Research On Transformerless PV Grid-Tied Inverter System

In recent years,the cost reduction of photovoltaic(PV)systems has made them a viable option for large-scale deployment of clean energy in the future.The main driving forces behind this trend are the continuous decrease in the cost of PV panels and the substantial subsidies provided by governments worldwide to promote clean energy.In PV applications,transformers are typically used to provide electrical isolation and voltage conversion between the input and output.However,traditional iron-copper transformers increase the weight,volume,and cost of the inverters,while also reducing efficiency and power density.To overcome the limitations associated with transformer-based inverters,transformerless inverters have been introduced.Taking into account the challenges and advantages of transformerless inverters,this dissertation conducts research,analysis,and design of two different types of transformerless inverter specifically for PV applications.The dissertation begins by summarizing the current types of transformerless PV inverter topologies and analyzes the half-bridge single-phase inverter and full-bridge single-phase inverter topologies.The losses of the system are calculated to address the issue of system losses,evaluate system efficiency,and compare and summarize the characteristics of various topologies.Further simulations of the system are carried out using software,and a thermal model is designed based on semiconductor device datasheets to improve accuracy.Among the challenging factors associated with these inverters are common-mode voltage,leakage current,number of semiconductor devices,cost,power density,and conversion efficiency.A novel zero-voltage switch control transformerless midpoint-clamped inverter is proposed to address the issues of common-mode voltage,leakage current,and efficiency.The circuit structure and unipolar sinusoidal pulse width modulation strategy of the proposed inverter are analyzed,and the operating principle and common-mode characteristics of the inverter are detailed.Based on this analysis,loss calculations and efficiency evaluations are conducted,and a comparison with traditional inverter is made to demonstrate the higher efficiency of the proposed solution.The proposed inverter is verified using MATLAB-SIMULINK and PLECS toolboxes,and experimental validation is carried out on a hardware prototype.The verification results show that compared with the traditional midpoint-clamped transformerless inverter,the proposed inverter has advantages such as maintaining common-mode voltage,suppressing leakage current,and low conduction losses.A variable DC-link multilevel inverter based on single-phase switched capacitors is proposed to address issues related to voltage levels,semiconductor device voltage stress,and power losses.The circuit structure of the proposed inverter is analyzed,and the operating principle and different operating states are detailed.Comparative analysis,thermal modeling,and design guidelines are also conducted.Finally,a five-level inverter with two switched capacitor units is tested to validate the performance of the proposed multilevel inverter under different operating conditions and the effectiveness of the proposed solution.The verification results demonstrate that compared to traditional single-input switched capacitor-based multilevel inverters,this inverter has advantages such as fewer semiconductor devices,lower overall voltage stress,higher power density,and greater flexibility.In summary,this dissertation studied and evaluated transformerless inverter according to various standards,and conducted comparative analysis of the schemes from different dimensions such as system common-mode characteristics,component quantity,power density,and efficiency.Based on this analysis,considering the challenges and advantages of transformerless inverters,this dissertation researched,analyzed,and designed two different types of transformerless inverters for photovoltaic applications.