BREAKER AND A HALF SUBSTATION SCHEME – BASIC INFORMATION AND TUTORIALS

The breaker-and-a-half scheme can be developed from a ring bus arrangement as the number of circuits increases. In this scheme, each circuit is between two circuit breakers, and there are two main buses.

The breaker-and-a half scheme, sometimes called the three-switch scheme, has three breakers in series between two main buses. Two circuits are connected between the three breakers, hence the term breaker and a half. This pattern is repeated along the main buses so that one and a half breakers are used for each circuit.

Under normal operating conditions, all breakers are closed, and both buses are energized. A circuit is tripped by opening the two associated circuit breakers. Tiebreaker failure will trip one additional circuit, but no additional circuit is lost if a line trip involves failure of a bus breaker.

  

Either bus may be taken out of service at any time with no loss of service. With sources connected opposite to loads, it is possible to operate with both buses out of service. Breaker maintenance can be done with no loss of service, no relay changes, and simple operation of the breaker disconnects.

The failure of a circuit will trip the two adjacent breakers and not interrupt any other circuit. With the three breaker arrangement for each bay, a center breaker failure will cause the loss of the two adjacent circuits.

However, a breaker failure of the breaker adjacent to the bus will only interrupt one circuit. Maintenance of a breaker on this scheme can be performed without an outage to any circuit.

Furthermore, either bus can be taken out of service with no interruption to the service. This is one of the most reliable arrangements, and it can continue to be expanded as required. Relaying is more involved than some schemes previously discussed. This scheme will require more area and is costly due to the additional components.

The breaker-and-a-half arrangement is more expensive than the other schemes, with the exception of the double breaker, double-bus scheme, and protective relaying and automatic reclosing schemes are more complex than for other schemes. However, the breaker-and-a half scheme is superior in flexibility, reliability, and safety.