A plant fits variable frequency drives on its pumps and fans, and the energy bill drops as promised. A year later the transformer runs warm at half its rated load, a neutral conductor shows signs of overheating, and a capacitor bank fails early. The drives did not malfunction. They did exactly what they were built to do.

Where harmonics come from

A drive does not draw a smooth sine wave. Its rectifier front end takes current in short pulses near the peak of each cycle. That pulsed current can be described as the 50 Hz fundamental plus a series of higher frequencies, with the fifth, seventh, eleventh and thirteenth harmonics being the largest on a typical six-pulse drive.

Those higher-frequency currents do no useful work. They still flow through every cable, busbar and transformer winding between the drive and the supply.

Where the energy goes

Transformers heat more. Eddy current losses rise with the square of frequency, so a fifth-harmonic current heats the core far more than the same fundamental current would. A transformer can reach its thermal limit well below its nameplate kVA.

Neutrals overload. Triplen harmonics, the third and its multiples, add together in the neutral instead of cancelling. On a four-wire system with many single-phase electronic loads, the neutral can carry more current than any phase.

Capacitors fail. A capacitor’s impedance falls as frequency rises, so it draws harmonic current preferentially. Worse, a capacitor bank and the transformer inductance can resonate near a harmonic frequency and amplify it. This is why correction in a plant full of drives needs detuning reactors, not plain capacitors.

What to measure before correcting

  • Total harmonic distortion of current at the main incomer, not only voltage
  • Which harmonic orders dominate, since the fix depends on the order
  • Neutral current against phase current on four-wire circuits
  • Whether any existing capacitor bank already has detuning reactors

Harmonics are cheap to measure and expensive to ignore. A power quality survey over a normal working week shows whether the problem is real before anyone quotes for a filter.

Frequently asked questions

What creates harmonics in a plant?

Anything that draws current in pulses rather than smoothly: variable frequency drives, rectifiers, UPS systems, LED drivers, welding sets, induction furnaces. The more of the plant’s load that sits behind power electronics, the higher the distortion.

What level of distortion is acceptable?

IEEE 519 and the utility’s own supply code give the limits, stated separately for voltage and current distortion at the point of common coupling. The figure worth acting on is the one measured at your incomer across a normal working week, not a single spot reading.

Do capacitors make harmonics worse?

They can. A capacitor bank and the supply transformer’s inductance form a resonant circuit, and if that resonance lands near a harmonic your plant produces, the harmonic current is amplified. Capacitors failing early, or fuses blowing with no obvious overload, is the usual sign.

Is a detuned reactor or an active filter needed?

A detuned reactor shifts the resonant point below the lowest significant harmonic, and is the normal answer where the job is protecting the capacitor bank. An active filter injects a correcting current, and is for cases where the distortion itself has to come down, for instance to meet a utility limit. The measurement decides which, not the catalogue.