Protective:
Protective relay work as a sensing device, it senses the fault, then known its position and finally, it gives the tripping command to the circuit breaker. The circuit breaker after taking the command from the protective relay, disconnect the faulted element.
By clearing the fault fast with the help of fast-acting protective relay and associated circuit breaker, the damage to the apparatus is reduced, and the resultant hazards like fire, the risk of the life are reduced, by removing the particularly faulted section.
But the continuity of supply is maintained, though remaining healthy section, by clearing the fault fast, fault arising time is reduced, and therefore the system can be restored to the normal state sooner. Hence the transient state stability limit of the system is greatly improved, permanent damage to the equipment is avoided, and the possibility of developing most simple fault such as single phase-to-ground into most severe fault such as double phase-to-ground fault is reduced.
The fault can only be reduced if the protective relay is reliable, maintainable and sensitive enough to distinguish between normal and abnormal condition. The relay must come into action whenever there is a fault and must not operate if there is no fault. Some relays are used for the protection of the power system. Some of them are primary relay meaning that they are the first line of defense. Such relays sense the fault and send a signal to the proper circuit breaker to trip and clear the fault.
The fault may not be cleared if the circuit breaker fails to open or relay maloperates. The relay failure is because of three reasons such as wrong setting, bad contacts and open circuit in the relay coil. In such cases,the second line of defense is provided by the backup relays. The backup relay has longer operating time, even though they sense the fault along with the primary relays.
To attain the desired reliability, the power system network is divided into two different protection zones. The overall system protection is divided into different protection zones. They are generator protection, transformer protection, bus protection, transmission line protection and feeder protection. The relay employed for protection of the apparatus and transmission lines are as follows
-Overcurrent Relays
-Under-frequency Relays
-Directional Relays
-Thermal Relays
-Phase Sequence Relays
-Negative phase sequence Relays
-Positive sequence Relays
-Distance or impedance Relays
-Phase Impedance Relay
-Angle Impedance Relay
-Ohm (or reactance ) Relay
-Angle Impedance Relay
-Mho’s Relay offset or Restricted Relay
-Pilot Relays
-Carrier channel pilot or Microwave pilot Relays
The protective relays do not eliminate the possibility of fault occurrence on the power system rather their circuit actions start only after the fault has occurred on the system. The main features of a good protective relaying are its reliability, sensitivity, simplicity, speed, and economy. For the sake of familiarity of protective relay, we have to understand some important terms.
Energizing Quantity– It is an electrical quantity which is a merger of voltage or current or either voltage or current alone, required for the operation of the relay.
Trip Circuit– It is the circuit that controls the circuit breaker for opening operation and comprises of trip coil, relay contacts, auxiliary switch battery supply, etc.
Characteristic Quantity– It is designed for determining the response of the relay. Some relays have a graded response to one or more quantity called characteristic quantity.