Modern commercial and industrial sites are no longer powered by simple resistive loads. Variable frequency drives (VFDs), LED lighting systems, uninterruptible power supplies (UPSs), switch-mode power supplies and other electronic equipment have fundamentally changed the nature of electrical demand. These loads introduce harmonic distortion into a power system - and if this factor is overlooked during generator specification, the consequences can range from nuisance tripping and overheating through to premature equipment failure and unreliable backup power when it is needed most.
For organisations investing in diesel generators as part of a standby or prime power strategy, understanding how harmonic distortion affects generator selection is not optional. It is an essential part of getting the specification right from the outset.
In an ideal power system, electrical current flows as a smooth sine wave at the fundamental frequency - 50 Hz in the UK. Harmonic distortion occurs when non-linear loads draw current in a non-sinusoidal pattern. This creates additional frequency components at multiples of the fundamental frequency, known as harmonics, which distort the waveform and introduce additional stress across the electrical system.
Common sources of harmonic distortion found on modern commercial and industrial sites include:
On utility grid supplies, the sheer size of the network means harmonic distortion is absorbed and diluted across a large generation capacity. A diesel generator operates as an isolated power island during a utility failure, meaning it must manage the full impact of harmonic-producing loads without the buffering effect of the wider grid.
When a generator is exposed to significant harmonic distortion, several technical problems can arise if the unit has not been correctly specified to manage those loads:
Alternator heating: Harmonic currents cause additional heating within the alternator windings. This increases thermal stress and can reduce the operational life of the alternator if the generator has not been rated to account for it.
Voltage waveform distortion: The generator's automatic voltage regulator (AVR) attempts to maintain a stable output voltage. Harmonic distortion makes this more challenging, potentially resulting in voltage instability that affects sensitive connected equipment.
Derating requirements: Generators operating with high proportions of non-linear loads typically need to be derated - meaning the effective usable capacity of the generator is lower than its nameplate kVA rating. A unit that appears correctly sized on paper may be inadequate once harmonic derating is properly applied.
Nuisance tripping and instability: Voltage and current distortion can trigger protective relays and circuit breakers unnecessarily, interrupting power to critical systems at exactly the moment reliable backup power is required.
A straightforward load calculation that simply adds up the nameplate ratings of connected equipment will not capture the harmonic characteristics of a site's electrical demand. Two sites with identical total connected loads can present very different challenges to a generator if one operates predominantly resistive loads while the other relies heavily on VFDs and electronic equipment.
Accurate generator specification for sites with significant non-linear loads requires a more detailed engineering assessment. This involves identifying the proportion of non-linear loads within the total electrical demand, calculating the expected total harmonic distortion (THD) levels and applying appropriate derating factors to ensure the selected generator can deliver stable, reliable power under realistic operating conditions.
At EA Power Systems, load analysis goes beyond surface-level calculations. Engineers evaluate the actual electrical characteristics of each site - including the nature, proportion and behaviour of non-linear loads - before recommending a generator specification. This process ensures that the selected unit is genuinely capable of supporting the site's operational requirements rather than simply matching a headline kVA figure.
Not all alternators respond equally to harmonic-rich loads. The selection of the alternator - including its winding configuration, pitch factor and subtransient reactance - has a direct bearing on how well a generator manages non-linear demand. Specifying an alternator with appropriate harmonic tolerance is a critical part of producing a robust generator solution for sites with high proportions of electronic loads.
This is one area where independent multi-brand sourcing offers a genuine advantage. Because EA Power Systems is not restricted to a single manufacturer's product range, equipment can be selected on the basis of technical suitability for the specific harmonic environment of each site. Whether that points toward a Perkins, Cummins, Volvo, Doosan or Baudouin-based solution, the recommendation is driven by engineering merit rather than manufacturer preference.
Where harmonic distortion levels are particularly high, additional mitigation measures may be appropriate alongside correct generator sizing. These can include:
