Research On Control Strategy Of T-type Three-level Dual PWM Converter

Compared with the traditional two-level PWM converter,three-level converter has been widely used because of its low output harmonic content,low switching frequency and high efficiency.The development of the three-level converter technology will improve the performance of the inverter,increase the utilization rate of the power supply,and reduce harmonic interference to the power grid.In this thesis,a T-type three-level dual PWM converter is taken as the research object.Theoretical analysis and control scheme research are carried out on the three-level converter’s dynamic performance,harmonic interference,neutral point potential balance,and common-mode voltage.Based on the analysis of the working principle of the three-level converter,the mathematical models of the rectifier side and the inverter side of the three-level converter are derived.The SVPWM algorithm and implementation method in the three-level 60 ° coordinate system are given.The reason of the neutral point potential fluctuation of the three-level converter is studied.In order to solve the problem that the traditional redundant small vector method can’t suppress the fluctuation of neutral point voltage under high modulation ratio and the virtual medium vector strategy can’t reduce the output common mode voltage of three-level converter under low modulation ratio,this thesis proposes a neutral point voltage balancing strategy based on hybrid variable virtual space vector(HV2SV).By eliminating the basic voltage vector with large common mode voltage,the virtual small vector and virtual medium vector are constructed reasonably,and the neutral point voltage fluctuation and common mode voltage are suppressed by adjusting the neutral point voltage regulation factor.In order to improve the dynamic performance of the three-level PWM rectifier system,the model predictive direct power control scheme of the three-level PWM rectifier is studied.At the same time,in view of the periodic harmonic interference caused by the harmonic and dead zone effect of the power grid to the three-level PWM rectifier,this thesis proposes a power prediction error compensation control strategy based on the open-loop repetitive control,which uses the repetitive control to compensate the periodic power prediction error,greatly improving the system’s ability of harmonic suppression,and eliminating the integral part in the repetitive controller.compared with the control scheme based on traditional repetitive control,it improves the system’s ability of accurate tracking.An experimental platform for T-type three-level dual PWM converter is designed based on DSP+FPGA,and the structure of the experimental platform is discussed.The simulation and experiment methods are used to verify the proposed neutral point voltage balancing strategy based on hybrid variable virtual space vector.The results of simulation and experiment prove the correctness and feasibility of the strategy.The simulation method is used to verify the correctness of the power prediction error compensation control strategy based on open-loop repetitive control structure.