Double Current Closed-loop Control And Improved NLC Modulation Applied In Grid-connected Pv System By MMC

Solar power,also named photovoltaic power(PV),in the early 1970 s,it has proved that PV can produce a lot of energy,without burning fossil fuels or nuclear reactors.PV has solved many power problems for human,and has the advantages of sustainability and no pollution.PV technology gradually develops in China.The grid-connected PV station becomes a key research direction in the future.Grid-connected inverter is the essential part of grid-connected PV system.MMC has become the most popular converter technique since its multilevel output voltage and low harmonic components.The thesis analyzes the key techniques of MMC applied in grid-connected PV system.Firstly,this paper summarizes the development background and research status of grid-connected PV system.The contents include DC/DC converter and its control strategies,the inverter and the key technologies of MMC.Next,the paper introduces the maximum power point tracking(MPPT)technique of the DC side in the grid-connected PV system.And then the paper analyzes the operating principles and the mathematical model of PV array,as well as the selection of the DC converter.In order to ensure the maximum output power of the PV array,this paper uses the conductance increment method to control the duty cycle of the Boost converter.In this paper,MMC acts as a grid-connected inverter.The MMC topology circuit,its operating principle and the capacitor voltage balance control are discussed in detail.This paper improves the conventional NLC technology of the MMC,so that the amplitude of the MMC output voltage can be adjusted to the change of grid voltage.The thesis proposes a new frame of PV system interfacing with the grid by 11-level MMC and LCL filter.The double currents feedback control are used to the whole system.In this new structure,not only the grid current is to be controllable,but also the grid-connected voltage quality is improved with the low THD.At last,through the simulation the paper can conclude that the improved NLC technique and the double currents feedback control can overcome the PV system's disadvantage of being vulnerable to changing weather and surrounding conditions,which obviously improves the feasibility of PV station operating in grid-connected condition.