Harmonics Explained: Causes, Effects & Mitigation
A clean power supply is a pure sine wave at the supply frequency. Real installations increasingly draw current that is anything but: variable-speed drives, LED lighting, computers, UPS systems and EV chargers all take current in pulses rather than smoothly, and those pulses distort the voltage waveform for everyone on the same supply. The distortion is described as harmonics — multiples of the fundamental frequency riding on the waveform — and it causes overheating, nuisance tripping and equipment failure that looks mysterious until you measure it. This guide explains where harmonics come from, what they do, and how to deal with them.
Where harmonics come from
Any load that does not draw current in proportion to the voltage is a harmonic source. The classic example is a rectifier: it draws current only near the peaks of the voltage wave, producing a pulsed current rich in harmonics. Six-pulse rectifiers, the workhorse of older drives and UPS systems, characteristically produce 5th and 7th harmonics; twelve-pulse arrangements cancel some of those and shift the problem to higher orders. Switch-mode power supplies in IT equipment and LED drivers draw similar pulsed currents, and while each one is small, thousands of them on one installation add up. Even loads that look innocent individually become a harmonic problem in aggregate — which is why modern commercial buildings, full of LED lighting and IT equipment, show distortion levels that would have been unusual a generation ago.
What harmonics do
Harmonics cause losses wherever current flows: extra heating in transformers, motors, cables and capacitors, because higher-frequency currents suffer greater skin effect and because harmonic currents do no useful work while still heating conductors. Transformers are particularly affected — harmonic currents increase eddy-current losses sharply, so a transformer serving a heavily distorted load may need derating or a special K-rated design. Motors suffer torque pulsations and extra heating that shorten insulation life. One effect surprises people: triplen harmonics (3rd, 9th, 15th and so on) add together in the neutral instead of cancelling, so a neutral conductor carrying only unbalanced fundamental current can be overloaded by harmonic current alone — a common cause of overheated neutrals in buildings full of single-phase electronic loads. Capacitors installed for power factor correction can resonate with the supply inductance at a harmonic frequency, amplifying that harmonic dramatically and destroying the capacitors — power factor correction on a distorted supply must be designed with harmonics in mind, not added blindly.
Measuring: THD and the harmonic spectrum
The headline number is total harmonic distortion (THD), the ratio of all harmonic content to the fundamental, quoted separately for voltage and current. Voltage THD describes how distorted the supply waveform is; current THD describes how distorted a particular load's draw is. But THD alone does not diagnose anything — two installations with the same THD can have entirely different problems — so a power quality analyser's harmonic spectrum, showing the size of each harmonic order, is the real diagnostic tool. Measure at the point of common coupling for the supply picture and at individual large loads to find the culprits. The power quality analyzer guide explains what to look for in an instrument for this work, and the power factor correction calculator covers the correction side, with the resonance warning above in mind.
Mitigation options, in order of effort
Start with the source where possible: twelve-pulse or active-front-end drives instead of six-pulse, and equipment specified with harmonic limits. Next, detune power factor correction with series reactors so the capacitor bank cannot resonate at a troublesome harmonic — cheap insurance that should be standard on any distorted supply. Passive harmonic filters, tuned to trap a specific harmonic order, suit stable loads with a dominant harmonic. Active harmonic filters measure the distortion in real time and inject cancelling currents, handling varying loads and multiple harmonics, at higher cost and complexity. For transformers, oversizing or K-rating handles the heating where filtering is impractical. The right answer is usually a combination, sized from measurements rather than rules of thumb, and verified by measuring again after installation — mitigation that is never verified is a hope, not a solution.