Relays
When failure occurs on any part of the system it must be quickly detected and disconnected from the system for two reasons. Firstly if fault is not cleared it may cause unnecessary interruption of service to the customers. Secondly the rapid disconnection prevents the damage and spreading of fault into the system. The detection of fault and disconnection of a faulty section can be achieved by relays in conjunction with circuit breakers.
The protective relay is a device, which detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system. Relays detect the abnormal condition by constantly measuring the electrical quantities that are different under normal and fault conditions. The electrical quantities, which may change under fault conditions, are voltage current, frequency and phase angle. After detecting the fault the relay operates to close the trip circuit of the breaker.
A typical relay circuit is shown in fig 2. This diagram shows one phase of a 3-phase system for simplicity. The relay circuit connections are divided into three parts:
i. First part is the primary winding of a current transformer, which is connected in series with the line to be protected.
ii. Second part consists of secondary winding of ct and relay operating coil.
iii. Third part is the tripping circuit, which may be either ac or dc. It consists of a source of supply, trip coil of the circuit breaker and the relay stationary contacts.
When a short circuit occurs at point F on the transmission line, the current flowing increases to an enormous value. This results in a heavy current flow through the relay coil causing the relay to operate by closing its contacts. This in turn closes the trip circuit of the breaker, making circuit breaker open and isolating the faulty section from rest of the system.
Written by arjun on October 30th, 2008 with
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