September 1, 2012

Describe the primary and back-up protection features that are provided for transmission lines.

A Primary and back-up protection: 
The primary protection is essential protection provided for protecting a machine. As a precautionary measure, an additional protection is generally provided and is called a “Back up protection”. The primary protection is the first to act and back-up protection is the next in the line of defence. Therefore, if primary protection fails, the back-up protection comes into action and removes the faulty part from one healthy system.  

When main protection is inoperative for purpose of maintenance, testing etc. The back-up protection acts like main protection. The back up 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 programme 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 centre.


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