Why VFDs Generate Harmonics
- Rectification Stage:
- VFDs convert incoming AC power to DC using a rectifier, typically using diodes or thyristors. This process draws current in pulses rather than a smooth sinusoidal wave, which introduces harmonic currents at multiples of the fundamental frequency.
- Nonlinear Load Behavior:
- As nonlinear loads, VFDs draw current in a way that does not match the sinusoidal voltage waveform. This results in harmonic currents superimposed onto the original waveform, creating harmonic distortion in the input power.
- DC Bus Capacitors:
- DC bus capacitors within VFDs are charged at the peaks of the AC input waveform, drawing sharp current pulses. This “peak charging” effect increases harmonic distortion.
- Switching Frequency:
- The variable frequency control achieved by VFDs requires high-speed switching devices, which can introduce higher-order harmonics, particularly when the switching frequency is close to harmonic frequencies.
Solutions to Mitigate Harmonics
- Input Line Reactors:
- Adding reactors (inductors) between the VFD and the power source smooths out the current waveform, reducing the amount of harmonic distortion.
- Harmonic Filters:
- Passive Harmonic Filters are designed to filter out specific harmonic frequencies. They are effective for reducing lower-order harmonics.
- Active Harmonic Filters dynamically monitor and counteract harmonic distortion across a range of frequencies, offering a more flexible and comprehensive solution to remove the harmonics induced by VFD to the input source.
- Multi-Pulse Rectifiers (12-Pulse, 18-Pulse, 24-Pulse):
- Multi-pulse VFDs use multiple rectifiers with phase-shift transformers to cancel out many harmonic frequencies, significantly lowering the harmonic impact on the input source.
- Active Front-End (AFE) VFDs:
- AFE VFDs use an IGBT-based rectifier that draws current more smoothly from the source, significantly reducing harmonics, often achieving harmonic distortion below 5%.
- Dedicated Harmonic Transformers:
- Using specially designed transformers, such as K-rated transformers, can handle higher levels of harmonic current without overheating, while helping isolate and reduce harmonics in sensitive power systems.
Using these solutions can bring a VFD system into compliance with harmonic standards, such as IEEE 519, helping maintain power quality and protect other equipment on the same power network.
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