Techology Research Of T-type Three-level Grid-connected Inverter

With the application of new energy power generation around the world become more and more widely, the technology of grid-connected power generation has became an important research direction. Compared to traditional two-level three-phase bridge inverter, three-phase three-level inverter has the advantages of lower output voltage harmonics, higher output waveform quality, lower switching loss, lower switching device voltage stress and the forth, and it is widely used in medium voltage high power applications. In this paper, the study is based on T-type three-phase three-level inverter topology which uses the space vector pulse width modulation, the emphasis of the study are the imbalance of DC-link midpoint potential and the leakage current in practical application.Firstly, this article discusses the principle of LCL-based T-type three-level grid-connected inverter, and gives a brief analysis on the topological structure and commutation process under different operating conditions. On this basis, this article makes a modeling analysis of T-type three-level circuit, which is used for the control of grid-connected current. Meanwhile, for the LCL filter, this article studies its advantages and disadvantages in the grid-connected inverter applications, analyzes the design and impact of its parameters, and gives the design process.Secondly, this article takes a focus on the problem of leakage current in these two SVPWM algorithms, based on seven-segment SVPWM, it analyzes the influence mechanism to leakage current of the connection between the DC-side capacitor midpoint and AC-side filter capacitor midpoint, and gives a detailed simulation and experimental validation; based on LMZVM/LMSVM, it analyzes the vector selection method of suppressing leakage current, and combined the strategy of shorting the DC-side capacitor midpoint and the AC-side filter capacitor midpoint and LMZVM/LMSVM, to achieve the goal of reducing the leakage current.Aiming at the problem of DC-side midpoint potential balance of T-type three-phase three-level inverter, this article takes the method of hysteretic control which is based on the reassign of action time of small vectors redundant state. when the potential deviates beyond the threshold, based on the deviation direction, by increasing the small vector action time which makes the midpoint potential change to normal direction, and decreasing the corresponding redundant small vectors action time to achieve the goal of balance of DC-side upper capacitor and lower capacitor. This method effectively improves the output phase voltage waveform and makes a comparison with LMZVM/LMSVM in controlling the balance of midpoint potential.Finally, this article focus on the methods of suppressing leakage current and controling midpoint potential balance, then verifies the feasibility of the proposed methods by building simulation platform and experimental equipments.