Research On Multi-objective Redundant Optimal Configuration Of MMC Submodules

Modularized multilevel converter(MMC),with its unique technical advantages,has become the main development trend in the field of high voltage direct current(HVDC),MMC sub-module(Sub-Module,SM)The capacitor voltage determines the output voltage of the DC side of the system.The switching of the bridge arm sub-modules also directly affects the working state of the MMC.The number of bridge arm sub-modules gradually increases with the increase of the system voltage level.There are module failures.In order to reduce various failures,it is necessary to configure corresponding redundant modules.Redundant submodules are an important basis for ensuring the stability of the entire system,but excessive redundancy will lead to increased costs,and the use efficiency of redundant submodules is reduced,increasing system losses,and does not have higher economic benefits;redundancy If the temperature is too low,the stability of the entire system cannot be guaranteed.Therefore,it is necessary to analyze the existing sub-module redundancy strategy and propose a benefit-optimized sub-module redundancy configuration method.Firstly,the topology,working principle and mathematical model of MMC are analyzed.Comprehensive analysis of the control strategy of the converter,mainly analyzing the advantages and disadvantages of different modulation methods,and analyzing the impact of MMC circulation on the entire system The PSCAD simulation model of the redundant operating state of the device lays the theoretical foundation.Secondly,combined with the MMC dynamic redundancy scheme,for traditional MMC control without considering the switching effect of redundant SM,an optimized modulation strategy for MMC redundant operation is proposed.In order to reduce the bridge arm current distortion and effectively suppress the internal circulating current,a circulating current suppression strategy is adopted;for the bridge arm sub-module capacitor voltage fluctuation,an improved capacitor voltage equalization control strategy is selected.The simulation was carried out under the PSCAD/EMTDC platform,and the simulation results verified the feasibility of the above strategy.Then,the redundancy structure and principle of MMC sub-modules are studied.With a single redundancy index as the objective function,the influence of the investment of redundant sub-modules on the MMC system is studied.In view of the traditional MMC reliability analysis,most of its sub-modules are considered to have a constant failure rate during operation,ignoring the impact of life loss.A reliability model of MMC redundant system based on two-parameter Weibull distribution considering component life loss is proposed and analyzed.The relationship between redundancy and MMC sub-module utilization and system loss is derived,and its mathematical analytical expression is derived.Finally,the traditional optimization method of redundant submodules cannot guarantee the balanced development of various indicators,so on this basis,a new configuration scheme of redundant submodules is proposed.Based on the reliability of the entire MMC system,considering the utilization of redundant sub-modules,system losses and other factors,a perfect configuration system of redundant sub-modules is constructed,in this way to maintain the stability and economy of the entire system.Using PSCAD/EMTDC simulation,the overall effectiveness of the configuration method is verified.