Research On Leakage Current Suppression Method Of Transformerless Three-phase Photovoltaic Inverter

In order to ease the urgent pressure brought by the energy and environmental crisis,it is imperative for us to develop new energy which represented by solar energy.As the main form of solar energy utilization,to improve the conversion performance of the core device photovoltaic inverter is of great significance to enhance the efficiency of the whole system and realize high quality electric power transmission.Among the many categories of inverters,the outstanding advantages of transformerless PV inverter have made it to be a research focus at this stage.but at the same time,the leakage current is generated because of the transformer eliminates consumes part energy delivered by the solar cells and seriously affects the conversion efficiency of the inverter,For the issues stated above,numerous research methods are from the hareware topology and modualation technology in this stage.The former one can complete the target of leakage current suppression,but it also makes cost,device volume and other defects increased.Although the latter one does not exist these defects,it usually causes the DC voltage utilization and output current quality decreased and other insignificances.Based on the modulation method,the correlation algorithm aimed at three-phase inverter leakage current suppression are systematically analyzed,and an appropriate improvement is made to realize the optimization for the leakage current and other related proformance of the inverter to a certain extent.The main content are as follows:(1)This dissertation introduces the working mode of the three-phase two-level inverter and diode-clamped three-level inverter at First.On the basis of fully understanding its working principle,a leakage current analysis model of transformerless three-phase PV grid-connected inverter with all the parasitic parameters is developed.Based on this mode,the root reason of the leakage current and the influence for leakage current caused by various parasitic parameters are analyzed and discussed.Thus proving the advantage of modulation scheme for the leakage current suppression.(2)For three-phase two-level and three-level inverters,the corresponding principle of each traditional SVPWM(Space Vector Pulse Width Modulation)algorithm is analyzed in detail,And from its principle analysis shows that the discontinuous five-stage and unipolar modulation mode have a certain suppression function on the leakage current.This has laid the foundation for the improvement of the modulation algorithm in following text.(3)Some of the existing leakage current suppression method based on the modulation were described in principle.The leakage current,DC voltage utilization and output harmonic distortion rate of various methods are compared and analyzed.And on this basis,an NSV-PWM leakage current modulation method for three-level inverter is proposed.This method achieves the goal of reducing the leakage current of PV inverter system without changing any hardware topological structures.And at the same time,it can also optimize the DC voltage utilization and harmonic content and other performances.(4)The dissertation builds a general simulation model of three-phase two-level and three-level inverters based on the Matlab/Simulink software platform,and the conclusions of this dissertation are simulated and analyzed.The simulation results indicate that the NSV-PWM algorithm has a balanced performance in all aspects which consistent with the results of the principle of the algorithm.