There’s a tendency to think of energy on the power lines in terms of its fundamental 60- or 50-Hz frequency—the way the voltage is supposed to be created with the turbines and generators at the power house. Sure, the current lags the voltage if there’s a reactive load. That’s “power factor,” right? But isn’t it still a matter of “real” and reactive components at 50 or 60 Hz? Yes and no. Unfortunately, that conceptualization is a bit oversimplified.
In power distribution, power-factor correction (PFC) has traditionally been understood in terms of adding (in general) capacitive reactance at points in the power distribution system to offset the effect of an inductive load. One could say “reactive” load, but historically, power engineers have been most concerned with motors as loads when dealing with power factor. Correction could take the form of a bank of capacitors or a “synchronous condenser” (an unloaded synchronous motor).
More broadly, PFC can also be needed in any line-powered apparatus that uses ac-dc power conversion. These applications can range in scale from battery chargers for portable devices to big-screen TVs. Cumulatively, their input rectifiers are the largest contributor to mains-current harmonic distortion.
Where does that harmonic distortion come from? One common misconception is that switching regulators cause harmonic power-factor components. Actually, they’re produced in the typical full-bridge rectifier and its filter capacitor, aided and abetted by the impedance of the power line itself.
In the steady state, the supply draws current from the line when the input voltage exceeds the voltage on the filter capacitor. This creates a current waveform that includes all the odd harmonics of the power-line frequency (Fig. 1).
Once the voltage crosses that point, the current is only limited by the source impedance of the utility line as well as by the resistance of the diode that is forward-biased and the reactance of the capacitor that smoothes out the dc. As the power lines exhibit non-zero source impedance, the high current peaks cause some clipping distortion on the peaks of the voltage sinusoid.
Harmonics get to be considered elements of power factor because of their relationship to the power-line frequency. As Fourier components, they cumulatively represent an out-of phase current at the fundamental frequency. In fact, one broad definition of power factor is:
where THD is total harmonic distortion.
The Problem with Power Factor
Whatever the cause, what’s actually so wrong with power factors less than unity? Part of the problem is economic. Another part has to do with safety. Whatever their phase relationships, all those superposed harmonic currents create measurable I2R losses as they’re drawn from the generator, through miles of transmission and distribution lines, to the home or workplace.
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