| High-voltage bushings are critical parts of transformers and circuit breakers,which play an important role in the safe and stable operation of power systems.More than 80% of bushings in-service are the Oil Impregnated Paper(OIP)condenser type and this design is analyzed in this work.Nearly 25%~30% transformer failures are caused by faults in high voltage bushings connected to transformers and about 40% bushing failure was caused by moistened and aging insulation defects.However,traditional measurements are insensitive to the various insulation defects of bushings,which can cause erroneous judgment.Therfore,distinguish and quantitative evaluation of moistened defects and aging defects of OIP bushing was realized by the frequency domain dielectric response technique,which play an important role in the safe operation of OIP bushing.This paper mainly carries out three aspects of research work: First,an equivalent geometric OIP condenser bushing model was designed and a laboratory OIP condenser bushing model was fabricated to simulate the moistened defects and thermal aging.Secondly,the frequency domain dielectric response characteristics of OIP bushing under different moistened status and aging degree were studied and the method of distinguishing moistened defects and aging defects was presented.Finally,the method of distinguishing moistened defects and aging defects was provided and the characteristics parameters of moistened defects and aging defects are extracted to quantitative evaluation of moistened defects and aging defects.The evaluation method of moistened defects and aging defects of OIP bushing was studied and verified.The main results of this paper are as follows:(1)A dielectric response test platform was established and an equivalent geometric OIP condenser bushing model was designed using COMSOL Multiphysics according to the principle of insulation structure.Then a laboratory OIP condenser bushing model was fabricated to simulate the moistened insulation defects and thermal aging of real OIP bushing and study the influence of moistened insulation defects and thermal aging on frequency domain dielectric response of OIP bushing insulation.(2)The dielectric response characteristics of the OIP bushing were obtained and moistened and aging defects is distinguished.Aging and moistened defects can be distinguished accoding to difference of dissipation factor frequency band on OIP main insulation and the change of capacitance frequency band on insulation oil.The dissipation factor of OIP bushing mian insulation increases obviously in the whole frequency range due to moisture ingress,while aging of OIP bushing model causes dissipation and capacitance increased mainly in the low frequency and has little impact in the middle and high frequency.Moreover,moistened insulation oil causes capacitance increased obviously in the low frequency and has little impact in the middle and high frequency,while the capacitance of aged insulation oil increases less in the whole frequency range.(3)The dielectric dissipation integral factor extracted from the measured dielectric response curves can be used to evaluate the moisture content and aging degree of the oil-paper condenser bushing model.Moreover,the quantitative relationships among the dielectric dissipation integral factor,moisture content and polymerization degree follow an exponential law.With the increase of moisture content,the dielectric dissipation integral factor increased exponentially.With the increase of aging degree,the dielectric dissipation integral factor decreased exponentially.(4)The effectiveness of the method for evaluating the moisture and aging state of OIP bushing based on frequency domain dielectric response is verified by the field test.The influence of the measured temperature and the measured voltage on the dielectric response of the main insulation and insulating oil of OIP bushing model is still analyzed.Moreover,a FEM model was built to demonstrate the equivalence between OIP bushing model and real OIP bushing and a condition assessment process of OIP bushing insulation was presented based on the frequency domain dielectric characteristics of OIP bushing model.Finally,the evaluation results of field FDS measurements initially verified the feasibility of the frequency dielectric response technique served as a tool to diagnosis faults. |
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