Harmonic distortion from nonlinear loads: heat, trips, and confused meters
A linear load at 60 Hz draws current that resembles voltage. A nonlinear load conducts in pulses: the rectifier and capacitor inside a variable-frequency drive, a switched-mode supply, or a LED driver turn on near the voltage peak. Those pulses decompose into harmonics — integer multiples of the fundamental. Transformers, neutrals and breakers still have to carry the true rms. A conversation that only cites kW will miss heat and false trips.
How the investigation is actually run
Plan the logging window against the process, not against the electrician’s shift. Capture at the transformer secondary first so you know the source impedance the plant actually has, then move downstream to the feeder that overheats or trips. Keep a written list of which capacitor banks and filters were in service. If a drive is the suspect, one capture at its line terminals during a speed change is worth more than a day of idle spectra. Photograph nameplates so the report does not mix frame sizes.
When you write the conclusion, separate thermal risk, mis-trip risk, and voltage-notch risk. They are not the same mitigation. A reactor that helps a six-pulse front end will not cool a triplen-loaded neutral. A filter specified from a screenshot without a mechanism will become another resonant actor. The report should also say what you did not measure — weekend load, a second transformer, a generator source — so the next visit is not a repeat of folklore. Harmonics work is slow on purpose: the waveform has to belong to a named operating state.
Total harmonic distortion is a ratio: how much of the rms is not the fundamental. Voltage THD and current THD are not interchangeable. Stiff sources can absorb ugly current with little voltage movement; a weak transformer or a long feeder can notch voltage enough to upset other equipment. Name which quantity you measured, where, and under which process state.
Symptoms
Overheated neutrals on three-phase four-wire feeders with single-phase IT and lighting are a triplen signature: 3rd, 9th and similar orders are in phase on all three lines and add in the neutral. Dry-type transformers that run hot at modest kVA, audible magnetostriction, or temperature trips with “light” load belong in the same file. Breakers that open while a cheap clamp reads below the handle rating often include harmonic heating of the thermal element — see nuisance breaker tripping.
Instruments that assume a sine wave mis-report power factor, demand, and sometimes direction of energy. An UPS on a distorted input may transfer or derate; that is a critical-power problem as much as a spectrum problem. See UPS behaviour under real load.
Causes
The cause is not “harmonics exist.” It is a source impedance plus a set of load spectra that add at a named point of common coupling. Six-pulse drives without line reactors, dense LED retrofits, and large rectifier fronts are usual suspects. Power-factor capacitors can resonate with an order until voltage distortion becomes a plant-wide issue rather than a local one.
A spectrum screenshot is not a root cause. The same 5th-harmonic current stresses a transformer differently than a feeder cable. Published limit tables are comparison tools; they do not replace naming the overheating or the mis-trip you actually have.
Investigation
Power-quality logging should capture voltage and current at the transformer secondary, on the suspect feeder, and if needed at drive terminals — same window, documented load. Report fundamental current, true rms, and dominant orders. Note capacitor banks in or out. One cycle during a process change beats a week of min/max without waveform.
Conclusions stay tied to the file: this neutral carries triplen current consistent with the single-phase electronic load on those panels; this transformer’s extra heat tracks current distortion under the weekday process, not outdoor temperature. Reactors, multi-pulse fronts, filters or load rearrangement come after the mechanism — not as a catalogue reflex. Related: interpreting power-quality data.