An active harmonic filter (AHF) is a parallel-connected power-electronic compensator that improves power quality by canceling harmonic current and adjusting reactive power in real time. Unlike a passive LC trap, it adapts to changing load and avoids resonance with system impedance. This article walks the principle from detection to injection, and explains why the semiconductor choice (IGBT vs SiC) sets the performance ceiling.
What an AHF does
The AHF connects in parallel to the distorted bus and continuously measures the load current. It computes the compensating current — equal in magnitude, opposite in phase to the unwanted harmonic and reactive content — and injects it, leaving the source with a near-sinusoid and a high power factor.
The working principle, step by step
1. Harmonic detection and separation. High-precision current sensors (Hall-effect, covering up to ~50 harmonic orders) sample the load current in real time. The signal is decomposed two ways: the instantaneous reactive power theory (p–q) transforms three-phase current into real and imaginary components, while FFT extracts each harmonic's amplitude and phase. The result is a reference current command matching the harmonic current in amplitude and opposite in phase — the "reverse harmonic."
2. Compensation-current generation. The core is a voltage-source inverter (VSI) built on high-frequency switching devices (IGBT or SiC). It injects the compensation current, which combines with the original harmonic current and cancels it.
3. Reactive-power compensation. The same inverter supplies leading or lagging current to offset the load's reactive component, raising PF toward unity (CHITEK LV AHF/SVG target 0.99+).
4. Closed-loop control and stability. The compensated grid current is monitored and compared to the reference; the inverter output is adjusted to hold accuracy.
5. Anti-resonance and adaptability. Active control avoids forming resonant circuits with grid impedance, handles broadband harmonics (3rd, 5th, 7th, high-order), and supports multiple parallel units for distributed compensation.
The compensation law
i_c* = i_L − i_s(fundamental)
Synthesized by a bridge switching at 15–20 kHz, the correction tracks within < 1 ms and pulls THDi from 30%+ down to < 5% across the 5th through 25th orders simultaneously — following the load as it varies. Left untreated, THDi at a nonlinear load often sits between 8% and 30%, against the IEEE 519 limit of THDi < 5% at the PCC.
The semiconductor bottleneck: IGBT vs SiC
IGBT: junction capped near 150°C, switching frequency typically under 20 kHz.
SiC MOSFET: bandgap 3.26 eV vs 1.12 eV, junction 200°C, ~10× breakdown field, 2–3× the switching frequency, ~50% lower switching loss. SiC widens the cancellation bandwidth (handles 13th-order and above), shrinks the cabinet, and runs cooler — the direction CHITEK's LV AHF platform is built on.
Engineering deployment checklist
1. Measure at the PCC with a PQ analyzer over a full duty cycle.
2. Place the AHF at the source — on the feeder feeding 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 added drives.
Bottom line
An AHF is a measurement→computation→injection loop that cancels distortion dynamically and without resonance. The principle is constant; the switch decides the bandwidth and footprint. CHITEK's LV AHF platforms are built around this IGBT→SiC upgrade path, specified from a real measurement.
What harmonic orders is your AHF actually canceling — and is its switch rated for the highest one on your bus?
CHITEK Technical Team
5 min read