Research On Electric Field Distribution Characteristics Of Uhvdc Dry Bushing Considering Low Frequency Dielectric Relaxation

With the vigorous construction of UHVDC transmission projects,UHVDC dry bushing as a key equipment,which is still the bottleneck restricting the development of UHVDC projects in China.In the process of operation and test,UHVDC dry bushing will not only bear the working voltage,but also bear the abrupt change voltage caused by polarity reversal and fault blocking.The main insulating material of its capacitor core is epoxy composite material.The low frequency dielectric relaxation under the combined action of temperature gradient and high field will have a great influence on the electric field distribution before and after the polarity reversal of the bushing.However,there is still lack of in-depth research on the influence of lowfrequency dielectric relaxation to the insulation characteristics of bushing at home and abroad.Therefore,it is of great significance to study the electric field distribution characteristics of epoxy composite materials for UHVDC dry bushing under polarity reversal voltage.Aiming at the temperature gradient,high field strength and polarity reversal operation conditions of UHVDC dry bushing in long-term operation,this thesis comprehensively considers the low-frequency dielectric relaxation processes such as σ polarization of epoxy composite impregnated paper,polarization of epoxy/impregnated paper interface,carrier dispersion,etc and studies the influence of low-frequency dielectric relaxation on the electric field distribution of bushing.Specifically completed the following contents:Because of field strength,temperature and polarity reversal voltage have a influence on the low-frequency dielectric relaxation process of impregnated paper,which includes lowfrequency polarization of epoxy/impregnated paper interface and carrier dispersion.The lowfrequency dielectric relaxation process of insulating corrugated paper was fitted and analyzed.At the same time,the variation rules of time constant,relaxation intensity and other characteristic parameters of different dielectric relaxation processes of insulating corrugated paper with time,field strength and temperature were obtained;and the change characteristics and expression forms of low-frequency time domain dielectric characteristics of epoxy impregnated paper were determined.Combined with the microstructure characteristics of UHVDC dry bushing insulation material,the distribution characteristics of AC and DC electric fields of single-layer epoxy impregnated paper at different temperatures were simulated and calculated.It was clear that the maximum field strength of epoxy composite material under AC/DC electric field appeared at the interface area of epoxy/impregnated paper.According to the actual size of UHVDC dry bushing,the finite element simulation analysis model is established,and the distribution of AC/DC electric fields of bushing under isothermal and temperature gradient is calculated.It is found that the temperature gradient has more obvious influence on the distortion of DC electric field distribution of bushing.Considering the influence of temperature gradient and low-frequency dielectric relaxation,the electric field of multi-layer epoxy impregnated paper composites under polarity reversal voltage was analyzed.It was found that the low-frequency dielectric characteristics greatly influence the electric field distribution of multi-layer epoxy impregnated paper composites and the maximum electric field strength shifted to the corrugated paper layer under polarity reversal voltage.Analyzing the influence of equal temperature and temperature gradient respectively,the electric field distribution in DC dry bushing under different polarity reversal time is calculated.It is found that the maximum field strength in bushing before and after polarity reversal is higher under temperature gradient;At the same temperature and temperature gradient,the maximum field strength area after polarity reversal will migrate from the flange attachment to the vicinity of the central conductive rod,which may accelerate the aging of the insulation material in this area combined with the higher temperature of the central conductive rod area.