Study On Impurities Effect On The Insulation Properties Of Mineral Oil And Vegetable Oil And Their Differences

During the operation of transformers,the generation of tiny impurities in insulating oil is inevitable.At present,impurities in insulating oil are considered to be one of factors that threaten the safe and reliable operation of large power transformers,which is also an important subject concerned by domestic and foreign researchers.Mineral oil and vegetable oil are widely used as liquid dielectrics in large transformers at home and abroad.However,the accumulation behavior of particles in two kinds of insulating oils is still unclear,and the law of influence of particles accumulation on the insulation strength of insulating oil is also not explicit.Moreover,the differences of the accumulation of particles in mineral oil and vegetable oil and their effects on insulation performance are rarely studied.Therefore,through the combination of experiments and simulation analysis,studying the accumulation characteristics of particles in mineral oil and vegetable oil under AC and DC voltage,as well as the impact of particles accumulation on the breakdown performance of insulating oil,can provide valuable reference for further understanding the transformer insulation fault caused by particles pollution and better management and control of oil quality.Firstly,current measurements of mineral oil and vegetable oil containing non-metallic particles(cellulose particles),copper particles,and mixed particles of fiber and copper under AC and DC voltage have been carried out to record the movement and accumulation behavior of particles in oil.This paper compares and analyzes the differences of particles accumulation state between mineral oil and vegetable oil,as well as establishes dynamics and accumulation simulation models of particles with different attributes.Secondly,the AC and DC breakdown voltages of mineral oil and vegetable oil containing particles with different concentrations were tested without pre-applied voltage and pre-applied voltage.The quantitative relationship between impurities concentration and AC or DC breakdown voltage has been constructed.Finally,the electrical conductivity and electric field distortion characteristics of oil caused by impurity pollution are analyzed.What is more,the difference of the impact of particles on the breakdown strength of mineral oil and vegetable oil is explored.The main achievements of this paper are as follows:(1)The movement and accumulation properties of particles in mineral oil and vegetable oil under AC and DC voltage have been obtained.The detailed process of the bridge formation of fiber particles,copper particles and mixed particles,as well as the corresponding relationship between particles accumulation and current of oil products are got through experiments.The particle accumulation morphology is divided into four stages according to characteristics of current variation.It is found that the DC voltage component and voltage action time are key factors for the bridge formation of particles.The velocity of particles movement and accumulation in vegetable oil is less than that in mineral oil.(2)A particles accumulation simulation model has been established and its accuracy has been verified by experiments.The simulation model of particles accumulation in insulating oil is constructed by using particles dynamics model,Fick's law and Laplace equation.The simulation parameters of the accumulation of fiber particles,copper particles and mixed particles in mineral oil and vegetable oil are proposed.The accuracy of the simulation model is verified by observing the particles accumulation behavior through experiments.Using the simulation model,particles accumulation behavior in oil and its influence on electric field distribution in oil can be studied under different voltages.(3)The quantitative relationship between particles concentration and insulation strength is obtained.The Weibull characteristic DC or AC breakdown voltage of insulating oil containing cellulose particles,copper particles and mixed particles decreases linearly with the increase of concentration,while the Weibull characteristic DC or AC breakdown strength of insulating oil containing mixed particles decreases exponentially.For the same concentration,breakdown strength with pre-applied voltage is lower than that without pre-applied voltage.The presence and accumulation of particles can increase the conductivity of oil and expand the range of electric field distortion,which seriously deteriorates insulation properties of insulating oil.(4)The reason why vegetable oil is superior to mineral oil in particles pollution resistance is revealed.Under the same experimental conditions,the saturated current of vegetable oil containing particles is lower than that of mineral oil,and the electric field distribution distortion in vegetable oil is less affected by particles accumulation than that in mineral oil,which makes breakdown voltage of vegetable oil higher than that of mineral oil.In addition,simulation results of particles accumulation behavior show that the kinematic viscosity of vegetable oil is higher than that of mineral oil,which is the reason for the difficulty of particles accumulation in vegetable oil.The results of molecular simulation analysis show that the interaction energy between vegetable oil and cellulose molecules is greater than that between mineral oil molecules and cellulose molecules,which is also one of main reasons for the difficulty of particle accumulation in vegetable oil.