Research On Non-Uniform Ageing Characteristics And Non-Destructive Evaluation Method Of Transformer Oil-Paper Insulation

As the main equipment of power system,oil-immersed power transformer is very important to maintain its safe and stable operation state.In view of the non-uniform ageing phenomenon of transformer main insulation after long-term operation and the problem that it cannot be quantitatively evaluated,this thesis starts with the non-uniform ageing of transformer main insulation,and studies the dielectric properties and non-destructive evaluation method of oil-paper insulation non-uniform ageing.In order to explore the influence of non-uniform ageing on the dielectric properties of oil-paper insulation in the frequency domain,according to the non-uniform ageing state,this thesis builds radial,axial and two directions non-uniform ageing oil-paper insulation models.The frequency domain dielectric response tests of the models are carried out,under variable temperature and oil replacement conditions.The frequency domain dielectric response variation rule of the non-uniform ageing models are analyzed.In addition,compared with the variable frequency domain dielectric spectrum of the uniform ageing models with the same structure,the influence of non-uniform ageing on the frequency domain dielectric response curve is analyzed.Considering the non-uniform ageing and the temperature gradient inside the transformer under operating conditions,the field strength distribution of the main insulation of the coaxial structure will be directly affected.And its influence on the field strength distribution of the main insulation is difficult to obtain intuitively through experiments.Therefore,in order to explore the influence of non-uniform ageing on the field strength distribution of the main insulation,this thesis establishes the two-dimensional simulation models of radial and axial main insulation according to the main insulation structure,and conducts the coupled simulation of temperature field and electric field.The thermal conductivity,frequency-domain dielectric response and electrical conductivity of several groups of oil-paper insulation samples with different ageing degree are tested at varying temperatures to obtain the material parameters required for the simulation.Compared with the simulation results of the field strength distribution in the unaged and uniform ageing states,the non-uniform ageing changes the field strength distribution of the main insulation of the transformer under operating conditions.The innermost oil gap of the main insulation near the hot spot area bears the highest field strength and needs to increase the insulation margin.To realize the non-destructive quantitative evaluation of the non-uniform ageing of the main insulation,the equivalent dielectric relaxation model of the oil-paper insulation with non-uniform ageing is constructed,and the quantitative evaluation method of the non-uniform ageing of the radial,axial and overall main insulation is proposed.The frequency domain dielectric response curves obtained by laboratory and field tests are calculated.The calculation results are compared with the physical and chemical performance test results of oil-paper insulation samples collected in laboratory and field tests.The results show that the relative error of the polymerization degree calculation is less than 4%,and the error of the moisture content calculation is less than 0.3%.The calculation results are consistent with the actual ageing state of the transformer.The accuracy of the evaluation method in this thesis and the validity of its engineering application is verified.