Harmonics / Voltage Distortion

Voltage distortion is any deviation from the nominal sine waveform of the AC line voltage. A similar definition applies for current distortion; however, voltage distortion and current distortion are not the same. Each affects loads and power systems differently, and each requires a different solution.

In discussing harmonic distortion we must also address several other parameters involved with distortion. Namely, what is a harmonic and how do we quantify it?

Fourier Analysis allows us to break down a distorted waveform into a set of sine waves with certain characteristics. The first characteristic deals with frequency. The distorted waveform repeats itself with some basic frequency. The sine wave associated with this frequency, which is usually 60 Hz, is called the fundamental. Each successive sine wave, or harmonic, of this particular set has a frequency that is an integer multiple of the fundamental. So, the 2nd harmonic has a frequency of 120 Hz, the 3rd is at 180 Hz, the 4th at 240 Hz, and so on.

The next characteristic is magnitude, also called the harmonic distortion factor. Each of these sine waves may have a different magnitude from others depending on the actual distorted signal. The magnitude is determined by a harmonic analyzer. Typically, the magnitude of each harmonic is represented as a percentage of the RMS value of the fundamental, not the total RMS of the distorted waveform. For example, let's say a voltage waveform has an RMS value of 120 volts, but the fundamental is only 100 volts. If the magnitude of the 3rd harmonic is 25 volts, then it has a 3rd harmonic distortion factor of 25 divided by 100, or 25%.

The aggregate effect of all harmonics is called the Total Harmonic Distortion, or THD. THD equals the RMS value of all harmonics divided by the RMS value of the fundamental, usually converted to a percentage.