Research On Low Voltage Ride Through Control Strategy Of Photovoltaic Grid-connected Power Generation System

In recent years,photovoltaic power generation has occupied a very important position in new energy power generation,and its penetration rate is increasing in the power grid.In the event of a grid failure,if the photovoltaic system is operated off-grid,the reliability of the grid will be threated.Nowadays,in the case of grid failure,many countries have issued relevant operational standards.The standard clearly states that when the voltage drop occurs,the photovoltaic power generation system needs to continue to be connected to the grid,and provide some reactive power to the grid,and finally restore the grid to normal operation,then achieve low voltage ride through.This paper mainly studies the low voltage ride-through technology in photovoltaic power generation system,analyzes and discusses the LVRT technology of photovoltaic power system under reactive power support control strategy and protection strategy of crowbar unloading circuit in DC side based on linear ADRC respectively.The correctness of theoretical research is verified by Matlab simulation.Firstly,in general,the mathematical model and basic control method of direct-driven permanent magnet photovoltaic power generation system are introduced,including mathematical model of solar cells and output characteristics of solar cells,photovoltaic power system maximum power point tracking.The mathematic model of grid-connected inverter is studied and the double closed-loop control strategy of voltage outer loop and current inner loop is introduced.The simulation results show that the double closed-loop control strategy can ensure that photovoltaic power generation system works in grid-connected state and operate stably under the condition of unity power factor.It provides mathematical model and theoretical support for the LVRT study of the photovoltaic power generation system.Secondly,the operation characteristics of photovoltaic grid-connected power generation system under symmetrical fault are analyzed.For the symmetrical fault,there will be overvoltage on the inverter side,and a reactive power support control strategy that can achieve low voltage ride through is studied.This strategy can achieve the purpose of suppressing the impulse current from the output side of the inverter,because it has a grid voltage feedforward control link,which can redistribute the active and reactive reference currents.Moreover,at this time,the operating mode of the inverter is the static reactive power compensation mode,and there is a certain amount of reactive power support to the power grid.Next,a simulation is performed to verify that the control strategy is feasible.Finally,for the symmetrical fault,when voltage drop depth is larger,there will be unbalanced power on the DC side.The existing protection scheme is outlined.According to the shortcomings of the existing scheme,a novel low voltage through technique of crowbar unloading circuit based on linear ADRC is proposed.a crowbar unloading circuit based on thelinear ADRC theory is proposed.The new low voltage ride-through technology can make the remaining unbalanced power on the DC side to be consumed.At the same time,when the inverter is in symmetrical fault,the reactive power control strategy running in the STATCOM mode is coordinated to it toform a coordinated control strategy,providing reactive power to the grid,which enhances the low voltage ride through capability of the photovoltaic power generation system,and the control strategy is verified by the simulation.