2 Power factor compensation 1

2 Power factor compensation 1 - Power Factor Compensation...

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Power Factor Compensation Electric loads, whether pure admittances, PVpf loads, or PQV loads, may have power factors that differ from 1. A unity power factor is however desirable for a number of reasons. One is that at a power factor of one, the device consumes only real power, the imaginary power being zero. The other is that with pf=1 the total current drawn by the device is there uniquely for the purpose of doing useful work. A device that consumes (or generates) reactive power requires a current flow, current that the supplying utility must send through its transmission system wires. Even a device that consumes zero watts but a non-zero amount of vars requires some current. This purely reactive current flow, although resulting in zero watt load consumption, nonetheless produces some real losses in the transmission lines leading to the load, in addition to causing network voltage stability problems. The transmission utility therefore has to supply the resulting transmission losses and must provide voltage stability resources, all of which cost money. Consequently, large electricity consumers who use up or generate vars are charged a rate for both reactive and real power, one in c/kVAr-h and the other in c/kWh. It is therefore economically important for consumers to adjust their loads to reduce their var requirements as much as possible, or equivalently to bring their power factor close to unity. Power Factor Compensation in Admittance or Impedance Loads If the load behaves as an impedance with ZRj X =+ , then the consumer can add a compensating device in series whose impedance is cc Zj X = . The combined impedance is then () ZZ Rj XX += + + , which if c =− , becomes purely resistive and therefore has unity power factor. The phasor diagram of the above load compensated to obtain a pf=1, could be denoted by, X X = V + I ' V + I V ' V c jX I
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Note that in the above phasor diagram, the angle between the voltage, V , and the current, I , is zero, which corresponds to a pf of unity. Note also that an alternative phasor diagram when the original load is inductive is, If the desired pf is not unity, but something less than unity, then the phasor diagram would be as shown below, first for a case of final lagging power factor, then, for the case of a leading power factor. In both cases, the original power factor is assumed to be leading.
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This note was uploaded on 01/27/2011 for the course ECSE 361 taught by Professor Franciscodgaliana during the Spring '09 term at McGill.

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2 Power factor compensation 1 - Power Factor Compensation...

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