Research On Capacitor Voltage Balancing Strategies For Stacked Multicell Converters

With the rapid development of power electronics technology,a variety of highvoltage,high-power variable-frequency speed control devices have been widely used in the industrial field.Stacked Multicell Converter has attracted attention from scholars at home and abroad because of easy extended to a higher level,high power density and easy modularization.However,due to the existence of DC-link capacitors and floating capacitors,its neutral-point voltage and floating capacitors voltage imbalance limit its application.As a result,this paper conducts a detailed analysis and research of the above issues.The basic composition and principle of the Stacked Multicell Converter are analyzed in detail,including the basic unit of topology and the overall topology,switching states,commutation circuit and traditional modulation strategies.On this basis,the mechanism of capacitor voltage fluctuation is analyzed,which lays a solid theoretical foundation for the analysis and proposal of modulation strategy and active balancing strategies.In view of the fact that phase disposition PWM(PDPWM)has better line voltage harmonic performance than phase-shifted PWM(PSPWM),but it can not realize the selfbalancing of the floating capacitors when applied to SMC.Based on the detailed analysis of the characteristics and shortcomings of the CRPWM strategy,one more general improved CRPWM modulation strategy is proposed.Furthermore,the effect of Balance Booster on the self-balancing characteristics is analyzed.In practical application,due to the numerous control objectives of five-level SMC,it is very suitable to use FCS-MPC.However,due to the lots of vectors of five-level SMC and the limited performance of the existing controller,the algorithm is difficult to be applied in practice.So a hierarchical control with single space-vector modulation for SMC is proposed.This method avoids the complicated ergodic optimization process through vector filtering vector positioning mechanism.At last,the voltage balance of capaicitors is ensured.at the same time,the better output current harmonic performance and the minimum common-mode voltage are also ensured.Matlab/Simulink software is used to simulate and verify the modulation strategy and active balancing strategy which are proposed in this paper.The experimental platform was designed and built,and the modulation strategies,BB and active balancing strategy which were mentioned in this paper are verified and analyzed.The simulation and experimental results prove the correctness of the above strategies and the rationality of BB device selection.