Sliding Mode Control Study Of Single-phase Photovoltaic Microgrid Two-stage Converters

In order to cope with the global warming crisis and the growing energy demand,it has become the consensus of mankind to vigorously develop renewable energy.Solar energy,as one of the clean and low-carbon renewable energy sources,has gained wide attention with its unique advantages.Photovoltaic microgrid is one of the effective forms to realize solar energy utilization.Among them,the two-stage photovoltaic microgrid includes a front-stage DC/DC converter and a back-stage DC/AC converter,which can realize independent control of the front and back stages and simplify the control algorithm.In order to improve the efficiency of the two-stage structure,the study of the maximum power point tracking(MPPT)of the frontstage DC/DC converter and the control strategy of the back-stage DC/AC converter has important theoretical significance and engineering value.Due to the strong nonlinear characteristics of photovoltaic power generation system,the traditional linear control cannot meet the high precision control requirements of the system when the solar irradiance,ambient temperature,load size and DC bus voltage change.Sliding mode control(SMC),which is robust to model errors and external disturbances and easy to implement,has been widely used for the control of nonlinear uncertain systems.In this paper,the Nonsingular Fast Terminal Sliding Mode Control(NFTSMC)in islanding mode and Integral Sliding Mode Control(ISMC)based on adaptive exponential convergence law in grid-connected mode are designed for the two-stage converter of single-phase photovoltaic microgrid.The details of the study are as follows:(1)For the two-stage single-phase Photovoltaic microgrid,firstly,the mathematical model of the photovoltaic module is established and the output characteristic curve of the studied photovoltaic array is analyzed;Then,the front-stage DC/DC converter is selected and the mathematical model of the front-stage Boost converter is established;Finally,filter selection,filter parameter design and mathematical modeling are performed for DC/AC inverters in islanding mode and grid-connected mode.(2)For the two-stage single-phase photovoltaic microgrid in islanding mode,NFTSMC is used for PV MPPT and islanding inverter control,respectively.To effectively improve the MPPT performance,the front-stage DC/DC converter adopts the Perturb and Observe(P&O)cascaded NFTSMC strategy: the outer loop P&O to obtain the photovoltaic reference voltage at the maximum power point,while the inner loop uses NFTSMC to make the PV output voltage track the reference voltage at the maximum power point;To provide high quality load voltage,the back-stage DC/AC converter uses the same NFTSMC as the front stage for regulating the load voltage to track its reference value.Finally,to balance the power of the single-phase islanding PV microgrid and to reduce the double fundamental frequency ripple on the DC bus voltage,dual-loop PI control is applied to the bidirectional DC/DC converter.Simulation results show that the proposed two-stage converter NFTSMC strategy is robust to various uncertainties and variations,thus enabling the islanding photovoltaic microgrid not only to obtain satisfactory MPPT performance but also to provide high quality AC power.(3)For two-stage single-phase photovoltaic microgrid in grid-connected mode,ISMC strategies based on adaptive exponential convergence law are used for Photovoltaic MPPT and grid-connected inverter control,respectively.To effectively improve the MPPT performance,the front DC/DC converter adopts a P&O cascaded adaptive exponential convergence law-based ISMC strategy: the outer loop uses classical P&O to obtain the photovoltaic reference voltage at the maximum power point,while the inner loop uses ISMC to make the PV output voltage track the reference voltage;To stabilize the DC bus voltage and provide high-quality grid-connected current,the grid-connected inverter also adopts a dual-loop control strategy: the outer voltage loop uses an adaptive exponential convergence law-based ISMC to keep the DC bus voltage constant and provide a reference value for the inner loop grid-connected control;The inner current loop also uses an adaptive exponential convergence law-based ISMC to ensure that the grid-connected accurately and quickly tracks the grid current reference value.The simulation results show that the designed ISMC not only provides good dynamic and steady-state performance of the grid-connected photovoltaic microgrid,but also enhances the robustness of the system and reduces the system chattering.