Harmonic analysis and filter design for medium-voltage multilevel PWM inverters

Multilevel PWM inverters have become the development trends of medium-voltage and high-power motor adjustable-speed drives (ASDs). Harmonic analysis of medium-voltage ASD output voltage is described first in this paper. Through lots of simulation and experimental results, the frequency spectrum characteristics of multilevel PWM voltage waveforms are concluded. It offers the academic fundamentals for the filter design. The use of an L-R-C filter is a simple and efficient method to mitigate the voltage surges effects. The filter parameters are related to the harmonic components of ASD output voltage and cable length. Furthermore it makes some problems more serious with the increase of motor capability. On the basis of considering some problems synthetically such as fundamental voltage drop, active power loss, current harmonics and resonance, this paper presents the parameter choice formulae that can be applied to medium-voltage and high-power ASDs output filter design.