One and half Bus System:
In Switchyard two most Bus bar schemes are very popular one is One and Half Breaker Bus System (3/2 bus) and Double Bus Bar arrangement. In high voltage system typically above 220 kV Switchyard, One and Half Breaker Bus System is mostly recommended because of it have many advantage over double bus system.
Advantage of one and half bus system:
- During any fault on any one of the buses, that faulty bus will be cleared instantly without interrupting any feeders in the system since all feeders will continue to feed from other healthy bus.
- It occupies less space.
- Maintenance free. Maintenance of more than one breaker is possible without any loss of service
- High stable operation.
- Very less Cost as compared with Two Main and Transfer Bus Scheme
In Switchyard, Bus are made of hollow tubular aluminum called IPS Tubes. Corona and electrostatic field performance is better for tube type bus; therefore, Aluminum Tubes are used for Buses.
One And Half Circuit Breaker Scheme
In One and Half Breaker Scheme, three breakers are connected between the two buses. Each Breaker is provided with two Isolators and two Earth Switches in each side of the breaker. These Isolators are provided to physically isolate the Circuit Breaker for maintenance. Earth Switch ES is provided to ensure that isolated portion is effectively earthed. Earth Switch shall be closed after the opening of Isolator.
Refer the diagram, Feeder2 can be charged from CB1 + CB2, CB3, and CB1+CB2+CB3. Like that feeder1 can be charged from CB3 + CB2, CB1, and CB1+CB2+CB3. You absorb that, for protecting two feeders, three circuit breakers are used. Thus, this kind of arrangement is called 3/2 i.e. One and Half Breaker scheme.
Lets prove one advantage of three and half bus system:
Let us assume a fault in Feeder-1. Due to the fault in the feeder1, the CB1 and CB2 open due to the fault. At that same time the feeder still in service from the bus 2. Thus the reliability of the power system increases.
Also if you want take maintenance on any breaker, you can charge the feeders via any bus 1 or 2. i.e you want to take maintenance of CB1, then you can charge the feeder 1 via CB2 and CB3. You can feed the power supply to any load under any one of the breaker under maintenance.