The Aging Assessment Of Oil-paper Insulation With Dielectric Spectroscopy

The transformer is one of the necessary equipment in power system,so its safe and stable operation is the key to ensure the safety of power grid.A large number of studies have shown that the transformer fault mainly comes from its insulation degradation.How to evaluate the exact aging condition of transformer insulation has become an urgent technical problem.Frequency domain spectroscopy has the advantage of non-destructive measurement,user-friendly control,with rich information and strong anti-interference ability.Thus it is widely used for condition evaluation of oil-impregnated transformer in recently years.But most previous studies is to analyze the qualitative relationship between dielectric response parameters and the insulation aging condition,few studies research the quantitative relationship between them.In order to solve this problem,this paper establish Cole-Cole model and study the relationship between the model parameters and the insulation aging condition.This paper designed multiple exponential piecewise fitting method surrounding dielectric response theory and extended Debye model.Taking a group of different aging degree transformer oil as an example,calculate the parameters of five different relaxation branches.Then conversing the time domain to frequency domain and calculating the frequency domain dielectric parameters(for example,complex capacitor,complex permittivity and dielectric loss factor).Based on these data,"Cole-Cole graph could be obtained,then analyzes the effect on Cole-Cole graph by the changes of the Debye model parameters and insulation aging degree.To find the relationship between parameters in Cole-Cole model and insulation aging degree,the nonlinear least squares fitting method is designed to extract the parameters.This paper also designed the oil-paper insulation aging and testing system and the dielectric spectrum tester ID AX 300 was used to test frequency domain data.Then analyzes how complex permittivity and dielectric dissipation factor change with the change of aging degree and test temperature,and extract the Cole-Cole model parameters.The algorithm calculating frequency domain dielectric parameters basing PDC current is proposed in this paper,to a certain,it solves that the time domain test method carry little information and frequency domain test cannot be achieved in very low frequency band.It is concluded that transformer oil-paper insulation conform Davidson-Cole model due to the distribution of the relaxation time.And parameter characterization of arc deviation degree of semicircle and relaxation time decreased with aging degree deepen,but the difference of steady-state permittivity and high frequency permittivity increased with aging degree deepen.The research on dielectric spectrum insulation diagnosis in this paper has a certain application value.