Analysis To Oil-paper Insulation State Of Transformer Using Modified Davidson-cole Dielectric Model

Oil-immersed transformer is at the core of power system operation.Oil-paper insulation,as the main insulation component of oil-immersed power transformer,is aging under the effects of temperature,moisture,oxygen,acid,electric field and mechanical force for a long time during operation,which leads to the decline of its mechanical behavior and insulation performance.Therefore,it is of great practical value in the power industry to accurately evaluate the aging and damp status of transformers,diagnose and evaluate the operation status and service life of transformers.Frequency domain dielectric spectroscopy is a new diagnostic method of dielectric response.Its measurement data are abundant.It is not easily disturbed by the on-site environment.And the measurement frequency band is narrow and the required test voltage is low.It is more conducive to on-site diagnosis of transformers.The polarizability,complex dielectric constant and dielectric loss factors measured by frequency domain dielectric spectroscopy can reflect the aging and damp degree of oil-paper insulation structure of transformer in varying degrees.However,in the current research on the evaluation and diagnosis of oil-paper insulation aging and damp state,the main method is to map the aging and damp state of oil-paper insulation by FDS spectral lines and conduct qualitative evaluation.There is a lack of deeper parametric analysis of frequency-domain dielectric spectrum,extracting Frequency-Domain Characteristic Parameters of oil-paper insulation,and quantitatively evaluating the aging and damp state of oil-paper insulation.Considering the effect of conductivity on polarization relaxation process of oil-paper insulation,an improved Davidson-Cole dielectric model is proposed by introducing DC conductivity and jump conductivity.The modified Davidson-Cole dielectric model parameters were used to diagnose and evaluate the moisture and aging status of oil-paper insulation.Because the improved Davidson-Cole dielectric model is an expression combines the real and imaginary parts of the complex permittivity with the model parameters,the complex permittivity spectrum measured by FDS is an irregular curve composed of several frequency-complex permittivity points,therefore,based on the least square method,the parameter extraction of the improved Davidson-Cole dielectric model is transformed into a non-linear optimization problem,and the parameters of the improved Davidson-Cole dielectric model are identified by using the hybrid frog leap algorithm.The characteristic parameters of Davidson-Cole dielectric model,such as relaxation time?,arc asymmetry?and DC conductivity?_dc,are identified.The accuracy of the identification results is verified by using particle swarm optimization and FDS curve reconstruction.The influence of characterization quantities M_c,C_k,DP and Av of oil-paper insulation state on the parameters of improved Davidson-Cole dielectric model was studied.The quantitative relationship between M_c,C_k,DP,Av and the parameters?,?and?_dcc of improved Davidson-Cole dielectric model was obtained by linear fitting method.The results show that there is a better exponential relationship between?and M_c,and a better exponential relationship between?and DP.There is a better triangular function relationship between?and C_k and Av.There is a better power function relationship between?_dcc and M_c,?.And a better power function relationship between?_dcc and C_k and DP.There is a good linear relationship between?and Av.Then the same FDS data and identification method used in this study.Unmodified Davidson-Cole dielectric model parameters were extracted.It is found that only the relaxation time?has a corresponding functional relationship with M_c,C_k,DP and Av,and the types of functions are consistent with the improved ones.But the goodness-of-fit of the function without improvement is less than that of the improved value.The validity of the improved Davidson-Cole dielectric model is verified in this study.And the concepts of frequency-temperature translation and translation factor are introduced in this study.The measured FDS curve of transformer oil-paper insulation is corrected by temperature.The interference of test temperature on the evaluation results of oil-paper insulation state is eliminated.Finally,a case study was carried out in the laboratory.It was found that the calculated value of the state characterization of oil paper samples was consistent with the measured value,which showed the reliability of the evaluation method in this paper.It is helpful to quantitatively study the oil-paper insulation state of transformer and diagnose the insulation state of transformer in practical engineering.