Common-mode Current Suppression Methods Of Transformerless Bridge-type Inverter Based On Switching Functions

With large-scale mining of coal, oil, natural gas and other fossil fuels, global energy crisis and environmental pollution have become increasingly prominent. As a new clean renewable energy, solar energy is gradually being exploited. Photovoltaic (PV) power generation has become the main trend of solar energy utilization. Many researchers are focusing on the transformerless grid-connected inverters due to the unique advantages of high efficiency, small size, light weight and low cost. Because there is no electrical isolation between the PV array and the grid, large common-mode leakage current will occur if the system is designed unreasonably. It not only increases the grid-connected current harmonics and the loss of the system, but also endangers the safety of equipment and persons. In order to realize the transformerless grid-connected inverter run safely, how to reliably suppress common-mode current becomes the key focus and issue for the researchers.Based on the analysis of the generating mechanism and circulation loop of the common-mode current, the suppressing common-mode current universal method starting from the topology and modulation strategy is studied in this thesis. The newly defined switching functions are proposed to overcome the deficiency of the existing switching functions. The output of the switching functions is extended from integer to fraction. The common-mode voltage expression is described based on the new switching functions and the common-mode voltage constant criterion is deduced. The novel modulation strategies have been proposed, which can suppress common-mode current effectively. It is realized through reasonable sort of switching sequences to inhibit the common-mode current without changing any hardware. The method is also applicable to the three-phase inverter to suppress the common-mode current.The novel modulation strategies based on the common-mode voltage criterion has been verified through the control of three typical single-phase transformerless H4, H5, H6PV inverter topologies. Simulation results have verified the effectiveness of the proposed common-mode voltage criterion based on the new switching functions. According to the criteria, a variety of new modulation strategies have been concluded. On the basis of simulation analysis, the hardware design and control software program have been done in this paper. At last, a lkW experimental platform was built to validate the common-mode characteristics and the differential-mode characteristics of the ransformerless inverter. The experimental results have verified the correctness or this heory. The grid-connected current waveform is good and the system can run reliably.