Question Bank for Power System Protection and Switchgear.

  1. Describe the basic components of protection system.
  2. State the essential qualities of an ideal protection system.
  3. Explain the concept of zones of protection with a schematic diagram.
  4. Define the terms: plug-setting multiplier, time-multiplier setting.
  5. State the mathematical relation between the current and operating time of an extremely inverse time over-current relay.
  6. Suppose, the actual fault current in case of a severe line to ground fault is 3000 A.
    Assuming the CT ratio to be 100/1, calculate the time of operation of an inverse definite minimum time (IDMT) relay for the above fault, if the plug-setting is 50%, and the time-multiplier setting is 0.2.
  7. State the reason for the operating time of IDMT relays becoming definite minimum at high values of current.
  8. Mention the differences between primary and back-up protection.
  9. In a three-stepped distance protection scheme of a line, indicate the reaches (zones of operation) of the first two steps/zones.
  10. State the universal (generic) relay-torque equation.
  11. Derive the trip law for a simple impedance relay using the universal relay-torque equation. Draw the characteristics of the impedance relay on the R-X plane.
  12. Compare the performance (in brief) of the different types of distance relays with respect to:
    • (i) effect of fault/arc resistance
    • (ii) effect of power swings
  13. Assume that there are three buses A, B, and C connected by a distribution feeder. It is required to design a suitable protection scheme for the feeder using definite time overcurrent (DTOC) relays. The DTOC relay at B provides primary protection to the section BC, and the DTOC relay at A provides back-up protection to the section BC.
    Suppose, a fault occurs in the section BC close to the remote bus C. In this situation:
    • (i) Mention the range of pick-up values of the DTOC relays at B and A.
    • (ii) Define overshoot time of a relay.
    • (iii) Assume that the operating times of the relay and circuit breaker at B are 0.15 seconds and 0.6 seconds, respectively. The overshoot time of the relay at A is 0.15 seconds. Determine the minimum operating time of the back-up relay at A. Explain the logic involved in the determination of the same.
  14. With a neat phasor diagram, derive the expression for the operating torque of a directional relay based on induction principle. State the expression for the maximum torque angle.