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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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.

1) 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.

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

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

4) 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.

5) 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.

25 thoughts on “One and Half Breaker Bus System”

    • Protection Scheme for for One ad Half breaker system mainly consists of: STUB Protection,LBB Protection and Busbar Protection.
      STUB Protection is nothing but a differential protection scheme. STUB here means a T section of One and Half Breaker scheme. This differential protection is meant for protection of the T section.
      In 400 kV One and half Breaker system, Main-1 and Main-2 protection scheme is adopted instead of Main and Back-up. This means there will be two relays of preferably two different make to protect the same zone. Like there will be two different STUB relays.

    • What do you mean by downstream fault with reference to one and half breaker system? As stub protection is zone protection and LBB is breaker failure protection.

  1. As per your fig. assume there is a generator connected at feeder-1. If there is any fault on generator or on feeder-1 then what should be the relay settings (operation time) of one and half breaker for better coordination with generator time grading? Assume generator time grading is one second.

    • I strongly recommend to use protection guidelines. But for the sake of understanding, in case of fault in generator, generator protection will operate which will trip GTCB if there is no GCB. The time grading will depend on the type of fault (read Class A, B and C tripping classification of Generator).
      Now, if there is any fault in feeder, then generator overall differential protection will actuate which don’t have any intentional time delay.

  2. In case generator overall differential fails. How we can give backup protection in one and half breaker scheme? Is it possible to take care it with time delayed over current and earth fault protection in one and half breaker scheme?

  3. I see one disadvantage in this system. In a scenario when one of the busses is out for maintenance, say Bus-2 then both the feeders would be fed by Bus-1. In this case if there is a fault in the feeder-1 then that would result in tripping of both breakers CB-A & CB-C resulting in loss of both feeders. This scenario may not be there in a double bus?

    • In this case only tie breaker will open if there is fault in a feeder fed by CB-C. Just think, opening of CB-C will isolate the fault if fault is in feeder. So CB-A won’t trip. If fault is in feeder-1, then yes, feeder-2 will also trip.

  4. Ok admin it’s worth full ,and I have doubt in case Bus-2 is take shutdown I want to test that busbar relay but my bus -2 feeder still get supply by tie breaker if I give a fault in bus -2 it’s trip the tie breaker or not.

    • In case of bus fault, all main breakers connected to the bus get tripped by Busbar protection.

  5. Sir I start my final year project on -“protection system for one and half switching scheme” .So what I do my project???? &&&&& How I draw load flow diagram of one and half switching scheme???

    • This is governed by the various types of clearances like phase to phase, sectional clearance and phase to ground clearance.

  6. High voltage networks such as 400kV usually transmit large MW of power and taking any feeder on an outage to perform circuit breaker maintenance or transfer a feeder from one bus to another could cause a huge power loss. Having a standby CB for every feeder could be too expensive to resolve this situation. Hence a 3rd circuit breaker (Tie Breaker) is introduced to serve as a common breaker for two feeders, hence the name 1.5breaker scheme. This enables substations to take a breaker out of service for periodic maintenance and also to switch feeders from one bus to another by using the Tie CB. Trust this explanation helps.


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