PROTECTION INTERVIEW QUESTIONS WITH ANSWERS

1. Where Does Negative Phase Sequence Relay Is Employed?

Negative sequence relay is employed for the protection of generators and motors against unbalanced loading that may arise due to phase to phase faults.

2. What Is The Operation Principle Of Differential Relay?

Differential relay operates when the phasor difference of two or more similar electrical quantities exceeds a pre-determined amount.

3. Why Distance Protection Is Preferred As Primary Protection Compared To Over Current Protection For Transmission Lines?

Distance relay is superior to over current protection for the protection of transmission lines. Some the reasons are faster protection, simpler coordination, simpler application, permanent settings without need for readjustment, less effect of the amount of generation and fault levels, fault current magnitude, permits the high line loading.

4. Why Biased Differential Protection Is Preferred Over Simple Differential Protection?

Biased differential relay is preferred because its operation is not affected by the trouble arising out of the difference in the CTs ratios for high values of external short circuit currents.

5. Where Impedance Relay, Reactance Relay And Mho Relays Are Employed?

The Impedance relay is suitable for the phase faults relaying for the lines of moderate lengths Reactance type relays are employed for the ground faults while Mho type of relays are best suited for the long transmission lines and particularly where synchronizing power surge may occur.

6. What Is Percentage Differential Relay?

It is a differential relay where the operating current required to trip can be expressed as a percentage of load current.

7. What Are The Main Functions Of Differential Relays?

• High speed operation; High sensitivity
• Adequate short circuit thermal rating
• Ability to operate operate with low values of voltage
• Burden must not be excessive
• There should be no voltage and current creep

8. What Is Meant By “relay Settings”?

Relay settings means actual value of the energizing or characteristic quantity at which the relay is designed to operate under given conditions.

9. Define Plug Setting Multiplier?

Plug Setting Multiplier and is defined as the ratio of fault current in the relay coil to the pick up value.

10. Where Is Directional Relay Used?

Directional relay are used when graded time overload protection is applied to ring mains and interconnected networks.

11. For What Type Of Fault Does Buchholz Relay Is Employed?

Buchholz relay provides protection only against transformer internal fault.

12. How Definite Time Lag Is Achieved In Attraction Armature Relays?

The instantaneous type attraction armature can be made a definite time lag or inverse time lag by using a oil dash pot, an air escapement chamber a clock work mechanism or by placing a fuse in parallel with it.

13. What Is Protective Relay?

It is an electrical device designed to initiate the isolation of a part of the electrical installation, or to operate an alarm signal, in the event of abnormal condition or a fault. In simple words relay is an electrical device that gives signal to isolation device (eg: Circuit Breaker) after sensing the fault and helps to isolate the fault system from the healthy electrical system

14. What Are The Different Relays That Employed For Protection Of Apparatus And Transmission Lines?

• Over current relay; Directional relay; Distance relay; Under Voltage relay; Under-frequency relay
• Thermal relay; Differential relay; Phase sequence relays; Pilot relays

15. How The Electrical Power System Protection Is Divided?

• Generator protection; Transformer protection; Busbar protection
• Transmission line protection and Feeder protection

16. How Relays Are Connected In The Power System?

The relays are connected to the power system through the current transformer (CT) or potential transformer (PT).

17. What Are Different Types Of Principles Of Operation Of Electromechanical Relays?

Electromechanical relays operate by two principles: Electro-magnetic attraction and electro-magnetic induction. In electromagnetic attraction relay plunger is drawn to the solenoid or an armature is attracted to the poles of the electromagnet. In case of electro-magnetic induction, principle of operation is similar to induction motor. Torque is developed by electromagnetic induction principle.

18. Action Carried Out By The Relay And Circuit Breaker During Fault Condition?

After the relay sensing the fault condition, relay operates and close the trip coils. The effect of this will be circuit breaker operate to open the contacts.