Analysis of the Unbalance Phenomenon Caused by the PWM Delay and Modulation Frequency Ratio Related to the CPS-PWM Strategy in an MMC System

The carrier phase-shift pulsewidth modulation (CPS-PWM) strategy is widely used in modular multilevel converters. However, the PWM delay and modulation frequency ratio, which are typically neglected in the design of the CPS-PWM strategy, are the primary factors that can cause an unbalance phenomenon in the submodules' capacitor voltages and thus deteriorate the system working performance. This study first proposed mechanism of the balance of the capacitor voltage related to the CPS-PWM strategy. A quantitative analysis of the relations between the PWM delay, modulation frequency ratio, and unbalance phenomenon is then provided based on the balance mechanism, indicating the root cause of the unbalance phenomenon related to the CPS-PWM strategy. Based on this analysis, the appropriate compensation methods are introduced into the system: an optimal design of the system parameters is used to mitigate the effect of the PWM delay. And an improved balance strategy, which combines the rotations of the carrier waves, the method regulating the phase-shift angle, and a state switcher with a balance-time calculator, is used to suppress the effect of a low modulation frequency ratio. The unbalance phenomenon caused by the PWM delay and low modulation frequency ratio, and the efficiency of the compensation methods to balance the capacitor voltage, are finally demonstrated via experiments.