Research On Modeling And Control Strategy Of The PV Power Generation System Via MMC-based DC Transmission

China is rich in solar energy resources,but the load centers are far from the power sources.Considering the randomness and volatility of solar energy,the power transmission structure using flexible DC transmission to collect and transmit photovoltaic energy has many advantages.Maintaining the stability of power and voltage under normal operation and different fault operation conditions is the key to the safe and stable operation of photovoltaic power generation system.Based on this,this paper carries out a simulation study on the control and protection of large-scale photovoltaic generation system via flexible DC transmission.This paper takes the PV generation system via modular multilevel converter(MMC)DC transmission system as the research object.Designed a circuit structure of the photovoltaic DC transmission system through MMC.The working principle,control strategies during normal and fault operation are researched and analyzed.Firstly,the structure of the PV generation system via MMC-based DC transmission system is expounded,and the working principle of the high-frequency isolated photovoltaic grid-connected inverter system and the half-bridge modular multilevel converter is analyzed,respectively.Secondly,for the photovoltaic power generation system,the control logic of the front-stage DC-DC converter,the maximum power point tracking control method and the double-loop control method of the DC-AC inverter are studied,and the grid-connected control of the photovoltaic system is realized.Aiming at the double-terminal modular multilevel DC transmission system,the double-loop grid-connected control method,the double-frequency circulating current suppression method,the modulation,and the capacitor-voltage equalization method of the double-terminal converter station under normal operating conditions are analyzed.The PSCAD/EMTP simulation model of the PV generation system via MMC-based DC transmission system is constructed,and the transient and steady state processes of photovoltaic power generation system,DC transmission system and photovoltaic DC transmission system under normal operation are respectively simulated and analyzed in detail.Thirdly,the fault coordination control method of photovoltaic station,sending-end MMC converter station and receiving-end MMC converter station without communication line is proposed.In this method,the AC and DC side voltages of the converter station is used to detect the remote or near-end fault,and corresponding control strategies are designed to realize the fault ride-through.Using the constructed simulation model,three-phase symmetrical faults with different voltage sags in the receiving-end power grid,three-phase symmetrical faults with 100% voltage sag in the sending-end power grid,and DC bipolar short-circuit faults are simulated respectively.The simulation results verify the correctness and effectiveness of the proposed control strategy.Finally,the research content of this paper is summarized and the future research is prospected.The simulation model built in this paper and the proposed control strategy of the photovoltaic station and the MMC converter station at the sending and receiving end under normal operation and different fault operation have laid a certain foundation for the further analysis,such as the dynamic characteristics of the photovoltaic DC transmission system through MMC and the optimization of the controller performance.