Requirements of Protective Relaying
The principle function of protective relaying is to cause the prompt removal from service of any element of the power system when it starts to operate in an abnormal manner or interfere with the effective operation of the rest of the system. In order that protective relay system may perform this function satisfactorily, it should have the following qualities:
(i) selectivity (ii) speed (iii) sensitivity (iv) reliability (v) simplicity (vi) economy
(i) Selectivity – It is the ability of the protective system to select correctly that part of the system in trouble and disconnect the faulty part without disturbing the rest of the system A well-designed and efficient relay system should be selective i.e. it should be able to detect the point at which fault occurs and cause the opening of the circuit breakers closest to the fault with minimum or no damage to the system. In order to provide selectivity to the system, it is a usual practice to divide entire system into several protection zones. When a fault occurs in a given zone, then only the circuit breakers within that zone will be opened. This will isolate only the faulty circuit or apparatus, leaving the healthy circuits intact. The system can be divided into the following protection zones:
(a) generators (b) low-tension switchgear (c) transformers (d) high-tension switchgear (e) transmission lines
(ii) Speed – The relay system should disconnect the faulty section as fast as possible for the following reasons:
(a) Electrical apparatus may be damaged if they are made to carry the fault currents for a long time.
(b) A failure on the system leads to a great reduction in the system voltage. If the faulty section is not disconnected quickly, then the low voltage created by the fault may shut down consumers’ motors and the generators on the system may become unstable.
(c) The high speed relay system decreases the possibility of development of one type of fault into the other more severe type.
(iii) Sensitivity - It is the ability of the relay system to operate with low value of actuating quantity. Sensitivity of a relay is a function of the volt-ampere input to the coil of the relay necessary to cause its operation. The smaller the volt-ampere input required to cause relay operation, the more sensitive is the relay. Thus, a I VA relay is more sensitive than a 3 VA relay. It is desirable that relay system should be sensitive so that it operates with low values of volt ampere input
(iv) Reliability - It is the ability of the relay system to operate under the pre determined conditions. Without reliability, the protection would be rendered largely ineffective and could even become a liability.
(v) Simplicity - The relaying system should be simple so that it can be easily maintained. Reliability is closely related to simplicity. The simpler the protection scheme, the greater will be its reliability.
(vi) Economy – The most important factor in the choice of a particular protection scheme is the economic aspect. Sometimes it is economically unjustified to use an ideal scheme of protection and a compromise scheme has to be adopted. The protective gear should not cost more than 5% of the total cost. However, when the apparatus to be protected is of utmost importance (eg. generator, main transmission line etc.), economic considerations are often subordinated to reliability