Comparison of phase and level shifted switching methods for a three-phase modular multilevel converter

Abstract

In this study, a three-phase, 10-kV DC, 50-Hz, modular multilevel converter is designed in PSCAD environment by using 10 sub-modules in each arm. For each submodule of the designed converter, half bridge structure consisting of two IGBTs, two reverse parallel connected diodes and one capacitor are used. The gate signals triggering the semiconductors are obtained by comparing sinusoidal reference signal and triangular carrier signals are used to compare the level shift and phase shift pulse width modulation (PWM) methods. Modulation methods are compared by changing the modulation index and carrier frequency in terms of switching cases, DC voltage utilization factor and output voltage harmonic content. Level shifting (LS) method is advantageous in terms of number of switching with respect to phase shifting (PS) methods while phase shifting methods is better in terms of direct current voltage utilization factor and harmonic content. In the PS-PWM method, it is seen that the number of switching is independent from the modulation index, whereas in the level shifting PWM methods, the number of switching changes with respect to the modulation index. Modular multi-level converters (MMC) provide a cost advantage in medium and high voltage applications compared to conventional two and three-level inverters because of the output voltages do not need to filter out low harmonic content due to the sinusoidal proximity of the output voltages.