Bus-bars are mostly used to interconnect the loads and sources of electrical power. It does the connection of incoming and outgoing transmission lines. It also does the connection between the generator and the main transformer of the power system. A bus-bar is a strip of copper or aluminum that conducts electricity within a switchboard, distribution board, substation or other electrical apparatus. Bus-bars are either flexible or rigid. Flexible bus-bars are made up of aluminum tubes and are supported by disc insulator strings on both sides with the gantries. Rigid bus-bars are made up of aluminum tubes and it is supported on post insulators.
The bus-bar size determines the maximum amount of current that can be safely carried. They are either flat strips or hollow tubes these shapes will allow heat to dissipate more efficiently due to their high surface area to cross-sectional area ratio.
When several feeders or generators operating at the same voltage have to be directly connected to each other so in that case bus-bars are used as the common electrical equipment bus-bars can provide operating flexibility, cost reduction or sufficient reliability
Bus-bar connection
Bus-bar could be supported on insulators or insulation could be surrounded it completely they are protected from accidental contact by a metal earthed enclosure. Neutral bus-bars could be insulated too. Bus-bars could be enclosed in metal housing in the form of bus-duct or busway segregated phase bus or isolated phase bus. Bus-bars could be connected to each other and to electrical apparatus by a bolted clamp or welded connections
How bus-bars can be rated
- Rated current– the RMS value of current which the bus-bars can carry continuously with temperature rise within a specified limit
- Rated voltage – it is the RMS value of voltage between lines for which the Bus-bar is intended
- Rated frequency – 50Hz for Indian system
- Rated short-time current – it is defined as the RMS value of the current which the bus-bar can carry temperature rise within specified limits for a specified duration
- Rated insulation level – insulation level is characterized by normal rated voltage, power frequency withstand voltage, lightning impulse withstand voltage, and switching impulse voltage
How to select Bus-bar
Some factors must be considered before selecting the bus-bar size such as normal current carrying capability, surface gradient, corona free performance. It must be selected by considering the electrical and mechanical stresses such as short circuit fault currents and it should be able to withstand the thermal stresses under short circuit the clamps and connectors are also stressed
What are the types of Bus-bar arrangements used in power system
Single bus bar system
This arrangement consists of one main bus which will be energized at all times and all the circuits will be connected to this, and this kind of arrangement only provides the least amount of reliability. Bus faults or failure of circuit breaker operation under fault condition could result in complete loss of the substation
Advantages of a single bus bar system
- Low cost
- Small area needed
- Easily expandable
- Simple in concept and operation
- Simple for the application of protective relaying
Disadvantages of a single bus bar system
- Lowest reliability
- Failure of a Circuit breaker or bus fault can cause the loss of entire substation
- Maintenance switching can complicate and disable some of the protective relay schemes
Sectionalized bus
It can be said as an extension of the single bus arrangement it consists of two or more single bus schemes and each of them will be tied together by the bus sectionalizing breakers. These breakers can operate open or closed according to the system requirements. If any bus fault or breaker failure occurs the affected bus section can be removed and prevents the total substation shutdown.
Advantages of sectionalized bus bar system
- Flexible operation
- Higher reliability than a single bus scheme
- Only one part of the substation will be a loss for a breaker failure or bus fault
Disadvantages of sectionalized bus bar system
- Higher cost than a single bus scheme
- Additional circuit breakers are needed for sectionalizing
- Sectionalizing can cause interruption of non-faulted circuits
Main and transfer bus
This configuration consists of two independent buses and the main bus is normally energized under normal operating conditions and all the incoming and outgoing circuits are fed from the main bus through their circuit breakers and switches. If a circuit breaker needs to be repaired then the circuit operation integrity can be maintained by the use of bypass and bus tie equipment
Advantages of main and transfer bus system
- Easily expandable
- Reasonable in cost
- Small land area is needed
Disadvantages of main and transfer bus system
- An additional circuit breaker is needed for the bus tie
- Circuit breaker failure or bus fault could cause the loss of entire substation
- Complicated switching is needed to remove a circuit breaker from service to maintenance
Ring bus
It is an extension of the sectionalized bus arrangements and it is accomplished by interconnecting the two open ends of the buses through another sectionalizing breaker and it would result in a closed-loop or ring with each bus section separated by a circuit breaker. For better reliability and operational flexibility, each section must supply only one circuit. If a fault occurs in a bus or in circuit breaker only limited bus section and circuit will be removed.
Advantages of a ring bus system
- Flexible operation
- High reliability
- Double feed to each circuit
- No main buses
- Circuit operation won’t be affected while isolation of bus section or circuit breaker for maintenance
Disadvantages of a ring bus system
- Each circuit needs to have its own potential source for relaying
- The configuration is limited to four circuit positions
Breaker and a half
This configuration consists of two main buses each normally energized, three circuit breakers are electrically connected between the buses. So in this arrangement, three circuit breakers are used for two independent circuits and by this configuration, the circuit breaker can be removed without interrupting any circuits
Advantages of the breaker and a half system
- Flexible operation
- High reliability
- Double feed to each circuit
- Bus fault doesn’t interrupt the service of the circuit
- All switching is done by a circuit breaker
Disadvantages of the breaker and a half system
- Each circuits needs its own potential circuit for relaying
- One and a half breaker is needed per circuit
Double breaker double bus
In this type of configuration two main buses are used, two circuit breaker is electrically connected between the buses, two circuit breakers are needed for each circuit. This type of configuration is limited to large generating stations because of the high cost.
Advantages of double breaker double bus system
- Flexible operation
- High reliability
- Double feed to each circuit
- All switching with circuit breaker
- During breaker failure, only one circuit will loss
Disadvantages of double breaker double bus system
- High cost
- Two circuit breaker is needed for each circuit
