Isolated-phase bus
Encyclopedia
In electrical engineering, isolated-phase bus (IPB), also known as Phase-isolated Bus (PIB) in some countries, is a method of construction for circuits carrying very large currents, typically between a generator
and its step-up transformer
in a steam or large hydroelectric power plant.
Each phase current is carried on a separate conductor, enclosed in a separate grounded metal housing. Conductors are usually hollow aluminum tubes or aluminum bars, supported within the housing on porcelain or polymer insulators. The metal housings are electrically connected so that induced current, nearly of the magnitude of the phase current, can flow through the housing, in the opposite direction from the phase current. The magnetic field
produced by this current nearly exactly cancels the magnetic field produced by the phase current, so there is almost no external magnetic field produced. This also limits the amount of force produced between conductors during a short circuit
. The external housings of the conductors remain at a low potential with respect to earth ground and are usually bonded to ground. Additionally, by enclosing the conductors in separate housings a high degree of protection from faults between the generator and the transformer is obtained. Since these conductors are between the generator and the first circuit breaker
, if a fault were to occur the only way to interrupt it would be to stop the generator, which would take considerable time and result in much damage to the generator or conductors.
Isolated-phase bus is made in ratings from 3000 amperes to 45,000 amperes, and rated for voltages from 5000 volts up to about 35,000 volts. In the larger current ratings, dry air is forced through the enclosures and within the tubular conductors to cool the conductors. Some items of switchgear, such as circuit breaker
s and isolating switches, are made in housings compatible with the isolated-phase bus. Accessories such as instrument transformers, surge arresters, and capacitors are also made in compatible housings. Due to the expense of its construction and the energy loss, isolated-phase bus is usually used in short segments; a large underground powerhouse may have isolated-phase bus up to about 50 metres or so to connect generators to transformers in an underground cavern.
A smaller form of isolated-phase bus is manufactured for use with direct-current circuits; this may be used in the field circuit of a generator.
Other types of bus are:
These are used at lower ratings or where adequate protection of the circuit by overcurrent devices is possible.
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
and its step-up transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
in a steam or large hydroelectric power plant.
Each phase current is carried on a separate conductor, enclosed in a separate grounded metal housing. Conductors are usually hollow aluminum tubes or aluminum bars, supported within the housing on porcelain or polymer insulators. The metal housings are electrically connected so that induced current, nearly of the magnitude of the phase current, can flow through the housing, in the opposite direction from the phase current. The magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
produced by this current nearly exactly cancels the magnetic field produced by the phase current, so there is almost no external magnetic field produced. This also limits the amount of force produced between conductors during a short circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
. The external housings of the conductors remain at a low potential with respect to earth ground and are usually bonded to ground. Additionally, by enclosing the conductors in separate housings a high degree of protection from faults between the generator and the transformer is obtained. Since these conductors are between the generator and the first circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
, if a fault were to occur the only way to interrupt it would be to stop the generator, which would take considerable time and result in much damage to the generator or conductors.
Isolated-phase bus is made in ratings from 3000 amperes to 45,000 amperes, and rated for voltages from 5000 volts up to about 35,000 volts. In the larger current ratings, dry air is forced through the enclosures and within the tubular conductors to cool the conductors. Some items of switchgear, such as circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
s and isolating switches, are made in housings compatible with the isolated-phase bus. Accessories such as instrument transformers, surge arresters, and capacitors are also made in compatible housings. Due to the expense of its construction and the energy loss, isolated-phase bus is usually used in short segments; a large underground powerhouse may have isolated-phase bus up to about 50 metres or so to connect generators to transformers in an underground cavern.
A smaller form of isolated-phase bus is manufactured for use with direct-current circuits; this may be used in the field circuit of a generator.
Other types of bus are:
- Metal-enclosed segregated phase bus
- Low and high impedance bus duct
- Bare flat or tubular bus bars
These are used at lower ratings or where adequate protection of the circuit by overcurrent devices is possible.