Where does Negative phase sequence relay is employed?
Answer: 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?
Answer: 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?
Answer: 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?
Answer: 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?
Answer: 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?
Answer: It is a differential relay where the operating current required to trip can be expressed as a percentage of load current
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