Research On Photovoltaic Grid- Connected Inverter Based On Sliding Mode Control

With the increasing of energy demand and the increasingly serious environmental pollution,solar energy as a clean energy has been attracted much attention,and photovoltaic power generation is an important way of solar energy utilization.In order to improve the efficiency of photovoltaic power generation system and the quality of grid-connected power,the pre-circuit MPPT control strategy which based on sliding mode control and the post-stage grid-connected control strategy for the rear circuit are proposed in this paper,so as to achieve safe and stable grid-connected operation.The main work of this paper includes the following aspects:(1)The basic theory of sliding mode control is analyzed,sum up the basic principle of sliding mode control.The basic design method of sliding mode controller and the design method of second-order sliding mode controller based on adaptive super twisting algorithm.The selection of sliding surface,control rate,the solution of parameters,the advantages and disadvantages of sliding mode control are analyzed.The causes of chattering of sliding mode controller and the general methods of reducing chattering are analyzed in detail,and proposes the method which using improved saturation function instead of symbolic function to reduce chattering.(2)Research on MPPT control strategy.The output characteristics of photovoltaic cells are analyzed,and by simulating the light intensity and temperature in real environment are simulated by simulation,the output characteristic curves of photovoltaic cells under different conditions are obtained.MPPT control algorithm is studied,In order to further improve the tracking accuracy of MPPT,several common MPPT algorithms,such as constant voltage tracking,conductance increment,disturbance observation,are analyzed and compared,and a MPPT algorithm based on improved sliding mode control is proposed.The simulation models of several common MPPT algorithms and MPPT algorithms of improved sliding mode control are built in Matlab/Simulink.The feasibility of the new control algorithm is verified by comparing and analyzing the simulation results.(3)Presented the topology of three-phase LCL photovoltaic grid-connected inverter,analyzed and established mathematical model.The state equation under the two-phase rotating coordinate system is derived according to the coordinate transformation,and the LCL filter parameters are designed.The principle,advantages and disadvantages of active suppression method and passive damping method are analyzed,useing the active damping method of capacitive current proportional feedback which used to suppress the resonance peak generated by LCL filter at a specific frequency.(4)Research on control strategy of grid-connected inverter.The principle and characteristics of several common grid-connected control strategies are analyzed,the double closed-loop PI control strategy and the parameter design of PI controller are analyzed and studied.Aiming at the problem of unsatisfactory dynamic response characteristics and robustness in traditional control,a double closed-loop sliding mode control strategy based on adaptive super screw algorithm is proposed.In this strategy,the strategy voltage outer loop adopts improved sliding mode control,and the grid current inner loop adopts sliding mode control of the adaptive super screw algorithm.The design of sliding mode controller is analyzed in detail,and its specific design process is given.The stability of the system is analyzed by using Lyapunov stability criterion.The anti-interference ability,dynamic response characteristics and the harmonic content of gridconnected current in photovoltaic system are studied by using MATLAB/Simulink,and the feasibility of the proposed control strategy is verified.(5)Software and hardware design of grid-connected inverter.An experimental prototype of 10 kW LCL grid-connected inverters is developed by using the DSP control chip of model TMS320F28335,the hardware and software of the whole system are designed,and the prototype is tested to verify the superiority of the proposed control strategy.The results of simulation and experiment show: The MPPT algorithm of improved sliding mode control is used in the front boost circuit to ensure the rapidity and accuracy of maximum power point tracking for photovoltaic cells.The double closed-loop sliding mode control strategy based on improved adaptive super screw algorithm is adopted in the back-stage inverters,which improves the robustness and dynamic response characteristics of the system and reduces the harmonic content of grid-connected current.The new control strategy proposed in this paper effectively improves the power generation efficiency and the quality of grid-connected power.