An Improved CPS-PWM Scheme-Based Voltage Balancing Strategy for MMC With Fundamental Frequency Sorting Algorithm

Abstract

In this paper, a new voltage balancing strategy based on improved carrier-phase-shifted pulsewidth modulation (CPS-PWM) scheme and fundamental frequency sorting algorithm (FFSA) is proposed. FFSA is a salient method which sorts the capacitor voltages at fundamental frequency and utilizes the different charging ability of driving signals to converge the capacitor voltages. Unfortunately, under the CPS-PWM scheme with high carrier frequency, 250 Hz or higher, FFSA cannot bring the capacitor voltage to the nominal value once the unbalance occurs, because there is a little difference remaining in the driving signal's charging ability. To avoid this disadvantage, an improved CPS-PWM scheme is proposed. In this scheme, driving signals with proper phase angle differences are selected to be dealt with logic and/or processing according to the sorting result of FFSA. With the logic processing, new driving signals with obviously different charging abilities are synthesized, which are assigned to the corresponding unbalanced submodules to achieve the balance of capacitor voltages. And several regions for logic processing are set up, where the proper phase angle difference is selected for each region in order to both guarantee the convergence speed and avoid the voltage oscillations. With the proposed scheme, FFSA can be adopted under CPS-PWM with high carrier frequency. Moreover, the merits of FFSA are inherited, such as the reduced computational burden and the elimination of arm current detection. Performance and effectiveness of the proposed strategy are validated by both simulations and experimental results.