In short. A non-linear load such as a UPS rectifier or a variable-frequency drive draws harmonic current, and that current flowing through the alternator’s sub-transient reactance distorts the voltage every other load on the bus sees. The alternator is checked for terminal voltage distortion, extra heating in windings and damper cage, and AVR stability. The remedies are a low-reactance alternator, a larger frame, or harmonic treatment at the load, chosen from the measured current spectrum.
Why a generator reacts differently from the grid
On DISCOM supply the same non-linear equipment usually causes no trouble, because the utility is a stiff source. The impedance behind the point of supply is small, so harmonic current produces little harmonic voltage. A diesel generator is a soft source by comparison. The sub-transient reactance X”d of a typical salient-pole alternator lies between about 10 and 20 percent on its own kVA base, which is several times the effective impedance of a transformer and feeder from the grid. The same harmonic current therefore produces several times the voltage distortion when the facility transfers to the generator.
The symptoms are familiar. The UPS drops to battery on input tolerance alarms. The AVR hunts because the voltage sensing sees a flattened or notched waveform.
How harmonic current becomes voltage distortion
A distorted periodic current can be treated as a fundamental at 50 Hz plus components at integer multiples. A six-pulse rectifier front end produces the 5th, 7th, 11th and 13th harmonics in significant amounts. Each harmonic current meets the alternator’s reactance at that frequency, which rises roughly in proportion to harmonic order.
V_h ≈ I_h × h × X”d
Here V_h is the harmonic voltage at order h, I_h is the harmonic current at that order, h is the harmonic number and X”d is the sub-transient reactance expressed in ohms on the alternator base.
Worked example (illustrative). A 500 kVA alternator with X”d of 14 percent supplies a 200 kVA six-pulse UPS. The UPS draws about 30 percent of its fundamental current as 5th harmonic, which is roughly 83 A against a 5th-order reactance of five times the base reactance. The resulting 5th-harmonic voltage is of the order of 4 to 5 percent, and when the 7th, 11th and 13th are added the voltage THD approaches 7 or 8 percent. The same UPS on a stiff utility supply would produce under 2 percent.
What the alternator suffers
Harmonic currents increase copper losses because resistance rises with frequency through skin effect, and they increase iron losses in the stator core. More importantly, non-triplen harmonics such as the 5th and 7th appear partly as negative-sequence components to the rotor, inducing currents in the damper bars and pole faces that heat the rotor.
| Effect | Mechanism | What to check |
|---|---|---|
| Voltage distortion | Harmonic current through X”d | Predicted THD against UPS and sensitive-load limits |
| Stator heating | Skin effect and additional iron loss | Manufacturer derating curve for the declared spectrum |
| Rotor heating | Negative-sequence and harmonic flux in damper cage | Continuous negative-sequence capability, usually 8 to 10 percent |
| Neutral overload | Triplen harmonics adding in the neutral | Neutral conductor rated at full phase current or higher |
| AVR instability | Distorted sensing voltage | True-RMS three-phase sensing, possibly a sensing filter |
Sizing the alternator for the load you actually have
There is no universal multiplier; the decision depends on the non-linear share, pulse number, input filtering and alternator design, and falls into three cases.
- Non-linear share under about 25 percent of the generator rating, with ordinary six-pulse equipment. A standard alternator at the normal kW sizing usually holds voltage THD within tolerable limits. Confirm with the alternator maker’s distortion estimate.
- Non-linear share between 25 and 60 percent. Choose an alternator with low X”d, typically 10 to 12 percent, which often means one frame size larger on the same engine. This is cheaper and more effective than oversizing the whole generator set.
- Non-linear share above 60 percent, as in a data hall where almost everything passes through UPS. Specify UPS with active front ends or 12-pulse rectifiers with input THD under 5 percent, and still select the alternator with the full spectrum in hand.
The engine is sized on kW and the alternator on kVA and distortion, so it is common for a non-linear application to end up with a larger alternator than the engine strictly needs, in the same way that motor-starting duty often does. How the frame, winding pitch and excitation system influence this is described under generator alternator architecture. The background on reactance figures is in reactance and short-circuit performance, and the wider harmonic picture is covered in harmonics and THD.
UPS and VFD specifics
UPS as a generator load
The generator sees the UPS input, not the UPS output nameplate. After an outage the rectifier carries the IT load plus battery recharge current, which can raise input demand by 10 to 25 percent for the recharge period. The behaviour of the pairing is dealt with in UPS and non-linear loads.
Variable-frequency drives
A VFD removes the direct-on-line starting surge, which helps the engine, but it replaces it with a continuous six-pulse harmonic current. Line reactors of 3 to 5 percent impedance on each drive are inexpensive and cut 5th-harmonic current substantially.
Capacitor banks and resonance
An APFC bank left in circuit on generator supply changes the harmonic impedance of the bus. The capacitance can resonate with the alternator reactance near the 5th or 7th harmonic and magnify voltage distortion sharply. Standard practice is to switch the APFC bank off on transfer to generator, or to use detuned reactors, and to check the point in a harmonic and resonance study when the bank is large.
Verification at commissioning
Measure, do not assume. A power-quality analyser on the generator terminals during a transfer to the real load gives voltage THD, current THD, neutral current and the individual harmonic orders in one record. Compare the figures against the alternator maker’s limits and the UPS input specification. Where the site load is not yet available, a resistive load bank proves the engine and the fundamental voltage regulation only, and a note to that effect belongs in the commissioning record.
Common mistakes
- Sizing the generator on total UPS output kVA and ignoring the input spectrum. The result is a set that passes the load-bank test and then trips the UPS to battery on every real transfer.
- Applying a flat oversizing multiplier such as 1.5 or 2 to everything. This wastes capital on linear loads and may still be insufficient for a heavily non-linear bus with a high-reactance alternator.
- Leaving the APFC bank connected on generator. Resonance can push voltage distortion far above the level the load alone would produce.
- Sizing the neutral at half phase current for a bus full of single-phase electronic loads. Triplen harmonics can take neutral current above phase current and overheat the conductor.
What to send us
Send the alternator nameplate or datasheet with X”d and X2 if you have them, the UPS or drive datasheets showing input current THD and pulse number, and a load list that separates linear from non-linear equipment. A power-quality recording from the existing generator or the utility supply is more valuable than any assumption. If you are planning a new set for a UPS-heavy site, send the proposed single-line diagram as well through the application engineering enquiry.
Questions engineers ask
How much should I oversize a generator for a UPS load?
There is no fixed multiplier. For a six-pulse UPS forming most of the load, selecting an alternator with sub-transient reactance of 10 to 12 percent, often one frame larger, is usually more effective than oversizing the complete set. For UPS with active front ends and input THD under 5 percent, ordinary kW sizing with a modest margin is often enough.
What voltage THD can a diesel generator tolerate?
Most alternator makers accept voltage THD at the terminals of around 10 to 15 percent for heating purposes, but the loads on the bus usually set a tighter limit. Many UPS and electronic loads want under 8 to 10 percent, and sensitive instrumentation may need under 5 percent. Design to the strictest connected load.
Why does my UPS go to battery when the generator takes over?
The commonest causes are input voltage distortion or frequency drift outside the UPS input window, caused by the generator’s higher source impedance and governor response. A larger or lower-reactance alternator, a wider UPS input window where the maker permits it, and a walk-in or recharge current limit on the rectifier normally cure it.
Do VFDs make generator sizing easier or harder?
Both. They remove the direct-on-line starting kVA, which eases the engine and alternator transient duty, but they add continuous harmonic current. Fit line reactors or DC chokes on drives of any size that will run on generator and check voltage THD at commissioning.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.