Research On Control Strategy Of MMC Based Grid-Connected Convert In Photovoltaic Plant

Currently,all countries around the world are facing challenges such as energy shortage and environmental deterioration.Therefore,pollution-free renewable energy shows an attractive prospect.Solar power generation has been promoted to a large extent because it does not generate pollution,has a sufficient power supply,and is widely applicable to distributed power systems.Modular multilevel converter(MMC)has been recognized as the potential topology to become the next generation of large-scale photovoltaic grid-connected inverters due to its modular design,expansibility,and fault-tolerance capabilities.Therefore,this thesis studies the modulation strategy of MMC grid-connected inverter in photovoltaic power plant,the control strategy of grid-connected current,the strategy of capacitor voltage equalization,and the start-up control of the MMC grid-connected converter in photovoltaic-power plant.Firstly,this thesis analyzes the working principle of MMC inverter and proposes the mathematical model of MMC.On basis of this,the fluctuation of capacitor voltage of inverter sub-module and the composition of circulating current are analyzed.This thesis further proposes a carrier phase-shift modulation method based on directly arm current control for MMC converter.The effectiveness of proposed modulation strategy has been verified through simulations.Secondly,this thesis studies the grid-connected current control strategy of MMC inverter.In order to improve the adaptability of the MMC grid-connected inverter to the AC grid,this thesis studies the directly arm current control strategy for asymmetric grid voltage.In order to improve the dynamic and steady-state performance of the system and effectively suppress the impact of grid voltage quality degradation,a composite control strategy based on repeated control and deadbeat control is proposed.This thesis also gives the parameter design method of the controller.The stability of the bridge arm current controller is analyzed in depth and the effectiveness is verified by simulation.In addition,this thesis studies the voltage balancing strategy of capacitor for MMC grid-connected converter.The voltage balancing strategy of capacitors consists of two layers,the arm voltage balancing and the individual voltage balancing.In this thesis,an individual voltage balancing strategy with a simplified algorithm is proposed,and the effectiveness is verified by simulation.At last,the start-up control strategy of MMC grid-connected inverters in photovoltaic power plants is studied.The charging process is divided into a controllable charging stage and an uncontrollable charging stage.In this thesis,the parameter selection of the current limiting resistor is studied on basis of the analysis of uncontrollable charging process.When uncontrolled charging period is over,the capacitor voltage is still lower than that of normal state.This thesis further proposes controllable charging strategy to ensure stable start-up of the MMC grid-connected inverter in photovoltaic power plant.According to the input status of the module,the modules are grouped to make multiple modules in the bypass state,and certain modules are charged to ensure that all modules can be smoothly charged to the rated voltage.The reliable start-up of MMC grid-connected inverter in the photovoltaic power plant is thus guaranteed.