Harmonic interference and suppression of frequency converter
Apr 29,2026
When frequency inverters (especially PWM type) are operating, they generate a large number of harmonics due to the nonlinear characteristics of the rectification and inversion stages, which can interfere with the power grid and surrounding equipment. The following is a systematic explanation from three aspects: causes, hazards, and suppression methods.
1.Causes of harmonic interference
The main circuit of a frequency converter typically consists of: a rectifier bridge (three-phase uncontrolled rectification) → a DC bus (capacitor filtering) → an IGBT inverter (PWM modulation)：
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Input-side rectification: The current waveform is pulsed and contains a large number of odd harmonics such as 5, 7, 11, and 13.
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Output-side PWM modulation: The output voltage/current waveform is a high-frequency switching pulse containing high-order harmonics (near the carrier frequency and its multiples).
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Distributed parameter effects: For long cables, the capacitance to ground amplifies high-order harmonic currents, generating voltage spikes.
2.The main hazards of harmonics
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Power grid: voltage distortion, reduced power factor, and increased reactive power.
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Other equipment on the same power grid: control equipment malfunctions, instrument errors, and communication interference.
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Motor: Additional heat generation, torque pulsation, and accelerated insulation aging (voltage spikes).
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Inverter itself: DC bus overvoltage or interference with the control board can cause malfunctions.
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Capacitors and transformers: Harmonic heating, abnormal noise, and shortened lifespan.
3.Harmonic suppression methods (common engineering techniques)
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AC reactors: Increase commutation impedance, smooth current waveform, and reduce THDi to 30-40%.
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DC reactors: suppress DC pulsation and improve the input current waveform, with better performance than AC reactors.
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Harmonic filters: When a passive LC filter is tuned to a specific harmonic frequency (such as the 5th or 7th harmonic), the THDi can be reduced to below 10%.
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Active power filter (APF): Actively generates reverse harmonic current to cancel out THDi to below 5%.
4.Typical configuration recommendations
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A single frequency converter, less than 30m from the motor, with sufficient grid capacity, and only a DC reactor (such as built into the frequency converter).
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Multiple frequency converters, grid sensitive (with PLC and instruments), each with an AC reactor and a passive filter installed at the main incoming line.
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Strict harmonic requirements (IEEE 519, GB/T 14549), active harmonic filter.
