Research On Improved Carrier Phase Shifting And Voltage Equalization Control Strategy In Modular Multilevel Converter

With the development and utilization of clean energy such as wind energy and solar energy,flexible DC transmission technology has attracted attention because it can effectively solve transmission problems such as long distances and large capacity.Modular Multilevel Converter(MMC)has the advantages of modular cascade structure,high waveform quality and low loss,and has become the most potential way in flexible DC transmission.Carrier phase-shift modulation is one of the common engineering application methods of modular multilevel converters.In this method,the switching frequency is fixed and the modulation wave follower characteristics are better,but as the number of levels increases,there will be a large number of proportional and PI controllers.Therefore,this paper proposes an improved carrier phase-shift modulation method.Instead of directly generating voltage equalization control signals for each sub-module through the controller,the carrier-phase-shift characteristic is used to obtain the total required number of sub-modules on each bridge arm at a certain moment,and then combined with the optimized sorting algorithm.This method can avoid the use of a large number of controllers,and achieve the goal of lower switching frequency and better voltage equalization effect.In addition,this modulation method maintains good modulation wave following characteristics,and is easy to combine with circulating current suppression,which can further optimize the voltage equalization effect and reduce system loss.This paper takes the MMC of the half-bridge submodule as the research object,and first introduces its topology,the working principle of the submodule,the single-phase equivalent circuit,and the calculation of important electrical quantities.Then,the principle,advantages and disadvantages of traditional carrier phase-shift modulation are analyzed.According to the problems of a large number of controllers,an improved carrier phase shift is proposed,and the modulation principle is introduced in detail.A 21-level model is built in MATLAB / simulink,and the capacitor voltage equalization effect,the three-phase voltage distortion rate change,and the IGBT switching frequency are compared and analyzed after the improvement.Simulations show that although improved carrier phase shifting has a better voltage equalization effect,there is a problem that the IGBT switching frequency of the sub-module is too high.In order to reduce the switching frequency of the IGBT of the sub-module,the maintenance factor method,state-based sequencing and incremental switching method are combined with improved carrier shift respectively.The basic principles of the two methods,the control process and the selection of important parameters are introduced in detail.The capacitor module voltage fluctuation and IGBT switching frequency are analyzed through simulation.Simulation results show that the combination of improved carrier phase shifting and optimized sequencing method can achieve better voltage equalization effect and lower switching frequency.Aiming at the MMC circulation problem,the internal circulation generation mechanism is analyzed mathematically,and it is proved that the root cause of the internal circulation is due to the fluctuation of the capacitor voltage of the submodule.According to the nature of the main harmonic component of the internal circulation being the double-frequency negative sequence,the double-frequency negative sequence rotation coordinate transformation is used to extract the harmonic components,and the circulation current suppression is achieved by controlling the d and q current command values.Then,an overall MMC control scheme is constructed,which includes three parts: improved carrier phase shift,voltage equalization,and circulating current suppression.Three-phase AC output voltage,current distortion rate and circulating current suppression effect is analyzed in simulation.Simulation results show that the improved carrier phase shifting combined with this circulating current suppression method has a better circulating current suppression effect.