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Effects of Harmonics in Power Factor Measurement
According to a special report by Power Magazine you should include the effects of harmonics in power factor measurement. Power factor, the ratio of active power to apparent power, is a familiar concept in power-system management. It determines how much energy–both work-producing (watts) and reactive (VArs)–is required to power a load. The relationships are shown clearly in the classic power-factor triangle.
But for many applications, the classic triangle is oversimplified. That’s because it does not take into account the effects of harmonic voltages and currents found in today’s power-distribution systems. Harmonics add a third dimension to the classic power-factor triangle, thereby increasing the apparent power required to do a particular amount of work. The presence of harmonics requires that you change the way you think about–and the way you measure–power factor.
When active power is divided by apparent power in the presence of harmonics, the result is known as total power factor (PF). The component of power factor not contributed by harmonics is known as displacement power factor (DPF). Note that PF and DPF are equal in completely linear circuits–such as a 208-V, 3-phase induction motor operating a blower–but are different in non- linear circuits, for example a variable-frequency drive controlling cooling-tower fans.
O&M personnel should understand three practical effects of the PF/DPF definitions: (1) The difference between PF and DPF readings is proportional to the degree of harmonics in the power distribution system; (2) a power meter must provide both PF and DPF readings in order to effectively troubleshoot systems with harmonics; and (3) manufacturers of nonlinear equipment often provide only a single power-factor specification for their equipment, and it may be unclear whether the specification refers to PF or DPF.
If PF and DPF differ by a factor of 10% or more, the difference is probably caused by harmonics. The degree of difference may also suggest a course of action, depending on the types of loads in the system.
Case 1: Predominantly linear systems. When PF and DPF are essentially the same value, motors or other linear loads dominate the circuit. In this case, low power factor can be compensated for with kVAr correction capacitance. Use caution in diagnosing problems involving both low power factor and harmonics, because kVAr capacitors may be only part of the solution. Even in systems with low levels of harmonics, kVAr capacitors applied improperly can cause resonant conditions that can lead to overvoltages.
Case 2: Predominantly nonlinear systems. When PF is significantly lower than DPF correct low power factor by applying line reactors directly to the sources of harmonic current or by using kVAr capacitor networks with series inductors to limit harmonic current in the capacitors. Always exercise caution in the use of kVAr correction capacitors and compensating filters to avoid resonance problems at harmonic frequencies and consult the capacitor manufacturer or an expert in filter design.
Case 3: Systems with kVAr capacitors already installed. When variable-frequency drives are added to existing motors, and kVAr correction capacitors are already installed, DPF can actually be overcorrected, causing current to lead voltage. Without system modifications, these new components might cause instability and overvoltage problems. Under these conditions take readings in the circuit to determine whether it is necessary to remove the kVAr correction capacitors.
Users can measure both PF and DPF with a single meter. The best ones show three views of the measured signal: a numeric reading of signal parameters, a visual display of the waveform, and a view of the entire harmonic spectrum.
Reprinted from Power Magazine, Copyright 1995.
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5044 B U Bowman Drive #102
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PHONE: (678) 546-6780
Last Updated: 07 Jun 2004
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