Protective Relays and their functional characteristics

An electric power system should ensure the availability of electrical energy without interruption to every load connected to the system. The most severe electrical failures in a power system are the shunt faults which are characterized by an increase in system current, reduction in voltage, power factor and frequency. 

"Protective relays and relaying systems detect abnormal conditions like faults in the electrical circuits and operate automatic switchgear to isolate faulty equipment/equipments from the system as quickly as possible."

Thus a protective relay is an automatic device which senses an abnormal condition in an electrical circuit and closes its contacts. These contacts in turn close the circuit breaker trip coil circuit, thereby it opens the circuit breaker and the faulty part of the line or equipment is disconnected from the rest of the system. 

The protective relays should operate only the concerned circuit breaker so as to disconnect the faulty equipment from the system as quickly as possible without affecting the healthy section. Relays are tested thoroughly to ensure that they will operate correctly and will assist the related circuit breaker to clear a fault, only within their specified zone. Figure 1 shows the wiring connection of Relay and Circuit Breaker.  

Fig.1: Wiring connection of Relay and Circuit Breaker.

The protective relays do not eliminate the possibility of fault occurrence. It can take action only after the fault has occurred. Situation would be ideal if the protection system could anticipate and prevent fault occurrence but this is next to impossible except where the original cause of fault creates some effect which can operate a protective relay. However Buchholz relay is one of such devices which can anticipate and prevent major faults.

Functional Characteristics of Protective Relays:

A protective relay should have certain qualities. These essential qualities are:

1.      Reliability – A protective should be reliable and must operate when it is required. Every component and circuit which is involved in the operation of the relay is vital and should be considered as a potential source of failure. Failure can be reduced by reliable design well supported by regular and thorough maintenance. Some features of design and manufacture which make relays inherently reliable are good contact material, high contact pressures, dust free enclosures, well braced joints and robust construction.

2.      Selectivity – The protective relay must be capable of selecting the part of the system which is faulty and should isolate it from the healthy one i.e. a relay should differentiate between the faulty part and the healthy part.

3.      Speed – A protective relay must be quick acting and fast otherwise may result in damage to the equipment and the system. The operating time of a relay is of the order of 30 to 100 ms, depending upon the fault level. Also the relays should not be extremely fast (less than 10 ms), otherwise it may result in undesired operation during transient faults such as lightning surges.

4.      Sensitivity – Sensitivity of a protective relay refers to the smallest value of actuating quantity at which the protective relay starts operating in relation with the minimum value of fault current in the protected zone.