An electrical power system consists of generators, transformers, transmission and distribution lines etc. The faults (short circuits) and other abnormal conditions often occur on above components. This abnormality causes a heavy flow of current in the power system, likely to cause damage to the equipment if suitable protective relays and circuit breakers are not provided.

A heavy short circuit –

Thus an uncleared heavy short circuit may cause the total failure of the system. Hence if a fault occurs in an element, an automatic protective device is needed to isolate the faulty element within a fraction of a second to regain the normal condition. A protective scheme includes circuit breakers and protective relays. The function of a protective relay is to monitor electrical quantities like current, voltage, phase angle and frequency of the system under normal and abnormal conditions. It is a device which senses abnormal conditions, detects and locates a fault and issues a command to the circuit breaker to disconnect the faulty element. Hence a protective relay is defined as, “It 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.”

Inadequate protection can lead to a major fault that could have been avoided, e.g. the thermal overload protection of motor prevents the overloading of motor and thereby the insulation failure is avoided. Damaged equipment needs time for repairs and replacement. By adequate protection, the damage can be eliminated or minimised. A fault in the equipment in the supply system leads to disconnection of supply to a large portion of the system. If the faulty part is quickly disconnected, the damage caused by the fault is minimal and the faulty part can be repaired quickly and the service can be restored without further delay. Better service continuity has its own merits. Thus the protective relaying helps in improving service continuity and its importance is self-evident.

Protective relays are provided to detect few abnormalities like over-speed of generators and motors, over-voltages, under-frequency, loss of excitation, overheating of stator and rotor of an alternator etc and issue alarm signals to alert operators or trip circuit breakers. It does not prevent the occurrence of a fault; rather it takes action only after a fault has occurred, however exceptions to Buchholz relay (a gas actuated relay).