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 is 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 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. A number of relays are used for the protection of the power system. Some of them are primary relay meaning that they are the first line of defence. 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 defence 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.
Operating Force or Torque – It is a force which tends to close the relay contacts.
Restraining Force or Torque – It is a force or torque, which oppose the torque and tends to interrupt the closure of the relay contacts.
Setting – It is an actual value of the energizing quantity at which the relay is made to operate under given conditions.
Power Consumption of a Relay – It is the value of power consumed by the relay circuit at the rated current or voltage and expressed in VA for AC and watts for DC.
Pick Up – A relay is said to pick up when it moves from the off position to the on position or the operation of the relay is called the relay pick-up.
Operating or Pickup Relay – It is the value of the actuating quantity (current or voltage) which is on the threshold above which the relay operates and closes it contacts. If the current in the relay is less than the pickup value, the relay does not operate, and the breaker operates by it remain in the closed position.
Drop Out or Reset Level – This is the value of the current or voltage, etc. below which a relay opens its contacts and comes back to its original position. The ratio of the drop-out voltage or reset value to the pick or operating value is called the drop-out or reset ratio.
Quick Value – It is given by the time which elapses between the instant when the current or voltage exceeds the pickup values at the time when the relay contacts are closed.
Reset time – It is given by the time which passes away between the instant when the current or voltage (actuating quantity) becomes less than the reset value at the time when the relay contacts are closed.
Seal-in-coil – This coil does not allow the relay contact to open when the current flow through them.
Overshoot Time – This is the time during which stored operating energy is dissipated after the characteristic quantity has been suddenly restored from a specified value to the value which it had at the initial position of the relay.
Fault Clearing Time- It is the time between the existence of a fault and at the time of final arc quenching in a circuit breaker is called the fault clearing time.
Breaker Time – It is the time between termination of fault and final arc quenching in a circuit breaker is called the breaker time.
Relay Time – The interval between the existence of fault and closure of relay contacts is called the relay time.
Reach – It is defined as the limiting distance covered by the protection, the fault beyond which are not within the reach of the protection and should be covered by the other relay.
Operating Principle of Protective Relays
The main principle employed in the operation of the relay is either electromagnetic attraction or electromagnetic induction. In an electromagnetic attraction, a plunger is drawn into a solenoid or an armature is attracted to the poles of an electromagnetic. Such relay can be operated either by AC or DC. In the case of the electromagnetic induction relays, the principle of induction motors is used, and the torque is developed by electromagnetic induction. Such Realy are operated by the AC quantities only.