In short. Nonlinear loads inject harmonic currents; those currents create voltage distortion across the source impedance. A generator has higher impedance than the utility, so the same loads distort voltage more. A study measures or models the injection, scans the network for resonance and compares results with limits.
Formula and physics
Vh = Ih × Zh
Source impedance rises with harmonic order, so higher harmonics see higher impedance. On a generator the subtransient reactance sets Zh.
Resonant order h ≈ √(Ssc ÷ Qcap)
If a capacitor bank resonates near a harmonic order that loads inject, the voltage and currents magnify and can damage equipment.
Study steps
- Identify harmonic sources: drives, UPS rectifiers, LED drivers, welding, arc furnaces.
- Collect injection spectra from datasheets or measurement.
- Model the source: utility, generator and transformer impedances.
- Run a frequency scan to find resonance points.
- Calculate voltage and current distortion at points of interest.
- Compare with limits and select mitigation. See filter studies.
Generator-specific points
- Alternators with lower subtransient reactance give lower distortion, but cost more.
- Harmonic currents heat the alternator damper windings, so derating may apply.
- Pitch (winding span) affects triplen harmonics and neutral currents.
- Generator sizing for UPS loads often follows harmonic limits, not kW. See UPS and nonlinear loads and harmonics.
Acceptance
| Measure | Meaning |
|---|---|
| THD(V) | Voltage distortion at the point of common coupling or bus |
| THD(I) / TDD | Current distortion relative to load or demand current |
| Individual harmonics | Limits on each order |
Illustrative example
A site with a 400 kVA generator feeds a 200 kVA six-pulse UPS. The study finds voltage THD above the UPS input limit when running on generator but within limit on utility. Options: an alternator with lower reactance, a 12-pulse input or an active front end, or a harmonic filter. The cheapest passing option is chosen after cost comparison.
What a harmonic study answers
- How much harmonic current do the nonlinear loads inject?
- What voltage distortion results at each bus, given the source impedance?
- Does any capacitor or cable create resonance at a harmonic frequency?
- Do the distortion levels stay inside limits such as those of IEEE 519 or the project specification?
Voltage distortion ≈ harmonic current × source impedance at that harmonic frequency
Why the generator is the weak point
A generator’s subtransient reactance is several times that of a transformer on a grid, so the same harmonic current creates much larger voltage distortion. Illustratively, a 6-pulse drive load that gives 4 percent voltage THD on the grid can give 12 percent or more on a generator of equal kVA.
Resonance
Resonant harmonic order h ≈ √(kVA short-circuit ÷ kVAr of capacitor)
Example: a 2,000 kVA short-circuit level with a 200 kVAr bank gives h ≈ √10 ≈ 3.2. This is close to the third harmonic and dangerous when harmonic sources exist.
Study cases
| Case | Why |
|---|---|
| Mains supply | Baseline |
| Generator supply | Worst-case distortion |
| Capacitor stages in and out | Resonance scan |
Measures
Passive filters, active filters, 12-pulse or active front-end drives, larger alternators and detuned capacitors. See harmonics and THD and filter studies.
Common mistakes
- Studying the utility case only.
- Adding capacitors without a scan.
- Using THD alone without checking the actual voltage limit.
What to send us
Send the drive and UPS list and any measurements. Send your application details.
Questions engineers ask
Why does my generator show more distortion than the mains?
Its source impedance is higher, so the same currents create larger voltage distortion.
What is resonance?
An inductive and capacitive pair that amplifies a certain frequency.
Do harmonics heat the alternator?
Yes, mainly the damper windings and iron.
Can I measure instead of model?
Measurements help calibrate, but a model is needed for the future case.
This guide gives general principles. Limits come from the applicable standards and equipment.