Research On Low Frequency Oscillation Suppression Method Of Photovoltaic Grid-connected Based On Three-order Sliding Mode Variable Structure Control

In recent years,with the rapid development of cross-regional construction of my country’s power system,the requirements for long-distance power transmission have increased day by day.At the same time,the vigorous development of new energy technologies has raised the research on the penetration rate and capacity of large-scale photovoltaic power stations to a new level.Because large-scale grid-connection and long-distance transmission are prone to low-frequency oscillations,in order to strengthen the stability of the power system,this paper proposes a third-order variable structure sliding mode control method to suppress low-frequency oscillations.The main research work is as follows:First,the mechanism of low-frequency oscillation is analyzed,and the mathematical modeling of photovoltaic grid-connected is designed,including synchronous generator model,power system stabilizer model,transmission line model,transformer model,load model,large-scale photovoltaic power plant model,etc.Then the topological structure of the LCL single-phase inverter is introduced,the mathematical model of the LCL filter is established,and the LCL filter parameters are designed based on the principle of minimizing the total inductance and the minimum change of the resonant frequency when the grid inductance changes.The stability of the system when the power grid is resistive,inductive,and capacitive is analyzed respectively.A brief analysis of the current commonly used low-frequency oscillation suppression strategies,and their advantages and disadvantages are compared.Because sliding mode control can continuously modify the state of the system to make it move according to the pre-designed sliding surface,and because it is invariant to parameter changes and external disturbances,a sliding mode control method is proposed to suppress low-frequency oscillations,and the strategy is research.Finally,the sliding mode control is applied to the design of photovoltaic gridconnected controller.In order to eliminate the steady-state error generated by the inverter,an integral link is added on the basis of the traditional proportionaldifferential second-order sliding mode.A third-order sliding mode surface is designed and applied to the suppression of low-frequency oscillation of photovoltaic grid-connected.And use Matlab/Simulink to build a third-order variable structure sliding mode control simulation model.By comparing with the system frequency when only the PSS device is attached,it is verified that the control can effectively suppress the low-frequency oscillation in photovoltaic grid-connected.