Active Harmonic Filter Product

Overview

An active harmonic filter cleans up the current distortion that modern loads dump into a facility's electrical system. Variable-frequency drives, UPS rectifiers, LED drivers, and server power supplies all draw current in non-sinusoidal gulps; the resulting harmonic currents — predominantly 5th, 7th, 11th, and 13th order for six-pulse rectifier loads — overheat transformers and neutral conductors, trip breakers spuriously, distort the voltage other equipment sees, and put a plant in breach of IEEE 519 limits at the utility connection. Passive LC traps tuned to single harmonics were the old answer; they detune as the network changes and can resonate with utility capacitance. An active filter instead measures the distortion and cancels it, harmonic by harmonic, in real time.

The principle is superposition. The filter sits in shunt on the bus, doing nothing to fundamental power flow. Three Load Current Transformers clamped around the load feeder report the actual load current; the DSP Controller subtracts the fundamental and commands the IGBT Power Stage to inject the inverse of everything that remains. Load harmonics and injected anti-harmonics sum to zero, and the upstream transformer sees a clean sinusoid.

How it works

The power stage is a three-level neutral-point-clamped bridge: one IGBT Power Module phase leg per phase, each driven by an isolated Gate Driver Board over the Fiber Optic Link. Three-level switching halves the voltage step per transition and pushes the effective ripple frequency up, which is what lets the filter synthesize accurate current at harmonics as high as the 50th order (2.5 kHz) while switching at 20 kHz. The bridge draws from the DC Bus, a split 750 V bank of Film Capacitors — film rather than electrolytic, because compensation current is nearly all ripple and electrolytics would cook; Balancing Resistors keep the two bus halves equal. No external DC source exists: the controller steals a small fundamental current from the grid to cover losses and hold the bus voltage.

Between bridge and bus sits the LCL Output Filter. The Converter-Side Inductor limits IGBT ripple current, the Filter Capacitor shunts 20 kHz residue to ground, and the Grid-Side Inductor isolates that capacitor from unknown grid impedance, with Damping Resistors flattening the LCL resonance. The result: compensation current passes, switching noise does not.

Control runs on a floating-point DSP (Microcontroller) fed by the ADC Front End, which samples nine channels — three load CTs, three internal Hall Sensor transducers on the injection legs, three grid voltages from the Voltage Divider Board — simultaneously every PWM cycle. A phase-locked loop tracks grid angle; the load current is transformed into the synchronous reference frame, where the fundamental becomes DC and is removed by a high-pass operation, or decomposed by FFT into individual harmonics that the commissioning engineer can enable selectively. The remaining reference drives a deadbeat current loop closing every 50 µs, giving sub-100 µs response to load steps — fast enough to track a DC drive through its firing-angle transients.

Connection and operation

At power-up the Precharge & Isolation circuit closes the Precharge Contactor first, charging the DC bus through Precharge Resistors and the IGBT body diodes; only then does the Main Contactor make the solid connection, with Semiconductor Fuses as last-resort bridge protection. Because the filter is a shunt device, any internal failure simply drops the contactor — the load keeps running, uncompensated, which is the failure mode plants want.

Capacity is allocated by priority. Harmonic cancellation takes first claim on the 100 A rating; whatever current headroom remains, the controller can spend on displacement power-factor correction (injecting leading or lagging fundamental reactive current up to 0.99 PF) and on rebalancing unequal phase loading, including neutral-current cancellation in 4-wire configuration. When one unit is not enough, up to eight parallel on a shared CT set and split the reference among themselves.

Losses of roughly 2 kW at full output leave through the Power Stage Heatsink under speed-controlled Blower Motor fans, with NTC Probes enforcing thermal derating before shutdown and a Dust Filter guarding the intake. The door-mounted HMI Display — an LCD Panel with Touch Digitizer — shows live waveforms and harmonic spectra before and after compensation, the commissioning view that proves THDi has dropped from a typical 30% to under 5%. A Door Interlock trips the main contactor if the cabinet opens energized.