Power system harmonics

Power system harmonics is an area that is receiving a great deal of attention recently. This is primarily due to the fact that non-linear (or harmonic producing) loads are comprising an ever-increasing portion of the total load for a typical industrial plant. Incidence of harmonic related problems is low, but awareness of harmonic issues can help to increase plant power system reliability. On the rare occasions that harmonics are a problem, it is either due to the magnitude of the harmonics produced or a power system resonance.

The term harmonic refers to a component of a waveform that occurs at an integer multiple of the fundamental frequency. Fourier theory tells us that any repetitive waveform can be defined in

terms of summing sinusoidal waveforms which are integer multiples (or harmonics) of the fundamental frequency.

It is important to understand that harmonics are a steady state phenomenon and repeat

with every 50 Hz cycle (60Hz in North America). Harmonics should not be confused with spikes, dips, impulses, oscillations or other forms of transients.

A common term that is used in relation to harmonics is Total Harmonic Distortion (THD). Another common term used is Distortion Factor (DF) which is essentially the same as THD.

Now that we have a basic understanding of the term harmonics, we can move into more detailed aspects such as what causes harmonics? Harmonics are caused by non-linear loads, which are loads that draw a non-sinusoidal current from a sinusoidal voltage source. Some examples of harmonic producing loads are electric arc furnaces, static VAR compensators, inverters, DC converters, switch-mode power supplies, and AC or DC motor drives. In the case of a motor drive, the AC current at the input to the rectifier looks more like a square wave than a sine wave.
How do harmonics affect our power system? Power system problems related to harmonics are rare but it is possible for a number of undesirable effects to occur. High levels of harmonic distortion can cause such effects as increased transformer, capacitor, motor or generator heating, incorrect operation of electronic equipment (which relies on voltage zero crossing detection or is sensitive to wave shape), incorrect readings on meters, incorrect operation of protective relays, interference with telephone circuits, etc. The likelihood of such ill effects occurring is greatly increased if a resonant condition occurs. Resonance occurs when a harmonic frequency produced by a non-linear load closely coincides with a power system natural frequency. There are 2 forms of resonance which can occur: parallel resonance and series resonance.