Research On A Single-phase Full-bridge Photovoltaic Grid-connected Inverter With Common-mode Current Suppression Capability

Energy shortage has become a problem that plagues the sustainable development of all countries in the world.Traditional fossil energy power generation will cause varying degrees of pollution to the environment.At the same time,national policies do not encourage individuals to use fossil energy.At present,single-phase photovoltaic systems mostly use non-isolated inverters to reduce volume and cost.One of the major technical problems of non-isolated inverters is that after removing isolation transformers,common-mode currents need to be suppressed,otherwise it may cause electromagnetic interference and even endanger personal safety.Traditional gridconnected inverters have many disadvantages,such as low efficiency,high harmonic content and large common-mode current.Based on the study of the current inverter circuit topology,an improved midpoint clamped Heric circuit topology is proposed to reduce the common-mode current of the non-isolated grid-connected photovoltaic inverters in small and medium power applications.In this thesis,the advantages and disadvantages of traditional disturbance observation method and variable step disturbance observation method are compared.A variable step disturbance observation method for photovoltaic cells is simulated and realized.The simulation results show that the variable step observation method has good dynamic response performance and strong anti-interference ability.In this thesis,the mechanism and harm of common-mode current in grid-connected inverters are studied.The suppression ability of full-bridge,H6-bridge,Heric topology and midpoint clamped Heric circuit to common-mode current is emphatically studied.An improved midpoint clamped Heric circuit topology is proposed,which has certain advantages in common-mode current suppression ability,converter efficiency and control strategy.Based on the state space averaging method,the mathematical model of Heric inverters is established,and the double-loop control strategy of voltage outer-loop current innerloop is designed.The control parameters of PI controller as voltage outer-loop and QPR controller as current inner-loop are designed and optimized,and the stability of the control strategy is analyzed.Because the third harmonic of grid-connected current is serious in non-isolated grid-connected photovoltaic inverters,based on the study of the generation principle of third harmonic of grid-connected current,an optimization method for DC bus voltage secondary ripple compensation is proposed and simulated.The simulation results show that the proposed control strategy effectively improves the grid-connected current waveform,and verifies the feasibility of the proposed method.In addition,this thesis designs a digital phase-locked loop based on DSP,and successfully realizes the gridconnected operation of the improved midpoint clamped Heric inverters.The simulation and experimental results verify the correctness of the theoretical analysis.An experimental prototype of low power inverters based on improved midpoint clamped Heric circuit is designed and developed.The experimental results verify the correctness of the theoretical analysis.