Power Balance Analysis And Control In Cascaded H-Bridge-Based Multilevel Converters

Cascaded H-bridge(CHB)-based multilevel converter has been widely used in medium and high-voltage motor drive,Flexible AC Transmission System(FACTS),photovoltaical converters,energy storage and many other applications,because of its outstanding characteristics,such as good modularity,high-voltage/power level,good harmonic performance.In the past decades,the function realization of CHB converters have been deeply studied,which mainly concerned the internal and external features at the low-frequency band(mainly the fundamental frequency).However,with the growth of the complexity and requirements of application scenarios,there arise some new issues to be solved,such as the second harmonic ripple,the DC voltage divergency under the non-ideal grid.These issues share the same underlying problem that is the inner power flow analysis and management of CHB-based converters.Regarding this problem,this thesis studies the inner power flows of a CHB.The main contents and contributions of this thesis are listed as follows:(1)At the low-frequency band,the second-harmonic ripple on the DC voltage is studied.A power decoupling control method based on negative-sequence current injection is proposed for the capacitor reduction in regenerative CHB-based motor drive converters.The proposed method can reduce the required DC capacitance by 10 times without adding any extra components.Both the open-loop and closed-loop controls for realizing the power decoupling method are developed and tested.(2)At the high-frequency band,the harmonic power unbalance phenomenon is discovered and revealed to be caused by the coupling of the output harmonic currents and the low-order switching harmonics of single H-bridges.Without appropriate control,the harmonic power unbalance may lead to individual DC voltage divergence of the CHB chain.Thereafter,an adaptive carrier frequency optimization method is proposed to solve this problem in the case of grid voltage distortion and harmonic current compensation scenarios.(3)Across the low-frequency and high-frequency band,the interaction between the individual voltage balancing control and the harmonic power unbalance is revealed.How the carrier frequency,the filter inductance,the output current amplitude,and the DC voltage can influence the load unbalance tolerance ability of a CHB converter are studied and then verified.(4)A low-cost hardware-based method is proposed for individual voltage balancing in a CHB under zero current mode,which is a small size and low power switching resister integrated with the local control board of the H-bridge cell.It solves the problem that the software-based DC voltage balancing control loses its ability when the output current is very low.