2. Flexibility and immunity are very less.
1. As we are using circuit breaker to divide a bus - bar into two sections fault on one section will not interrupt power supply on the other section only few loads will have lac of power supply.
2. The fault level can be reduced by adding a current limiting reactor.
Disadvantages of sectionalized single bus - bar arrangement:
1. We are using extra isolators and circuit breakers so that the cost will be high.
Double bus arrangement:
Double bus arrangement has two bus bars and the incoming feeders and outgoing feeders are connected in parallel to both buses with the help of isolators. By closing the isolator switch we can connect the feeders either to bus - bar 1 or to bus - bar 2 . We can divide the load among two buses with the help of isolator switches by closing the isolator switch that is connected to bus - bar 1 and feeder the load can be connected to bus - bar 1 and by closing the isolator switch connected to bus - bar 2 and feeder the load gets connected to bus - bar 2. This can be shown in following figure:
We have a bus coupler breaker which is used for bus transfer operation. When we need to transfer load from one bus to other bus we need to close the bus coupler first and then close isolators of the associated bus to which load is to be connected and open the isolator switch coupled to fault bus and then open the bus coupler breaker.
Advantages of double bus system:
1. It has greater flexibility.
2. During fault conditions there is no interruption of power supply to load.
Disadvantages of double bus system:
1. We cannot transfer load from one bus to other bus without interruption of power supply for few minutes.
Double bus double breaker arrangement:
In double bus double breaker arrangement we connect a feeder in parallel to both buses with the help of two circuit breakers and isolator switches instead of bus coupler as shown in the following figure:
Here we energize both the feeders and divide feeders among both the buses but we can connect desired feeder to desired bus at any time for this purpose we need to close the isolator and then circuit breaker associated with the required bus - bar and later open the circuit breaker and then isolator from which is has to be disconnected.
Advantages of double bus double breaker system:
1. During fault conditions load can be transferred to one bus so there will not be interruption in power supply.
2. Here we are not using a bus coupler so there will not be much delay in power supply while closing circuit breaker to transfer load from one bus to other bus.
3. High flexibility.
Disadvantages of double bus double breaker system:
1. The number of circuit breakers used are high so cost is very high.
2. Maintenance cost will also be high.
So this type of arrangement is used very rare.
One and a half breaker arrangement:
Because of high cost of double bus double breaker arrangement we use one and a half breaker arrangement. Here the two feeders are fed through their corresponding bus - bars and these two feeders are coupled by a third circuit breaker called tie - breaker as shown in the following figure:
During normal conditions all the three circuit breakers are closed and the both circuits operate in parallel and power is fed to feeders from the two bus - bars. If fault occurs on one bus bar then with the help of second bus - bar feeder circuit breaker and tie breaker power is fed to feeders. This means each feeder breaker has to be rated to feed both feeders which are coupled by tie breaker.
Advantages of one and a half breaker system:
1. There will be no interruption of power in case of fault because all the feeders can be transferred to other healthy bus immediately.
2. Additional circuits can be easily added to the system.
3. Cost is less compared to double bus double breaker arrangement.
Disadvantages of one and a half breaker system:
1. This arrangement is complicated because during fault two circuit breakers are to be opened.
2. Maintenance cost is high.
Main and transfer bus arrangement:
In main and transfer bus arrangement we have two buses one is main bus and the other is transfer bus. With the help of isolator switches it is connected to the transfer bus which are called bypass isolators and with the help of circuit breakers and isolator switches it is connected to the main bus. There is also bus coupler as shown in the following figure:
In normal conditions the feeders are fed through main bus but during fault conditions load is transferred to the transfer bus. In order to transfer load from main bus to transfer bus we need to close the bus coupler first and then close by pass isolators of feeder to be connected to transfer bus and open the isolator switch of feeder coupled to main bus and then open the bus coupler breaker.
Advantages of main bus and transfer bus system:
1. No interruption of power supply because in case of fault load can be shifted to transfer bus.
2. Load can be divided into two groups since they can be feed from either of the buses.
Disadvantages of main bus and transfer bus system:
1. Two bus - bars are used which increases the cost.
Double bus arrangement with by pass isolators:
Double bus system with bypass isolators is combination of double bus system and main and transfer bus system. Here the feeders are connected to both the buses with the help of isolators during fault conditions loads can be transferred to healthy bus by closing isolators of feeders associated to healthy bus and opening isolators of feeders associated to faulty bus. The connection diagram is given below:
A bus coupler is also provided so while transferring load to healthy bus close the bus coupler breaker first and close the isolator of feeder to which it has to be transferred and open the isolator switch of feeder coupled to fault bus and then open the bus coupler breaker.
Advantages of double breaker system with by pass isolators:
1. No interruption of power supply because in case of fault load can be shifted to transfer bus.
2. Load can be divided into two groups since they can be feed from either of the buses.
Disadvantages of double breaker system with by pass isolators:
1. cost is high as we are using two bus bars and extra isolator switches.
2. Complex in nature.
Ring main arrangement:
Ring main arrangement provides double feed to each feeder circuit. Here the end of the bus bars is returned upon themselves to form a ring. Hence it is called main ring arrangement. This arrangement is shown in the following figure:
Here if one circuit beaker is damaged it is opened and the feeder can be supplied from the other circuit breaker which is near to it.
Advantages of ring main system:
1. Since each feeder is fed from two circuit breakers even if fault occurs in one system the feeder can be fed from other system so there will be no interruption of power supply under fault conditions.
2.The effect of fault is localized to that section alone and the rest of the section continues to operate normally.
Disadvantages of ring main system:
1. There will be difficulty to add any new circuit in the ring.
2. Over loading problems may occur.
Mesh arrangement:
In mesh arrangement between the mesh formed by bus bars circuit breakers are installed as shown in the following figure:
From the node point of mesh circuit is tapped. We need to open two circuit breakers when the fault occurs so that protection can be obtained but switching is not possible.
Advantages of mesh system:
1. Provides protection against fault.
2. For substations having large number of circuits this arrangement is suitable.
Disadvantages of mesh system:
1. It doesn't provide switching facility.
2. Not suitable for all type of substations.
These are the different sub station layouts or bus bar arrangements.
In this post we have learnt about electrical bus -bar and types of bus - bar arrangements and its advantages and disadvantages.
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