Guides for the Plant Engineer
Power Factor and Harmonics
Another way to state this definition is to say that PF is a measure of the efficiency of utilization of a power distribution system. The closer PF is to unity, the more of the electrical system capacity is being used to do useful work.
With linear loads, PF depends on the phase relationship between the current sine wave and the voltage sine wave. When these two waves are in phase, PF is unity and there is no system capacity wasted.
Linear loads such as resistance heaters and incandescent lights are 100% efficient in converting real power into heat and have a PF of unity.
Induction motors, in addition to real power, require reactive power, measured in kilovolt amps reactive (kVAR). The reactive current that flows in the system creates a magnetic field that enables the motor to operate, but does not contribute to the work done by the motor.
The PF for a system powering only linear loads is called the "displacement power factor" (the "DPF" on your meter). Unless the loads are pure resistance, this PF will be less than unity.
If harmonic currents are introduced into a system, the true power factor is always lower than the displacement power factor. For example, the displacement power factor for a computer is close to unity, usually about 0.95, whereas the true power factor, which includes harmonics, is around 0.7. For both linear and non-linear loads, the result of extra current that does no real work, whether reactive current or harmonic current, is a reduced capability for the system to support useful loads. In effect, much of the transformer, wiring, and circuit breaker capacity is wasted carrying reactive or harmonic currents.
Many utilities make the customer pay for wasted system capacity by charging a "power factor penalty," which is based on the numerical power factor. The further this number deviates from unity (the more system capacity being wasted), the larger the penalty.
If your utility is measuring true power factor and charging you accordingly, it could benefit you to investigate harmonic mitigation to increase your power factor and reduce the penalties.
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