Low-frequency Ripple Suppression Based On Active Power Decoupling For Modular Multilevel Converter Sub-module Capacitors

With the continuous development of economy and technology,the global demand for clean energy and electricity is rising rapidly,and there is an increasingly urgent need for the development of technologies in application areas including grid-connected renewable energy,grid-connected distributed generation,and island power supply.High voltage direct current(HVDC)technology can be used in these areas in the future because it overcomes the limitations of long-distance AC transmission and ensures stable transmission performance with low electrical losses.In recent years,Modular Multilevel Converter(MMC)technology has been favored by major manufacturers of HVDC equipment for its modular design,high efficiency and low output harmonics,and is expected to achieve more deployments in HVDC projects,so a more in-depth study of MMC systems has important theoretical value and engineering applications.In this paper,the MMC system is used as the main research object,and the main objective of the research work is to reduce the sub-module capacitance voltage ripple.Firstly,based on the single-phase inverter MMC topology,the basic working principle of MMC is clarified,the mathematical model is analyzed and established,and the basic control objectives and control system architecture based on single-phase inverter MMC are clarified;the mechanism of sub-module capacitance voltage and power ripple generation during MMC operation is derived,and it is shown that there are low-frequency ripple voltages in the submodule capacitance voltage with fundamental frequency and diphasic frequency as the main components.The theoretical analysis of this part is the basis of the overall research work.Secondly,from the perspective of MMC sub-module topology reconfiguration,an MMC sub-module reconfiguration scheme based on inductive energy storage is proposed by combining the active power filtering decoupling technology in single-phase rectifiers.Starting from the ripple power of the submodule,the basic working principle of the topology is analyzed,and the low-frequency power pulsation in the submodule capacitor is dissipated by reasonably controlling the current of the energy storage inductor.The multi-objective control including MMC output current,average sub-module capacitor voltage,circulating current,sub-module capacitor voltage equalization,and inductor current is realized,and the corresponding controller parameters are designed.For the single-phase inverter MMC model,the PLECS simulation software verifies that the proposed scheme can effectively suppress the sub-module capacitor voltage ripple from both steady-state and dynamic perspectives,and the system response is fast.Again,since the capacitor as an energy storage device is more beneficial to improve the system power density,a reconfiguration scheme of the MMC sub-module based on the symmetric capacitor structure is proposed to reduce the voltage ripple on the DC side of the sub-module.By controlling the two capacitor voltages in the sub-module,the AC components in the two voltages are of the same magnitude and opposite direction,and the sum of the direct flows is the conventional sub-module capacitor voltage to achieve a constant sub-module DC side voltage.The energy fluctuation in the symmetrical capacitor is analyzed to derive the selection principle of symmetrical capacitor parameters in the submodule.Based on the topology,the relationship between the variables of the topology is analyzed,specifying that the topology is uncoupled from the conventional MMC sub-module circuit,and the overall system model is established to clarify the coupling relationship between the variables of the system;based on the system model,the voltage ripple suppression strategy is designed.Based on the single-phase inverter MMC model,it is verified by PLECS simulation software that the proposed scheme can suppress the low-frequency ripple of the capacitor voltage well without increasing the MMC bridge arm current and loop current,and can significantly improve the system power density compared with the conventional MMC.Finally,two active power decoupling circuits are built with the existing experimental platform to verify the sub-module ripple suppression,system steady-state performance,dynamic response and other performance in small power experiments.