Protective Relays Interview Questions and Answers:
1. 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.
2. 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.
3. 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.
4. What are the fundamental requirements of protective relaying ?
Ans. The fundamental requirements of protective relaying are speed, selectivity, sensitivity, reliability, simplicity, and economy.
5. Define the terms ‘sensitivity’ and ‘selectivity’ of a relay.
Ans. Sensitivity of a relay means its capacity to operate reliably under the actual conditions that produce the least operating tendency while the selectivity means the ability to determine the point at which the fault occurs and select the nearest circuit breaker in the system to trip so that the fault is cleared with minimum or no damage to the system.
6. What is meant by “relay setting” ?
Ans. Relay setting means actual value of the energizing or characteristic quantity at which the relay is designed to operate under given conditions.
7. What is meant by operating time of a protective relay ?
Ans. The operating time of a protective relay means the time which elapses between the instant when the actuating quantity exceeds the pick-up value to the instant when the relay contacts close.
8. What is meant by ‘relay time’ ?
Ans. Time interval between occurrence of fault and closure of relay contacts is called the relay time.
9. How are relay time, breaker time and fault clearing time related ?
Ans. Relay time plus breaker time equals the fault clearing time.
10. What are the shortcomings in electromagnetic relays ?
Ans. The vibration of relay armature at double the supply frequency, due to pulsating nature of electromagnetic force, causes the relay to hum and produce noise and also is a source of damage to the relay contacts. This leads to sparking and unreliable operation of the relay operative circuit contacts due to make and break of the circuit.
11. How definite time lag is achieved in attraction armature relays ?
Ans. The instantaneous type attraction armature relays can be made a definite time lag or inverse-time lag by using an oil dash pot, an air-escapment chamber a clockwork mechanism or by placing a fuse in parallel with it.
12. How induction cup type construction is superior to induction disc type ?
Ans. Induction cup type construction develops more efficient torque than either shaded pole or watt-hour meter constructions. Such relays are very fast in operation and may have an operating time of less than 0.01 second.
13. What is Buchholz relay ? Which equipment is protected by it ?
Ans. Buchholz relay is a gas actuated relay and is used for the protection of power transformers of ratings exceeding 500 kVA.
14. For what type of fault Buchholz relay is employed ?
Ans. Buchholz relay provides protection only against transformer internal faults.
15. Define PSM ?
Ans. PSM stands for plug setting multiplier and is defined as the ratio of fault current in relay coil to the pick-up value.
16. What are the main features of directional relays ?
Ans. Directional relays must have the following features: (i) High speed operation, (ii) high sensitivity, (iii) adequate short-time thermal rating, (iv) ability to operate with low values of voltage (v) burden must not be excessive and (v) there should be no voltage and current creep.
17. Where is directional relay used?
Ans. Directional relays are used when graded time overload protection is applied to ring mains and interconnected networks.
18. What is distance protection ?
Ans. Distance protection is the name given to the protection, whose action depends upon the distance of the feeding point to the fault point. It is non-unit type protection and the protection zone is not exact. It can be used as primary as well as backup protection.
19. In what way a distance relay is superior to overcurrent protection for the protection of transmission lines
Ans. Distance relay is superior to overcurrent protection for the protection of transmission lines. The reasons are faster protection, simpler coordination, simpler application, permanent setting without need for readjustments, less effect of amount of generation and fault levels, fault current magnitude, permits the high line loading.
20. Why directional feature provided for impedance relay cannot be used for reactance relay ?
Ans. The directional feature provided for impedance relay cannot be used for reactance relay because the reactance relay will trip even under normal operating conditions at or near unity power factor. The reactance type distance relay needs a directional unit that is inoperative under normal load conditions.
21. Why reactance type relay is very suitable as a ground relay for ground fault ?
Ans. Reactance type relay is very suitable as a ground relay for ground fault because its reach is not affected by fault impedance (i.e., arcing resistance).
22. For what type of protection will you recommend (i) impedance relay (ii) reactance relay and (iii) mho relay ?
Ans. The impedance type relay is suitable for phase fault relaying for lines of moderate length; reactance type relays are preferred for ground fault relaying while mho type relays are best suited for long lines and particularly where a severe synchronising power surge may occur.
23. What is meant by a differential relay ?
Ans. A differential relay is defined as the relay that operates when phasor difference of two or more similar electrical quantities exceeds a predetermined amount.
24. Where differential protection principle is employed ?
Ans. Differential protection principle is employed for the protection of generators, generator-transformer units, transformers, feeders, large motors and bus-bars.
25. Why is percentage differential relay named so ?
Ans. Percentage differential relay is known by this name because the operating current required to trip can be expressed as a percentage of load current.
26. Why a biased differential relay is preferred over a simple differential relay ?
Ans. Biased differential relay is preferred over a simple differential relay because its operation is not affected by the trouble arising out of difference in CTs ratios for high values of external short-circuit currents.
27. Where is negative phase sequence relay employed ?
Ans. Negative phase sequence relay is essentially employed for the protection of generators and motors against unbalanced loading that may arise due to phase-to-phase faults.
28. How a negative phase sequence relay is made to respond to the flow of zero-sequence currents also
Ans. The negative phase sequence relay is made to respond to the flow of zero-sequence currents also by providing additional winding on the central limb of the upper electromagnet of the relay connected in the residual circuit of three line CTs.