Research On Moisture Transient Distribution In Oil-Paper Insulation Under Composite Electric Field And Temperature Field

The stable operation of converter transformer is an important guarantee to maintain the safety of the power grid.Oil-paper insulation is the main form of insulation,and moisture can seriously reduce the electrical strength of oil-paper insulation.The operating environment of converter transformers is different from traditional transformers.During long-term operation,the presence of a composite electric field changes the distribution of moisture in the main insulation.At the same time,when checking the moisture content in the main insulation after shutdown,different detection times cannot guarantee that the oil-paper insulation in the main insulation reaches equilibrium only under the action of temperature.By now the traditional oil-paper insulation moisture balance curve/equation is no longer applicable.It is urgent to propose a non-destructive evaluation method to accurately evaluate the moisture content of the main insulation inside the converter transformer.In order to study the transient diffusion and balance law of moisture in oil-paper insulation of converter transformers during long-term operation,this thesis fully considers the moisture diffusion mechanism under AC/DC composite electric field and temperature field.Iterative calculation is used to simulate and analyze the transient diffusion process of moisture,and single and double sided moisture transient diffusion models under composite electric field and temperature field are constructed.Simulation shows that the thickness of oil-immersed cardboard the activation energy will have an impact on the transient distribution of moisture inside the oil-immersed cardboard.Under the action of a composite electric field temperature field,the direction of water diffusion inside the cardboard is related to the direction of the electric field,and the distribution of electrical resistivity within the cardboard micro elements directly affects the migration rate of water.The migration rate of water inside the oil-immersed paperboard is faster than when it is only affected by the temperature field,and the steady-state moisture content inside the paperboard is higher when the water balance at the oil-paper interface is reached.In addition,for the experimental part,this thesis adopts frequency domain dielectric response detection technology to conduct moisture evaluation tests on oilpaper insulation under different measurement temperatures,aging states,and moisture contents.NSGA-Ⅱ algorithm was applied to solve the test results,and the parameters of the extended Debye model of oil paper insulation were obtained.Quantitative evaluation of moisture content in oil-paper insulation has been achieved.At the same time,in the experiment of transient distribution of moisture in oilpaper insulation,according to the "secondary equilibrium" law of the frequency domain dielectric response curve,it was found that as the test temperature increased,when the moisture in the oil-paper insulation model reached the first steady-state equilibrium,the amount of moisture change in the oil-immersed cardboard gradually increased.The higher the test temperature,the more obvious the moisture change;Under the same magnitude of electric field stress,high temperature will promote the second steady-state equilibrium of water in the oil-paper insulation model.