Electricity: power supply or regulation systems – For reactive power control – Using impedance
Patent
1996-06-04
1998-04-07
Sterrett, Jeffrey L.
Electricity: power supply or regulation systems
For reactive power control
Using impedance
323210, G05F 170
Patent
active
057368384
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to power factor correction in single or polyphase alternating current systems, and, more particularly, to a method and apparatus for effecting near-instantaneous power factor correction in a variable reactive load.
2. Description of the Related Art
Producers and consumers of large amounts of electrical power have long sought to optimize the use of power to avoid energy waste. One way to optimize the use of electrical power is to maximize the power factor associated with a reactive electrical load. A reactive load is one which has inductive or capacitive components. An alternating current power source generally provides a sinusoidal voltage and the load draws a sinusoidal current. In a purely resistive load, the source voltage and source current are in phase and the power factor is equal to one. In an ideal purely reactive load (zero resistance), the source voltage and source current are 90.degree. out of phase, and the power factor is equal to zero. The power factor has a fractional value between zero and one for loads which are a combination of resistive and reactive components. The use of electrical power is optimized when the power factor is equal to one.
The power factor is the ratio of the average power required by the load to the apparent power delivered by the source. The power factor thus indicates the ratio of useable power delivered to the load to the amount of power which must be generated at the source. With a low power factor, substantially more line losses occur relative to the useful power delivered to the load, than with higher power factors approaching unity.
When a load requiring a given average power has a relatively high power factor, the power generator need not supply as much excess power to operate the load properly. When the power factor is low, the generator must supply a relatively large amount of excess power, which is accompanied by higher losses in the power lines. The power factor ranges from zero to one depending on how far out of phase the instantaneous voltage and current are at the load. In equation form, the power factor (PF) is expressed as PF=cos (.crclbar..sub.v -.crclbar..sub.i), where .crclbar..sub.v is the phase angle of the voltage and .crclbar..sub.i is the phase angle of the current at a given time.
A typical industrial load may be a bank of induction motors or other expensive machinery. The load requires a certain amount of average power to operate properly. Usually, an industrial load has an overall reactance which is characterized by resistive and inductive components. The power factor in such a load is said to be lagging, since the current through the load lags behind the voltage applied to the load in such a case. This is termed a lagging power factor. In a capacitive load, the current leads the voltage, resulting in a leading power factor.
The apparent power is the product of the root mean square source voltage and the root mean square source current. The apparent power is normally stated in volt amperes (VA) or kilovolt amperes (kVA) in order to distinguish it from average power, which is measured in watts (W).
A low power factor at the load means that the power company's generators must be capable of delivering more current at constant voltage, and they must also supply power for higher line losses than would be required if the power factor of the load were high. Since high line losses represent energy expended in heat and benefit no one, the power company often will insist that a plant maintain a high power factor, typically 0.90 lagging, and will adjust their rate schedule to penalize users that do not conform to this requirement. However, installations that require large amounts of power have a wide variety of load conditions, which may include loads which vary with time.
For instance, consider a single 25 horsepower walking beam oil pump motor. Such motors are used to drive the "see-saw" type oil pumps commonly seen in many oil fields, which are known as walking beam
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Dove Donald C.
Kent Howard D.
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