The Study Of Cable Insulation Water-tree Aging Detection Technology Under The Variable Frequency Power Supply

XLPE power cable has become the mainstream of the current urban power supply grid,to operate the grid normally.In order to power grid can be normal operation and save unnecessary loss,it is very necessary that carrying out in-depth research about insulation diagnostic technique and relative Standard.Water-tree degradation is the dominant mode for XLPE power cable insulation,it influences service life of power cable directly.Effective diagnosis of water-tree degradation condition of XLPE cable is the premise of assuring XLPE cable operating safely.Under standard pure sinus voltage,the ideal insulation medium loss current is small,waveform distortion will not occur and have no harmonic components;Water-tree aged XLPE insulation has characters of nonlinear conductance,loss current will be distort,then can measure the harmonic component.Thus it can be seen that harmonic components method can diagnose water-tree degradation extent of XLPE power cable.Firstly,we use the test system which has successfully research and development for a lot of field test and briefly introduces the typical cases.According to the water tree aging degree is divided into different without water tree aging,mild water tree aging and a heavy water tree aging.On the basis of it,we deal with the field experiment data of 108 cables in Suzhou industrial park.Using loss factor method and loss current harmonic component method condition evaluation respectively.Evaluation results show that loss current harmonic component method more strict than loss factor method,more sensitive with the mild water tree diagnosis.As this paper use variable frequency power supply as a testing system of incentive power,how to different frequency of the test results into standard frequency will become urgent problem to be solved.By reading Chinese and foreign literature to solve nonlinear equations characterized conductivity of water-tree regions conductivity.Water-tree aging model had been established on the bases of it using Comsol software and simulation analysis.By simulation,we obtain some response current waveform of different water tree aging degree and.different applied excitation amplitude,frequency.And then according to the incentive voltage waveform get response current decomposition,and get loss factor and loss current harmonic component information.From the simulation results,the testing result in a sure frequency is difficult to directly convert the standard frequency test results at Different degree of aging experiment.In order to solve this problem,this paper put forward the convert method under nonlinear circuit parameters from the test results.This method can be test under any frequency in frequency resonant pressure frequency range.Processing the voltage and response current signal and getting the nonlinear circuit parameters.And then through the nonlinear circuit parameters obtained the loss factor and loss current harmonic information under the standard frequency.Simulation results have verified the feasibility of the proposed method of this study.