Research On The Control Strategy Of MMC Series Structure Microgrid System Based On Micro-source Redundancy

The failure of some microsources or half-bridge converters in a Modular Multilevel Converter(MMC)series structured microgrid system will lead to asymmetric operation of the system,resulting in output voltage and current distortion and increased loop current,which will reduce the reliability of the system operation.Therefore,this thesis investigates a microsource redundancy control strategy for this system to improve the system reliability.First,the structure of the MMC series microgrid system,the s tructure of the microsource submodules and their internal power relations are analyzed,and the structure of the hybrid energy storage system used to maintain the voltage stability of the DC chain of the microsource submodules and its control method are pr esented.According to the structural characteristics of the system,its working principle,various operation modes of the micro-source sub-module are described and the mathematical model of the system is established.The two modulation methods applicable to this system are compared and analyzed,and the simulation verification of the hybrid energy storage voltage stabilization effect and modulation method is carried out to lay the foundation for the study of the system micro-source redundancy control strategy.Second,a dynamic redundancy control strategy based on microsource power ordering is proposed for the system containing redundant microsource submodules.However,since the output power of the wind power and photovoltaic microsources connected in the s ystem is volatile,and the energy is mainly concentrated in their low-frequency components,while the energy of the middle and high-frequency components is low,an ensemble empirical modal decomposition algorithm is used instead of a first-order low-pass filter to decompose the original power of the microsources.Among them,the low-frequency component can be used as the effective output power of the microsource and for the microsource sub-module power sorting,and then for the system microsource dynamic re dundancy control,and the medium and high frequency components are used for the power leveling reference commands of the battery and supercapacitor in the hybrid energy storage respectively to achieve a better leveling effect.Then,the specific implementation steps of the dynamic redundancy control strategy based on micro-source power ordering proposed in this thesis are elaborated.That is,during the non-fault operation of the system,the dynamic redundancy control of microsources is realized through the ensemble empirical modal decomposition of the original power of microsources,the selection of microsource submodules and the expansion and distribution of control signals;during the fault operation of the system,the faulty microsource submodules are removed first,and then operated according to the non-fault operation state.It is shown that the strategy can effectively reduce the distortion rate and asymmetry of output voltage and current,while keeping the number of bridge arm voltage output levels an d the maximum voltage amplitude constant,as well as reducing the virtual DC bus voltage fluctuation,and by decomposing the original output power of microsources before ranking the power magnitude,it can realize the efficient utilization of energy.Finally,the asymmetric operation characteristics of the system after the exhaustion of the redundant microsource submodules are specifically analyzed.Accordingly,an asymmetric control strategy is proposed by adjusting the reference value of the DC chain voltage of the redundant microsource submodule and reconfiguring the modulated waveform while ensuring that the main components of the loop current remain unchanged.It is shown that the strategy can effectively suppress the fundamental components and higher h armonic components in the circulating current,reduce the distortion rate of the system three-phase line current,and reduce the asymmetry of the system fault phase upper and lower bridge arm currents.