Definition: An electrical bus bar is defined as a conductor or a group of conductor used for collecting electrical energy from the incoming feeders and distributes them to the outgoing feeders. In other words, it is a type of electrical junction in which all the incoming and outgoing electrical current meets. Thus, the electrical bus bar collects the electrical energy at one location. When the fault occurs in any section of the bus bar, all the circuit equipment connected to that section must be tripped to give complete isolation in the shortest possible time.
The bus-bars used in electrical substations usually have rectangular cross section bars, but they can be in another shape also, as round tubes, solid round bars, or shape tubes. The bus-bar are usually made up of aluminum. Aluminum have several advantages over copper such as higher conductivity, lower cost, excellent corrosion resistance, etc.
There are several types of bus bar arrangements, and the choice of particular arrangement depend on different factors such as system voltage, the position of a substation in the system, reliability of supply, flexibility and cost. The following are the electrical considerations governing the selection of any one particular arrangement.
- The bus bar arrangement is simple and easy in maintenance.
- It must have the provision of extension with the load growth.
- The installation should be as economical as possible, keeping in views the needs and continuity of supply.
- There is the availability of alternative arrangements in the in the event of the outage of any of the apparatus.
In a small and medium sized station, where, the shutdown can be permitted at times, single bus bars can be used. But in a major plant, an elaborate bus bar arrangement is essential so that the duplicate supply is always available in case of the fault, which might otherwise cause interruption of supply to a large area. Different type of electrical bus bars is explained below.
Single Bus-Bar Arrangement
This is the simplest arrangement consisting of a single set of a bus bar for the full length of the switchboard and to this set of bus bars are connected to all the generators, transformer and feeders as illustrated in the figure. The advantage of single bus bar arrangements are
- It has low initial cost.
- It requires less maintenance
- It is simple in operation
- One of the disadvantages of such type of bus bar arrangement is that when the fault occur on the bus-bars whole of the supply is affected, and all the healthy feeders are disconnected.
- The single bus bar arrangement provides the least flexibility and immunity.
Single Bus-Bar Arrangement With Bus Sectionalized
The bus-bar may be sectionalized by a circuit breaker and isolating switches so that a fault on one part does not cause a complete shutdown. In a sectionalized bus bar arrangement only one additional circuit breaker is required which does not cost much in comparison to the total cost of the bus bar system.
- In this type of arrangement, the faulty section can be isolated without affecting the supply of other section or sections.
- In such type of bus bar arrangement, one section is completely shut down for maintenance and repair without affecting the supply of other sections.
- In this bus bar arrangement, the fault level can be reduced by adding a current limiting reactor.
Disadvantages of Single Bus-Bar Arrangement with Sectionalization
- Additional isolators and circuit breaker are used in the sectionalized part of the circuit breaker which increases the cost of the bus-bar.
Main and Transfer Bus Arrangement
Such types of arrangements used two bus bars, known as main bus-bar and transfer bus-bar used as an auxiliary bus bar. Each generator and feeder may be connected to either bus-bars with the help of bus coupler which consist of a circuit breaker and an isolating switch. In this arrangement, a bus coupler is usually used so that change over from one bus-bar to the other can be carried out under load condition. The following steps may be taken while transferring the load to the reserve bus. These steps are
- Close the bus coupler so as to make the two buses at the same potential.
- Close isolators on the reserve bus.
- Open isolators on the reserve bus.
The load is now transferred to the reserve or auxiliary bus and the main bus is disconnected.
- It ensures the continuity of supply in case of bus fault. When the fault occurs on one bus, the entire load can be shifted to the other bus bar.
- Repair and maintenance can be carried out on the main bus without interrupting the supply as the entire load can be transferred to the auxiliary bus.
- Each load can be supplied from either bus, and the infeed and load circuit may be divided into two separate groups if required.
- The testing and maintenance of feeder circuit breakers can be done by putting them on a spare bus, thus keeping the main bus undisturbed.
- The maintenance cost of bus substation is lowered, and bus potential can be used for relays.
Disadvantages of Main and Transfer Bus Arrangement
- In such type of arrangements two bus bars are used which increases the cost of the system.
- The bus is maintained or expanded by transferring all of the circuits to the auxiliary bus depending upon the remote backup relays and breakers for removing fault of the circuit.
Double Bus Double Breaker Arrangement
In such type of arrangement two bus bars and two circuit breakers are employed. This arrangement does not require any bus coupler and permits switch over whenever desired without arrangement.
- This type of arrangement provided maximum reliability and flexibility as the fault and maintenance interrupt the supply to the minimum.
- There is continuity in supply because if a circuit breaker can be opened for repairs, then the load can be shifted on the other circuit breaker easily.
Disadvantages of double bus Double breaker
- In such type of arrangement two buses and two circuit breakers are used which increases the cost of the system.
- Their maintenance cost is also very high.
Because of its higher cost, such type of bus-bars is seldom used in substations.
Sectionalized Double Bus Bar Arrangement.
In this type of bus, arrangement sectionalized main bus bar is used with the auxiliary bus bar. In this arrangement, any section of the bus bar can be isolated for maintenance, while any section may be synchronized with any other through the auxiliary bus- bar. Sectionalization of the auxiliary bus bar is not required and would increase the cost.
In this arrangement, three circuit breakers are required for two circuits. The number of circuit breakers per circuit comes out to be one and a half. Such type of arrangement is preferred in large stations where power handled per circuit is large.
- It provides large security against loss of supply as a fault in a bus will not interrupt the supply.
- In such type of arrangement, the additional circuits are easily added to the system.
- The bus potential can be used as a supply for relays.
Disadvantages of One and a Half Breaker Arrangement
- In this arrangement, there are complications in the relaying system because at the time of fault two breakers are to be opened.
- The other drawback of this arrangement is that for maintenance of circuit breaker, two breakers are to be opened. Hence the line will be operating with one circuit breaker from bus only.
- Their maintenance cost is very high.
Ring Main Arrangement
- In this arrangement, the end of the bus bars is returned upon themselves to form a ring. Hence it is called main ring arrangement.
- Such type of arrangement provides great flexibility as each feeder is supplied by two paths. Hence the failure of supply does not cause any interruption of the supply.
- In such type of arrangement, the effect of fault is localized to that section alone and the rest of the section continues to operate.
- In this arrangement, a circuit breaker can be maintained without interrupting the supply.
Disadvantages of Ring Main Arrangement
- In such type of bus-bar, there are difficulties in addition to any new circuit in the ring.
- There is the possibility of overloading of the circuits on the opening of any section of the breakers.
- There is the necessity of supplying the potential to relays separately to each of the circuits.
In such type of arrangement, the circuit breakers are installed in the mesh formed by the buses. The circuit is tapped from the node point of the mesh. Such type of bus arrangement is controlled by four circuit breakers.
When a fault occurs on any section, two circuit breakers have to open, resulting in the opening of the mesh. Such type of arrangement provides security against bus-bar fault but lacks switching facility. It is preferred for substations having a large number of circuits.