| With the shortage of traditional energy and the increasing environmental pollution,the development and utilization of renewable energy has become the hot spot of people's research,Solar photovoltaic power generation is one of the most potential and rapidly developing clean energy.With the increase of photovoltaic permeability,the impact on the power grid is also growing,which in the power grid disturbance or failure,the PV system suddenly off the grid will bring serious consequences to the grid,and low voltage ride through(LVRT)technology as a powerful support for power grid failure recovery has become a key issue in the development of photovoltaic power generation.In order to ensure photovoltaic power generation can still be connected during voltage sag,domestic and international grid-connected standards require photovoltaic grid-connected power generation system should have certain low voltage ride through capacity.Firstly,this paper introduced and established the mathematical model of photovoltaic cell,maximum power point tracking(MPPT)control module and grid-connected inverter,and established the relevant simulation model in Matlab/Simulink software.The output characteristic curves of the PV cells and the simulation results of the MPPT are obtained by simulation.The validity and correctness of the model are verified,which provides the basis for the research of low voltage ride through..Secondly,the finite control set-model predictive control(FCS-MPC)of the inverter is studied.By analyzing the basic principle of FCS-MPC algorithm,a model of finite control set-model predictive control of three-phase grid-connected inverter is established.The algorithm can predict the control variables and the output of the system corresponding to all the switch states in the next period of time according to the mathematical model of the inverter and the state information of the system in each sampling period,and according to the cost function to select the switch state that optimizes system's performance acting on the inverter,in advance to correct the system bias in order to get the desired performance.The control method has the advantages of simple and easy to understand,no need of modulator,internal complete decoupling and fast dynamic response.The simulation results show that the control can achieve the unit power factor operation,the output current is in phase with the grid voltage to meet the grid-connected requirements.Finally,this paper puts forward the LVRT control strategy based on FCS-MPC for the low voltage ride through problem of PV grid-connected system.The domestic and foreign low voltage ride through standards are introduced.And the operation characteristics of the grid-connected inverter during the grid failure are analyzed.Then,the influence of grid failure on PV grid-connected power generation system is described,which provides a theoretical basis for low voltage ride through control.The simulation results show that the voltage sag detection method based on dq phase-locked loop can accurately and quickly detect the three-phase symmetrical voltage sag and provide the basis for the operation of low voltage ride through control.During voltage sag caused by grid fault,the photovoltaic cell is no longer using MPPT control and is converted into a reference power tracking control mode to reduce the active power delivered by the PV cell to the inverter.At the same time the inverter uses a control strategy with reactive power compensation based on FCS-MPC.The active current and reactive current reference value are reallocated to limit the overcurrent of the inverter and provide a certain amount of reactive power to support the grid voltage recovery.The simulation results show that the LVRT control strategy can suppress the increase of the DC side bus voltage,limit the increase of the output current of the inverter,and output a certain reactive power for the recovery of the grid voltage during voltage sag,achieved low voltage ride through. |
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