Analysis and Reduction of the Output Voltage Error of PWM for Modular Multilevel Converters

Standard pulsewidth modulation (PWM) schemes for modular multilevel converters (MMC) have neglected deviations of the module voltages from the mean module voltage of an MMC arm as well as the dynamic change in the module voltages since their last measurement and during the switching period. This induces an output voltage error. Large output voltage errors disturb the higher level closed-loop control of the MMC, such that control targets cannot be met anymore. The output voltage error can be reduced using advanced PWM methods that predict and include the dynamic change of the individual module voltages during a switching period in the calculations for the duty cycle. This paper shows measurement results obtained from an experimental medium-voltage prototype, proving that advanced methods lead to much lower output voltage errors than the standard methods, but cannot fulfill the expectations raised by the simulations documented in the literature. In order to overcome the limitations of the old method, a new method that includes the semiconductor voltage drop and parasitic resistances is introduced and evaluated with measurements in this paper. Furthermore, simulation results showing the influence of the considered modulation methods on the closed-loop current control performance are presented.