Study On Insulation Ageing Properties Of XLPE Cable Based On Ultra-low Frequency Dielectric Spectrum And U-I Characteristics

XLPE cable is one of the core equipment of urban medium and low voltage power transmission and distribution system.Its safe and reliable operation is of great significance to the stability of urban power supply and distribution system.Frequency-domain dielectric spectrum(FDS)method and 0.1Hz ultra-low frequency(VLF)method are commonly used in the diagnosis of XLPE cable insulation ageing.Among them,the test frequency band used by the traditional FDS method is narrow and high,as a result,the difference of dielectric loss of insulating materials in different ageing states is indistinguishable.Meanwhile,the ultra-low frequency method of 0.1Hz has low reliability for the insulation evaluation of slight ageing,and it is difficult to avoid the occasional errors caused by the application of single frequency point to the dielectric loss evaluation of cable insulation characteristics.Therefore,in view of the limitations of the traditional FDS and 0.1Hz-VLF method,the influence of different ageing on the insulation performance of XLPE cable was studied through ultra-low frequency dielectric spectrum(1m Hz～1k Hz),and the ageing state characterization method of cable insulation based on ultra-low frequency dielectric spectrum was proposed,which has important engineering significance for the diagnosis and evaluation of cable insulation state.In this paper,XLPE cable insulation samples with different ageing states were prepared by accelerated thermal aging test and accelerated water tree test,and the ageing states of the samples were preliminarily characterized by physical and chemical properties such as carbonyl index,crystallinity and AC breakdown characteristics.Then the ultra low frequency dielectric loss detector(i FDS)based on the identification system was used to measure the ultra-low frequency dielectric spectrum of the ageing samples.Finally,based on the influence of U-I characteristics of insulating materials on ultra-low frequency dielectric loss,the U-I hysteresis curve was proposed to characterize the ageing state of cable insulation,and the relevant characteristic parameters were extracted.The main conclusions of this paper are as follows:(1)Through ultra-low frequency dielectric spectrum test experiments,it is found that the dielectric loss value of XLPE insulation sample increases sharply when the test frequency drops to the characteristic frequency with the corresponding frequency of tanδ=0.5 as the characteristic frequency.The more serious the aging sample is,the higher the characteristic frequency is when the dielectric loss increases sharply,which is attributed to the combined action of DC conductance and interface polarization.Ultra-low frequency dielectric spectrum is sensitive to the change of XLPE insulation state and has strong anti-interference,which can effectively detect the change of insulation performance at the early stage of aging.(2)The limitations of U-I characteristic curve in the evaluation of XLPE cable insulation aging state are analyzed,and the U-I hysteresis curve containing the amplitude and phase information of excitation voltage and response current is proposed as the evaluation characteristic curve of cable insulation aging.When the insulation state changes,the U-I hysteresis curve mainly has three characteristic changes,namely,the degree of deflection of the long axis,the curve area and the degree of curve distortion.Among them,the deflection degree of the long axis changes in the whole insulation aging process,while the curve area and distortion occur in the late insulation aging period.(3)Based on the influence characteristics of U-I characteristics on ultra-low frequency dielectric spectrum,the deflection Angle change rate(RDA)and curve deformation rate(RCD)of U-I hysteresis curve were proposed as the characteristic quantity to characterize the aging of cable insulation.Among them,1MHz characteristic quantity is more suitable for the characterization of cable insulation performance.Firstly,the 1MHz characteristic parameter curve showed different variation rules under thermal aging and water tree aging,which could be used as a criterion to distinguish aging types.Secondly,with the increase of thermal aging degree,RDA gradually increases from 1 to 4.74,which can evaluate the aging status of cable insulation.In the early and middle stages of thermal aging,the RCD value is stable around 1,and rapidly increases to 2.59 in the late stage of thermal aging,which can be used as a characteristic parameter to distinguish mild aging from severe aging.Both RDA and RCD increase with tanδ in the aging of water tree,which can be used to identify the different stages of cable insulation in the aging of water tree.