Primary and Backup Protection

Protective Relaying (Part8)      
       The protection provided by the protective relaying equipment can be categorized into two types as :
1. Primary protection
2. Backup protection.
       The primary protection is the first line of defence 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 in protective relay
3. Failure in tripping circuit
4. Failure in d.c. tripping voltage
5. Loss of voltage or current supply to the relay
       Thus if the backup protection is absent and the main protection fails then there is a possibility of severe damage to the system.
       When the primary protection is made inoperative for the maintenance purpose, the backup protection acts like a main protection. The arrangement of backup protective scheme should be such that the failure in main protection should not cause the failure in back up protection as well. This is satisfied if back up relaying and primary relaying do not have anything common. Hence generally backup protection is located at different stations from the primary protection. From the cost and economy point of view, the backup protection is employed only for the protection against short circuit and not for any other abnormal conditions.
1.1 Concept of  backup Relaying
       Consider the backup relaying employed for the transmission line section EF as shown in the Fig.1.
Fig. 1  Backup relaying
       The relays C, D, G and H are primary relays while A, B, I and J are the backup relays.
       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.
       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.
       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.
       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.
1.2 Methods of Backup Protection
       The various methods used for the backup protection are classified as,
1. Relay Backup Protection : In this scheme, a single breaker is used by both primary as well as backup protection but the two protective systems are different.
2. Breaker Backup Protection : In this method, separate breakers are provided for primary and backup protection. Both the types of breakers are at the same station.
3. Remote Backup Protection : 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 Co-ordinated Backup Protection : In this method, primary protection is at various stations. There is a central control room and backup protection for all the stations is at central control room. Central control continuously inspects the load flow and frequency in the system. If any element of any part of the system fails, load flow gets affected which is sensed by the control room. The control source consists of a digital computers which decides the proper switching action. The method is also called centrally controlled backup protection.

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