Study On Fractional Order Sliding Mode Control Strategy For LCL Photovoltaic Grid Connected Inverter

With the rapid development of social economy and science,environmental problems and energy crisis are increasing.As a result,the new energy generation technology represented by photovoltaic power generation technology is concerned.Grid connected inverter is the core link between DC power supply and AC power grid.It has great impacts on the power quality and energy conversion efficiency.In this paper,a fractional order sliding mode control strategy is designed for the current control of photovoltaic grid connected inverter.This paper studies the weakening of chattering in sliding mode control,and the numerical realization of fractional order system.Main achievements of this dissertation are as follows:(1)For the problem that the model of three phase LCL grid connected inverter has strong coupling and nonlinearity,and is susceptible to external disturbance,the sliding mode control method is proposed.Sliding mode control system does not depend on the internal parameters of the control object,and has strong robustness and strong anti-interference ability.It can realize the high precision control of inverter grid connected current.(2)For the problem of system chattering in traditional sliding mode control,the fractional order sliding mode control is proposed.The fractional calculus operator is introduced to increase the parameter freedom of the control system and optimize the controller flexibility.By using the memory characteristics of fractional calculus operator,the continuity of sliding mode control is enhanced and the switching gain of sliding mode control is optimized to reduce the chattering of the system.(3)To realize the numerical value of fractional calculus operator,the optimal Oustaloup filter algorithm is proposed.The traditional approximate algorithm of fractional operators generally has the problem that the computational process is complicated and the approximate accuracy is not ideal.The Oustaloup filter algorithm has simple structure and good approximation accuracy in particular frequency domain.By adding a specified filter,the optimal Oustaloup algorithm can be obtained to further improve the numerical approximation accuracy and broaden the approximate frequency width.The amplitude frequency response characteristic waveform of the optimal Oustaloup algorithm shows the superiority of the method.(4)Finally,the fractional order sliding mode control is designed for three phase LCL grid connected inverter.In view of the coupling in the mathematical model of the inverter,the inverse system method is used for linear decoupling,and the sliding mode surface and sliding mode control law are designed according to the subsystems obtained after decoupling.The simulation model of the control system of three phase LCL grid connected inverter is built in Simulink.Simulation results show that the fractional order sliding mode control strategy designed in this paper can suppress chattering and improve the performance of the controller.