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Power System Interview Questions and Answers

Power system interview questions and answers


Power System Interview Questions and Answers

Q.1. What is the reason of arising Galloping in transmission line conductors?

Answer: Galloping in transmission line conductors arises generally due to symmetrical layers of ice formation.

Q.2. What do you mean by sag in transmission line?

Answer: The transmission line is not fully stretched but it is allowed to sag down. The difference in levels between point of support and the lowest point on the conductor is called sag.

Q.3. On what basis is the insulation level of a 400kv, EHV overhead transmission line decided?

Answer: At transmission line voltages up to 230kv, the insulation level is dictated by the requirement of protection against lightning.

In EHV, switching surges cause higher over voltages than lightning and are therefore mainly responsible for insulation level decision.

Q.4. The making and breaking currents of three phase ac circuit breakers in power system are in which form?

Answer: The making and breaking currents of three phase ac circuit breakers in power system are in instantaneous value and rms value respectively.

Q.5. Which relay is used for protecting a transformer against all internal faults?

Answer: A Buchholz relay is used for protection of a transformer against all internal faults.

Q.6. What are P-V curves?

Answer: P-V curves are useful for conceptual analysis of voltage stability and for study of radial systems. The method is also used for large interconnected network, where P is the total load in an area and V is the voltage of a critical bus. P could be the power transferred over a transmission line.

Q.7. What do you mean by grading of cables?

Answer: The stress in the insulation is maximum at the conductor surface and minimum at the sheath. To avoid the breakdown of the insulation, it is necessary to have uniform distribution of stress all along the insulation.

The process of obtaining uniform distribution of stress in the insulation of cables is called grading of cables.

Q.8. Which type of connection is employed for current transformers for the protection of star-delta connected 3-phase transformer?

Answer: Delta-star. In case of star-delta transformer, the current on the two sides differ in phase by 30 degree. This is compensated by connecting current transformer in delta on the star side and in star on the delta side of the transformer.

Q.9. What do you understand by matched lines?

Answer:  When a transmission line is terminated through a resistance equal to its surge impedance the wave does not suffer reflection and, therefore, such lines could be said to be of infinite length. Such lines are also called as matched lines and the load corresponding to this is known as surge impedance loading or natural impedance loading.

Q.10. What are pumped storage plant?

Answer: Pumped storage plants are an special type of power plant which work as ordinary hydro power plants for part of the time and when such plants are not producing power, they can be used as pumping stations which pump water from tail race to the head race. During this time, these plants utilize power available from the grid to run the pumping set.

Q.11. Which relay has the capability of anticipating the possible major fault in a transformer?

Answer: Buchholz relay is the gas detector relay used to protect transformers which operates when the oil level in the conservator pipe of a transformer is lowered by the accumulation of gas caused by a poor connection or by an incipient breakdown of insulation.

Q.12. What is the difference between steady state and transient state stability of power system?

Answer: The steady state stability limit refers to maximum power transfer that is possible with small changes in power flow or gradual disturbance without losing stability.

The transient stability refers to the maximum power transfer that is possible for given amount of sudden or large changes in power disturbance without loss of stability.

Q.13. Why mostly air blast circuit breakers are susceptible to current chopping?

Answer: Current chopping mainly occurs in air-blast circuit breakers because they retain the same extinguishing power irrespective of the magnitude of the current to be interrupted. When breaking low currents with such breakers, the powerful de-ionizing effect of air-blast causes the current fall abruptly to zero well before the natural current zero is reached.

Q.14. What is “expanded ACSR”?

Answer: “Expanded ACSR” are conductor composed of filler between the inner steel and the outer aluminum strands to increase the overall diameter of the conductor.

Q.15. Why it is desirable to have a high power factor of the system in case of HVDC transmission?

Answer: In case of HVDC transmission it is desirable to have a high power factor of the system for the following reasons –

  • For a given current and voltage of the thyristor and transformers, the power rating of the converters is high.
  • The stresses on the thyristor and damping circuits are reduced.
  • For the same power to be transmitted the current rating of the system is reduced and also the copper losses in the ac lines are reduced.
  • In the ac lines the voltage drop is reduced.

Q.16. What do you mean by breaking capacity of a circuit breaker?

Answer: The breaking capacity of a breaker is the product of the breaking current and the recovery voltage.

Q.17. What do you mean by equal area criterion for stability?

Answer: According to equal area criterion, the system is stable if the area under Pa (accelerating power) – δ curve reduces to zero at some value of δ. In other words, the positive (accelerating) area under Pa – δ curve must be equal the negative (decelerating) area.

Q.18. What is Ferranti effect?

Answer: When a long line is operating under no load or light load condition, the receiving end voltage is greater than the sending end voltage. This is known as Ferranti effect.

Q.19. What requirements should be met in order to have transients in an electrical system?

Answer: In order to have transients in an electrical system the following requirements should be met –

  • Either inductor or capacitor or both should be present.
  • A sudden change in the form of a fault or any switching operation should take place.

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