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Substation Interview Questions Answers

Updated on January 2, 2014

What is hybrid substation?

Answer: Hybrid substation is a combination of both conventional air insulated substation (AIS) and Gas Insulated Substation (GIS). Some of the bays in the substation are gas insulated (enclosed in modules filled with SF6 gas) and some of the bays are air insulated type (open to atmosphere). The design of hybrid substation is based on the convenience, local conditions, availability of the area and cost

Why Earthing Grid is Required in Substation?

Answer: In electrical substation in order to limit the step potential and touch potential to sufficiently safe values and to control the influence of the short circuit currents on telecommunication installations. A Strong earthing network is provided. This is provided to effectively earth the systems with high earth fault currents

What are the different current transformers used in substation?

Answer: Current transformers in substations are generally used for reducing or stepping down a.c currents from higher value to lower value for the purpose of measuring, protection and control purposes. There are two classes of current transformers.

Measuring current transformers:

They are used for measuring purposes.They are used with ammeters, watt meters, KVA meters and kWh meters for reducing line current to 1 ampere or 5 amperes

Protective current transformers:

They are used with over-current protection, earth fault protection, differential protection, impedance protection

What is the power frequency spark-over voltage?

Answer: Power frequency spark-over voltage is the r.m.s value of the power frequency (50Hz or 60Hz) voltage applied between the line and earth terminals of the arrester and earth which causes spark-over of the series gap

What is Follow Current with respect to Surge Arrester?

Answer: Follow current is the current which flows from the connected power source through lightning arrester following the passage of discharge currents

Which is the basic busbar scheme employed in 220kV and 400kV substations?

Answer: In case of 220kV and 400kV substations double main busbar scheme or one-and-half breaker schemes are generally employed

Function of Isolators in Substation?

Isolators are disconnecting switches which are used for disconnecting of the circuit under no load conditions. Isolators are placed in the substation such a way that any part of the circuit can be isolated from the live parts for maintenance purposes. Isolators plays an important role during maintenance of the substation. Isolators should be always operated under no load condition. Isolators should be open after opening the circuit breaker and closed before closing the circuit breaker contacts.

Function of earth switches in substation?

Earth Switches in substation are employed to connect the conductors to the earth thereby discharging any charging currents due to capacitance. Earth switches are employed for the protection of operating personnel. Earth switches are operated along with isolators. When isolators opens, earth switches immediately connects the conducting parts to ground


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    • profile image


      15 months ago


    • profile image

      mohammed basheer 

      2 years ago

      Good article

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      3 years ago

      Thanks mr.tawhid117. I like isolater explainatuon.

    • profile image


      5 years ago

      dear kiran, thank you very much for your post. it is a very excellent effort. i like to share some extra information from my experience:

      1. o/c protection ct and differential ct are installed with different cores. so, for feeder protection there are 2 ct's: metering & o/c - e/f protection and for transformer incoming there are 3 ct's: metering, o/c - e/f protection and differential protection.

      2. isolators are for back-up safety purpose also. because, there is 3-5 mm gap between the contacts inside a breaker cylinder. so, due to aging or malfunction, a CB may show "OFF" and breaker opening sound is also heard, but it is possible that one phase is still in contact. that's why, we use isolators to be sure of disconnected circuit by our own eyes.


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