Protection and Switchgear Important Questions and Answers

1. Mention the essential features of the power system protection.

Answer:

  1. Speed and time,
  2. Selectivity,
  3. Sensitivity,
  4. Stability,
  5. Simplicity and Economics,
  6. Reliability

2. Explain the need for overlapping the zones of protection.

Answer:

  1. The circuit breakers are located in the connections to each power system element.
  2. This provision makes it possible to disconnect only the faulty element from the system

3. Classify the different types of overcurrent relays based on the inverse time characteristics.

Answer:

  1. Static instantaneous Overcurrent Relay
  2. Directional Static Overcurrent Relay
  3. Inverse Time Overcurrent Relay

4. Explain the basic difference between the measurement and protection CT’s.

Answer:

  • Measuring current transformers –  used in conjunction with ammeter, wattmeter etc.
  • Protection current transformers – used in association with relays, trip coils, pilot wires etc.

5. What is the need for multiple breakers in circuit breaking?

Answer:

When the higher voltage operation to need small rating or multiple breakers is reduced arc in the contacts.


6. List out the types of faults in power system.

Answer:

  • Symmetrical faults.
  • Unsymmetrical faults.
  • Single phase to ground (L-G) fault.
  • Two phase to ground (2L-G) fault.
  • Phase to phase (L-L) fault.
  • Open circuited phases.
  • Winding faults.
  • Simultaneous faults.

7. Mentioned different types of overcurrent relay.

Answer:

  • Definite-time overcurrent relay
  • Instantaneous overcurrent relay
  • Inverse-time overcurrent relay
  • Inverse definite minimum time overcurrent relay
  • Very inverse-time overcurrent relay
  • Extremely inverse-time overcurrent relay.

8. What are the advantages of static relays over electromagnetic relays?

Answer:

  • Low burden on C.Ts and P.Ts
  • Static relay consume less power
  • Fast response and long life
  • Less maintenance due to absence of morning parts.
  • Compact in size and greater sensitivity.

9. Why are current transformers required in protection schemes?

Answer:

  • Current transformers are used to reduce the heavy current flowing in an element of a power system to low values.
  • Besides reducing the current level, the C.T. also isolates the relay circuit from the primary circuit which is a high voltage power circuit and allows the use of standardized current rating for relays.

10. What are the different methods of high resistance are interruption?

Answer:

  • Lengthening the arc
  • Splitting of arc
  • Cooling of arc.

11. What are problems encountered in DC circuit breaking?

Answer:

The arc becomes unstable and the difference in voltage is supplied by inductance L. For decreasing values of current this voltage is negative and it tries to maintain the arc.


12. How does circuit breaker differ from a switch?

Answer:

Circuit breaker is designed that it can be operated manually under normal conditions and automatically under fault conditions whereas switch will not operate under faulty conditions.


13. What are chief requirements of the contact material for vacuum circuit breaker?

Answer:

  • Contacts are required to be travelled by small distance.
  • Contacts must be of less weight such that many repeated operations can be performed with this type.

14. Define zone of protection.

Answer:

A protective zone covers one or at the most two elements of a power system. The protective zone covers are planned in such a way that the entire power system is collectively covered by them is known as zone of protections.


15. What are the causes of faults in power system?

Answer:

  • Internal causes of the equipment.
  • Heavy short circuit current may causes damage to damage equipment or other element of the system due to over heating and high mechanical forces set up due to heavy current.
  • Deterioration of insulation.

16. Mentioned any two applications of differential relay.

Answer:

  • It can be differentiate the heavy load conditions and minor load conditions.
  • Fault current can be easily identify the relay.
  • Protection of generator and generator transformer unit protection of large motors and bus bars.

17. What are the features of directional relay?

Answer:

A directional power relay which operated when the power in the circuit flows in a particular direction. This it requires to sense the system voltage as well as the system current.


18. Give the limitations of Merz Price protection.

Answer:

Since neutral earthing resistance are often used to protect circuit from earth fault currents, it becomes impossible to protect the whole of a star connected alternator. If an earth-fault occurs near the neutral point, the voltage may be insufficient to operate the relay. Also it is extremely difficult to find two identical CT’s. In addition to this there always an inherent phase difference between the primary and the secondary quantities and a possibility of current through the relay even when there is no fault.


19. Why bus bar protection is needed?

Answer:

  1. Fault level at busbar is high.
  2. The stability if the system is affected by the faults in the bus zone.
  3. A fault in the busbar causes interruption of supply to a large portion of the system network.

20.What is RRRV?

Answer:

It is the rate of rise of restricting voltage, expressed in volts per microsecond. It is closely associated with natural frequency of oscillation.


21. What do you mean by current chopping?

Answer:

When interrupting low inductive currents such as magnetizing currents of the transformers shunt reactor, the rapid deionization of the contact space and blast effect may cause the current to be interrupted before the natural current zero. This phenomenon of interruption of the current before its natural zero is called chopping.


22. What is dielectric test of a circuit breaker?

Answer:

Dielectric test is used for schering bridge to identify the continuation faults, internal insulation failure improve the power factor and loss if angle detection and dielectric losses.


23. What is surge absorber? How do they differ from surge diverter?

Answer:

It is a protective device which reduces the steepness of wave front of a surge by absorbing surge energy.

Surge Absorber: It eliminates the surge, by absorbing the surge energy.

Surge Diverter: It eliminates the surge, by diverting the surge to earth.


24. Define the term ” Insulation Coordination”.

Answer:

As per IEEE 1313.1 the insulation co-ordination is the selection of insulation strength consistent with expected over voltages to obtain an acceptable risk of failure.


25. Explain the secondary of current transformer should not be open.

Answer:

Secondary of CT should not be open:

If the secondary of CT is left open, due to infinite impedance the current through secondary becomes zero and hence ampere turns produced by secondary which oppose primary ampere turns becomes zero. Since there is no counter mmf unopposed primary mmf produce high flux in core. This causes excessive core loss, and heavy induced emfs on primary and secondary side. This damages the insulation of winding. So the secondary of CT should not be kept open.


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