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

With the growing demand for energy from all over the world,solar energy is becoming increasingly used as a clean and renewable energy in various fields.PV systems consist of transformer grid-connected PV system and transformerless grid-connected PV system.The transformers provide galvanic isolation,and thus eliminats leakage current.However,the isolated transformer will increase the cost,volume and weight of PV system,and also affect the efficiency of the system.Thus,Transformerless grid-connected PV systems are attractive due to its low cost and high efficiency.However,the leakage current is one of the main problems of the transformerless PV inverters.The leakage current not only affects the quality of the grid current,but also harms human safety and also has EMI problem.In recent years,many interesting sigle-phase PV inverter topologies have been reported to deal with the leakage current problems.And there is a well-known conclusion that the clamping topology is better than the unclamping one in terms of leakage current reduction.In a single-phase photovoltaic grid-connected system,the output side is pulsating ac power,while the three-phase system has a constant ac power on the output side and a small dc-side capacitance.This subject takes three-phase dc-bypass inverter as the main research object,solves the leakage current problem by improving the circuit structure and modulation strategy,improves the power quality,and improves the reliability of the system.This article first determines the research content and research methods of this topic on the premise of introducing the current research status at home and abroad.Then,analysing the key factors that affect the leakage current,and establishing the common-mode model of the dc-bypass topology by considerring the switch junction capacitance.The effect of suppressive capacity was analyzed theoretically.The analysis results show that the conclusion of three-phase dc-bypass topologies is different from that of single-phase dc-bypass topologies.In order to further suppress the leakage current,this thesis proposes a new topology that increases the common mode loop impendence and a new modulation method that can reduce the range of the common mode voltage.The new scheme is simulated and verified.The results show the correctness and effectiveness of the new scheme for suppressing leakage current.Finally,the hardware circuit of the three-phase dc-bypass topology was built,and the control circuit based on the TMS320F28335 DSP and Xilinx XC6SLX9 FPGA was designed to experimentally verify the effect of the clamp switches on the suppression of leakage current and the proposed new scheme.The experimental results verify the correctness and feasibility of the theoretical analysis and the new scheme.