Research Of Power Theory And Direct Power Control Method Based On Cascaded H-bridge Converters

With the continuous development of active distribution networks,multilevel converters have gradually replaced two-level converters as the first choice for high-voltage and large-capacity applications.In several topologies of multilevel converters,cascaded H-bridge multilevel converters have the advantages of simple implementation,high reliability,and low harmonic content.So,the cascaded H-bridge multilevel converters have been widely used in static synchronous compensators,solid state transformers,active power filters and other fields.This paper focuses on single-phase cascaded H-bridge multilevel converters to achieve sinusoidal AC side current and balanced DC side capacitor voltage for research purposes.The working principle and control method analysis,power control strategy research,stability analysis and experimental verification are carried out.The main sections of the paper are as follows:(1)Analysis of working principle and control method.The topology and the commonly used modulation strategies of the cascaded H-bridge multilevel converter are analyzed.The mathematical models of various coordinate systems are established based on the coordinate transformation and the description of the switch function.The vector(dq)control method of the traditional single-phase cascade H-bridge multilevel converter is analyzed,and the places where it can be optimized have been pointed out.(2)Power control strategy research.The single-phase cascaded H-bridge multilevel converter power control strategy and the power-based dc-link capacitor voltage balance strategy in two-phase ??and three-phase abc coordinate systems are proposed.The effectiveness of the control strategy is verified by simulation.(3)Stability analysis.The stability of the two-phase ?? power control strategy is analyzed by Bode plot(the stability of the three-phase abc power control strategy can be analyzed in the same way).(4)On the single-phase cascaded H-bridge multilevel converter experimental platform with TMS320F28377D as controller,the two-phase ?? power control strategy and power-based dc-link capacitor voltage balancing strategy are verified from both in steady states and transient states.