28 November 2011

Power System Protection and Relaying:Question and Answers

What is protective relay?
Answer: It is an electrical device designed to initiate the isolation of a part of the electrical installation, or to operate an alarm signal, in the event of abnormal condition or a fault. In simple words relay is an electrical device that gives signal to isolation device (eg: Circuit Breaker) after sensing the fault and helps to isolate the fault system from the healthy electrical system

What are the different relays that employed for protection of apparatus and transmission lines?
Answer: The relays that are usually employed for protection of transmission lines include 

  • Over current relay
  • Directional relay
  • Distance relay
  • Under Voltage relay
  • Under-frequency relay
  • Thermal relay
  • Differential relay
  • Phase sequence relays
  • pilot relays


How the electrical power system protection is divided?
Answer: The overall system protection is divided into 

  • Generator protection
  • Transformer protection
  • Busbar protection
  • Transmission line protection and
  • Feeder protection


How relays are connected in the power system?


Answer: The relays are connected to the power system through the current transformer (CT) or potential transformer (PT). 

What are different types of principles of operation of electro-mechaical relays?
Answer: Eletro-mechanical relays operate by two principles. Electro-magnetic attraction and electro-magnetic induction. In electromagnetic attraction relay plunger is drawn to the solenoid or an armature is attracted to the poles of the electromagnet. In case of electro-magnetic induction, principle of operation is similar to induction motor. Torque is developed by electromagnetic induction principle

Action carried out by the relay and circuit breaker during fault condition?
Answer: After the relay sensing the fault condition, relay operates and close the trip coils. The effect of this will be circuit breaker operate to open the contacts.


1 comment:

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