In short. Non-linear loads such as drives, UPS rectifiers, LED drivers and chargers draw distorted current, which distorts voltage. Generators are more sensitive than the grid because their source impedance is higher. Screen the share of non-linear load against the source and decide whether a study, filtering or a larger alternator is needed.
What harmonics are
Harmonics are currents or voltages at multiples of the fundamental frequency, for example 250 Hz (fifth) and 350 Hz (seventh) on a 50 Hz system. Total harmonic distortion (THD) measures how much of the waveform is not the fundamental.
Typical sources
- Variable-speed drives and soft starters.
- UPS and battery chargers with rectifiers.
- LED lighting and switch-mode power supplies.
- Arc furnaces and welders.
- Inverters for solar and BESS.
Why generators suffer
A generator’s subtransient reactance is much higher than a utility’s equivalent source impedance. The same harmonic current therefore produces a larger voltage distortion. Effects include voltage waveform distortion, overheating of damper windings, AVR misoperation and nuisance trips.
Screening indicators
| Indicator | Concern level (illustrative) |
|---|---|
| Non-linear load as % of generator kVA | Above roughly 30 per cent deserves attention; above 50 per cent needs a study |
| Measured voltage THD at generator terminals | Above limits in the applicable standard, often around 5 per cent |
| Six-pulse drives or older UPS | Higher harmonic current than modern low-harmonic designs |
| Capacitor banks present | Resonance risk |
| Symptoms | Hot neutrals, flickering, tripping of sensitive equipment, voltage regulator hunting |
Limits come from standards such as IEEE 519 and IEC 61000, plus the generator and equipment manufacturers’ guidance.
Mitigation options
- Larger alternator or one with lower subtransient reactance.
- Line reactors or DC chokes on drives.
- 12-pulse or active front-end rectifiers.
- Passive or active harmonic filters. See harmonics and mitigation.
- Detuned capacitor banks.
- Segregating sensitive loads.
When a study is needed
- A large share of non-linear loads on a generator or weak supply.
- Capacitor banks plus non-linear loads.
- Utility limits to meet.
- Symptoms already present.
See harmonic studies and resonance analysis and harmonics and THD.
Worked illustration
A 500 kVA generator supplies a 300 kVA UPS (60 per cent of the generator) with a six-pulse rectifier. Voltage THD at the generator terminals measures 11 per cent, and the AVR hunts. Options are a larger alternator, an input filter or a modern UPS with low input distortion. The measure-first approach found the problem and chose the cheapest fix, a UPS input filter, instead of upsizing the generator.
Quick harmonic risk screen
Nonlinear load ratio = nonlinear load kVA รท source kVA
| Ratio | Risk (indicative) | Action |
|---|---|---|
| Below 15 percent | Low | Routine, measure after commissioning |
| 15 to 30 percent | Moderate | Check alternator, consider filters or low-distortion drives |
| Above 30 percent | High | Harmonic study, oversized alternator or filters |
Generators are more sensitive than transformers on a stiff grid because of higher subtransient reactance.
Worked example (illustrative)
A site has a 500 kVA generator with 200 kVA of six-pulse drives and UPS: ratio 40 percent. High risk. Voltage THD could be 10 to 15 percent without mitigation, depending on drives. Options: active front-end drives, 12-pulse rectifiers, active filter, or alternator of 1.5 times the rating with lower reactance.
Warning signs
- Flat-topped or distorted voltage waveform on generator.
- Overheating alternators or neutral conductors.
- Nuisance trips of sensitive electronics.
- Capacitor failures.
Measure before deciding
A power analyser at the incomer, logged on generator and mains, gives THD and spectrum. See harmonics and THD and harmonic studies.
Common mistakes
- Sizing generators on kVA of the load only.
- Installing capacitors without checking harmonics.
- Measuring only current, not voltage THD at the source.
What to send us
Send the non-linear load list and any measurements. Send your application details.
Questions engineers ask
What is THD?
Total harmonic distortion, the ratio of harmonic content to the fundamental, for voltage or current.
Why do generators suffer more than the grid?
Higher source impedance turns harmonic current into more voltage distortion.
Can a UPS upset a generator?
It can, through input current distortion and step loading. Check the UPS input specification.
Do I need a filter?
Only if measured or studied distortion exceeds limits or causes problems.
Thresholds are illustrative. Use applicable standards and manufacturer guidance.