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Harmonics

Service reliability and quality of power have become growing concerns for many facility managers, especially with the increasing sensitivity of electronic equipment and automated controls. There are several types of voltage fluctuations that can cause problems, including surges and spikes, sags, harmonic distortion, and momentary disruptions. Harmonics can cause sensitive equipment to malfunction and other problems, including overheating of transformers and wiring, nuisance breaker trips, and reduced power factor.

What Are Harmonics?

Harmonics are voltage and current frequencies riding on top of the normal sinusoidal voltage and current waveforms. Usually these harmonic frequencies are in multiples of the fundamental frequency, which is 60 hertz (Hz) in the US and Canada. The most common source of harmonic distortion is electronic equipment using switch-mode power supplies, such as computers, adjustable-speed drives, and high-efficiency electronic light ballasts.

Harmonics are created by these ―switching loads‖ (also called “nonlinear loads,‖ because current does not vary smoothly with voltage as it does with simple resistive and reactive loads): Each time the current is switched on and off, a current pulse is created. The resulting pulsed waveform is made up of a spectrum of harmonic frequencies, including the 60 Hz fundamental and multiples of it. This voltage distortion typically results from distortion in the current reacting with system impedance. (Impedance is a measure of the total opposition—resistance, capacitance, and inductance—to the flow of an alternating current.) The higher-frequency waveforms, collectively referred to as total harmonic distortion (THD), perform no useful work and can be a

significant nuisance.

Harmonic waveforms are characterized by their amplitude and harmonic number. In the U.S. and Canada, the third harmonic is 180 Hz—or 3 x 60 Hz—and the fifth harmonic is 300 Hz (5 x 60 Hz). The third harmonic (and multiples of it) is the largest problem in circuits with single-phase loads such as computers and fax machines. Figure 1 shows how the 60-Hz alternating current (AC) voltage waveform changes when harmonics are added.