The circuit breaker scheme used at a substation provides for varying degrees of reliability and maintainability on both the input and output sides. Each additional circuit breaker provides greater reliability and flexibility in maintaining the bus energized during a fault or during maintenance. However, the cost also increases with each circuit breaker. Hence, the selection of a particular substation scheme depends on safety, reliability, economy, and simplicity. The most commonly used circuit breaker schemes are:
• The single-bus, shown in Figure a
• Double-bus/double-breaker, shown in Figure b
• Main-and-transfer bus, shown in Figure c
• Breaker-and-a-half, shown in Figure d
• Ring bus, shown in Figure e
Of these designs, the single-bus scheme costs the least; however, it possesses rather low reliability because the failure of the bus or any circuit breaker results in a total shutdown of the substation. The most expensive arrangement is the double-bus/double-breaker scheme. Each circuit is provided with two circuit breakers, and thus, any breaker can be taken out of service for maintenance without disruption of service at the substation. In addition, feeder circuits can be connected to either bus. The main-and-transfer bus requires an extra breaker for the bus tie between the main and the auxiliary buses.
The breaker-and-a-half scheme provides the most flexible operation with high reliability. The relaying and automatic reclosing, however, are somewhat complex.
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| Figure: Substation bus and breaker arrangements: (a) single-bus; (b) double-bus/double-breaker; (c) main-and transfer bus; (d) breaker-and-a-half; (e) ring bus. |