In a power system consisting of generators,
transformers, transmission and distribution
circuits, it is inevitable that sooner or later
some failure will occur somewhere in the sys-
tem. When a failure occurs on any part of the
system, it must be quickly detected and discon- nected from the system.
There are two principal reasons for it. Firstly, if the fault is not cleared quickly, it may cause unnecessary interruption of service to the customers. Secondly, rapid dis-connection of faulted apparatus limits the amount of damage to it and prevents the effects of fault from spreading into the system.
The detection of a fault and disconnection
of a faulty section or apparatus can be achieved by using fuses or relays in conjunction with cir-cuit breakers. A fuse performs both detection and interruption functions automatically but its use is limited for the protection of low-voltage circuits only. For high voltage circuits (say above 3·3 kV), relays and circuit breakers are employed to serve the desired function of automatic pro-tective gear. The relays detect the fault and supply information to the circuit breaker which per- forms the function of circuit interruption. In thischapter, we shall focus our attention on the various types of relays and their increasing use for the
protection of power system.
Protective Relays
A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to ioslate the defective element from the rest of the system.
The relays detect the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. The electrical quantities which may change under fault conditions are voltage, current, frequency and phase angle. Through the changes in one or more of these quantities, the faults signal their presence, type and location to the protective relays. Having detected the fault, the relay operates to close the trip circuit of the breaker.
This results in the opening of the breaker and disconnection of the faulty circuit.
(i) First part is the primary winding of a current transformer (C.T.) which is connected in series with the line to be protected.
(ii) Second part consists of secondary winding of C.T. and the relay operating coil.
(iii) Third part is the tripping circuit which may be either a.c. or d.c. It consists of a source of supply, the 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 in the line 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 the circuit breaker open and
isolating the faulty section from the rest of the system. In this way, the relay ensures the safety of the
circuit equipment from damage and normal working of the healthy portion of the system.