| With the shortages such as environmental issues,shortage of resources,grid security and reliability probiems increasingly exposed,the clean and new distributed generations(DGs)and distributed generation technology are being developedrapidly.However,the grid needs to take certain restrictions and isolations because of the negative effects produced by the DG units,which greatly weakens the efficiency of the DGs.Microgrid is proposed in this case,aiming to solve the contradiction between the grid and the DGs,and fully dig to the grid and users' benefit brought by the DGs.As a basic power electronic interface unit in microgrid,the reasonable and effective control of the microgrid inverter is a significant research area and directly determines the safe and stable operation of microgrid.This paper focuses on the smooth switching comprehensive control strategy of the photovoltaic-storage microgrid.Firstly,mathematical model and output characteristics of photovoltaic cell are analyzed.Based on the analysis,a photovoltaic maximum power point tracking(MPPT)improved algorithm combined Lagrange quadratic interpolation with variable-step conductance increment method is proposed and verified under the MATLAB/Simulink.Then,to ensure the output of power of photovoltaicsystem,PQ control system which includes outer power loop and inner current loop control system is designed based on analysis of the principle of PQ control method.Secondly,the mathematical models of three-phase cascaded voltage source inverter based on the three-phase coordinate system,?? two-phase static coordinate system and dq two-phase rotating coordinate system are explained and the transformational matrix among the models are inferred.To ensure the stability of system voltage and frequency under island mode,energy storage system is set as the main control unit.V/f control system and droopcontrol system combined droopcontroller with outer voltage loop and inner current loop controller are designed based on analysis of the principle of V/f control method and droop control method.Finally,comprehensive analysis of peer-to-peer control and master slave control,a comprehensive control strategy ofphotovoltaic-storage microgrid system is proposed based on the analysis of photovoltaic system control and energy storage system control.The mothed is as follows.PQ control is always used on photovoltaic system to ensure the output of power of photovoltaicsystem.Droop control method is used on energy storage system to finish plug and play underthe grid-connected mode.The frequency and voltage of microgrid system are detected and judged in order to determine the control mothed of energy storage systemunder the island mode.The state self-adaptive switch method of controller is used,when the control mothed of energy storage system is switching between droop control and V/f control method.Finally,a comprehensive control system composed by PQ control system,droop control system,V/f control system and state self-adaptive switch system is designed.The simulation results show that comprehensive control strategy can realize microgrid system stable control of voltage and frequency and reduce the switching times of energy storage inverter. |
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