Substation bus schemes

An electric substation is a junction point of transmission lines or power generators and an electric system. Substations are equipped with power transformers to change voltage level.

Configuration of an electric substation is usually implemented by using one of the following bus schemes:

  • Single bus
  • Main bus and transfer bus
  • Double bus double circuit breaker
  • Double bus single circuit breaker
  • Ring bus
  • Circuit breaker and Half

ESMOGrid users can add any of these substation models to a one-line diagram and size circuit breakers for a safe substation operation. Rectangular symbols on power lines represent protection devices, like circuit breakers or fuses. Contact resistance of a circuit breaker is modeled by changing resistance of a power line with the circuit breaker. Characteristics of protection devices (selectivity charts and others) can be also modeled, this will be shown in further articles. In this article we will present examples of substation bus schemes implemented with ESMOGrid.

Single bus

The main advantage of a single bus configuration, is simple and the least expensive implementation compared to other schemes. However, the reliability of this system is low, because if a fault occurs at bus 2 (in Fig.1), this will result in a full outage of the substation. In ESMOGrid rectangular symbols on power lines represent protection devices, for example, circuit breakers, mini-circuit breakers, fuses or others.

Fig. 1 Single bus scheme

Fig. 1 Single bus scheme

Main bus and transfer bus

The system is energized from the “Main bus” (in Fig.2). In case lines “Main bus – Bus 1” or “Main bus – Bus 2” are disconnected for maintenance purposes, transfer bus is energized and loads can continuously operate. This improves system availability, however, if a short circuit appears at the “Main bus”, the whole substation will be disconnected.

Fig. 2 Main bus with transfer bus scheme

Fig. 2 Main bus with transfer bus scheme

Double bus double breaker

In this bus scheme (Fig. 3) are two buses that are normally energized. Any circuit breaker can be disconnected for a maintenance without interrupting power supply to loads, as each load can be fed from one of two buses. What is more, a short circuit at bus “Main 1” or “Main 2” does not interrupt power supply, because, the failed bus can be isolated from the rest of the system.

Fig. 3 Double bus double breaker scheme

Fig. 3 Double bus double circuit breaker scheme

Double bus single breaker

In this bus scheme (Fig. 4) “Bus 1” and “Bus 2” are energized from “System bus”. Both buses are interconnected with a tie breaker that is normally closed, loads can be fed from one of two power lines connected to different buses. Fault at one bus requires isolation of it, while loads are switched to another bus.

double bus_single breaker

Fig. 4 Double bus single circuit breaker scheme

Ring bus

 The ring bus scheme is depicted in Fig. 5. In this scheme buses and interconnections between them forms a closed loop. Each of the loads is fed from two lines, therefore, disconnection of one line doesn’t affect a power supply. If a short circuit at one of buses occurs, two circuit breakers around the bus will trip, however, other circuits will continue to operate. The advantages of this scheme is high flexibility and reliability, however, relay coordination is more complex.

Fig. 5 Ring bus scheme

Fig. 5 Ring bus scheme

Breaker and half

As with the “Ring bus” scheme, in this scheme (Fig. 6) any of the breakers can be opened and removed for maintenance, without interrupting a power supply to loads. This system can operate continuously even if a short circuit at one of buses occurs, because only the failed bus is disconnected. The worst scenario for this type of connection is a failure at a middle segment (between bus 1 and bus 2 or bus 3 and bus 4), because this will cause tripping of breakers adjacent to main buses and disconnection of two loads at once.

Fig. 6 Circuit breaker and half scheme

Fig. 6 Circuit breaker and half scheme

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