• 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 (Basic fundamental requirements):

1. Selectivity or discrimination

2. Reliability

3. Sensitivity

4. Stability

5. Fast operation (Speed)

6. Simplicity

7. Economy

8. Adequateness

Fig. 1.5 Single line diagram of a typical power system

1. Selectivity or discrimination:-

• “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” OR “It is the quality of a protective relay by which it is able to discriminate between a fault in the protected section and the normal condition or transient conditions like power surges or inrush of a transformer’s magnetizing current”. Also, it should be able to distinguish whether a fault lies within its zone of protection or outside the zone.
• When a fault occurs on a power system, only the faulty part of the system should be isolated and the healthy part should be left intact. This can be illustrated by referring to the single line diagram of a typical power system shown in figure 1.5. The relay is also able to discriminate between a fault and transient conditions like power surges or inrush of a transformer magnetising current either by its inherent characteristics or with the help of an auxiliary relay. Hence, a protective relay must be able to discriminate between two conditions such as instantaneous tripping condition and no operation or a time delay operation condition.

2. Reliability:-

• “It is the ability of the relay system to operate under the predetermined conditions”. A protective system must operate reliably when a fault occurs in its zone of protection. The failure of a protective system may be due to failure of protective system elements such as protective relay, circuit breaker, P.T., C.T., wiring battery etc. Hence for high reliability, more attention should be given to the design, installation, maintenance and testing of these elements. Apart from reliability, the system should be robust and simple. A typical value of reliability of a protective scheme is 95 %.

3. 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 (or amperes) input to the coil of the relay necessary to cause its operation. A protective relay should be sufficiently sensitive to operate when the magnitude of operating current exceeds the pick-up value. The smaller value of input required causing relay operation, the more sensitive is the relay. Thus, a 1 VA relay is more sensitive than a 3 VA relay.

4. Stability:-

• A protective system should remain stable even when a large current is flowing through its protective zone due to an external fault, which does not lie in its zone. The concerned circuit breaker is supposed to clear the fault. But the protective system will not wait indefinitely if the protective scheme of the zone in which fault has occurred fails to operate. After a present delay the relay will operate to trip the circuit breaker.

5. Speed (Fast operation):-

• The relay system should disconnect the faulty section as fast as possible for the following reasons:

Electrical apparatus may get damaged or get burned if they are made to carry the fault currents for a long time.

Reduction in the system voltage which shutdown consumers, loss of industrial loads and also motors & the generators on the system may become unstable.

The high speed relay system decreases the possibility of development of one type of fault into another more severe type.

• Hence, the operating time of the protective system should not exceed the critical clearing time to avoid the loss of synchronism. The operating time is half cycle or one cycle and for distribution systems it is more than one cycle. 

6. Simplicity:-

• The relaying system should be simple so that it can be easily maintained. As we discussed already, reliability is closely related to simplicity. The simpler the protection scheme, the greater will be its reliability.

7. Economy:-

• This is the most important aspect while deciding the protection scheme. As a rule, the protection scheme should not exceed 5 % of total cost. But sometimes it is economically unjustified to use an ideal scheme of protection and a compromise method has to be adopted.

8. Adequateness:-

• There can be abnormal conditions and providing protection against every abnormal condition is economically impossible. However, the protection provided for any machine should be adequate. The adequateness of protection is judged by considering the following aspects:

Rating of the protected machine

Location of the protected machine

Probability of abnormal condition due to internal and external causes

Cost of machine

Continuity of supply as affected by failure of supply

• For low voltage machines / equipment, at the remote end of the system, an elaborate and costly protective system is not necessary.