In short. Harmonics are currents and voltages at multiples of the supply frequency, created by nonlinear loads. They heat transformers, cables and neutrals, distort voltage and upset generators. Measure first, then mitigate with reactors, filters or better equipment choice.
Physics to effect
Nonlinear load → distorted current → current flows through source impedance → distorted voltage → heating, nuisance trips, resonance. Generators have higher source impedance than the grid, so the same load distorts a generator more. See harmonics and THD.
Common sources
- Variable frequency drives.
- UPS rectifiers. See UPS and nonlinear loads.
- LED drivers, computers and chargers.
- Welders and arc devices.
Effects
| Equipment | Effect |
|---|---|
| Transformers | Extra heating; derate or use K-rated |
| Neutrals | Triplen harmonics add in the neutral |
| Capacitors | Resonance risk. See APFC |
| Generators | Voltage distortion and rotor heating |
| Electronics | Mis-operation and trips |
Mitigation options
- Line reactors or DC chokes on drives.
- Passive filters tuned for dominant harmonics.
- Active harmonic filters for varying spectra.
- Multi-pulse or active-front-end rectifiers.
- Oversizing or derating the source.
See harmonic studies and filter studies.
Measure
Log current and voltage THD and the spectrum at the point of common coupling and at the generator. Compare against agreed limits.
Illustrative example
A site with many drives sees generator voltage THD rise during runs. Line reactors on the drives and an upstream filter reduce THD and stop random trips. Before-and-after measurements prove the result.
Measuring harmonics properly
- Use an analyser rated for harmonic measurement up to a suitable order.
- Measure at the point where the generator or transformer connects to the loads.
- Record at light, typical and peak load.
- Record voltage and current distortion separately.
- Note the dominant orders, such as the 5th, 7th and 11th for six-pulse rectifiers.
Why generators suffer more
Voltage distortion ≈ Harmonic current × Source reactance at that harmonic
A generator’s subtransient reactance is much higher than a transformer’s, so the same harmonic current produces more voltage distortion. For a set feeding mostly drives or UPS, the voltage can be distorted enough to upset other loads. This is why generator sizing for nonlinear loads often includes extra margin or filtering. See UPS and nonlinear loads.
Mitigation comparison
| Method | Strength | Limitation |
|---|---|---|
| Line reactors | Simple, low cost | Partial reduction |
| Passive tuned filter | Targeted at specific orders | Needs study; can resonate |
| Active filter | Adapts to varying spectrum | Higher cost |
| Multi-pulse or active front end | Reduces at source | Equipment choice at purchase |
| Oversized source | Lowers source impedance effect | Cost and space |
Neutral and triplen harmonics
Single-phase electronic loads produce third-harmonic currents that add in the neutral instead of cancelling. In some offices the neutral current can exceed the phase current. Size neutrals and transformers with this in mind.
Verification
After mitigation, repeat measurement under comparable load. Keep a before-and-after table. Compare against the limits agreed with the utility or defined in the standard you follow.
The master engineering question
Which loads produce the harmonics, which equipment suffers, and where is the best place to treat them?
What to send us
Send single-line diagram and measurements. Send your application details.
Questions engineers ask
Is THD always bad?
High THD at a weak source can harm; small at a stiff source may be fine.
Do stabilisers fix it?
No.
Why is a generator more sensitive?
Higher source impedance converts current distortion to voltage distortion.
Passive or active filter?
Depends on spectrum stability and budget; study it.
Values are illustrative. Follow applicable standards.