Primary and Back-up Protection in Power System

back up protection

Primary and Back-up Protection

Primary protection (Main protection) is the essential protection provided for protecting an equivalent/machine or a part of the power system. As a precautionary measure, addition protection is generally provided and is called Backup Protection. In this post, we will look into the difference between primary & backup protection, advantages of backup protection, and methods of backup protection.

How Protective Relays Work?

If any fault occurs in the protected area, the primary protection act first. If primary protection fails to act, the back-up protection comes into action and removes the faulty part from the health system. 

Advantages of Back-up Protection

Back-up protection is provided for the following reasons
  • If due to some reason, the Main protection fails, the Back-up protection serves the purpose of protection. 
  • Main protection can fail due to failure of one of the components in the protective system such as a relay, auxiliary relay Current Transformer, PT, trip circuit, circuit-breaker, etc. If the primary protection fails, there must be an additional protection, otherwise, the fault may remain uncleared, resulting in a disaster.
  • When main protection is made inoperative for the purpose of maintenance, testing, etc. the Back-up protection acts like main protection. 
  • As a measure of the economy, Back-up protection is given against short-circuit protection and generally not for other abnormal conditions. The extent to which back-up protection is provided depends upon economic and technical considerations, 
The cost of back-up protection is justified on the basis of the probability of failure of individual component in the protection system, cost of the protected equipment, the importance of protected equipment, location of protected equipment, etc.

Methods of Back-up Protection

The methods of back-up protection can be classified as follows :
  1. Relay Back-up
  2. Breaker Back-up
  3. Remote back-up
  4. Centrally Coordinated Back-up

Relay Back-up

Same breaker is used by both main and back-up protection, but the 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 being in the same station

Remote back-up

The main and back-up protections provided at different stations and are completely independent.

Centrally Coordinated Back-up

The system having central control can be provided with centrally controlled back-up. Central control continuously supervises the load flow and frequency in the system. The information about load flow and frequency are assessed continuously. 

If one of the components in any part of the-system fails, (e.g. a fault on a transformer, in some station) the load flow in the system is affected. The central coordinating station receives information about the abnormal condition through high-frequency carrier signals. 

The stored program in the digital computer determines the correct switching operation, as regards severity of fault, system stability,

Back up Protection by Time Grading Principle

The current is measured at various points along the current path, e.g., at source, intermediate locations, consumer end..The tripping time at these locations are graded in such a way that the circuit-breaker/fuse nearest the faulty part operate first, giving primary protection. The’circuit breaker/fuse at the previous station operates only as back-up.
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Time Grading
Referring to the figure, the tripping time at station C, B and A are graded such that for a fault beyond C breaker at C operates as primary protection. Meanwhile, the relays at A and B also may start operating but they are provided with enough time lag so that the circuit-breaker at B operates only if the circuit-breaker at C does not.

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