Non-advanced VFDs (Variable Frequency Drives) generate harmonics on the input power source primarily due to their rectification process and nonlinear current draw. Below is an explanation of this issue and some common solutions for mitigating these harmonics.
Why VFDs Generate Harmonics
1. Rectification Stage:
o 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.
2. Nonlinear Load Behavior:
o 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.
3. DC Bus Capacitors:
o 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.
4. Switching Frequency:
o 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
1. Input Line Reactors:
o Adding reactors (inductors) between the VFD and the power source smooths out the current waveform, reducing the amount of harmonic distortion.
2. Harmonic Filters:
o Passive Harmonic Filters are designed to filter out specific harmonic frequencies. They are effective for reducing lower-order harmonics.
o 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.
3. Multi-Pulse Rectifiers (12-Pulse, 18-Pulse, 24-Pulse):
o 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.
4. Active Front-End (AFE) VFDs:
o AFE VFDs use an IGBT-based rectifier that draws current more smoothly from the source, significantly reducing harmonics, often achieving harmonic distortion below 5%.
5. Dedicated Harmonic Transformers:
o 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.
