Power System Protection Interview Questions
Q. Mention some of the applications of Directional Over-Current relays?
Double end fed power system
Single end fed system of parallel feeders
Ring main feeder system
Q. Explain differential protection?
A. Differential protection is based on the fact that any fault within an electrical equipment would cause the current entering it, to be different, from that leaving it.
Thus, we can compare the two currents either in magnitude or in phase or both and issue a trip output if the difference exceeds a pre- determined set value.
Q. What are the different types of faults in a transformer?
A. The internal faults include phase faults, ground faults, inter-turn faults, tap-changer failure, and leakage oil from tank.
The external faults include system phase faults, system ground faults, over loads and over fluxing.
Q. What is inrush current?
A. the initial value of flux is zero when the transformer is energized. The, the flux reaches a peak value in half a cycle.
Since power transformers operate near the knee of the saturation curve, a flux demand drives the transformer core deep into saturation, causing it to draw a very large magnetizing current with a peaky non-sinusoidal waveform.
The magnetizing current is, therefore, very high, of the order of 8 to 30 times the full-load current. This current is known as inrush current.
Q. what is Buchholz relay and the significance of it in to the transformer?
A. Buchholz relay is a device which is used for the protection of transformer from its internal faults, it is a gas based relay.
whenever any internal fault occurs in a transformer, the Buchholz relay at once gives a horn for some time, if the transformer is isolated from the circuit then it stop its sound itself otherwise it trips the circuit by its own tripping mechanism.
Q. What is the difference between megger test equipment and contact resistance meter test instruments?
A. Megger test equipment used to measure cable electric resistance, conductor continuity, phase identification.
Contact resistance meter test instruments used to measure low resistance like relays, contactors.
Q. What is different between resistance grounding system and resistance earthing system?
A. In resistance grounding system the neutral point of the load is connected to the ground to carry the residual current in case of unbalanced conditions.
Resistance earthing system is done in an electric equipment in order to protect the equipment in occurrence of fault in the system.
Q. What is meant by knee point voltage?
A. Knee point voltage is the voltage at which a CT gets saturated. It is one of the important factors taken into consideration for selecting a current transformer.
Q. What is reverse power relay?
A. Reverse Power flow relay are used to stop the flow of power from grid to station generator when the generating units are off.
Q. When voltage increases then current also increases then what is the need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only?
A. No. We can’t sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage (As most of the loads are non-linear in nature).So, the over voltage protection & over current protection are completely different.
Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition, earth fault thereby reducing the system failure & risk of fire. So, for a better protection of the system. It should have both over voltage & over current relay.
Q. Where are the lighting arrestors placed in distribution lines?
A. Near distribution transformers and outgoing feeders of 11kv and incoming feeder of 33kv and near power transformers in sub-stations.
Q. What is the difference between surge arrestor and lightning arrestor?
A. Lightning arrestor is installed outside and the effect of lightning is grounded, whereas surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.
Q. In three pin plug why earth pin diameter is higher than other two pin?
A. Because Current flow in the conductor is inversely proportional to the conductor diameter.
So if any short circuits occur in the system first high currents bypassed in the Earthling terminal. (R=Pl/a area of the conductor increases resistance value decreases)