Over-current protection is usage of magnitude relays since it picks up once the magnitude of current exceeds some setting values. There are four forms of over current relays.
1. Instantaneous Over-Current Relay
2. Definite Time Over-Current Relay
3. Inverse Time Over-Current Relay
4. Direction Over -Current Relay
Instantaneous Over-Current Relay
Instantaneous over-Current Relays operation criteria is simply current magnitude (without time delay). This sort is applied to the outgoing feeders from the substation. The association diagram of fast over current relay is shown above.
Definite Time over Current Relay
In this sort, 2 conditions should be fulfilled for operation (tripping), Current should exceed the setting value and therefore the fault should be continuous a minimum of for a time capable time setting of the relay. Fashionable relays could contain quite one stage of protection every stage includes every own current and time setting. The association diagram of definite time over current relays with internal and external timers are as shown just below.
Definite time over current relay is that the most applied form of Over current. It’s used as:
1.Backup protection of distance relay of cable with time delay capable fourth stage of distance relay that is 2.5 second in 220 KV lines, and 1.5 second in 66 KV lines.
2.Backup protection to differential relay of power Transformer with time delay capable 2.0 second in 220/66 KV transformers, and 1.1 second within the 66/11 KV transformers.
3.Main fortification to outgoing feeders and bus couplers with regulating time delay setting.
Inverse Time over Current Relay:
In this type of relays, operational time is reciprocally modified with current. So, high currents can operate over current relay quicker than lower ones. Totally different currents of inverse time sort are outlined as normal inverse, terribly inverse, very inverse of these varieties are shown below.
Direction over Current Relay
The directional over current relay is associate over current relay operates in on direction of current flow and blocks within the other way. 3 condition should be gratified for operation the previous 2 conditions that are current magnitude and time delay should be thought of and therefore the twenty third condition that is additional during this sort is that the radial asymmetry. The radial asymmetry of current flow is known mistreatment voltage as a reference of direction. The association diagram of current circuit and voltage circuit of directional over current Relays. Almost, this sort is put in on the low aspect of power transformer.