The Facts of Power Factor

A load or circuit with significant loads that have rectified inputs will most likely have a true power factor (W/VA) that is less than displacement power factor (cosine of angle between voltage and current).

Power factor is another parameterthat is affected by power quality phenomena, particularly distortion and imbalance. This creates even more confusion about the term "true power factor." Power factor is a measure of how efficiently a load uses the electricity or how much energy is consumed by the load versus how much the electricity provider must deliver. This has been defined as real power divided by apparent power, watts / volt-amperes, or W / VA.

Until the proliferation of rectified input type loads (also called switching power supplies, or electronic or nonlinear loads), most electrical loads were resistive and/or inductive loads, such as heaters, incandescent lights and electrical motors. Whereas the voltage and current may not have been exactly in phase, both were nearly sinusoidal in their waveshapes, having only fundamental frequency components present. Hence, real power being equal to Vrms * Irms * cosine of the angle between V & I (called theta) and apparent power being Vrms * I rms reduced down to Power Factor equal to Cos (angle theta). The electric utility or generator must supply the apparent power, since different loads on the circuit use power differently. People then assumed that this was the "real" formula for PF, and revenue meters used such for billing purposes. Since real power will typically be less than apparent power, the billing or rate structures often include a penalty factor to recoup the difference between the real power consumed and the apparent power that is delivered.

As rectified input loads began to become the norm, the current waveshape in particular lost its sinusoidal shape, becoming rich in other harmonic frequency components. SCR-gated loads conducted current only during part of the voltage waveform. Even if the fundamental frequency components were in-phase, the real power was no longer just the Vrms * I rms * cos (theta), since each of the harmonic voltages and currents could have a different value for theta. Not surprisingly, watts become a lower value compared to apparent power, since the purpose of the rectified input switching power supplies and SCR-gated loads was to reduce the real power being consumed. But the apparent power, Vrms * Irms, was still the same. So, surprising as it may be to some people, the power factor become smaller.

In one example, a utility customer owed a PF penalty after replacing an old electromechanical meter with one that called PF with the traditional W/VA method. The customer refused to pay since the loads hadn't changed, and the utility ended up going back to the manufacturer to complain that the meters didn't work right. They worked fine; they were just providing power factor as originally defined, W/VA, instead of the displacement power factor version. To pacify the utility and ultimately the customer, the manufacturer changed the meter back to output displacement power factor.

The figures below show the current harmonic distortion for an office complex with significant rectified input loads, such as computers, printers, fax machines and the HVAC system. This nearly 60 percent distortion contributes to the true power factor being around -0.83, while the displacement power factor is much closer to unity, at -0.95. EC

BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.


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