Introduction to Bus Bar arrangement
The Bus bar arrangement is one of the vital elements in electric power stations and substations. The term bus is derived from the word omnibus which means collector of things. Thus the electrical bus bar is the collector of electrical energy at one location. Bus bars are convenient means of connecting switches and other equipment into various arrangements.
Bus-bar used in substations are usually bare rectangular cross-section bars. Aluminum is used for the construction of bus bars as it has several advantages over copper such as higher conductivity on a weight basis, lower cost for equal current capacity, and excellent corrosion resistance. For proper reliable electrical connections aluminum buses are coated with silver. The most common sizes of bus-bars are 40 *4mm; 40*5mm;50*5mm; 50*6mm; 60*8mm; 80*8mm; 100*10mm.
Whenever the fault occurs in the bus bar, considerable damage and disruption of supply will occur and system stability is adversely affected. Hence, the proper protection of the bus bar is needed.
Types of Bus-bar
There are several types of bus-bar arrangements. The choice for the particular arrangements depends upon system voltage, the position of the substation in the system, reliability of supply, flexibility, and cost. The factors for the selection of the bus bar arrangements are:
- Maintenance possibility without interruption of the power supply.
- Provision of extension with the load growth.
- In case of an outage, other alternative arrangements must be possible.
- Load and local conditions.
The various bus bar arrangements are as follows:
- Single Bus-bar arrangement
- Single Bus-bar arrangement with bus sectionalization
- Main and transfer Bus Arrangement
- Double Bus Double Breaker Arrangement
- Sectionalized Double Bus Arrangement
- One –and –a Half Breaker Arrangement
- Ring Main Arrangement
- Mesh Arrangement
Single Bus-bar arrangement
- This is the simplest arrangement consisting of a single set of bus bars for the full length of the switchboard and a set of generators, transformers, and feeders.
- The main drawback of this system is that in case of a fault in the bus bar whole of the system is collapsed and all the healthy feeders are disconnected.
- It is employed for switchboards, small and medium size substations, small power stations, and DC stations.
Single Bus-bar arrangement with bus sectionalization
- This bus bar is sectionalized by a circuit breaker and isolating switches so that a fault on one part does not cause a complete shutdown.
- Sectionalization of the bus bar is done in the large substations where the large units are installed.
- There are basically 2 or 3 numbers in the section of the bus bar.
- In case of the occurrence of a fault on any section of the bus bar. the faulty section can be isolated without affecting the supply of other sections.
- One section can be completely shut down for maintenance and repairs without affecting the supply of other sections.
- Adding a current limiting reactor between the sections the fault level(MVA) can be reduced thereby the circuit breakers of lower capacity can be used.
Main and transfer Bus Arrangement
- This arrangement is done where the loads and continuity of supply are to be done.
- This provides additional flexibility, continuity of supply and allows periodic maintenance without total shutdown.
- It consists of two bus bars as the main bus bar and the transfer bus bar used as an auxiliary bus bar.
- The power from the main bus can be totally transferred to the auxiliary bus bar as needed through the bus coupler since there will be a high degree of reliability.
- While transferring the load to the reserved bus
- Close the bus coupler.
- Close isolators on the reserved bus.
- Open isolators on the main bus.
- The cost index for this type of arrangement is 1.6.
- Repair and maintenance can be carried out on the main bus without the interruption of the supply.
- The maintenance cost of the substation is lowered.
Double Bus Double Breaker Arrangement
- Two circuit breakers are employed for each circuit.
- This type of arrangement doesn’t require any bus coupler.
- This type of arrangement can permit switch-over from one bus to another bus without interruption.
- Costly and maintenance cost is high.
- Provides maximum flexibility and reliability.
- In case of maintenance of a circuit breaker load can be shifted to another circuit breaker.
Sectionalized Double Bus Arrangement
- Duplicate bus bars are used with the main bus bar in sections connected through a bus coupler.
- Any section can be isolated for maintenance.
- Any section may be synchronized with any other through the auxiliary bus bar.
One –and –a Half Breaker Arrangement
- This arrangement is the improvement of the Double bus double breaker arrangement and reduces the number of circuit breakers.
- It needs three circuit breakers for two circuits.
- This arrangement is used in important large stations where the power handled per circuit is large.
- Provides high security against loss of supply.
- The main drawback of this type of system is complications in relaying the system because, at the time of the fault, two breakers are to be opened.
- The maintenance cost is higher.
Ring Main Arrangement
- Higher reliability.
- Less number of circuit breakers is needed.
- Difficulties in addition to any new circuit in the ring.
- Possibilities of overloading of the circuit in the opening of any section of the breaker.
- The necessity of supplying demand potential to relay separately to each of the circuits.
- This arrangement provides greater flexibility as each feeder is supplied by two paths, so the failure of the section doesn’t cause any interruption of supply.
- Economical use of circuit breaker in the substation.
- Circuit breakers are installed in the mesh formed by the buses.
- When the fault occurs in any section two circuit breakers have to open resulting in the opening of the mesh.
- Provides security against bus-bar faults.
- Lacks switching facility.
- Needs less number of circuit breakers than required by one–and–a –half breaker arrangement.
- Used in substations having a large number of circuits.