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The objective of power system protection is to isolate a faulty section of electrical power system from rest of the live system so that the rest portion can function satisfactorily without any severer damage due to fault current.
Actually circuit breaker isolates the faulty system from rest of the healthy system and this circuit breakers automatically open during fault condition due to its trip signal comes from protection relay. The main philosophy about protection is that no protection of power system can prevent the flow of fault current through the system, it only can prevent the continuation of flowing of fault current by quickly disconnect the short circuit path from the system.
Types of protection:
- In generator sets: the main purpose of using protective relays is for ensuring that no damage comes to the alternators. Many transformers can be damaged due to internal failures, and these relays protect against them too.
- In high voltage transmission network: In this case, the protection levels include the entire plant and also the public too. When there is an occurrence of any overload, the device is disconnected.
- In overload and backup for distance: These protective relays are used as cater to the instantaneous over current and time over current
- Earth fault protection: This detection allows catering any imbalance in a three phase circuit. If the imbalance is more than predetermined calculation, the circuit would break.
- Distance protection: this type of protection is useful for long lines. The protection offered in this case is that of the current differential protection.
- Backup protection: The intention of protective control is to cater to the affected area and not the entire system. Through local backup protection, it is possible to target the affected area instead of the complete plant.
Functional Requirements of Protection Relay
- Reliability
The most important requisite of protective relay is reliability. They remain inoperative for a long time before a fault occurs; but if a fault occurs, the relays must respond instantly and correctly.
- Selectivity
The relay must be operated in only those conditions for which relays are commissioned in the electrical power system. There may be some typical condition during fault for which some relays should not be operated or operated after some definite time delay hence protection relay must be sufficiently capable to select appropriate condition for which it would be operated.
- Sensitivity
The relaying equipment must be sufficiently sensitive so that it can be operated reliably when level of fault condition just crosses the predefined limit.
- Speed
The protective relays must operate at the required speed. There must be a correct coordination provided in various power system protection relays in such a way that for fault at one portion of the system should not disturb other healthy portion. Fault current may flow through a part of healthy portion since they are electrically connected but relays associated with that healthy portion should not be operated faster than the relays of faulty portion otherwise undesired interruption of healthy system may occur. Again if relay associated with faulty portion is not operated in proper time due to any defect in it or other reason, then only the next relay associated with the healthy portion of the system must be operated to isolate the fault. Hence it should neither be too slow which may result in damage to the equipment nor should it be too fast which may result in undesired operation.
Function of Protective Relaying
The functions of protective relaying include the following:
- To sound an alarm or to close the trip circuit of circuit-breaker so as to disconnect a component during an abnormal condition in the component, which include over-load, under-voltage, temperature rise, unbalanced load, reverse power, under-frequency, short circuits, etc.
- To disconnect the abnormally operating part so as to prevent the subsequent faults, e.g. over-load protection of a machine protects the machine and prevents insulation failure.
- To disconnect the faulty part quickly so as to minimize the damage to the faulty part, e.g. If a machine is disconnected immediately after a winding fault, only a few coils may need replacement. If the fault is sustained, entire winding may get damaged and the machine may be beyond repairs.
- To localise the effect of fault by disconnecting the faulty part, from the healthy part, causing least disturbance to the healthy system.
- To disconnect the faulty part quickly so as to improve the system stability, service continuity and system performance. Transient stability can be improved by means of improved protective relaying.
Faults cannot be avoided completely. They can be minimized.
Protective relaying plays an important role in minimizing the faults, and also in minimizing the damage in the event of faults.[/vc_column_text][/vc_column][vc_column width=”1/3″][/vc_column][/vc_row][vc_row][vc_column width=”2/3″][vc_column_text]AUTHORS
1.Bunty B. Bommera
2.Dakshata U. Kamble[/vc_column_text][/vc_column][vc_column width=”1/3″][/vc_column][/vc_row]