Protection against excess current was the earliest protective system developed and used by utilities. Over current protection is the protection in which the relay picks up when magnitude of current exceeds the pickup level. The over current relays are connected to the system by means of current transformer.
Over current relays are of following types:
1. Instantaneous over current protection
2. Definite time over current protection
3. IDMT (Inverse Definite Minimum Time) over current protection
4. Directional Over current protection
Over current protection has wide range of applications. The over current protection is provided for the flowing:
1. Motor Protection
For motor above 1200HP, it is used against overloads and short circuits in stator windings of motor. For small/medium size motors where cost of CT’s and relays is not justified are protected by thermal relays for overload protection and HRC fuses for short circuit.
2. Transformer Protection
For small power transformers (less than 5 MVA) over current protection is used as main protection. For bigger transformers over current protection is used as backup protection.
3. Line Protection
For 11KV feeders, over current protection is used a main protection. For 132KV lines over current protection is used as backup protection.
