The principal 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 :
In order to provide selectivity to the system, it is a usual practice to divide the entire system into
(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 the fault occurs and cause the opening of the circuit breakers closest to the fault with minimum or no damage to the system. This can be illustrated by referring to the single line diagram
of a portion of a typical power system shown in Fig. 21.2. It may be seen that circuit breakers are located in the connections to each power system element in order to make it possible to disconnect only the faulty section. Thus, if a fault occurs at bus-bars on the last zone, then only breakers nearest to the fault viz. 10, 11, 12 and 13 should open. In fact, opening of any other breaker to clear the fault will lead to a greater part of the system being disconnected.
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
It may be seen in Fig. 21.2 that there is certain amount of overlap between the adjacent protection zones. For a failure within the region where two adjacent zones overlap, more breakers will be opened than the minimum necessary to disconnect the faulty section. But if there were no overlap, a failure in the region between zones would not lie in either region and, therefore, no breaker would be
opened. For this reason, a certain amount of overlap* is provided between the adjacent zones.
(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 con- sumers’ 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-amperes 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 1 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 condi-tions. Without reliability, the protection would be rendered largely ineffective and could even be- come a liability.
(v) Simplicity.
The relaying system should be simple so that it can be easily maintained. Reli-
ability 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 method has to be adopted. As a rule, the protective gear should not cost more than 5% of total cost. However, when the apparatus to be protected is of utmost importance (e.g. generator, main transmission line etc.), economic considerations are often subordinated to reliability.