Low Frequency Versus High Frequency PWM in Medium Voltage, High Power, Higher Level Inverters: THD, Harmonic Filtering, and Efficiency Comparison

One of the main advantages of multi-level inverters (MLI) is their ability to achieve high power quality and high efficiency power conversion. With the emergence of wide-band-gap (WBG) devices, the tendency has grown towards using high switching frequencies to improve converters’ output power quality and minimize switching harmonic filters footprint. While high-frequency switching finds significant success in low and medium-power applications, it is not the best practice to reduce the filter size when applied to medium voltage (MV) high power (HP) inverters implemented with high voltage levels. In this paper, we show that the least filtering requirements for MV HP higher level inverters is achieved using the modulation strategy with the least switching, i.e., the staircase modulation. The paper compares the harmonic filter requirements and power losses for low-frequency and high-frequency pulse-width-modulated MLI. The analysis shows that low-frequency switching not only achieves the lowest losses, but also produces the lowest line-to-line voltage total harmonic distortion (THD), which allows eliminating both voltage and current harmonic filters in higher level inverters. Simulation results are provided to verify the theoretical analysis on the THD, and experiments on a 1 kW, 600Vdc MLI are presented to compare efficiency for low- and high-frequency modulation.