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 convert er’s efficiency and reliability. Commonly used DC/AC converters, based on the carrier- frequency 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 switch ing 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 sinu - soidal voltage with low total harmonic distortion (THD) and symmetrical variable phase shifts to control the output voltage amplitude. The principles of the sequential selec - tive harmonic elimination for MMC topology and amplitude control are described with two examples. The first example is the industrial-frequency DC/AC converter comply - ing 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. voltages. A simple algorithm for the control circuitries used to eliminate harmonics and regu late output voltage from zero to maximum maintaining stable phase is discussed. A simple expression for THD of the output voltage vs. the number of eliminated harmonics, derived from simulation results, is provided for design evaluation against IEEE 519 standard require -ments. The application of this method to the NMR transmitters operating in the high-temper ature environment eliminates the most dangerous output current harmonics and shows more than twice the gain in the life expectancy. This method was validated for NMR downhole log ging equipment, and two patents were granted.