Research On Soft-switching Topology Construction Method For High-frequency Transformerless Inverters With Full Power Factor Range

Grid-connected inverters in distributed photovoltaic power generation systems have developed from isolated to non-isolated type to reduce system cost and improve efficiency.Transformerless grid-connected inverters(TLI)have been researched for years and the key technologies of leakage current and DC component suppression have matured.Using wide bandgap(WBG)devices is an effective means to further improve the conversion efficiency and power density of TLIs and has attracted wide attention.However,there are some challenges,such as switch oscillation and switching stresses in practical application of WBG devices,which may jeopardize the reliable operation and EMI suppression of TLIs.By actively softening switching processes of WBG devices,soft-switching techniques contribute to stable and efficient operation of TLIs as well as high power density.Firstly,soft-switching topology construction method for TLIs with full power factor(PF)range is studied in this paper based on the analysis of TLIs’ modulation strategy at reactive power operation.This method focuses on the construction of two-quadrant resonance networks with wide applicability.The construction procedure includes resonance trajectory planning,resonant cell construction and auxiliary component integration in order to simplify the resonance network structure,from which both zero-current-switching(ZCS)and two types of zero-voltage-transition(ZVT)two-quadrant resonance networks are obtained.Secondly,the application method of ZCS resonance network in full-bridge TLIs is proposed,and a new family of switching loss-free TLIs with full power factor range(SLF-TLIs-FPF)is then harvested.The representative SLF-HERIC-FPF topology is selected from SLF-TLIs-FPF and researched in detail,including resonance timing and operation principle in both positive and negative power regions,resonance parameter design method,voltage and current stresses of power devices,differential-mode and common-mode voltage performances,et al.The calculation results based on loss model illustrate the advantage of SLF-HERIC-FPF in terms of power device losses.Furthermore,simulation and experimental results are presented and verify that the proposed SLF-HERIC-FPF can achieve SLF target and constant common-mode voltage performance under different PF conditions.Thirdly,the application method of ZVT resonance network Ⅰ/Ⅱ in full-bridge TLIs is proposed,and two families of ZVT-TLIs with full power factor range(ZVT-TLIs-FPF)are then harvested.The representative ZVT-HERIC-FPF topology applying ZVT resonance network I and ZVT-H6-I-FPF topology applying ZVT resonance network II are selected from two families of ZVT-TLIs-FPF and researched in detail,respectively.Based on operation principle analysis,it is pointed out that the proposed ZVT resonance network Ⅰ and Ⅱ are equivalent in terms of soft-switching implementation.A resonance parameter design method is proposed,which can minimize additional losses of ZVT resonance networks.In circuit performance analysis,the influence of ZVT resonance networks on voltage and current stresses of power devices,differential-mode and common-mode voltage performances is also evaluated,and the advantage of the proposed ZVT-TLIs-FPF in circuit performances is illustrated by comparison with several hard-switching topologies and other ZVT topologies.Furthermore,the experimental results of ZVT-HERIC-FPF and ZVT-H6-Ⅰ-FPF verify the soft-switching operation,circuit performances and efficiency advantage under different PF conditions.The research results of this paper have laid a solid foundation for the development of high frequency and high power density TLIs.