In short. Harmonics are currents at multiples of 50 Hz drawn by non-linear loads such as UPS rectifiers, variable-speed drives and LED drivers. Total harmonic distortion, THD, expresses their size relative to the fundamental. A diesel alternator has three to five times the source impedance of a utility transformer, so the same harmonic current produces far more voltage distortion on generator. The remedies are a low-reactance or oversized alternator, low-harmonic loads, or filtering.
What harmonics are and where they come from
A linear load such as a heater or an induction motor draws a sinusoidal current. A non-linear load draws current in pulses near the voltage peaks where its rectifier conducts, and that waveform is a 50 Hz fundamental plus harmonics at 150, 250, 350 Hz and so on. Six-pulse rectifiers in older UPS systems and drives produce mainly the 5th and 7th harmonics. Single-phase electronic loads such as computers and LED drivers produce strong 3rd harmonics, which add in the neutral rather than cancel.
THD = (square root of the sum of the squares of all harmonic components, divided by the fundamental component) x 100 percent
Here the components are RMS values of either voltage or current at each harmonic order above the first, and the fundamental is the 50 Hz component. Current THD and voltage THD are different quantities and must always be labelled. A load with 30 percent current THD on a stiff grid may produce only 2 percent voltage THD; the same load on a generator can produce 10 percent or more. State the measurement point and operating condition with any THD figure.
Why generators suffer more than the grid
Voltage distortion is harmonic current multiplied by source impedance at each harmonic frequency. A DISCOM distribution transformer has an impedance of around 4 to 6 percent on its rating with a high fault level behind it, so the grid is stiff. A diesel alternator’s sub-transient reactance, which governs its response to harmonic currents, is typically 10 to 20 percent on its own rating, and the alternator is usually sized closer to the load than the transformer is. The source impedance seen by the harmonics is therefore several times higher, and voltage THD rises in proportion. A plant that is unremarkable on grid can see flat-topped voltage and nuisance trips the moment it transfers to generator.
What harmonics do to a generating set
| Effect | Mechanism | Symptom on site |
|---|---|---|
| Alternator heating | Extra copper and iron losses, induced currents in the rotor damper cage | Winding temperature alarms at loads well below rating |
| Voltage distortion | Harmonic current through alternator reactance flattens the waveform | UPS rejects input, drives trip on DC bus ripple, electronic timers misbehave |
| AVR sensing error | Average or peak sensing AVRs misread distorted waveforms | RMS voltage drifts, instability, hunting |
| Neutral overload | 3rd harmonic currents add in the neutral | Neutral hotter than phases, neutral-earth voltage, nuisance earth fault trips |
| Resonance | Power factor capacitors and alternator reactance form a tuned circuit near a harmonic order | Capacitor failures, severe voltage distortion at one order |
Resonance is often missed. A capacitor bank that is harmless on grid can resonate with the alternator’s reactance at the 5th or 7th harmonic and amplify distortion. Any correction left in service on generator must be checked for this, as discussed in the power factor guide.
Limits that are commonly used
There is no single statutory THD limit for a private generator bus in India, so projects adopt recognised references. IEEE 519 sets voltage THD at the point of common coupling to 5 percent for systems below 1 kV, with individual harmonics below 3 percent, and current limits that depend on the ratio of short-circuit current to load current. IEC 61000-2-4 class 2 allows 8 percent voltage THD in industrial plant. Alternator manufacturers state what their machines accept, often as a maximum load current THD. A working target of 5 percent voltage THD at the generator bus and 8 percent at the worst sub-board is defensible for most sites.
