Sequential Selective Harmonic Elimination and Outphasing Amplitude Control for the Modular Multilevel Converters Operating with the Fundamental Frequency

Abstract

With the growing use of DC voltage for power transmission (HVDC) and DC links for efficient AC motor drives, the R&D efforts are directed to the increase of DC/AC converter's efficiency and reliability. Commonly used DC/AC converters, based on the carrierfrequency pulse-width modulation (PWM) to form a sinusoidal output voltage with a low level of higher harmonics, have switching time and switching loss issues. The use of multimodule multilevel converters (MMC), operating with the fundamental switching frequency and phase-shift control to form the ladder-style output voltage, reduces switching losses to minimum while keeping the low level of higher harmonics in the output voltage. The discussed sequential harmonic elimination method for MMC, using identical power modules operating with 50% duty cycle and fundamental frequency, is based on the combination of the multiple fixed phase shifts to form a ladder-style sinusoidal voltage with low total harmonic distortion (THD) and symmetrical variable phase shifts to control the output voltage amplitude. The principles of the sequential selective harmonic elimination for MMC topology and amplitude control are described with two examples. The first example is the industrial-frequency DC/AC converter complying with THD requirements of IEEE 519 2014 standard without the output filter. The second example is a high-frequency converter, used as a transmitter, loaded with the resonant antenna, where the evaluation criteria are decreasing of the transmitter losses and increasing of the reliability or life expectancy at elevated temperature.