Research On Photovoltaic MPPT And VSG Inverter Based On Full-order Terminal Sliding Mode Control

Photovoltaic(PV)microgrid distributed generation is an effective method to address the energy crisis,and research on its key technologies is of great significance.This thesis focuses on the maximum power point tracking(MPPT)control technology of PV generation system at the front end and the DC-AC inverter control strategy at the back end,conducting in-depth research.The incremental conductance(INC)method is a widely used MPPT control technology in engineering due to its high stability and simple control structure.However,traditional INC algorithms have fixed step sizes,resulting in low tracking response speed and poor accuracy.Optimizing the step size can improve the performance of the INC control.The introduction of power electronic devices in the grid-connected process of PV systems reduces grid inertia and damping,enhancing stability during load fluctuations and regional faults.The virtual synchronous generator(VSG)control technology can simulate the operation of synchronous generators in PV distributed systems,thus improving the stability of microgrid power systems.During the operation of the photovoltaic microgrid system,both the PV array and inverter power electronic devices are subject to component parameter perturbations and external disturbances.Therefore,robust control is required for the front-end MPPT control system and the photovoltaic virtual synchronous generator(PV-VSG)control system to ensure their effectiveness.Sliding-mode control(SMC)can eliminate the impact of uncertainties on PV-VSG,improving control accuracy and stability.However,traditional sliding-mode control inevitably introduces chattering in the control signal,which reduces control accuracy and response speed.The adoption of chattering-free full-order terminal sliding-mode(FTSM)control can eliminate the influence of chattering.In this thesis,based on the proposed front-end improved variable step size INC algorithm and the back-end PV-VSG inverter control strategy,a robust tracking control system based on FTSM is designed for PV distributed generation systems.This system aims to eliminate the influence of uncertainties on the system and improve the photovoltaic distributed system’s generation efficiency and grid stability.The main research contents of this thesis are summarized as follows:(1)For PV-VSG MPPT control technology,an improved variable step size INC algorithm based on FTSM is proposed.By combining the PV cell P-U curve,the INC step size is calculated based on the voltage fluctuation near the steady-state operating point to reduce voltage fluctuations near the steady-state operating point of the INC algorithm.The step size compensation factor is optimized by exploiting the angular relationship between PV cell output power variation and voltage variation,reducing the step size increment when encountering large variations in light intensity and temperature,and avoiding steady-state output fluctuations.To ensure the stable tracking of the given voltage by the DC-DC circuit using the INC control,a robust control algorithm based on sliding-mode voltage feedback loop is designed.To address the chattering issue in traditional sliding-mode control,chattering-free FTSM optimization is applied to the sliding-mode control design,enabling the Boost circuit to track the reference voltage provided by the improved variable step size INC algorithm quickly and accurately,thus improving the efficiency of the photovoltaic power source.Simulation results validate the superiority of the proposed control methods.(2)Considering the characteristics of the photovoltaic microgrid system,the overall PV-VSG system is designed,and an adaptive virtual inertia algorithm is proposed.Virtual rotor,virtual governor,and virtual excitation regulator are designed to simulate the operating characteristics of synchronous generators.Based on this,an adaptive virtual inertia is introduced to reduce the fluctuations in active power and frequency of the system output.The design of a DC energy storage section is implemented to maintain a constant DC bus voltage,providing frequency and voltage support for PV-VSG during grid-connected and islanded operation.By analyzing the transfer functions and establishing the mathematical constraints of the state of charge(SOC)on the system’s virtual damping and inertia,the parameter optimization range is determined,and the overall PV-VSG design is completed.Simulation results validate the rationality and effectiveness of the system design.(3)For the PV-VSG inverter,a voltage-current closed-loop control algorithm based on FTSM is designed.Firstly,the mathematical model of the inverter based on the dq-coordinate system is established.Taking into account the parameter perturbations of filtering inductance and capacitance caused by temperature and frequency variations and external disturbances,both voltage outer-loop system with unmatched uncertainties and current inner-loop system with matched uncertainties are established.For unmatched uncertainties,FTSM-based virtual control with backstepping method is designed for voltage tracking control of the inverter,improving control accuracy and eliminating control signal chattering.Simulation results validate the effectiveness of the inverter control algorithm.(4)Finally,a grid-connected inverter experimental platform is built to verify the feasibility and effectiveness of the virtual synchronous generator system design and the FTSM tracking control algorithm for the inverter.