Sizing the alternator for non-linear load
The traditional rule was to size the alternator at 1.5 to 2.5 times the kVA of a six-pulse UPS or drive load. Modern practice is more selective. Establish the true current THD of each non-linear load at its operating point from the manufacturer; an IGBT front-end UPS may draw under 5 percent, a six-pulse drive 30 to 40 percent, a six-pulse UPS with passive filter 7 to 10 percent. Then select an alternator with sub-transient reactance below about 12 percent, a 2/3 pitch winding that suppresses the 3rd harmonic, and class H insulation run at class F temperature rise for thermal margin. Oversizing the alternator alone by one or two frames is usually enough for mixed loads and does not raise the engine’s minimum load. The alternators and non-linear loads guide gives the manufacturer’s view, and the UPS and non-linear loads guide covers the load side.
Reducing the harmonics at source
Where the alternator cannot be changed, reduce the current distortion with active front-end or 12-pulse rectifiers on new equipment, 3 to 5 percent line reactors on existing six-pulse drives, or an active harmonic filter at the sub-board. Passive tuned filters must be reviewed for resonance with the alternator just as capacitors are.
Measuring and interpreting THD
Measure with a power quality analyser that records voltage and current THD, the spectrum to at least the 25th order and neutral current, at the generator terminals and the main non-linear sub-boards, on grid and on generator, at light and normal load. A voltage THD that rises from 2 percent on grid to 9 percent on generator confirms a source impedance problem; high current THD with acceptable voltage THD means the alternator is coping but its heating margin should be checked. Keep the spectra with the commissioning records.
A worked example (illustrative)
An IT park building in Gachibowli has a 500 kVA set feeding a 200 kVA six-pulse UPS without input filter, 150 kVA of HVAC drives and lighting. On grid the main board shows 2.5 percent voltage THD. On generator the UPS alarms on input quality and two chiller drives trip; measurement shows 11 percent voltage THD at the generator bus with a dominant 5th harmonic, on a standard alternator with sub-transient reactance near 16 percent. Two remedies were costed: a 650 kVA low-reactance alternator, or 5 percent line reactors on the drives and an input filter on the UPS, estimated to bring current THD at the bus from about 28 percent to under 12 percent and voltage THD to around 5 percent. The load-side option cost less and also improved power factor on grid. The decision used measured spectra and the alternator manufacturer’s harmonic data.
Common mistakes
- Quoting a THD figure without saying whether it is voltage or current or where it was measured, so no one can tell whether the alternator or the load is at fault.
- Sizing the alternator on the UPS output kVA, which ignores input distortion and battery recharge current and leaves the alternator hot and the waveform flat.
- Leaving power factor capacitors or passive filters in circuit on generator without a resonance check, which can amplify a harmonic rather than absorb it.
- Assuming the grid measurement represents generator operation, so a plant that is clean on DISCOM supply trips on the first outage.
What to send us
Send a list of the non-linear loads with make, rating and rectifier type for each UPS and drive, the generator and alternator nameplates and the alternator data sheet if you have it, any power quality recordings on grid and on generator, and the symptoms, such as UPS alarms, drive trips or overheating. A photograph of the main board showing the neutral conductor and any capacitor bank is also useful. Send your application engineering enquiry.
Questions engineers ask
What THD is acceptable on a generator?
A common working target is 5 percent voltage THD at the generator bus, in line with IEEE 519 for low-voltage systems, and up to 8 percent at industrial sub-boards. The alternator manufacturer’s harmonic capability and the input tolerance of UPS and drives set the real limits for each site.
Why does my UPS work on grid but alarm on generator?
The generator’s source impedance is several times higher than the grid transformer’s, so the UPS rectifier’s harmonic current distorts the generator voltage far more. The UPS sees a flat-topped waveform outside its input tolerance and goes to battery.
How much should I oversize the alternator for UPS load?
For older six-pulse UPS without filters, 1.5 to 2 times the UPS kVA was traditional. For modern low-harmonic UPS with input THD under 5 percent, little oversizing is needed beyond the recharge allowance. Establish the actual input THD and ask the alternator manufacturer for the permitted non-linear proportion.
Does a 2/3 pitch winding matter?
Yes for single-phase electronic loads. It largely cancels the 3rd harmonic voltage and reduces neutral current. Most industrial alternators use it; confirm it on the data sheet.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 19 October 2026.