| Recently,with the expansion of domestic photovoltaic(PV)industry and the increase of its application forms,PV system is more significant for power grid.If grid voltage drops due to faults or disturbances,the sudden off-grid of PV system will further affect the stability of grid.Therefore,it is of great practical significance to study Low Voltage Ride-Through(LVRT)technology of PV generation system.In this thesis,the model of two-stage grid-connected PV generation system,control strategy under unbalanced voltage sag,LVRT strategy of PV system and considering DC-side with storage and load unit are further studied.The main work and research results are as follows.Firstly,the output characteristics of PV cells are analyzed through the mathematical model.A variable step disturbance observation method relevance to illumination conditions is proposed to realize max power point track.A two-stage photovoltaic grid-connected power system model is built by using the double-loop grid-connected control strategy of voltage outer-loop and current inner-loop.Based on the power equation of photovoltaic system under normal and drop conditions,the relationship between DC side voltage and grid-side voltage sag is deduced.Secondly,by analyzing the influence of voltage negative sequence component on the operation of two-stage PV system under unbalanced voltage conditions,the positive and negative sequence components of voltage and current are separated by using the second-order generalized integral method to deal with negative sequence component of grid-side voltage.The positive and negative sequence reference current control technology and secondary notch filter method are studied to suppress the second-harmonic fluctuation of active power on the grid side and the negative sequence component of the output current of the inverter.The effect of corresponding control strategy is verified by simulation.Thirdly,based on technical standard of LVRT,the relationship between voltage sag depth and reactive power demand and dynamic active reference current limitation are established to generate reactive power and realize current limitation.Then the control strategy of active power feed-forward(APFF)is proposed to solve the problem that the DC-side bus voltage rises sharply because of voltage sag on the grid side.The operational reference active power of inverter is fed to DC/DC converter of PV array to stabilize the DC-side capacitor voltage.The feasibility of APFF mode to control DC-side voltage fluctuation during LVRT is verified by simulation under various operating conditions.Finally,the model of two-stage PV system model whose DC-side contains storage and load unit is built,a control strategy of LVRT based on the power limit of converter is proposed.Except the single signal of DC bus voltage change within the voltage hierarchical control mode,the power limit of converter and depth of voltage sag are added to control the operation mode of each unit.It is faster for micro-grid to realize power balance and stabilize DC bus voltage to finish LVRT of PV system with storage and load unit.The simulation can verify the feasibility of control strategy based on power limit of converter. |
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