| XLPE(Cross-linked Polyethylene) power cables are widely used in urban distribution system, power plants, substations, and industrial and mining enterprises. Cable insulation in operation will be fading gradually with the synergistic influence of heat, electricity, machinery, water, high-energy radiation and other factors, resulting in the reduction of the reliability and lifespan of cables. At present, the major development is focused on cable insulation aging mechanism and single factor aging model with the two factors of electricity and heat. The research of multi-factors synergies on the aging characteristics of cable insulation is not enough. In addition, there is less related accumulated aging data of cable insulation, which hinders the research on the condition assessment of cable insulation aging with a variety of operating conditions. Compared with the traditional qualitative analysis, the quantitative assessment of cable aging condition has more practical value. Therefore, XLPE test samples are used to carry out accelerated aging test, extract the aging characteristics, and establish the XLPE insulation aging assessment model with the action of multi-factors coordination. The main achievements in this paper are as follows:①Based on IEC60502.2-2005, IEC 60811, GB/T 1048.1-2006, and other regulations, the dumbbell like and flake XLPE film samples are prepared. A series of tests are designed, which are the test scheme for the heat aging test of cross-linked polyethylene, immersion-thermal aging test, electro-thermal aging with various water contained. The aging experiment under multi-factor coordination is carried out. The elongation of test samples, the frequency domain dielectric spectrum, and space charge distribution data at different aging time are studied.②After thermal aging test and immersion-thermal aging test, analysis is carried out on the trend of the elongation and the curve of frequency domain spectroscopy of XLPE sample in the dumbbell shape and disk shape. It is shown in the results that the dielectric property in the range of low-frequency(0.01~1Hz) can better reflect water content and the aging degree of the insulating materials. In addition, it is revealed by the analysis results of the space charge distribution characteristics of the disk shape XLPE sample after having the electro-thermal aging test that the space charge density of the samples increases obviously when the temperature and electric field rise. The negative charge amount of charge accumulated in the samples reduces and the peak of the negative charge density is more obvious with the rise of the water content.③The dielectric loss interval integral value and the amount of space charge is selected as the aging characteristics of XLPE based on lots of experiment data. Nonlinear regression method is used to establish the relational model of the aging time, aging factors and aging characteristics respectively. According to time-temperature superposition principle, the convert relational model is established between aging time and aging temperature with the reference temperature of 120℃. The convert relational model is established between aging time and different aging field strength with the reference field strength of 50 k V/mm.The relational model is based on the combination of aging time, aging factors and aging characteristics with the aging time convert relational model between aging temperature and aging field strength. The aging condition assessment model on cross-linked polyethylene insulation is established under the action of multi-factors coordination. Obtain the modified function for the aging assessment model through the analysis on the aging characteristics of coupling relationship error between its calculated values and measured values. At last, data of the XLPE specimens both in the accelerated aging test of laboratory and XLPE cable operation data is used to verify the effectiveness of the proposed assessment model. |
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