Study On Photovoltaic Grid-connected Generation System With Single-phase Non-isolated Inverter

The increasing consumption of traditional fossil energy has brought about global problems such as energy shortage and environmental pollution.Therefore,the rational development and utilization of renewable energy has gradually become a significant means for countries to solve energy and environmental problems.Photovoltaic power generation is one of the most important utilization forms of green and environmentally friendly solar energy.Since photovoltaic panels directly output direct current,in order to transmit power to the grid for grid-connection,an inverter is also required to convert direct current into alternating current.Compared with isolated grid-connected inverter,non-isolated grid-connected inverter has smaller volume,lighter weight,lower cost and higher efficiency,which is the development direction of grid-connected inverter in the future.However,it faces the problem of high-frequency leakage cuurrent,which will increase the loss of the system,cause potential safety hazards to the equipment and even endanger personal safety.In this paper,the photovoltaic grid-connected power generation system with single-phase non-isolated inverter is taken as the research object,and the topology of non-isolated inverter and the control strategy of photovoltaic grid-connected power generation system are analyzed and studied in detail.The generation mechanism and influencing factors of common-mode leakage current of non-isolated photovoltaic grid-connected inverter are analyzed,and an equivalent circuit for leakage current analysis is constructed,and the relevant ways to suppress leakage current are discussed.The inverter topologies with low leakage current which were proposed by domestic and foreign scholars are researched,according to the evolution law between topologies proposed by scholars,Heric topology is transformed and a new Neutral-Point-Clamped topology is obtained at last.Then the working mode of new topology is analyzed,it is theoretically explained that it disconnects the electrical connection between the photovoltaic cell and the grid during the freewheeling stage,and the midpoint-to-ground voltage of the two bridge arms is clamped to half of the DC bus voltage through the midpoint clamp branch,so as to keep the common mode voltage constant,thus the leakage current could be suppressed.A simulation model of the new topology is built to verify its ability to suppress leakage current.Adopting a two-stage inverter structure,and the maximum power point tracking control and inverter grid-connected control are implemented at the pre and post stages respectively.The Boost converter circuit is used as the pre stage-DC/DC circuit to achieve the boost function and impedance matching,and improved variable step perturbation and observation method is used to realize Maximum Power Point Tracking.The post stage uses the new Neutral-Point-Clamped topology as the DC/AC converter,and the single-phase phase-locked loop based on a Second-Order Generalized Integrator is used to complete the phase-locking of the grid voltage so that the grid-connected current and the grid voltage can remain at the same frequency and phase.As-for the grid-connected control,a double closed-loop control strategy of voltage outer loop and current inner loop is adopted,a Proportional-Integral regulator is used in voltage outer loop to stabilize DC bus voltage,a Proportional-Integral regulator with grid voltage feedforward is used in current inner loop to reduce the influence of grid voltage on current fundamental tracking and harmonics,and the LC filter is used to eliminate higher harmonics near the switching frequency.Finally,a simulation modelis built to verify the feasibility of the new topology application in photovoltaic grid-connected power generation system.