Insulation Properties Of The Winding And Current Lead For HVDC Superconducting Fault Current Limiter

In high voltage direct current(HVDC)power systems characterized with small fault damping and large short-circuit current,the converters are incapable of resisting strong impact current,and it is harder to cut out the DC fault current as the current has no zero-crossing point.The superconducting fault current limiter(SFCL)can effectively limit the fault current due to the excellent properties such as low impedance in normal operation and much higher impedance under fault condition respectively,which has great significance to the development of HVDC system.The insulation system of the SFCL is challenged by the extreme environments such as high voltage and low temperature.The regional distortion of electric field caused by the accumulation of charge together with the thermal stress caused by the mismatch of thermal expansion coefficient in the temperature changeable condition can accelerate the failure of insulation.In this paper,the research on the insulation of HVDC SFCL winding and the current leads is carried out.By the methods of surface molecule structure modification and nano-inorganic filling,the properties of the insulation material of the winding and current lead are modified separately.And the tests of charge characteristics and breakdown properties of the insulation material were carried out.These studies are expected to provide theoretical experimental basis for the design of cryogenic insulation of the SFCL.The main research work and achievements are as follows:(1)According to the short-circuit fault simulation model of the ±200 kV flexible HVDC transmission system,the required impedance value of the SFCL which can make the short-circuit current suppression rate greater than 35% is acquired.It is discovered that when short-circuit fault happens between the two poles of the DC exit bus,the SFCL withstands pulse overvoltage considerably higher than the rated voltage.And the waveforms of pulse overvoltage and current are acquired.(2)By surface molecular structure modification of the polypropylene laminated paper(PPLP)and polyimide(PI)insulation of the superconducting winding and the measurements of the chemical structure characteristics and element components of the insulating materials before and after the modification,it is found that surface molecular structure modification can replace the H atom in the surface molecules of the material with F atom,and a fluorination layer with a thickness of several microns is formed on the material surface.By calculating the trap distribution based on the surface potential decay and isothermal discharge current,it is found that surface molecular structure modification can reduce the trap density of the material,causing the decrease of the trapped charges and effectively suppressing the charge accumulation.Due to the strong electronegativity of the element F,the fluorination layer can trap large number of electrons and form a shielding layer on the material surface to inhibit the injection of the charges into the bulk,which improves the distribution of the electric field and enhances the breakdown strength of PPLP and PI.(3)Based on the problems of the current lead insulation materials,the physical blend method was used to prepare the epoxy resin/AIN nanocomposite insulating material,and it is found that filling AIN nanoparticles increases the thermal conductivity and decreases the thermal expansion coefficient of the nanocomposite insulation.Using the surface charge measurement system under DC and pulse voltages,the influences of the pulse voltage on the surface potential characteristics of the nanocomposite insulation under DC voltage is acquired.It also reveals that filling AlN nanoparticles increase the density of deep trap in the nanocomposite insulation and improve the breakdown resistance of the epoxy resin/AIN nanocomposite insulation.