Study On Nonlinear Characteristics Of Transformer Oil–Paper Insulation Dielectric Response And Its Time–Frequency Conversion Method

Transformers are crucial pivot equipment for normal operation of the power grid.Their safe and reliable operation is of great significance to the stability of the power system.Dielectric response measurement technology has the advantages of no damage to the insulation,rich insulation information,and easy implementation.At present,it is the most effective technology for the diagnosis of the insulation state of oil-immersed power transformers.The dielectric response test can be achieved by time domain and frequency domain methods.Due to different insulation states,transformer oil–paper insulation will exhibit different nonlinear characteristics in the test,affecting dielectric response test results.In addition,for test process,the shortcomings of poor anti-interference ability in the time domain test and the long time consuming of low frequency band in frequency domain test limits the application of the dielectric response method;for evaluation process,the evaluation strategy of combining the time domain and frequency domain dielectric results can more accurately characterize the insulation state of the transformer.At present,the time–frequency domain conversion method is often used to obtain the time domain and frequency domain dielectric response information from one test.However,the conventional time–frequency domain conversion method cannot be equivalent to a nonlinear oil–paper insulation system,resulting in conversion errors.Therefore,to study the effect of different insulation states on the nonlinear characteristics of transformer oil–paper insulation,and to propose a time–frequency domain conversion method of dielectric response effective for both linear and nonlinear insulation,have important reference value in engineering application for measurement and evaluation of the dielectric response technology for transformer oil–paper insulation.In this thesis,a dielectric response test platform capable of realizing special irregular waveform excitation output is first established.Oil–paper insulation samples with different moisture contents and aging degrees are prepared through natural moisture absorption and accelerated thermal aging experiments.Then,the influence of moisture and aging of oil–paper insulation on dielectric response test results was tested.The nonlinear coefficients were extracted to characterize the degree of nonlinearity,and a microscopic explanation of the causes of the nonlinear phenomenon was put forward.Finally,in view of the limitations of the conventional linear time–frequency domain conversion method for nonlinear oil–paper insulation,a dielectric response time–frequency domain conversion method with both linear and nonlinear characteristics is proposed.The effectiveness of this method is verified by oil–paper insulation samples in laboratory and field transformers.The main results obtained in this thesis are:(1)It has been found through experiments that moisture and aging of the oil–paper insulation can cause the generation of odd harmonic components in the response current,resulting in nonlinear characteristics.The increase of the moisture level of the oil–paper insulation makes the nonlinear coefficient of the u–i characteristic curve in the form of a concave power function,showing forward nonlinear characteristics.The dielectric loss is positively correlated with excitation voltage amplitude.It is considered that this nonlinearity mainly caused by the diffusion of water molecules and the increase of free ions.Aging makes the nonlinear coefficient of the u–i characteristic curve in the form of a convex power function,showing reverse nonlinear characteristics.The dielectric loss is inversely related to excitation voltage amplitude.It is considered that this nonlinearity mainly caused by weakening of the effective electric field by the space charges and the blocking effect of cellulose chain.(2)The limitation of the time–frequency domain conversion method based on extended Debye model and Fourier transform applied to nonlinear oil–paper insulation system is analyzed.When the oil–paper insulation exhibits nonlinear characteristics due to moisture and aging,the two conventional time–frequency domain conversion methods are no longer effective.The frequency domain spectrum of tanδ in low-frequency band converted from the time domain dielectric spectrum are compared with those by the standard sweep test and results in a large error,which fails to meet the accuracy requirements of the test and affects the accuracy of the dielectric response test and evaluation results.(3)Hammerstein–Wiener equivalent mathematical model with both linear and nonlinear characteristics is proposed as a time–frequency domain conversion bridge for the dielectric response of oil–paper insulation.Hammerstein–Wiener model adds input and output non-linear links before and after the dynamic linear link,respectively,and can be equivalent to both linear and nonlinear insulation systems.According to the test results of oil and paper insulation with different levels of moisture and aging,it is found that Hammerstein–Wiener model can effectively characterize linear and nonlinear oil–paper insulation.The time–frequency conversion accuracy of the dielectric response is more than 90%,and the accuracy and reliability of the conversion results are guaranteed.Several different grades of transformers are used as experimental objects to verify the effectiveness of the method for filed appliance.