Switching Schemes or Busbar Arrangements in Substation
Burbars are the important elements in the electrical substation. Bus bars acts as nodal point in the substation which connects different incoming and outgoing circuits.
Substations present in the power system performs various operations depends on the application such as stepping up the voltage, stepping down the voltage, high voltage transmission and switching stations to route the power to desired load center.
Busbars in Substation:
Busbars used in the substations are generally rectangular or circular cross section bars. These busbars can be either solid or hollow structures. Hollow circular cross section busbars are employes in EHV substations to reduce the corona effect.
Switching schemes implies different methods employed to connect electrical circuits in the power system to transfer the electrical power in reliable manner. Switching schemes helps in deliver the electrical power to power system if any part of the system is faulty or under maintenance.
Substations use different types of busbar arrangements or switching schemes depends upon the application, reliability of the supply and cost of installation. In every substations busbars plays a common role to connect different circuits. However switching is possible in the power system with the help of circuit breakers and isolators.
Considerations for selection of busbar arrangement:
Different types of busbar arrangements are employed based on the voltage, reliability of the supply, flexibility in transmitting power and cost. The other aspects considering in designing the busbars arrangements are:
- Simplicity in the design
- Maintenance of different elements without interruption in the power supply
- Future expansion feasibility
- Economical in cost of installation and operation
Different busbar arrangements:
Some of the switching schemes are busbar arrangements employed in the substations are listed below:
- Single Bus-bar arrangement
- Double Main Bus-bar scheme
- Main and Transfer bus-bar scheme
- One and half breaker scheme
- Ring Main arrangement scheme
Single Bus-bar scheme:
This is the simplest busbar scheme available which consists of single set of busbars connected to the generators, transformers and load feeders. All the feeders are connected by circuit breaker and set of isolators. This arrangement helps to remove the connecting elements (Generators, transformers, etc ) for maintenance by opening the circuit breaker contacts and further opening the isolators.
- This busbar arrangement enjoys less cost of installation
- Less maintenance
- simple operation
- Fault on the busbar all the feeders connected to the busbars should be disconnected
- when Busbar is under maintenance total supply and all feeders should be disconnected
- Least flexibility and reliability
Double Main Busbar Scheme:
Normally in double main busbar scheme each circuit is connected to both the buses. In some cases half of the circuits can be connected and operated on each bus, in these cases bus or circuit breaker failure would cause loss to half of the circuits. In double main busbar arrangement one or two breakers can be provided for each circuit. Double main busbar and double breaker scheme provides high reliability in the case of fault or outage of one of the breaker.
- Any circuit can be taken out of circuit for maintenance
- Flexibility in connecting the feeder circuit to either of the busbars
- Most expensive
- Loose circuits connected to busbar when fault occurs on the busbar
Main and Transfer Busbar Scheme
Main and Transfer busbar scheme is similar to single busbar arrangement with additional transfer bus connected. Tie circuit breaker is provided to tie both the main and transfer bus. During normal operation all the circuits are connected to the main bus. When circuit breaker connected to the circuit (transmission line) is required to trip for maintenance, tie circuit breaker connecting the main and transfer bus is closed. The relay protection for the circuits connected to the transfer bus is taken care by the tie circuit breaker.
- Low initial cost
- Any breaker can be taken of circuit for maintenance
- Requires one extra breaker for bus tie
- Switching is somewhat complicated when breaker is under maintenance
One and Half breaker Busbar scheme:
In One and half breaker scheme, two circuits are connected between the three circuit breakers. Hence One and Half breaker name was coined for this type of arrangement. Under normal operating conditions all the breakers are closed and both the busbars are energized. Any Circuit fault will trip two circuit breakers and no other circuit will be affected in this arrangement. When a busbar fault occur only breakers adjacent to busbars trips and no circuit will lose power. Two busbars can also be taken out of service with out affecting the power flow if the power source circuit ( alternator circuit) and receiving circuit (transmission line) available in the same bay.
- Most flexible operation possible
- High reliability
- Bus failure will not remove any circuit from service
- High cost
- Relaying is somewhat complicated since the middle breaker must responsible for both the circuits on either direction and should operate
Ring busbar scheme
In this ring main busbar scheme arrangement, breakers are connected in ring and circuits are connected between the breakers. There will be same number of circuits as the number of breakers in the arrangement. During normal operation all the breakers are closed. During circuit fault two breakers connecting the circuit trips. During breaker maintenance the ring is broken but all the lines remain in service.
- Low cost
- Flexible operation for breaker maintenance
- Any breaker can be taken out of service without interrupting load
- Power can be fed from both the direction
- Fault occur during maintenance will break the ring
- Relaying is complex
- Breaker failure during fault will trip one additional circuit