Glossary term
Harmonic Distortion
Distortion caused by signal or power components at integer multiples of a fundamental frequency.
Definition
metricHarmonic distortion is waveform distortion produced by harmonic components at integer multiples of the fundamental frequency.
In an ideal sinusoidal AC system, voltage or current contains only the fundamental frequency. Nonlinear loads and nonlinear devices introduce additional frequency components at 2, 3, 4, and higher integer multiples of the fundamental. The resulting waveform departs from a pure sine wave. Harmonic distortion is commonly quantified with total harmonic distortion (THD), individual harmonic magnitudes, and spectral analysis.
Harmonic distortion is the departure of a waveform from a pure sinusoid because energy appears at frequencies that are integer multiples of the fundamental. In a 50 Hz power system, the 3rd harmonic is 150 Hz, the 5th is 250 Hz, and the 7th is 350 Hz. In signal and audio systems, the same principle applies: a nonlinear device driven by a tone generates additional tones at multiples of the original frequency.
The most common summary metric is total harmonic distortion:
where X_1 is the RMS value of the fundamental component and X_n are RMS harmonic components. The result is usually reported as a percentage. Voltage THD and current THD must be distinguished because their causes and consequences differ.
Sources
Harmonics are produced by nonlinear behaviour. In power systems, major sources include rectifiers, variable-speed drives, switched-mode power supplies, saturated transformers, arc furnaces, LED drivers, inverters, and other power electronic converters. These devices draw current in pulses or switch rapidly, creating harmonic current that flows through network impedance and creates harmonic voltage distortion.
In electronics and communications, harmonic distortion can come from amplifier nonlinearity, clipping, mixer behaviour, magnetic saturation, quantization, or insufficient filtering. In mechanical and acoustic measurement, harmonics may indicate nonlinear stiffness, impacts, rubbing, or sensor saturation.
Effects
Harmonic distortion can overheat transformers, motors, cables, neutral conductors, capacitors, and filters. It can cause nuisance trips, measurement errors, torque ripple, acoustic noise, vibration, reduced power factor, and interference with control or communication circuits. In grids with high converter penetration, harmonics can interact with resonance conditions and protection settings.
Triplen harmonics, especially the 3rd, are important in three-phase four-wire systems because they can add in the neutral conductor rather than cancel. High neutral current can overheat conductors even when phase currents appear acceptable.
Measurement and mitigation
Harmonic analysis is usually performed with Fourier methods, power-quality analyzers, oscilloscopes, or spectrum analyzers. Correct measurement requires adequate sampling rate, windowing, synchronization, bandwidth, and RMS definitions. Reporting only THD can hide a dangerous individual harmonic, so engineering reports often include both total and per-harmonic limits.
Mitigation methods include passive filters, active filters, multi-pulse rectifiers, line reactors, transformer phase shifting, better converter modulation, power-factor correction designed for harmonic environments, and system impedance management. A common mistake is adding capacitors for power-factor correction without checking harmonic resonance; this can amplify distortion rather than reduce it.