Study On Leakage Current Reduction For Transformerless Three-Phase Three-Level Photovoltaic System

Photovoltaic(PV) power generation is one of the most effective solutions for today’s energy crisis. Many researchers are focusing on the transformerless grid-connected inverters which as an important part of PV grid-connected power system, due to the unique advantages of high efficiency, small size, and low cost. Because there is no electrical isolation between the PV array and the grid, large common-mode leakage current will occur, which increases the grid-connected current harmonics and the loss of the system. In order to realize the transformerless grid-connected inverter safely run, how to reliably suppress common-mode current becomes one of the key focus and issue for the researchers.Based on the analysis of three-phase three-level Neutral Point Clamped(NPC) PV grid-connected inverter, two simplified three-level Space Vector Pulse Width Modulation(SVPWM) algorithms are analyzed and compared. With the purpose to balance neutral-point potential for NPC three-level inverter, a method called the zero-sequence voltage injection is proposed which can be applied to both Sinusoidal Pulse Width Modulation(SPWM) and SVPWM, according to the results on simulation.Based on the common-mode current reduction of transformerless PV system, inverters topology and modulation strategy are improved. For the NPC, the three-phase neutral-point-clamped inverter(3x NPC), different connection of filters, improved LCL filter are discussed. The common mode models are given. The change of the ground current is proposed as the parameters vary. For the two-stage structures,the influence of the different front-end boost converters and modulation on the ground current is analyzed. Finally, the suppression effect of several topologies on the common mode current is compared and analyzed by the simulation results.Focus on the suppression problem of common mode current, which in transformerless NPC three-level PV inverter, the common-mode voltage is reduced or the inverters topology is improved. Based on the analysis of improved topology with capacitive voltage dividers on the common mode current in two-level inverter, the capacitive dividers are joined in DC bus positive and negative side in NPC three-level PV grid-connected inverter. The common mode models are given, common mode current is reduced, and midpoint potential fluctuation is controlled.The reduction of common mode current can be done to reduce common-mode voltage, according to the improved SPWM and SVPWM modulation strategies. Finally, the suppression effect of improved modulation strategies on the common mode voltage is compared and analyzed by the simulation results.According to the theoretical analysis and simulation model, a 1kW experimental platform has been done in the thesis. Modiation strategy and the control methods are verified by the experimental platform results.