There are numbers of incoming and outgoing feeders and transformers connected together in an electrical substation. But practically it is not possible to connect all the feeders and transformers together at a particular point or node. An electrical busbar is nothing but an extended version of a node.
That means when we draw a circuit diagram on a paper we connect a number of electrical elements together at a node. Obviously it is not practically possible to do the same in an actual electrical power system. The solution is the extended node. In other words in our drawing if we extend the node to a straight line and collect all the elements at different positions of the node then the circuit does not lose its actual behavior. And then theoretically we can refer to the straight line as the bus of the circuit. Now, you got the basic idea of an electrical busbar system.
Definition of Electrical Busbar
An electrical bulb is nothing but an equipotential conductor to feet to which we connect all the incoming and outgoing lines together in a substation or in any other similar system.
The practical electrical busbars are not so simple as we have told till now. Because it has to facilitate shutdowns of different parts of the system with the minimum sacrificing continuity, reliability, and flexibility of the power system. Because of this reason, the design of the electrical bulb system becomes a little bit complicated then it is expected.
Types of Electrical Busbar Systems
Depending on the design, there are different types of busbar systems.
1. Single busbar with a sectionalizer
2. Double busbar system
3. Double breaker busbar system
4. Breaker and a half scheme
5. Main and transfer bus system
6. Double bus with bypass isolators
7. Ring bus system
Single Busbar with a Sectionalizer
This is the most basic and simple bus bar arrangement. Also economically it is the cheapest one. This arrangement of busbar has two sections connected with a sectionalizer circuit breaker with associated isolators. All the elements connected to the single bus system are eventually distributed among two sections. For example, if there are 4 feeders and 2 power transformers connected to that bus then their arrangement will be as shown in the figure below.
Double Busbar System
In the double bus scheme, there are two identical main buses. Both of these buses can feed any of the feeders. In general, we divide the feeders into groups and connect one group to one particular bus. But in case of breakdown of any of the buses, we can shift all the feeders together to the healthy bus.
Where the flexibility of the system is more essential than the cost incurred, we use a double busbar system. Normally we provide a bus coupler between these two buses. The bus coupler facilitates the online changeover of a feeder from one bus to another. When the bus coupler is in on condition we can easily operate the isolator of any feeder without switching off the associated circuit breaker of the feeder. Because the bus coupler provides a parallel path to the feeder during the changeover. But the double busbar arrangement does not permit the breaker maintenance without interrupting the power flow through the feeder. This is one drawback of the system.
Double Breaker Scheme
This is one of the costliest arrangements. This is a costlier and improved version of the double bus scheme. In a double bus scheme, we have two isolators per feeder for connecting it to the buses. But in case of a double breaker scheme, we use circuit breakers instead of only isolators. So the maintenance of breakers without interrupting the feeder becomes possible in this scheme.
There are two main buses. Each of the buses connects one feeder through a circuit breaker. That means one feeder associates with two circuit breakers. But we do not keep the switch on of both the breakers at a time. Either of the breakers remains in on condition to provide a path to the feeder from the associated busbar.
The double bracket scheme provides two main advantages. In this scheme, we can connect one feeder to any of the two main buses through the associated circuit breaker. Again, we can take up the maintenance activities of any of the breakers without interrupting the power flow of the feeder. In that case, the other breaker remains in on condition to provide continuity of the supply.
The main disadvantages of this scheme are this double bus scheme is quite costly and it needs quite a wider space in the substation.
It provides extreme flexibility to the system. In the modern era, the number of connected substations to the power system is quite high. Therefore, even the total blackout of any substation merely affects the system flexibility. Hence the power system network of the modern era may not need such a costly bus bar scheme.
Breaker and a Half Scheme
This is the modification of the costliest double breaker bus system. Hindi scheme in this game in this scheme each feeder associated with its own main circuit breaker and a spare circuit breaker. Two feeders share the same spare circuit breaker. Therefore in this scheme, one pair of feeders is associated with three circuit breakers – two main breakers and one spare breaker. In other words, we can say that one feeder shares one and a half circuit breakers of the system. And this is the reason the scheme has got such a name.
