A rectifier does not draw current as a sine wave. It draws it in short, steep pulses near the voltage peak, and the Fourier series of that pulse train is the fundamental plus the 5th, 7th, 11th, 13th and higher orders. Those orders flow back toward the source as harmonic current, heat transformers and cables, and distort the voltage every other load on the bus sees. This article covers how the distortion is measured, what it costs, why fixed filters fall short, and how a CHITEK active harmonic filter cancels it at the point of origin.
How Harmonic Distortion Is Quantified
The standard metric is total demand distortion at the point of common coupling (PCC):
THDi% = √(Σ I_h², h = 2…50) / I_L × 100%
IEEE 519 sets the limit by short-circuit ratio (I_SC/I_L) and by load size — a typical C&I bus with I_SC/I_L below 20 is held to roughly 5% TDD. IEC 61000-3-2 and 61000-3-12 govern equipment emission; GB/T 14549 covers the Chinese grid equivalent. Measure current with a class-A analyzer over a full duty cycle, not a single snapshot.
Where the Distortion Comes From
- Six-pulse rectifiers — the dominant source in drives, UPS and DC chargers; characteristic orders are 5, 7, 11, 13 at roughly 1/h of fundamental.
- Single-phase switch-mode supplies — LED lighting, IT loads and office equipment inject 3rd and triplen orders that accumulate in the neutral.
- Arc and induction furnaces — broadband, rapidly varying distortion plus reactive swing.
- Regenerative drives and elevators — active front ends switch at high frequency and add interharmonic content.
- Capacitor banks — not a source, but an amplifier: they shift the resonance point down toward a characteristic order.
What It Costs
- Transformer heating — eddy and stray losses rise with the square of frequency; a K-rated unit derates where a standard one fails.
- Cable and neutral overload — triplen harmonics sum in the neutral and can exceed phase current.
- Motor and generator stress — negative-sequence and harmonic fields add rotor heating beyond nameplate assumptions.
- Capacitor failure — dielectric loss and resonance current shorten bank life and can rupture cans.
- Power factor penalties — distortion power depresses true PF, so the utility bills for kVA the plant never used.
- Protection misoperation — distorted zero-sequence current trips residual relays and nuisance breakers.
Why Fixed Methods Fall Short
Detuned reactors and passive filters are tuned to one order at a fixed impedance. They work only at design load, they can resonate with the source impedance when the network changes, and they cannot follow a load that swings through the day. A switched capacitor bank corrects average displacement PF but cannot cancel current harmonics or rebalance per phase. A static var generator handles the reactive and balancing side dynamically; pairing it with an active harmonic filter covers the waveform side. Neither fixed technology can do both as the load moves.
How an AHF Cancels Harmonics at Source
The CHITEK AHF is a voltage-source converter that measures load current and synthesizes the inverse of its harmonic content:
i_c* = i_L - i_s(fundamental)
An IGBT or SiC bridge switching at 15-20 kHz tracks that reference within < 1 ms and drives THDi from 30%+ down to < 5%, with selective orders cancelled up to the 50th. Because the correction is closed-loop, it follows the load instead of assuming it. That is the practical meaning of "at source": the counter-current is injected at the offending feeder, so harmonics never propagate to the transformer.
Where the SVG Fits
A CHITEK SVG injects or absorbs reactive current per phase, independently, to hold voltage and power factor near unity: sub-cycle response, continuous ±1% regulation, PF at 0.99+. It cancels the negative-sequence component a fixed bank cannot touch and carries the reactive burden so the AHF can concentrate on waveform correction. In a combined LV AHF + SVG platform, one controller coordinates both — no fixed tuning, no resonance risk, adaptive phase balancing as the load varies.
Sizing: A Worked Example
Assume a 1000 kVA transformer, 400 V, feeding a 600 A nonlinear load with measured THDi of 28% and displacement PF of 0.86.
- Harmonic current: 600 × 0.28 ≈ 168 A RMS of distortion to cancel.
- Required filter rating: 168 A ÷ derating factor (typically 0.8–1.0 by order mix) ≈ 170–210 A — select the next standard size.
- Reactive demand: 600 × √(1 − 0.86²) ≈ 306 kvar — size the SVG to that, plus headroom for swings.
- Add 15-25% margin for future single-phase and EV load.
State the assumptions; a filter sized to nameplate load rather than measured current is the most common specification error.
IGBT vs SiC
Conventional LV AHF and SVG units use IGBTs capped near 150 °C, switching typically below 20 kHz. SiC MOSFETs run at 200 °C junction, carry roughly 10× the breakdown field, switch at 2-3× the frequency and cut switching loss by about 50%. The higher switching frequency widens control bandwidth, shrinks the output filter and cabinet, and improves cancellation of high-order and interharmonic content. That is the direction the CHITEK active harmonic filter and SVG platform is built on.
| Aspect | IGBT | SiC MOSFET |
|---|---|---|
| Junction temperature | ~150 °C | ~200 °C |
| Switching frequency | < 20 kHz | 2-3× higher |
| Switching loss | baseline | ~50% lower |
| Cabinet size | larger | smaller |
Engineering Deployment Checklist
- Measure phase voltages and currents at the PCC over a full duty cycle; log THDi, TDD, PF and unbalance.
- Place the AHF/SVG at the offending feeder, close to the nonlinear load, not at the main incomer.
- Audit existing capacitor banks for resonance and mistuning before commissioning the active unit.
- Check weak-grid margin and size the converter for the worst-case voltage dip.
- Verify after commissioning: THDi against the IEEE 519 limit at the PCC, PF at 0.99+, and no relay misoperation.
- Leave 15-25% headroom for added single-phase load.
Typical Application Profiles
- EV charging hubs — single-phase charger clusters unbalance the bus; the SVG rebalances per phase while the AHF cancels rectifier harmonics.
- Arc-furnace and steel plants — violent, unbalanced reactive swings; AHF + SVG stabilize voltage and waveform together.
- C&I plants with mixed single/three-phase load — a combined unit cuts heating, neutral overload and PF penalties.
- Data centres and UPS rooms — high-order and interharmonic content from switch-mode supplies, corrected at the distribution board.
Bottom Line
Harmonic distortion is a source problem: it originates at the rectifier and propagates outward, so it is best cancelled where it starts, adaptively, with a closed-loop converter. A low-voltage active harmonic filter paired with a static var generator restores waveform quality and balance without fixed tuning or resonance risk. CHITEK's LV AHF + SVG platforms are specified from exactly this measurement-first discipline. What is your measured THDi at the PCC — and which feeder is carrying it?