Generally, the term power factor represents the ratio of power actually used to the power supplied and this varies with the losses encountered in a particular system. All inductive circuits within a distribution system require current for the purpose of the excitation of magnetic field. This is applicable to induction motors, transformers, induction furnaces, welding plants, induction regulators, fluorescent electric clocks etc.
Power factor can be corrected so as to have an improved power factor. The usual method of making a system capacitive is achieved through the introduction of static capacitor which consist of chlorinated diphenyl impregnated paper dielectric elements in a sealed case, into the circuits either in the load-source or adjacent to the inductive plant.
Due to the fact that these electrostatic capacitors take a leading current they can be used to compensate for the lagging currents of the inductive circuit. When connected in circuit the capacitors act as a reservoir for energy which can be interchanged between the dielectric field of the capacitor and the magnetising needs of the inductive plant.
Other methods which can be used for power factor correction include synchronous motors and synchronous condensers. Synchronous motors are excited by direct current and do not therefore impose a lagging current for magnetising purposes on the system. These machines are intended mainly for situations where constant speeds are crucial over a wide range of loads, but can also be operated at power factors between unity and 0.8 leading.This feature enables the system to generally benefit an improved power factor result.
However, unless the speed control properties are essential, the high cost of these machine would be quite uneconomic. Synchronous condensers are used purely for situations where larger amount of corrective kVAr are required and carry no mechanical load. These are not usually considered for normal industrial purposes.
Control of Capacitors
The control of static capacitor banks is carried out by means of contractor equipment which in turn is controlled by sensing relay. Basically, a single phase current and voltage supply is applied to the relay such that at unity power factor the current and voltage vectors are displaced by 90 degrees. Changes in this angular displacement either lagging or leading are sensed by the relay which then switches the contractors which connect the reactive kVAr required for correction of the power factor within or outside the system. The control relay can be single or multi-stage based on the extent of the capacitor equipment in use.
In a smaller installation the capacitor band would probably be located adjacent to the incoming supply. In progressively larger installation the capacitors would be positioned at different load centres. However, in practice it is more economical to group capacitor ban together using multi-stage control, possibly in a sub-station.
Individual correction should then be limited for example to motors of 37 kW and above. In this case control relays would not be required as capacitors would be switched in and out with the operation of the motor. Individual correction can also be an advantage with welding plant.
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