Research On Influence Mechanism And Suppression Strategy Of Distribution Capacitance Of Double Split Transformer In Photovoltaic

MW-class photovoltaic power plants are generally composed of many photovoltaic power generation units.Analysis of the operating characteristics of photovoltaic power generation units is the basis for studying the operating characteristics of large-scale photovoltaic power plants.The photovoltaic power generation unit is usually composed of two 500 k W grid-connected inverters and a 1MVA double-split boost converter,due to price issues,radial boost double split transformers are generally selected in the project.The radial transformer and the axial transformer are quite different in the distributed capacitance parameters of the low-voltage winding.There is a large distributed capacitance between the two low-voltage windings of the radial boost transformer.Difficulties in the common operation of dual inverters and high-frequency common-mode circulation problems in photovoltaic power generation unit systems often occur in the project.Therefore,it is necessary to study the influence mechanism and suppression strategy of the distributed capacitance of the double split transformer on the photovoltaic power generation unit.The paper focuses on three aspects: the equivalent modeling of photovoltaic power generation unit,the effect of double-split transformer distributed capacitance on the DC voltage of the inverter,and the generation mechanism and suppression strategy of high-frequency common-mode circulating current of photovoltaic power generation system,the main research contents are as follows:(1)Based on photovoltaic power generation unit system,equivalent modeling of the grid-connected inverter mathematical model,double split transformer model and photovoltaic array distributed capacitance model.Lay a theoretical foundation for the subsequent discussion on the mechanism of the influence of the distributed capacitance of the three-phase radial double split transformer on the photovoltaic power generation unit.(2)Equivalent model of photovoltaic power generation unit based on distributed capacitance between low voltage windings of double split transformer.The influence mechanism of the distributed capacitance between the low voltage coils of the radial dual split transformer on the dual inverters is analyzed.The results show that,there is a leakage current loop between two inverters connected in parallel on the two low-voltage sides of the transformer;the increased DC bus voltage of the delayed grid-connected inverters in the photovoltaic power generation unit makes it difficult to operate the dual inverters together;and verified the correctness of mechanism analysis through simulation.Finally,a solution was proposed,and simulation and experimental verification were carried out.(3)Based on the common-mode model of the photovoltaic power generation unit,the generation mechanism of the high-frequency circulation in the photovoltaic power generation unit system is analyzed.For two commonly used modulation strategies SPWM and SVPWM,The characteristics of high-frequency circulation in one carrier cycle of two modulation strategies are analyzed in detail.Then the influence of the distributed capacitance between the low voltage coils of the double split transformer on the high frequency circulating current is analyzed.Finally,the correctness of the mechanism analysis is verified by simulation.(4)For the problem of high-frequency circulation in photovoltaic power generation system,a scheme for suppressing high frequency current of the inverter output filter is given.And verify the effectiveness of the suppression scheme through simulation.