The Research On Transformerless Three-phase Three-level Grid-connected Inverter And Leakage Current

Photovoltaic grid-connected system as an important part of the new energy power generation system has spread more and more widely. As the link between the solar energy and the grid, inverter is an indispensable power conversion device. In contrast with isolated two-level inverter, transformerless three-phase three-level inverter has a lot of outstanding merits, has received widespread attention. However,in the system of transformerless three-phase three-level inverter, the leakage current is generated when the high frequency common-mode voltage imposes on the distributed capacitance because of the existence of distributed capacitance. The existence of leakage current causes many problems, so the leakage current must be suppressed. In present,leakage current suppression method of three-level inverter is a hot area of research, which is also the main research content in this paper.Firstly, the principles of three-level SVPWM modulation and neutral voltage balance control were introduced in this paper, as well as the design process of the closed loop control, which established the theoretical foundation for the follow-up study.Then the causes of leakage current in transformerless three-phase three-level grid-connected inverter system were analyzed in this paper. Based on the construction of common and differential mode models, the cause of low frequency component of common mode source was analyzed. Then the reason of large current virtual value and bridge-side current low-frequency resonance of the existing leakage current suppression method of improved LCL filter was analyzed. An improved leakage current suppression method was proposed, which changed the modulation method on the basis of improving the hardware circuit. The proposed method achieves the goal of leakage current inhibition and the line current reduction at the same time.At last, based on the MATLAB/Simulink environment, a 5 k W inverter model was designed and set up, the correctness of the proposed method has been proved by simulation. The three level grid inverter experimental platform based on DSP + FPGA control circuit was set up, experimental results were consistent with theoretical analysis and simulation results, which demonstrated the effectiveness of the leakage current suppression method proposed by this paper.