The protection provided by the protective relaying equipment can be categorized into 2 types as:
- Primary protection
- Backup protection
The main protection or primary protection is the first line protection which provides quick-acting and selective clearing of a fault within the boundary of the circuit section or element it protects. The main protection is provided in each section of an electrical installation.
The backup protection provides the back up to the main protection whenever it fails in operation or its cut out for repairs. The backup protection is essential for the proper working of the electrical system. The backup protection is the second line of defense which isolates the faulty section of the system in case the main protection fails to function properly. The failure of the primary protection occurs because of the failure of the DC supply circuit, current or voltage supply to relay circuit, relay protective circuit or because of the circuit breaker.
The backup protection may be provided either on the same circuit breaker which would be normally opened by the main protection or in the different circuit breaker. The backup protection is mainly used where the main protection of the adjacent circuit is unable to backup the main protection of the given circuit. Sometimes for simplification, the backup protection has a low sensitivity and operated over a limited backup zone.
The backup protection is provided as the main protection. It can fail due to many reasons like:
- Failure in circuit breaker
- Failure in protective relay
- Failure in tripping circuit
- Failure in d.c. tripping circuit
- Loss of voltage or current supply to the relay
When main protection is inoperative for purpose of maintenance, testing etc. The back-up protection acts like main protection.
The backup protection can be classified as:
– Relay back-up : Same circuit breaker is used by both main and back-up protection, but protective systems are different. Separate trip coils may be provided for the same-breaker.
– Breaker Back-up: Different breakers are provided for main and back up protection, both the breakers are being in the same station.
– Remote back-up: Main & back up protection provided at different stations and are completely independent.
-Centrally coordinated back-up: Here system is having central control and it can be provided with centrally controlled back-up. The central coordinating station receives information about the abnormal conditions through high frequency carrier signals. The stored program in digital computer determines the correct switching operation as regard severity of faults, system stability etc. Main protection at various stations and back-up protection for all stations is at central control center.
1.Bunty B. Bommera
2.Dakshata U. Kamble