The Study On Dielectric Response Characteristics Of Oil-film Insulation In Power Capacitor After Electrical And Thermal Aging

As the key equipment for filtering and reactive power compensation in power system,the good insulation state of power capacitor is the reliable guarantee for the safe and stable operation of power system.However,in the decades long working cycle of capacitors,not only frequent switching will make the capacitors subject to switching overvoltage,but also long-term exposure to the outside,the influence of direct sunlight will also lead to the high temperature rise in the capacitor.The internal insulation of power capacitor,oil-film dielectric will degrade or even fail under the action of factors such as electric and thermal aging,which leads to the failure of capacitor.It seriously threatens the operation of power grid.Based on the above problems,it is particularly necessary to study the failure mechanism of internal insulation of the capacitors.Finding a high-quality,efficient aging degree detection technology and extracting the relevant characteristic quantity as the diagnosis criterion.In this paper,from the perspective of engineering practice,aiming at the problem of how to grasp the insulation state change of capacitor insulation under the cumulative effect of electric and thermal stresses.Firstly,the polarization-depolarization current method（PDC）is selected as the insulation detection method of capacitor.In order to quantify the deterioration of internal insulation of capacitor,the Debye model of oil-film insulation was identified by using matrix pencil algorithm.Based on the dielectric response theory,the insulation diagnosis parameters,insulation resistance R₀and aging factor tanβ,are proposed in time and frequency domain respectively.In order to simulate the cumulative aging process of capacitors in the field,the impulse voltage generator and temperature control box are used as electrical and thermal accelerated aging platforms,respectively.The oil-film dielectric samples were prepared by using oil cup,polypropylene film and benzyltoluene to simulate the internal insulation of capacitor.The PDC curves of samples under different aging cycles were measured,and the variation of diagnostic parameters was analyzed.The experimental results show that,the PDC curve of the sample is raised by the electric stress and the thermal stress respectively.The insulation resistance of the sample decreases greatly when the impact number reaches 300,while the insulation resistance of the sample decreases almost linearly due to the thermal aging.The deterioration of oil-film dielectric makes the spectrum curve of tanβshift up,especially at high frequency.Furthermore,a new oil-film sample was made by using three electrode structure to investigate the influence of oil and film aging on depolarization current curve and Debye model parameters,respectively.The results show that the film aging mainly affects the middle part of the depolarization current curve.The oil aging will make the first half of the depolarization current curve move up.From the perspective of Debye model,only when the film is aged,the R_iand C_ivalues of the larger time constant branch and the intermediate time constant branch in Debye model will increase and decrease with the continuous aging of the film,respectively.While the R_iand C_ivalues of the branch corresponding to the small time constant do not change significantly.When the insulating oil is aged merely,the R_ivalue of small time constant branch in Debye equivalent circuit will decrease and C_ivalue will increase,which is just opposite to the change result of film aging.The parameters of intermediate time constant branch have the same change law,but the change range is small.The above parameters of large time constant branch can be considered almost unchanged.Finally,the experimental results are explained in combination with the microscopic changes of oil-film dielectric.It can be considered that,PDC method can effectively and accurately reflect the insulation state of oil-film dielectric,R₀and tanβcan quantify the change of insulation performance more carefully,which can provide reliable theoretical support for insulation diagnosis and failure mechanism research of field capacitor.