A protection system that isolates a faulty component must possess certain qualities in order to function properly. These qualities are
- Selectivity or Discrimination
- Sensitivity
- Reliability
- Stability
- Fast operation
Essential Qualities of Protection
Let us understand each quality in detail.
1. Selectivity or Discrimination
The selectivity or discrimination of a protective relay refers to how well it can distinguish between a fault in a protected section and the normal condition. The protective relay should have the ability to distinguish if a fault is within its zone of protection or outside it. It is desirable to isolate only the faulty part of a power system when a fault occurs. It is important that no healthy part of the system is deprived of electrical power and is, therefore, left intact. The relay should be able to differentiate between faults and transient conditions such as inrush of a transformer’s magnetizing current or power surges.
2. Sensitivity
It is important that the power system equipment be sufficiently sensitive so that equipment can operate reliably when the fault condition is just crossing the predefined limit.
3. Reliability
In quantitative terms, reliability can be expressed as a probability of failure. Components present in the protective system are also prone to failure. Thus, every component and circuit in the fault clearance process must be regarded as a potential failure source. An inherently reliable design supported by regular and thorough maintenance can minimize the risk of failure.
Reliability is the most important requirement for protective relays. Normally, a relay remains inoperative for a long time before it develops a fault, but it must act rapidly and correctly once a fault occurs.
4. Stability
Protective systems should remain stable even when a large current is flowing through their protective zones as a result of an external fault that is not in their zone. The concerned circuit breakers should clear the fault immediately. Although the protective system will not wait indefinitely if the zone in which the fault occurs is unable to detect the fault. A relay will operate after a predetermined delay to trip the circuit breaker.
5. Fast operation
It is essential that a protective system is fast enough to isolate the faulty element of a system as soon as possible to minimize damages to the equipment and to ensure system stability. A protective system operating time should not exceed the critical clearing time to prevent loss of synchronism. If fault currents are carried for a long time, electrical equipment may be damaged. The voltage will drop gradually resulting in crawling and overloading of industrial drives with a persistent fault. Because of these reasons, protective relays need to be quick-acting.
As the fault persists for a shorter period of time, a greater amount of load can be transferred between two points on the power system without loss of synchronism.
