. The balanced three-phase fault, though its occurrence is quite rare (only about 5 per cent of the total), imposes most severe duty on circuit breakers and is, therefore, used in determination of the circuit breaker rating.

7. What are the sources of short-circuit power in a power system ?

Ans. Mainly there are three sources viz synchronous generators, synchronous motors and synchronous condensers and induction motors, which can feed into the short circuit.

8. What is a protective relay ?

Ans. Protective relay is an electrical device designed to initiate isolation of a part of an electrical installation, or to operate an alarm signal, in the event of an abnormal condition or a fault.

9. What are the functions of a protective relay ?

Ans. The main functions of protective relays are to detect the presence of faults, their locations and initiate the action for quick removal from service of the faulty section of the system. Relays operate to trip the power circuit breakers to disconnect the faulty section.

10. What are the essential elements of a protective relay ?

Ans. The essential elements of a protective relay are (i) sensing element (ii) comparison element, and (iii) control element.

11. What is a zone of protection ?

Ans. A part of the system protected by a certain protective scheme is called protective zone or zone of protection. The entire power system is covered by several protective zones and no part of the system is left unprotected. The protection zone usually contains one or at the most two elements of the power system.

12. What is meant by primary protection and backup protection ?

Ans. The main protection is the first line of defence and ensures quick acting and selective clearing of faults within the boundary of the circuit section or element it protects. Main protection as a rule, is provided for each section of an electrical installation.

Backup protection is the name given to a protection which backs up the main protection whenever the latter fails in operation or is cut out for repairs etc.

Backup protection is important to the proper functioning of a good system of electrical protection since cent per cent reliability not only of the protective scheme but also of the associated CTs. PTs and circuit breakers cannot be guaranteed. It is the second line of defence which functions to isolate a faulty section of the system in case the main protection fails to function properly.

13. What are the basic requirements of a protection system ?

Ans. The basic requirements of a protection system are essentially reliability and simplicity, selectivity and discrimination, sensitivity, speed of operation and economy.

14. Why current transformers are required in a protective relaying scheme ?

Ans. The primary circuit currents which are of high magnitudes are to be reduced to values suitable for relay operation and current transformers are used for this purpose. In addition to this, CTs provide insulation against the high voltage of the power circuit and protect the apparatus and the operating personnels from contact with the high voltages of the primary circuits.

15. Why potential transformers are required in a protective relaying scheme ?

Ans. It is not possible to connect the potential coils of the protective devices directly into the system in case of high voltage systems. It is, therefore, necessary to step down the voltage and also to insulate the protective equipment from the primary circuit (high voltage power circuit). This is achieved by employing potential (or voltage) transformers (PTs).