Analysis Of The Factors Influencing The Electrification Of Oil Flow In Transformer Main Insulation And Its Suppression Method

In oil immersed power transformers,when transformer oil flows through the surface of insulation pressboard,oil flow electrification will occur.This phenomenon causes the electric field distortion and partial discharge in the insulation structure of transformer,thus damaging the insulation of the transformer.In order to suppress the phenomenon of oil flow electrification,the main methods currently used are improving the transformer structure design and adjusting the oil temperature,insulation material moisture,and other suppression methods.These methods have the disadvantage of increasing manufacturing costs or complexity.Therefore,this thesis proposes a method of using modified insulating oil paper to suppress oil flow electrification.This method can effectively suppress oil flow electrification,save resources and reduce costs,and can also be easily and conveniently applied to the transformers.In response to the suppression of oil flow electrification in the main insulation structure,this thesis prepared nano Ti O₂ modified pressboard and benzotriazole（BTA）modified insulation oil to analyze their modification mechanisms.The rotating disk system is used to simulate the operating conditions of the transformer main insulation,and the oil flow electrification current of modified oil paper is measured,and its influencing factors are analyzed.Through COMSOL Multiphysics,the simulation model of the main insulation structure of the transformer is established,and the electric field distortion in the transformer oil duct after the oil flow electrification is simulated.The inhibition effect of the modified materials on the oil flow electrification current and charge accumulation is verified,which provides theoretical and technical support for restraining the oil flow electrification of the main insulation for the transformer to improve the service life of the transformer.The main work and conclusions of the thesis are as follows:（1）According to the oil flow electrification current-charge density model,a rotating disk system is designed and built,the oil flow electrification current is measured,the charge density accumulated at the pressboard is calculated,and the relevant influencing factors are analyzed.As the speed of the oil flow and the contact area of the oil paper increase,the charged current of the oil flow will rapidly increase.As the temperature increases,the charged current of the oil flow shows a trend of first increasing and then decreasing.The charge density accumulated on the pressboard is less affected by the flow rate,and gradually decreases with the increasing of the oil temperature;（2）Nano TiO₂ modified pressboard and BTA modified insulating oil are prepared,and oil flow electrification current measurement experiments are conducted.For nano-Ti O₂ modified pressboard,only 1 wt%nano-Ti O₂ modified pressboard can reduce the oil flow electrification current and the accumulated charge density on the pressboard.For BTA modified insulating oil,with the increasing of BTA concentration in the oil,the oil flow electrification current and the accumulated charge density on the pressboard gradually decrease.After comprehensive consideration,1 wt%nano Ti O₂modified pressboard and 100 mg/L BTA modified insulation oil are selected as the methods to suppress oil flow electrification.At ambient temperature,compared with the unmodified main insulation system,the oil flow electrification current measured by this suppression method is about70.6%lower,and the accumulated charge density on the pressboard is about 69.8%lower;（3）The simulation model of the main insulation structure of the transformer is established,and several physical fields such as electrostatic field,temperature field and laminar flow field are coupled,and the corresponding equations are solved.For the main insulation system using 1wt%nano Ti O₂modified pressboard and 100 mg/L BTA modified insulation oil,after the occurrence of oil flow electrification,the extreme value of electric field distortion at the winding is only 83.4%of the original one,while the extreme value of electric field at the oil channel is 75.2%of the original one.