What Are Harmonics — and Why They Threaten a Modern Low-Voltage Bus

Harmonics are currents and voltages at integer multiples of the fundamental 50 or 60 Hz, riding on top of the ideal sine wave your equipment expects. On a modern low-voltage (LV) bus they are not an edge case but the default condition — produced by the very equipment that defines a contemporary plant: variable-frequency drives, rectifiers, UPS systems, LED lighting, and PV inverters. Left unchecked, they erode efficiency, age equipment, and push a facility past IEEE 519. This article explains what harmonics are, where they come from, and how a low-voltage active harmonic filter (AHF) cancels them.

Where Harmonics Come From

A harmonic is simply a frequency at an integer multiple of the fundamental. A nonlinear load draws current that is not proportional to the applied voltage, so its current waveform is distorted — and Fourier analysis resolves that distortion into the fundamental plus a stack of harmonics.

The classic source is a six-pulse rectifier — the front end of most VFDs and UPS units. It draws current in discrete lumps, generating the characteristic orders n = 6k ± 1: the 5th, 7th, 11th, and 13th. Left untreated, total demand distortion (THDi) at the load often sits between 8% and 30% — well above the IEEE 519 recommendation of THDi < 5% at the point of common coupling (PCC).

Why Harmonics Are a Threat

• Overheating — harmonic current raises copper loss with the square of RMS current and core loss with flux harmonics, so transformers and motors run hotter than their nameplate assumes.
• Nuisance trips and breaker stress — the extra RMS current pushes protective devices toward operation.
• Neutral overload — triplen harmonics (3rd, 9th, 15th) add in the neutral of a four-wire system and can exceed phase current.
• Sensitive-load malfunction — PLCs, sensors, and servers misread a distorted bus.
• Compliance and cost — exceed IEEE 519 and the utility can levy penalties or refuse a connection.

Why an AHF, Not a Fixed Filter

A passive LC trap is tuned to fixed orders and simply sits there. On a bus whose load changes by the minute, a fixed filter cannot track the load — and its shunt capacitance can resonate with grid impedance and amplify other harmonics.

An active harmonic filter works the opposite way: it monitors the load continuously and injects a counter-current that cancels the distortion at the source.

The Compensation Law

i_c* = i_L − i_s(fundamental)

Synthesized by a bridge switching at 15–20 kHz, that correction tracks within < 1 ms and pulls THDi from 30%+ down to < 5% across the 5th through 25th orders at once — following the load as it varies. It also supplies reactive current and helps balance phases.

IGBT vs SiC: The Thermal Ceiling

Most LV AHFs use IGBTs with a junction capped near 150 °C. Silicon-carbide (SiC) MOSFETs widen the bandgap (3.26 eV vs 1.12 eV), tolerate 200 °C, switch at 2–3× the frequency, and cut switching loss by roughly 50% — a smaller, cooler cabinet with headroom for higher-order cancellation. That is the direction CHITEK's LV AHF platform is built on.

Engineering Deployment Checklist

1. Measure at the PCC with a power-quality analyzer over a full duty cycle.
2. Place the AHF at the source — on the feeder that supplies the nonlinear cluster.
3. Audit capacitor banks for resonance before commissioning.
4. Size on measured TDD referenced to peak demand.
5. Leave headroom (15–25%) for drives added later.

Typical Application Profiles

• Industrial plants — VFDs dominate; the AHF restores < 5% and cuts transformer heating.
• Data centers — UPS rectifiers stack the 5th and 7th; the AHF protects uptime.
• Commercial buildings — servers, LED, and HVAC drives; the AHF removes neutral overheating.

Bottom Line

Harmonics come from the nonlinear loads you already run. A low-voltage AHF cancels them at the source — measurably, within IEEE 519. CHITEK's LV AHF platforms are specified from exactly this measurement-first discipline.

What THDi is your bus actually logging — and is it below 5% at the PCC?