What are harmonics in electricity?

 Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency, which in the U.S. is 60 Hertz. If the first fundamental frequency is 60 Hz, then the second is 120 Hz, and the third is 180 Hz. Here are a few examples of issues that might be related to harmonics.

• Flickering lights are a common symptom of a power quality issue. A potential source of flickering lights is machinery with rapid fluctuations in load current or voltage. These machines include large motors when they are starting up, machinery with cyclo-converters such as rolling mill drives and mine winders, as well as machines that use static frequency converters such as AC motors and electric arc furnaces.
• Overheated transformers and tripped breakers could be a sign of harmonic issues, which occur when non-linear loads that draw current in abrupt pulses, rather than in a smooth sinusoidal manner, cause harmonic currents to flow back into other parts of the power system.

Total Harmonic Distortion

The state of the harmonics in the system can be expressed in many ways and the first is the Total Harmonic Distortion or THD. The THD is the sum of all the harmonic effects; usually this is measured up to the 50th multiple of the fundamental frequency of the power system (60 Hz), at 3kHz or according to some guidance the 40th multiple (2.4kHz). This value of THD in terms of power quality health is most often applied to the voltage. Guidance states that the voltage harmonic effects should be less than 8 percent relative to the fundamental. Values above the stated 8 percent should be investigated further.

The first level of investigation would be to identify the percentage of each individual harmonic, 2nd, 3rd, 4th, 5th—up to 50th. This is indicated either live on a measurement instrument or on a chart from logged and downloaded data—this is visualized as a “harmonic spectrum.”