Although the scheme has its own complication. When we take a shutdown of any of the main circuit breaker of any feeder, the feeder gets supply through the associated spare circuit breaker. This supply comes through the main circuit breaker of the other feeder of the pair. In that condition, the protection scheme of the other feeder has to take care of the diverted feeder through the spare breaker. Therefore auto or manual changeover or protection scheme becomes necessary during the circuit breaker changeover operation. It makes the protection scheme of a breaker and half scheme quite complicated. This is the reason this scheme also could not become quite popular in practice.
Main and Transfer Bus System
This is another alternative to the double busbar system. In other words, it is the improved version of the double busbar system. The double busbar system provides flexibility to the system during maintenance of any of the two buses. But it does not provide any flexibility for the maintenance of a feeder circuit breaker without interrupting the flow of power through the feeder. To overcome the drawback, the main and transfer bus system comes into the picture. This scheme provides flexibility for the maintenance of a feeder circuit breaker with the continuation of power through the feeder.
In this scheme, there is a transfer bus coupler. The transfer bus coupler can couple the main bus with the transfer bus as when required. Normally the transfer bus is in dead condition. But during the maintenance of a feeder circuit breaker, we can divert the corresponding feeder to the transfer bus through the associated transfer bus isolator. Then we switch on the transfer bus coupler circuit breaker to electrify the transfer bus. After that, we switch on the transfer but isolator associated with the feeder to connect the feeder with the transfer bus. At that certain condition, the feeder shares the load through both the main circuit breaker and the transformer circuit breaker.
After that, finally, we switch off the main circuit breaker. As soon as we switch off the main circuit breaker the total load of the feeder starts passing through the transfer bus circuit breaker bay. Therefore the power from the main bus passes through the transfer bus circuit breaker to the transfer bus then to the diverted feeder. As a result, the continuity of the supply of the feeder remains uninterrupted.
The main and transfer bus scheme is the most common busbar scheme used in power systems. But the diversion of the feeder through the transfer bus requires a significant number of isolator operations. Therefore there is always a chance of electrical hazard if the sequence of switching operations is not maintained properly. To eliminate human mistakes during operation, the system requires the proper switching interlocking scheme. Obviously it adds extra expenditure to the system.
Double Bus with Bypass Isolators
This is the combination double bus and transfer bus system. Out of the two main buses, we can use one as the transfer bus if required. This busbar scheme provides flexibility for the maintenance of both buses as well as circuit breakers without any discontinuity of the supply.
In the scheme, each feeder requires an additional bypass isolator. Obviously this makes the layout of the scheme a little bit complicated. But the scheme has significant flexibility.
For taking the shutdown of one main bus, we first need to switch on the bus coupler to electrically couple two main buses. Then we need to close other bus side isolator of each of the feeders connected to the bus going under the shutdown. The feeders connected to the bus going under shutdown, now become connected with both of the buses. Finally, we open the isolator of the feeders towards the bus going under the shutdown. After opening the bus coupler circuit breaker we isolate the bus for the shutdown.
Similarly for taking the shutdown of the circuit breaker of a certain feeder, we need to close the bypass isolator first. Then we close the bus coupler isolators. After that, we switch on the bus coupler circuit breaker. Finally, we open the main circuit breaker. Lastly, open the bus side and breaker side isolators to isolate the circuit breaker from the system for maintenance purposes. Now the feeder gets power through the bus coupler circuit breaker. Also, we need to shift the protection scheme of the feeder to the bus coupler breaker.
Ring Bus System
The mesh bus system or ring bus system is the most flexible system. This system provides double sources to each feeder connected to it. That means each of the feeders associates with two circuit breakers. Therefore the opening of one of the circuit breakers does not affect the continuity of flow through the feeder. Hence availing the shutdown for maintenance purposes to any particular circuit breaker becomes quite easier in this system. Another advantage of the ring bus system is that it does not require any separate busbar protection scheme. This is because the protection scheme of each feeder covers the associated section of the system. Also economically this scheme is quite cheaper than the double bus system.