Substations Interview Questions and Answers:

1. State any four types of substations according to the service ?

Ans. The substations, according to the service rendered are: Transformer substations, Switching substations, Converting substations and Frequency changing substations.

2. State the relative merits of indoor and outdoor substations.


Merits of Indoor Substations. Less requirement of space, less maintenance and control cable length; protection from lightning; flexibility in installation, no dust and dirt.

Merits of Outdoor Substations. No building requirement, short erection time; Easy fault finding due to visibility of equipment; Easy repair work; Easier installation extension; Availability of sufficient space between equipment’s.

3. What are the factors governing the selection of site for substations ?

Ans. Factors governing the selection of site for a substation are :

1. Nearness to load centre 2. Availability of suitable and sufficient land 3. Availability of communication facilities 4. Drainage facilities 5. Availability of essential amenities to the staff. 6. Pollution free atmosphere 7. Facility for future extension 8. Distance from airport, military or police rifle training centre.

4. List out the equipment used in a substation.

Ans. Equipment used in a substation are Transformer(s); Bus-bars; Circuit breakers; CTs; PTs; Insulators; Isolators; Switchgear (fuses, air-break switches etc.,) Protective relays; Surge arresters; Cables, Firefighting equipment; Batteries; Earthing arrangement, Wave traps, Reactors etc.

5. What are the functions of panels and firefighting equipment in a substation ?

Ans. Panels or cubicles in a substation are provided to support the various equipment like meters, relays etc. It saves the horizontal space and facilitates the supervisory staff to record readings easily. All the panels/cubicles are properly grounded.

Special type of fire extinguishers are installed in a substation to quench electric fire in case of a fire accident because ordinary fire extinguishers or water cannot be used to quench electric fire as it is dangerous. They are provided in accessible position and every one in the substation must know low its operation.

6. What is flexible bus ?

Ans. Flexible bus consists of flexible ACSR or all aluminium alloy stranded conductors supported by strain insulators from each end.

7. Name the interlockings provided with isolators.

Ans. The interlocking provided with isolators are :

  1. Interlocking between three poles for simultaneous operation.
  2. Interlocking with circuit breakers.

8. In what way is an isolator different from an air-break switch ?

Ans. Isolators are used to connect or disconnect the line only under no-load condition while air-break switches are used for making and breaking of the circuit under loaded condition. Air-break switches are provided with arc control devices.

9. Why are isolators necessarily provided on the supply side of the circuit breakers ?

Ans. Isolators are necessarily provided on the supply side of the circuit breaker in order to ensure isolation of the circuit breaker from the live parts for the purpose of maintenance.

10. Give the sequence of operation during opening and closing of a circuit.

Ans. Sequence of operation of circuit breaker, isolator and earthing switch during opening/closing of a circuit are given below.

While Opening: Open circuit breaker, open isolator and then close earthing switch if provided.

While Closing: Open earthing switch, close isolator and then close circuit breaker.

11. What do you understand by switchgear ?

Ans. The apparatus employed for switching, controlling and protecting the electrical circuits and equipment is known as switchgear.

12. What is a circuit breaker ?

Ans. Circuit breaker is a mechanical device designed to close or open contact members, thus closing or opening an electrical circuit under normal or abnormal conditions.

13. How does a circuit breaker differ from a switch ?

Ans. A circuit breaker is a device that interrupts the abnormal or fault currents and in addition performs the function of a switch.

14. What for isolating switches are provided along with circuit breakers ?

Ans. Isolating switches are provided for separating the disconnected element from the live portion of the system for the purposes of safety.

15. What is difference between primary and secondary batteries ?

Ans. Batteries are either primary or secondary. Primary batteries can be used only once because the chemical reactions that supply the current are irreversible. Secondary batteries, sometimes called storage batteries or accumulators, can be used, recharged and reused. In these batteries, the chemical reactions that provide current from the battery are readily reversed when current is supplied to the battery.

16. Lead acid battery can be charged where as primary cell cannot be charged. Why ?

Ans. Primary cells can be used only once because the chemical reactions that supply the electric current are irreversible but in lead acid battery, the chemical reactions that provide the current are readily reversed when current is supplied to the battery.

17. In what form is energy stored in a secondary battery ?

Ans. Energy is stored in the form of electrical energy in a secondary battery.

18. Why Faure process is much suitable for manufacturing of negative plates rather than positive plates?

Ans. Faure process is much suitable for manufacturing of negative plates because negative active material (sponge lead) is quite tough (not hard and brittle as lead peroxide is) and moreover, it undergoes a comparatively negligible change in volume during charging and discharging processes.

19. Why in grouping of lead acid cells the negative group is made to have one plate more than positive group ?

Ans. The negative group of the plates contains one more plate than that of the positive group so that when interleaved both the end plates become the negative. This is done so that all the positive plates can work equally well on both sides. If a positive plate were used at one end, one of its sides would remain inactive, when the other side expands, and thus the plate is likely to buckle.

20. Do we use ac or dc to charge a battery ?

Ans. For charging of a battery dc is essential.

21. Why should the lead acid cells not be allowed to stand completely discharged ?

Ans. Lead acid cells shall not be allowed to stand completely discharged because the lead sulphate formed on the plates crystallizes and the proper reforming of the plates becomes difficult.

22. What are the factors that affect the battery capacity.

Ans. The capacity of battery depends upon several factors, principal among which are area of plate surface, quantity, arrangement and porosity of the active material used in the manufacture of plates, quantity and specific gravity of the electrolyte used and the porosity of the separators, rate of charge and temperature.

23. Why the lead acid battery should not be allowed to discharge beyond 1.75 V ?

Ans. The lead acid battery should never be allowed to discharge beyond 1.75 V otherwise lead sulphate will be formed on the electrodes which is hard, insoluble and increases the internal resistance of the battery.

24. Why is the watt-hour efficiency of a cell lower than its ampere-hour efficiency ?

Ans. Watt-hour efficiency is given as

Watt-hour efficiency

and Vd is always lower than Vc, therefore ηWH is always lower than its ηA-H.