EE1302 Protection & Switchgear B.E Question Bank : niceindia.com

Name of the College : Noorul Islam College of Engineering
University : Anna University
Degree : B.E
Department : Electrical & Electronic Engineering
Subject Code/Name : EE 1302 – Protection & Switchgear
Year : III
Semester : V
Document Type : Question Bank
Website : niceindia.com

Download :https://www.pdfquestion.in/uploads/ni…SWITCHGEAR.pdf

NICE Protection & Switchgear Question Paper

Two Marks Question & Answers :
1. What are the functions of protective relays :
To detect the fault and initiate the operation of the circuit breaker to isolate the defective element from the rest of the system, thereby protecting the system from damages consequent to the fault.

Related : Noorul Islam College of Engineering EE1301 Power Electronics B.E Question Bank : www.pdfquestion.in/2995.html

2. Give the consequences of short circuit :
Whenever a short-circuit occurs, the current flowing through the coil increases to an enormous value. If protective relays are present , a heavy current also flows through the relay coil, causing it to operate by closing its contacts.The trip circuit is then closed , the circuit breaker opens and the fault is isolated from the rest of the system. Also, a low voltage may be created which may damage systems connected to the supply.

3. Define protected zone :
Are those which are directly protected by a protective system such as relays, fuses or switchgears.If a fault occurring in a zone can be immediately detected and or isolated by a protection scheme dedicated to that particular zone.

4. What are unit system and non unit system? :
A unit protective system is one in which only faults occurring within its protected zone are isolated.Faults occurring elsewhere in the system have no influence on the operation of a unit system.A non unit system is a protective system which is activated even when the faults are external to its protected zone.

5. What is primary protection? :
Is the protection in which the fault occurring in a line will be cleared by its own relay and circuit breaker.It serves as the first line of defence.

6. What is back up protection? :
Is the second line of defence , which operates if the primary protection
fails to activate within a definite time delay.

7. Name the different kinds of over current relays :
Induction type non-directional over current relay,Induction type directional over current relay & current differential relay.

8. Define energizing quantity :
It refers to the current or voltage which is used to activate the relay into operation.

9. Define operating time of a relay :
It is defined as the time period extendind from the occurrence of the fault through the relay detecting the fault to the operation of the relay.

10. Define resetting time of a relay :
It is defined as the time taken by the relay from the instant of isolating the fault to the moment when the fault is removed and the relay can be reset.

11. What are over and under current relays? :
Overcurrent relays are those that operate when the current in a line exceeds a predetermined value. (eg : Induction type non-directional/directional overcurrent relay, differential overcurrent relay)whereas undercurrent relays are those which operate whenever the current in a circuit/line drops below a predetermined value.(eg: differential over-voltage relay)

12. Mention any two applications of differential relay :
Protection of generator & generator transformer unit; protection of large motors and busbars .

13. What is biased differential bus zone reduction? :
The biased beam relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected zone. It is essentially an over-current balanced beam relay type with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force.

14. What is the need of relay coordination? :
The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest possible time causing minimum disturbance to the system. Also, if a relay fails to operate, there should be sufficiently quick backup protection so that the rest of the system is protected. By coordinating relays, faults can always be isolated quickly without serious disturbance to the rest of the system.

15. Mention the short comings of Merz Price scheme of protection applied to a power transformer :
In a power transformer, currents in the primary and secondary are to be compared. As these two currents are usually different, the use of identical transformers will give differential current, and operate the relay under no-load condition. Also, there is usually a phase difference between the primary and secondary currents of three phase transformers. Even CT’s of proper turn-ratio are used, the differential current may flow through the relay under normal condition.