Numerical Calculation On The Influence Of The Water Content In Oil Over The Insulation Properties Of Bushing

Moisture intrusion which may lead to insulation deterioration is one of the main factors for the failure in oil-filled bushing.The internal structure of capacitor type bushing is multi-layered oil-paper insulation medium,moisture diffusion process can be very completed in this structure.This article compared moisture diffusion process under different temperatures by using eight-layered oil-paper insulation structure model.The simulation model was used to simulate the change of electro-thermal coupling field under damp conditions,which is of great significance to the research of bushing failure mechanism.In this paper,three aspects were studied.Firstly,the water transfer model was designed and the experiment of moisture diffusion was carried out under four different temperatures and different moisture content.Experimental results show that the correlation between temperature and moisture migration rate is the highest,each 10 ℃drop in temperature,diffusion time doubles.Cross-layer diffusion is the dominant moisture diffusion form,compared to the diffusion within pure paper,cross-layer structure has a greater influence on moisture diffusion,for the same diffusion distance,nearly five times of diffusion time will be needed.Secondly,the electric parameters and thermal parameters of oil-immersed insulating paper and insulating oil are tested for the simulation.In this paper,the dielectric constant,the tangent of the dielectric loss Angle,the volume resistivity,the specific heat capacity,and the thermal conductivity coefficient of oil-immersed insulating paper were measured under different moisture concentrations and different temperatures.Results showed that as the temperature increases,the dielectric constant of oil-immersed insulation paper increases after the first decreases,and around 80 ℃has a peak value.The dielectric loss index has a rising trend,the volume resistivity index declines with temperature,the specific heat capacity and thermal conductivity have an increasing linear relationship with temperature;As the moisture content increases,the dielectric constant and dielectric loss tangent increase exponentially,volume resistivity index has a decreasing trend exponentially,and the comparing results of moisture content of 1%and moisture content over 2%of the oil-immersed insulation paper,the volume resistivity index with 1%moisture content is about two orders of magnitude higher than other moisture content.Generally speaking,the whole dielectric properties of oil-immersed insulating paper would change rapidly as the moisture content changes.The specific heat capacity and thermal conductivity showed a linear increase trend with the increase of moisture content.Finally,through COMSOL finite element simulation software,the influence of the moisture diffusion process on the electro-thermal coupling field in oil-paper insulation structure in bushing is simulated.Result shows that when moisture migrate from the outside to the inside of the bushing,the moisture diffusion process near the conductor is obviously faster than elsewhere due to a higher temperature.When the moisture arrives to the root of the bushing,concentration "collapse" could be seen in the central part.Moisture migration rate has a high correlation relationship with temperature,but low correlation with concentration,thus the moisture diffusion rate is still in the distribution of temperature gradient mainly.Long time diffusion process of the moisture would lead to "collapse" in moisture distribution,which would bring corresponding"collapse" in conductivity distribution.Meanwhile,the radial maximum electric field reaches 1 kV/mm within electric field concentrated area,which is beyond the design margin.