There are many different electrical bus system schemes available but selection of a particular scheme depends upon the system voltage, position of substation in electrical power system, flexibility needed in system and cost to be expensed.
The Main Criterias to be Considered During Selection of one Particular Bus – Bar Arrangement Scheme
- Simplicity of system.
- Easy maintenance of different equipments.
- Minimizing the outage during maintenance.
- Future provision of extension with growth of demand.
- Optimizing the selection of bus bar arrangement scheme so that it gives maximum return from the system.
Some very commonly used bus bar arrangement are discussed below-
Single Bus System
Single Bus System is simplest and cheapest one. In this scheme all the feeders and transformer bay are connected to only one single bus as show.
Advantages of Single Bus System
- This is very simple in design.
- This is very cost effective scheme.
- This is very convenient to operate.
Disadvantages of Single Bus System
- One but major difficulty of these type of arrangement is that, maintenance of equipment of any bay cannot be possible without interrupting the feeder or transformer connected to that bay.
- The indoor 11 KV switch boards have quite often single bus bar arrangement.
Single Bus System with Bus Sectionalizer
Some advantages are realized if a single bus bar is sectionalized with circuit breaker. If there are more than one incoming and the incoming sources and outgoing feeders are evenly distributed on the sections as shown in the figure, interruption of a system can be reduced to a reasonable extent.
Advantages of Single Bus System with Bus Sectionalizer
If any of the sources is out of the system, still all loads can be fed by switching on the sectional circuit breaker or bus coupler breaker. If one section of the bus bar system is under maintenance, a part load of the substation can be fed by energizing the other section of the bus bar.
Disadvantages of Single Bus System with Bus Sectionalizer
- As in the case of a single bus system, maintenance of equipment of any bay cannot be possible without interrupting the feeder or transformer connected to that bay.
- The use of isolator for bus sectionalizing does not fulfill the purpose. The isolators have to be operated ‘off circuit’ and which is not possible without total interruption of bus-bar. So investment for bus-coupler breaker is required.
Double Bus System
- In double bus bar system two identical bus bars are used in such a way that any outgoing or incoming feeder can be taken from any of the bus.
- Actually every feeder is connected to both of the buses in parallel through individual isolator as shown in the figure.
By closing any of the isolators, one can put the feeder to the associated bus. Both of the buses are energized, and total feeders are divided into two groups, one group is fed from one bus and other from other buses. But any feeder at any time can be transferred from one bus to other. There is one bus coupler breaker which should be kept close during bus transfer operation. For transfer operation, one should first close the bus coupler circuit breaker then close the isolator associated with the bus to where the feeder would be transferred and then open the isolator associated with the bus from where the feeder is transferred. Lastly, after this transfer operation, he or she should open the bus coupler breaker.
Advantages of Double Bus System
Double Bus Bar Arrangement increases the flexibility of system.
Disadvantages of Double Bus System
The arrangement does not permit breaker maintenance without interruption.
Double Breaker Bus System
In double breaker bus bar system two identical bus bars are used in such a way that any outgoing or incoming feeder can be taken from any of the bus similar to double bus bar system. The only difference is that here every feeder is connected to both of the buses in parallel through individual breaker instead only isolator as shown in the figure. By closing any of the breakers and its associated isolators one can put the feeder to respective bus. Both of the buses are energized, and total feeders are divided into two groups, one group is fed from one bus and other from other buses similar to the previous case. But any feeder at any time can be transferred from one bus to other. There is no need for bus coupler as because the operation is done by breakers instead of isolators. For transfer operation, one should first close the isolators and then the breaker associated with the bus to where the feeder would be transferred, and then he or she opens the breaker and then isolators associated with the bus from where the feeder is transferred.
One and A Half Breaker Bus System
This is an improvement on the double breaker scheme to effect saving in the number of circuit breakers. For every two circuits, only one spare breaker is provided. The protection is however complicated since it must associate the central breaker with the feeder whose own breaker is taken out for maintenance. For the reasons given under double breaker scheme and because of the prohibitory costs of equipment, even this scheme is not much popular. As shown in the figure that it is a simple design, two feeders are fed from two different buses through their associated breakers, and these two feeders are coupled by a third breaker which is called tiebreaker. Normally all the three breakers are closed, and power is fed to both the circuits from two buses which are operated in parallel. The tiebreaker acts as a coupler for the two feeder circuits. During the failure of any feeder breaker, the power is fed through the breaker of the second feeder and tiebreaker, therefore each feeder breaker has to be rated to feed both the feeders, coupled by the tiebreaker.
Advantages of One and A Half Breaker 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.
Disadvantages of One and a Half Breaker Bus System
This scheme is much expensive due to investment for third breaker.
Main and Transfer Bus System
This is an alternative of a double bus system. The main conception of Main and Transfer Bus System is, here every feeder line is directly connected through an isolator to a second bus called transfer bus. The said isolator in between transfer bus and feeder line is generally called bypass isolator. The main bus is as usual connected to each feeder through a bay consists of the circuit breaker and associated isolators at both sides of the breaker. There is one bus coupler bay which couples transfer bus and main bus through a circuit breaker and associated isolators at both sides of the breaker. If necessary, the transfer bus can be energized by main bus power by closing the transfer bus coupler isolators and then breaker. Then the power in transfer bus can directly be fed to the feeder line by closing the bypass isolator. If the main circuit breaker associated with the feeder is switched off or isolated from the system, the feeder can still be fed in this way by transferring it to transfer bus.
Switching Operation for Transferring a Feeder to Transfer Bus from Main Bus without Interruption of Power
- First close the isolators at both side of the bus coupler breaker.
- Then close the bypass isolator of the feeder which is to be transferred to transfer bus.
- Now energized the transfer bus by closing the bus coupler circuit breaker from remote.
- After bus coupler breaker is closed, now the power from the main bus flows to the feeder line through its main
- breaker as well as bus coupler breaker via transfer bus.
- Now if the main breaker of the feeder is switched off, total power flow will instantaneously shift to the bus coupler breaker, and hence this breaker will serve the purpose of protection for the feeder.
- At last, the operating personnel open the isolators at both sides of the main circuit breaker to make it isolated from rest of the live system.
So, it can be concluded that in Main and Transfer Bus System the maintenance of circuit breaker is possible without any interruption of power. Because of this advantage, the scheme is very popular for 33 KV and 13 KV system.
Double Bus System with Bypass Isolators
This is a combination of the double bus system and main bus and transfer bus system. In Double Bus System with Bypass Isolators either bus can act as main bus and second bus as transfer bus. It permits breaker maintenance without interruption of power which is not possible in a double bus system, but it provides all the advantages of the double bus system. It, however, requires one additional isolator (bypass isolator) for each feeder circuit and introduces slight complication in system layout. Still, this scheme is best for an optimum economy of the system, and it is the best excellent choice for 220 KV system.
Ring Bus System
The schematic diagram of the system is given in the figure. It provides a double feed to each feeder circuit, opening one breaker under maintenance or otherwise does not affect supply to any feeder. But this system has two major disadvantages. One as it is a closed circuit system it is next to impossible to extend in future and hence it is unsuitable for developing systems. Secondly, during maintenance or any other reason, if any one of the circuit breaker in ring loop is switched off, the reliability of system becomes very poor as because closed loop becomes opened. Since at that moment for any tripping of any breaker in the open loop causes interruption in all the feeders between the tripped breaker and open end of the loop.