The breaker-and-a-half configuration, typically used at extra-high-voltage (EHV) stations, consists of two buses, each normally energized. Electrically connected between the buses are three circuit breakers and, between each two breakers, a circuit, as shown in Figure F.2. In this arrangement, three circuit breakers are used in a bay for two independent circuits; hence, each circuit shares the common center circuit breaker, so there are 1.5 circuit breakers per circuit. The breaker-and-a-half configuration provides for circuit breaker maintenance, since any breaker can be removed from service and isolated without interrupting any circuit. Additionally, faults on either of the main buses cause no circuit interruptions. Failure of a circuit breaker results in the loss of two circuits if a common breaker fails and only one circuit if an outside breaker fails. It is important to balance circuits in the bays, for example, source lines coming into the right-hand side of bays and load lines leaving the left-hand side of bays.
The main advantages of this scheme include the following:
• A bus fault does not interrupt service on any circuit, and circuit breaker failure causes loss of only one or two circuits;
• Flexible operation;
• High reliability; and
• Double feed to each circuit.
The primary disadvantages of this scheme include the following:
• One-and-a-half breakers are required per circuit;
• Relaying is complex, since the center breaker has to respond to faults of either of its associated circuits, and since currents from two sources must be measured for all circuits; and
• Each circuit must have its own potential source for relaying.
RING BUS CONFIGURATION
For stations having three to five circuits, a ring bus is often used. As more circuits are added, the configuration may evolve to a breaker-and-a-half arrangement. Figure F.3 shows a three-circuit ring bus that is based on Figure F.2 but with the bottom bay and three breakers and one bay-two circuit removed. A maintenance outage of a circuit breaker or circuit causes an “Open ring.” For open-ring operation, a subsequent circuit outage may cause outage of additional circuits.
The advantages of this scheme include:
• Low cost—only one circuit breaker per circuit; and
• Flexibility to evolve to a breaker-and-a-half arrangements as more circuits are added.
The disadvantages of this scheme include:
• Reduced reliability in open-ring operation; and
• Temptation to add circuits without evolution to a breaker-and-a-half arrangement.
DOUBLE BREAKER-DOUBLE BUS CONFIGURATION
The double breaker-double bus configuration consists of two main buses, each normally energized. Electrically connected between the buses are two circuit breakers and, between the breakers, one circuit, as shown in Figure F.4. Two circuit breakers are required for each circuit.
In the double breaker-double bus configuration, any circuit breaker can be removed from service without interruption of any circuits. Faults on either of the main buses cause no circuit interruptions. Circuit breaker failure results in the loss of only one circuit.
Because of high cost, the double breaker-double bus configuration is usually limited to large generating stations. The additional reliability afforded by this arrangement over the breaker-and-a-half scheme usually cannot be justified for conventional transmission or distribution substations. Occasionally, at a generating station, one bay of a breaker-and-a-half arrangement is used as a double breaker-double