Harmonic Distortion (THD)
Harmonic distortion is the deformation of the voltage/current waveform by components at multiples of the 50 Hz fundamental — injected by electronic loads like VFDs, UPS and inverters — quantified as Total Harmonic Distortion (THD).
Why it matters on an MEPF project
Modern plants are wall-to-wall electronics, and every rectifier front-end pays its power back in harmonics. The symptoms are famously misdiagnosed: neutral conductors running hot on triplen harmonics, capacitor banks failing young (or resonating), transformers derating thermally, sensitive electronics resetting, and meters/relays behaving oddly. "Power quality problem" is usually this, measurable in an afternoon.
Management is measurement-first: a power-quality logger at the incomer and major boards identifies the spectrum and the sources. Fixes ladder up — line reactors on drives, detuned PF banks (essential once drives are significant), active/passive filters for hard cases, and oversized neutrals in triplen-rich distributions. IEEE 519 is the commonly referenced benchmark for acceptable distortion at the point of connection.
How it's specified in practice
| Parameter | Typical / working position | Governing reference |
|---|---|---|
| Sources | VFDs, UPS rectifiers, LED drivers, solar inverters, welders | load electronics |
| Signature symptoms | Hot neutrals, capacitor deaths, nuisance trips, audible transformers | field diagnosis |
| Measurement | THD-V / THD-I logged at incomer + key boards | IEEE 519 commonly referenced |
| Mitigation ladder | Reactors → detuned banks → filters; neutral sizing | power-quality engineering |
Common mistakes
- Adding plain PF capacitors into a distorted network — resonance roulette.
- Blaming the DISCOM before logging your own spectrum.
- Half-sized neutrals in triplen-heavy lighting/IT distributions.
- Fixing a symptom board while the source drive stays unfiltered.
Related on this site
One of 68 working definitions in the MEPF & solar engineering glossary. The buying side of the same decisions is in the buyer's guides; the numbers are in the calculators.
Frequently asked
What THD level is acceptable?
Commonly benchmarked against IEEE 519-style limits at the point of common coupling — but the practical question is whether your equipment misbehaves. Measure, compare, and mitigate where symptoms and limits agree.
Do solar inverters cause harmonics?
Modern inverters are clean by specification, but plant-level interactions (many inverters, long cables, existing distortion) merit a post-commissioning PQ check — we include one on solar projects with significant drive populations on the same bus.
Standards referenced: IEEE 519 (commonly referenced) · CEA supply-quality context