Protection Relays Interview Questions & Answers

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.

What is the operation principle of differential relay?

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

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.

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.

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.

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.

What are the main functions of Differential Relays?

Differential Relays must have the following features:

• 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

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.

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.

Where is directional relay used?

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

For what type of fault does Buchholz relay is employed?

Buchholz relay provides protection only against transformer internal fault.

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.