Insulation Stress Soften And Optimization Design For Medium Voltage High-Frequency Transformer

Distributed renewable resources based future power system is the fundamental to achieve carbon peaking and carbon neutrality goals under the new development philosophy,in which energy router is the key infrastructure to achieve power management and optimization.Medium voltage high-frequency transformer(MVHFT)is one of the key components to achieve galvanic isolation in energy routers.Compared with bulky line-frequency transformers,it provides hundred times higher power density with the same insulation capability.Thus,there are three difficulties to be emphasized for its design,including marginal insulation capability,high electro-thermal stress and complicated design optimization.To ensure high reliability and long lifespan of MVHFT,the key challenge is to achieve multi-field stress grading with limited space.Previous researches on medium voltage high-frequency transformer have not dealt with main wall insulation and terminal structure design to meet the requirement of medium voltage insulation.At the same time,existing design methodology fails to resolve the contradiction between coupled fields’ stress grading and performance optimization.Therefore,the objective of this study was to investigate the design methodology of medium voltage high-frequency transformer stress grading and performance optimization with high power density requirement.The proposed methodology is essential for energy router demonstration with high withstand voltage,high efficiency and high power density,contributing to next generation power router research and application.The structure of this paper is shown as below:Firstly,this paper proposed a double-shielding-layers structure to resolve partial discharge issue of MVHFT.In this section,the characteristics of electrical stress of MVHFT are illustrated first,via E-field analysis of cascaded H-bridge based power router.Then,by modelling the electrical stress distribution in inner and outer shielding layers,the principle of partial discharge mitigation and shielding material parameter selection is proposed.Next,the mechanism of high frequency AC losses on shielding layer is investigated.A shielding structure is proposed to mitigate the AC losses generated by shielding layer.Finally,the methodology of inner shielding layer potential selection and structure design is proposed,to half the electrical stress.Secondly,a high-K material based terminal stress grading methodology is proposed,to resolve the challenge of triple junction on MVHFT terminals.In this section,the mechanism of E-field distortion and stress soar is investigated firstly.Then the surface impedance re-structure method is proposed to release the stress on terminals.Measurement on dielectric constant of stress grading materials with wide frequency,temperature and E-field domain characteristics was done in this section,to establish the stress distribution model of terminal covered by high-K materials.The proposed structure is validated under line-frequency and lightning strike voltage.The E-field is released efficiently on terminal at wide temperature range.The peak field strength in air is reduced to half,mitigating the breaking-down and surface corona on terminals.Thirdly,an optimization design method of MVHFT coupling Numerical Analysis(NA)and Finite Element Analysis(FEA)is proposed,to meet the requirement of multi-objective optimization,including power loss,thermal stress and power density.The proposed design method combining the said two analyzing approaches,to achieve rapid and accurate calculation.It provides a rapid rough global optimization at first step,and detailed local optimization based on FEA model furtherly.The said optimization approach leads to 15% winding loss reduction,and 45% thermal performance estimation accuracy increase.Finally,the MVHFT manufacture and validation process was defined in this paper,with a10Kv/100 Kw transformer demonstrated.Based on insulation structure and optimization methods mentioned above,MVHFT manufacture steps and key elements are summarized in this section,to ensure electrical,magnetic and thermal performance.The demonstration can achieve power density of 11.2k W/L,efficiency of 99.6% and temperature rise less than 75℃.From insulation perspective,it can pass 42 k V line-frequency withstand voltage test,85 k V lightning impulse test,and 13 k V partial discharge test.The said MVHFT has already been used in10 k V/2MW solid state transformer system,completing long time operation under rated voltage and power.