Power Factor Correction
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Wikipedia-Article "Power Factor Correction"
Power factor correction (PFC) is a technique of counteracting the undesirable effects of electric loads that create a power factor (p.f.) that is less than 1, (or unity). PFC returns the power factor of an electric AC power transmission system to very near unity by switching in or out banks of capacitors or inductors which act to cancel the inductive or capacitive effects of the load. For example the inductive effect of motor loads may be offset by locally connected capacitors. Electronic control of load current can be used for PFC as well.
When an electric load has a p.f. lower than unity, it uses both real power and reactive power, whereas loads that have p.f. of unity use only real power. Work done by the load uses only real power, however the reactive power must still be supplied. More current is required to deliver the same amount of useful energy to loads having a non-unity power factor, which wastes energy.
Energy losses in transmission lines increase with increasing current. Power companies therefore require that customers, especially those with large loads, maintain the power factors of their respective loads within specified limits or be subject to additional charges. Engineers are often interested in the power factor of a load as one of the factors that affect the efficiency of power transmission.
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Electric industry aspects
PFC is desirable because the source of electrical energy must be capable of supplying real power as well as any reactive power demanded by the load. This can require larger, more expensive power plant equipment, transmission lines, transformers, switches, etc. than would be necessary for only real power delivered. Also, resistive losses in the transmission lines mean that some of the generated power is wasted because the extra current needed to supply reactive power only serves to heat up the power lines.
The electric utilities therefore put a limit on the power factor of the loads that they will supply. The ideal figure for load power factor is unity (that is, a purely resistive load). Real loads deviate from this ideal. Electric motor loads are phase lagging (inductive), therefore requiring capacitor banks to counter this inductance. Sometimes, when the power factor is leading due to capacitive loading, inductors (also known as reactors in this context) are used to correct the power factor. In the electric industry, inductors are said to consume reactive power and capacitors are said to supply it, even though the reactive power is actually just moving back and forth between each AC cycle.
Unity power factor gives rise to the greatest utilization of the plant. Electric utilities measure reactive power used by high demand customers and charge higher rates accordingly. Some consumers install power factor correction schemes at their factories to cut down on these higher costs. The units of reactive power is volt-amperes reactive (VAr).
Application
Passive power factor correction
The most common type of PFC is passive power factor correction. It is achieved by complementing an inductive or a capacitive circuit with a (locally connected) reactance of opposite phase. For a typical phase lagging p.f. load, such as large inductive motors, this would consist of a capacitor 'bank' in the form of several parallel capacitors at the power input to the device.
Synchronous condenser power factor correction
- For more details on this topic, see Synchronous condenser.
A synchronous condenser is fundamentally an AC synchronous motor that is not attached to any driven equipment. It is started and connected to the electrical network. It operates at full leading power factor and puts VARs onto the network as required to support a system’s voltage or to maintain the system power factor at a specified level. The condenser’s installation and operation are identical to large electric motors.
Increasing the devices field excitation results in its furnishing magnetizing power (kilovars) to the system. Its principal advantage is the ease with which the amount of correction can be adjusted.
Active power factor correction
A modern type of PFC is called active power factor correction, often abbreviated as A-PFC. This employs electronic high-frequency switching to control the load current in a manner similar to a switched-mode power supply. The effect is to synchronize the current demand to be closely in phase with the AC voltage waveform. Active PFC can achieve a power factor of 0.95. The use of A-PFC is mainly for small-scale electronic loads with the intent to diminish harmonic noise injected back into the power line. Inexpensive electronic power supplies that use diode bridge rectifiers draw current in pulses in an otherwise non-linear fashion (i.e., not as a resistive load would) which causes harmonic noise to be generated in the power transmission line. This disturbs the power transmission to other devices. Several government regulatory agencies now require that electronic loads have power factor close to unity. The European Union requires new mains-powered devices to include some form of power factor correction.
Active PFC can add more cost to the manufacture of the equipment, and may be reflected in the price; however, integrated circuits are becoming available which perform the PFC function at low cost.
Generally, PC power supply manufacturers promote their employment of Active PFC, so it is likely to be quoted should it be featured in the product.
References
- A. K. Maini "Electronic Projects for Beginners", "Pustak Mahal", 2nd Edition: March, 1998 (India)