Modeling And Simulation Of Connecting Photovoltaic Power Station To AC Power Grid Via DC Grid

In recent years,the global installed photovoltaic capacity has been increasing,and the centralized power stations can exert their resource advantages and reduce the generation cost,which has obvious advantages in the proportion of photovoltaic power stations.The centralized power stations are mainly distributed in the northwest region,and it is difficult to consume the electricity locally,so they need to be connected to the grid for transmission.Compared with AC power transmission,DC power transmission does not have the problems of frequency jitter and phase synchronization,and it also has advantages in transmission loss and reactive power compensation.Therefore,this paper takes photovoltaic power station grid-connected through DC power grid as the research object to study the converter topology and control strategy of grid-connected converter in detail.Firstly,the full-bridge isolated dc booster converter with active clamping function is adopted in this paper to boost the outlet voltage of photovoltaic modules and connect it to the DC distribution network of ±30kV.Adopts Modular Multilevel Converter(MMC)to connect the DC and AC system,each phase bridge arm of MMC consists of multiple child modules cascade,solves the problem of voltage withstand of switching devices in voltage source converters,On this basis,The Capacitance Complementary Submodule(CCSM)is designed and the working principle of CCSM is elaborated,which solved the problem of the large number of sub-module switching devices,which causes high engineering design cost.Secondly,this paper analyzes the physical model and output characteristics of photovoltaic cells,and the mathematical model of photovoltaic array is derived.The working mode of the full-bridge isolated booster converter is analyzed in detail,and the mathematical model is esablished according to the state-space average method,and the Maximum power point tracking(MPPT)and constant DC voltage control are realized.This paper deduces the mathematical model of MMC's three-phase static coordinate system and two-phase synchronous rotating coordinate system,and analyzes MMC's double closed-loop control strategy,nearest level modulation and submodule capacitor voltage balance control strategy.Thirdly,the steady-state simulation model of the photovoltaic power station's DC power grid connected to the AC system is esablished in this paper.Through the simulation waveform of MPPT,switch duty ratio and DC voltage value of the steady-state boost converter,the rationality of the DC circuit and the effectiveness of the controller are proved.Simulation results on DC voltage,AC voltage,AC current,active power and reactive power of MMC converter show the effectiveness of MMC controller.Finally,CCSM and Clamp Double sub-module(CDSM)are used to verify the steady-state operation effect of CCSM.In order to verify the isolation capability of CCSM under DC bipolar short-circuit fault,Half Bridge sub-module(HBSM),CDSM and CCSM are compared,which proves the practical significance of CCSM in isolating DC fault and reducing engineering cost.