Switchgear And Protection — Winter 2021

What do you understand by a zone of protection? Discuss various zones of protection for a modern power system. Why do adjacent protective zones overlap?

What do you understand by 30˚ and 60˚ connections with reference to directional overcurrent relays?

The percentage differential protection of transformer will not operate during an external fault. Explain and justify with suitable figure.

With suitable waveforms, explain the problem of magnetizing inrush in case of transformer protection. Explain how percentage differential relay with harmonic restraint can overcome this problem.1

Explain the effect of arc resistance on Mho relay reach and the directional property of Mho relay with the help of characteristics on R-Xplane.

Explain capacitor voltage transformer. Give its comparison with electromagnetic potential transformer.

07 Figure 1 Two relays Rand Rare connected in two sections of a feeder, with CT ratios12 of 1000/5 Aas shown in Figure 1 above. The plug settings of relay Rand R12 are 150% and 200% respectively. Operating time characteristic of both relays at TMS=1 is given in Table 1 below. Table 1 PSM 2 4 5 6 8 10 12 15 Operating 10 5 4 3.6 3 2.8 2.6 2.2 1.6 Time (s) The time multiplier setting of relay Ris 0.3. The time grading scheme has a1 discriminative time margin of 0.5 seconds between the relays. Athree-phase short circuit at Fresults in a fault current as shown in figure. Find the actual operating times of Rand R .12 What is the time multiplier setting of R ?2

Explain the necessity of directional overcurrent relay in case of single end fed parallel feeders and radial feeders.

Briefly explain how the CT ratio should be selected for differential protection of bus-bars.

Explain in brief selectivity, sensitivity, discrimination and speed with reference to protective relaying.

In relation to circuit breaker arc interruption, explain the recovery rate theory and energy balance theory. Also with suitable figure, define restriking voltage and recovery voltage.

What is back up protection and why is it needed?

Compare Air Blast Circuit Breaker, SF Circuit Breaker and Vacuum Circuit6 Breaker with reference to their arc quenching mechanisms.

Explain three stepped distance protection of transmission line.

Briefly explain various types of faults and abnormal conditions in transformers.

Explain unbalanced loading and loss of excitation as abnormal operating condition of alternator.

With the help of characteristics on R-Xplane, compare between performance of impedance and reactance relays in terms of: i) Directional property exhibited by the relay; ii) Performance of relay during power swing.

Explain the difference between CT cores used for measurement and those used for protective relays.

Explain the basic schematic of differential protection of bus-bars. Explain its behavior with reference to internal and external faults, clearly explaining the reason of maloperation during external faults.

List out various stator and rotor faults of generator. Explain transverse differential protection of generator winding.

Explain how to select short-time current rating of CT and accuracy class of CT while selecting a CT.

With schematic, explain protection from phase fault and ground fault inside the three phase induction motor.

Explain the problem associated with unbalanced supply voltage and the method of protection from the same for a three phase induction motor.

Draw and explain the general block diagram of numerical relay.

Explain the concept of adaptive relaying.