One and Half Breaker Bus System

In Switchyard different Bus Bar arrangements are used for evacuation of power generated but the two most used schemes are One and Half Breaker Bus System and Double Bus Bar arrangement. In high voltage Switchyard like in 400 kV Switchyard One and Half Breaker Bus System is used due to many advantages of this scheme. The advantages of this scheme will be dealt latter in this post.

Bus Bar arrangement is nothing but a combination of Bus and Circuit Breaker. Normally in Switchyard, Bus are made of hollow tubular aluminum called IPS Tubes (here IPS stands for Iron Pipe Size, a code for selection of tubes). Corona and electrostatic field performance is better for tube bus, therefore Aluminum Tubes are used for Buses.

Now coming to One and Half Breaker Bus System, carefully observe the figure below.

In the figure above, CB stands for Circuit Breaker, LA for Lightening Arrestor, ES for Earth Switch and DS for Disconnect Switch also called Isolator.

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• Difference between Isolator and Circuit Breaker

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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. 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.

One bay of three breakers is called Diagonal in One and Half Breaker Scheme. In the figure, thus two Diagonals are shown. A feeder is connected in between the two Breakers CB-A and CB-C & CB-B and CB-C. Notice that three Breakers are used in this scheme to protect two Feeders and therefore it is called 3/2 i.e. One and Half Breaker scheme.

Let us consider some interesting aspects of One and Half Breaker Scheme. Let us assume a fault in any one feeder say in Feeder-1. In this case protection shall open the CB-A and CB-C & send Direct Trip signal to the Remote station through PLCC to isolate the fault. Mind that even though CB-A and CB-C are open, Feeder-2 is still in service and fed by Bus-2. Thus One and Half Breaker scheme increases the reliability of Power System.

You may love to read Power Line Carrier Communication (PLCC).

Let us assume that, we need to take maintenance of CB-B of Diagonal-1. So we will open the Breaker CB-B and will isolate it by opening Isolators and closing Earth Switch ES. Again, notice that none of the feeder will be out of service rather Feeder-2 will be fed through CB-A and CB-C. The most important advantage of this scheme is that you can take one Bus out for maintenance without interrupting power supply in Feeders.

Advantages of One and Half Breaker Scheme are as follows.

a)    Its high security against loss of power supply to feeders which makes it particularly suitable for Switchyard associated with generating Stations and those in which large quantum of power is handled through individual circuits. This involves minimum outage for maintenance.

b)   Area requirement is less in this arrangement compared to Two Main and Transfer Scheme.

c)      Cost is also less compared to Two Main and Transfer Bus Scheme.

d)     Either bus may be taken out any time without loss of service to the feeders. Maintenance of more than one breaker is possible without any loss of service. In this system no isolator operation is required for changing over from one bus to other, as all three breakers remain closed under normal operation.

e)      In case of bus fault, power supply to the connected feeders continues from the other bus with no power interruption required for isolation of the faulty bus. This advantage is not available with Two Main and Transfer Bus Scheme as in case of a bus fault all associated breakers will trip resulting in interruption of power supply to affected feeders. However, failure of center breaker only reduces the flexibility of uninterrupted changeover of feeders from one bus to the other.