High Output Voltage Precision PWM for Modular Multilevel Converters

Recently, modular multilevel conterters (MMC) are also considered for medium voltage (MV) applications, where the number of modules and the switching frequency are relatively low compared to high voltage (HV) applications. In this context, standard modulation methods can cause relatively large output voltage errors. This effect is amplified even more, if the value of the module capacitances is decreased to reduce cost and volume of the MMC. Large output voltage errors disturb the higher level closed loop control of the MMC significantly, such that control targets (e.g. keeping the circulating current zero) could not be met anymore. This paper analyses the output voltage error induced by the standard level shifted PWM method and proposes three approaches to reduce the error. All four methods are compared with respect to the error for different switching frequencies and arm currents. The effects on the closed loop performance of a current controller are investigated with time domain simulations.