Backup Protection of Busbar:
Backup Protection of Busbar is the simplest of all to protect the buses with the aid of backup protections of the connected supplying elements which should respond to any fault appearing on the buses.
When no separate bus protection is provided but distance protection is provided for the feeders connected to the bus, it is possible to cover the bus-bars within zone 2 reach of the distance relays.
Referring to Fig. 13.9, the bus A is covered in the second step of distance protection B. Thus, in the event of fault on bus A, the distance protection B will operate. The operating time of the second step can be of the order of 0.4 second. In such a system the protection is slow and there can be unwanted disconnection of all incoming parallel circuits. Distance protection is widely employed for the protection of transmission lines, hence it is quite economical to use the same for bus protection.
Above scheme may be quite satisfactory for small switchgear installations but for large and important installations a separate bus-zone protection is provided.
Referring to Fig. 13.9
- The local overcurrent protection at station A provides the primary protection to bus-zone A.
- The remote overcurrent protection or impedance protection at station B provides a backup protection to bus-zone A. In case protection ‘a’ fails protection ‘b’ provides a backup.
- Local overcurrent protection of incoming lines or feeders at station B provides primary protection to bus B.
Drawbacks:
The drawbacks of such backup protection of busbar are:
- delayed action
- disconnection of more circuits in case there are two or more incoming lines and
- exact discrimination not possible.
Bus backup protection may also mean that in case the breaker fails to operate for a fault on the outgoing feeder, then it must be regarded as a bus fault. It should then open all breakers on that bus. Such a backup protection can be provided with appropriate time delay through a timer.