Research On Control Strategies For Stacked Multicell Converters

Multilevel converter is widely adopted in medium-and high-power applications,such as Motor Drive,High-Voltage Direct Current.Flexible AC Transmission Systems,etc.Meanwhile,as a new topology of multilevel converter,Stacked Multicell Converter has attracted more and more attention from scholars at home and abroad.However,the neautral-point and flying capacitor voltage are easy to be imbalanced due to the presence of DC-link capacitors and flying capacitors.As a result,this paper conducts in-depth analysis and research on the above issues.Basic principles of Stacke Multicell Converter are discussed in detail,including converter topology,switching states,implementation and self-balancing characteristics as well as pre-charging strategy.Based on the analysis above,we analyzed a rotation-carrier modulation strategy in order to eliminate the poor output harmonic performance of the traditional phase-shifted modulation method.The traditional self-balancing strategies for Stacked Multicell Converters may not act properly due to the undesired factors in practical applications,including dead time,limitation of control accuracy,control delay,parasitic parameters,and load assymetric,etc.Aiming at this issue,three active flying capacitor voltage balancing strategies are analyzed,which are logic-form equation method,cost function method,and P controller method.The validity and reliability of the three algorithms are verified by simulation compared with the traditional phase-shifted modulation strategy.As for the imbalance of neutral-point voltage,due to the voltage coupling between DC-link capacitors and flying capacitors,the dynamic model of the both two capacitors are discussed first.Then a joint control strategy based on optimal zero-sequence are proposed.It solves the problem of small zero-sequence selection space and confused coupling problem of the ACVB(Active Capacitor Voltage Balancing)method.The optimal zero-sequence method calculates the compensation current of neutral-point and inject the corresponding zero-sequence voltage to the three-phase modulation signals.To balance the voltage fluctuation of flying capacitors,the duty cycle of certain switching states is changed quantitatively.Simulation shows that when modulation index is higher and power factor is lower,the above two methods share the same balance effect.However,the effect is diffent when modulation index is lower,with optimal zero-sequence shares a better effect.What's more,the dynamic performance is different.The physical platform of five-level Stacked Multicell Converter was designed and built.And all the balance strategies which were metioned in this paper are verified and analyzed.The experimental results prove the correctness and effectiveness of the above strategies.