Study On Dielectric Properties Of Insulation Medium In Valve Side Bushing Of Converter Transformer

As the core power equipment in the HVDC transmission system,the converter transformer plays an important role in inverter and electric rectification.The valve side bushing of the converter transformer not only has to withstand the effects of the DC electric field and the AC electric field,but also withstands the polarity reversal electric field.At the same time,with a large number of harmonics,this poses a great threat to the insulation performance of the valve side bushing of the converter transformer.In recent years,the frequency domain dielectric spectroscopy method is used to diagnose the insulation performance of the valve-side dry bushing of the converter transformer,which can easily find potential faults and insulation defects.This subject is based on the converter transformer dry casing epoxy resin impregnated paper insulation as the research object.This study aimed to investigate the effects of temperature and aging on the characteristic parameters of dielectric spectra of epoxy resin impregnated paper insulation.The variation of dielectric properties of epoxy resin impregnated paper insulation in different stages of partial discharge(PD)development was revealed under the action of long-term AC stress.A broadband dielectric spectra test platform and a PD experimental platform was established.A bushings model was prepared and an aging test was performed.Variation of the complex permittivity and dielectric loss factors of epoxy resin impregnated paper at different temperatures and different degrees of aging was revealed.The dielectric spectra test results were fitted using the Havriliak-Negami(HN)relaxation model.The characteristic parameters were extracted,and the influences of temperature and aging on the characteristic parameters of the HN model were analyzed.Scanning electron microscopy and Fourier transform infrared spectroscopy were used to assess changes in microscopic morphology and functional groups at different stages of PD development to reveal the influence of long-term AC stress on the dielectric properties of epoxy resin impregnated paper insulation.The results show that in the low frequency range,temperature and aging exert greater influences on the complex permittivity.In the high frequency range,temperature and aging have smaller effect.The relaxation time and temperature satisfy the relationship of the Arrhenius equation,and the relaxation strength increases exponentially with increasing temperature.The relaxation time τ,the relaxation intensity △ε,the shape parameter α,and the shape parameter β can characterize the aging degree of paper insulation.A linear relationship between relaxation and aging times,and an exponential relationship between relaxation intensity and aging time were revealed.As the PD progresses,the insulation performance deteriorates;under continuous bombardment by high-speed electrons,the network structure of the fibre surface and the epoxy molecular chain are destroyed,causing the weakening of the binding force between the molecular chains,and the short-chain structure formed by cracking has a stronger steering ability.Under the continuous action of PD,the inside of the long-chain molecule breaks and oxidises,and a large number of small molecules,such as free radicals and monoglucose,are formed.The decrease in the molecular weight of the polymer leads to the weakening of the interaction between the molecules,resulting in an increase in polarisation,a large dielectric constant,and an increase in the dielectric loss factor.