Primary and Backup protection in a Power System

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Primary and Backup protection in a Power System

There is always some possibility of a circuit breaker failure. For this reason, it is usual to supplement primary protection with other systems to ‘back-up’ the operation of the main system and ensure that nothing can prevent the clearance of a fault from the system.

The protection provided by the protective relaying equipment can be categorized into two types as :
1. Primary protection
2. Backup protection.

1. Primary 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 primary protection is the first line of defense and is responsible to protect all the power system elements from all the types of faults. The backup protection comes into play only when the primary protection fails.

The backup protection is provided as the main protection can fail due to many reasons like,
  1.  Failure in circuit breaker
  2. Failure of CTs or PTs operation
  3. Failure in protective relay
  4. Failure in tripping circuit
  5. Failure of DC supply to the tripping Circuit
  6. Loss of voltage or current supply to the relay

2. Backup protection

Consider the backup relaying employed for the transmission line section EF as shown in the Fig. ,The relays C, D, G and H are primary relays while A, B, I and J are the backup relays.

primary and backup protection

  • The backup relaying often provides primary protection when the primary relays are out of service for repairs.
  • It is obvious that when the backup relay operates, the larger part of the system is disconnected.
  • Normally backup relays are tripped if primary relay fail. So if the primary relay E fails to trip, then backup relays A and B get tripped. The backup relays and associated backup relaying equipments are physically apart from the faulty equipment.
  • The backup relays A and B provide backup protection for fault at station K. Also the backup relays at A and F provide the backup protection for the faults In line DB.
  • When the given set of relays provides the backup protection for several adjacent systems elements then the slowest primary relaying of any of those will determine the necessary time delay of the given backup relays.
  • The important requirement of backup relaying is that it must operate with sufficient time delay so that the primary relaying is given a chance to operate.
  • When fault occurs, both the type of relays starts relaying operation but primary is expected to trip first and backup will then rest without having had time to complete its relaying operation.

Methods of back-up protection

The methods of back-up protection can be classified as follows :
  1. Relay Back-up : in this method , Different breakers are provided for main and back-up protection, both the breakers being in the same station.
  2. Breaker Back-up : In this method, separate breakers are provided for primary and backup protection. Both the types of breakers are at the same station.
  3. Remote back-up :  In this method, separate breakers are provided for primary and backup protection. The two types of breakers are at the different stations and are completely isolated and independent of each other.
  4. 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 is 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 programme in the digital computer determines the correct switching operation, as regards severity of fault, system stability,

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