Harmonics

Event

Harmonics are sinusoidal voltage or current waveforms having frequencies that are some integer multiplier higher than the designed supply frequency (fundamental component, usually 50-Hz or 60-Hz). Harmonics combine with the fundamental voltage or current and result in waveform distortion. (Example)

Cause

Non-linear loads on the system that inject harmonic current into the power system usually cause harmonics. The voltage distortion results as these currents cause nonlinear voltage drops across the system impedance. Some of these non-linear loads are adjustable speed drives, computers, DC drives and battery chargers.

Problem

Harmonics may cause overheating of transformers, rotating equipment, and current-carrying conductors, as well as fuses. Deteriorated electrical system components, and metering inaccuracies can also result. The distortion can cause the miss-operation of PLC’s and other electronic equipment.

The major concern is overheating of transformers along with parallel and series resonance.

Monitoring

Harmonics may be easily seen on an oscilloscope with a minimum scanning frequency of 5-kHz or on a harmonics analyzer. Most modern electric power recording equipment can provide a measure of harmonic level such as Total Harmonic Distortion (THD), expressed as a percentage. The highest harmonic order measurable by an instrument is determined by the sample rate per cycle. For example a 128 samples per cycle (7.68 kHz for 60Hz power systems) can read up to the 63rd harmonic.

Mitigation

Removal or isolation of in-house non-linear sources, transformer configuration, passive or active filters, derating transformers, isolation transformers, line reactors

Best Source(s) of Information

IEEE 519