The Study on the Assessment Method of Polarity Reversal Lifetime for the Insulation Materials of HVDC XLPE Cables

In this study, an assessment technique to determine the polarity reversal characteristics of LCC Type HVDC XLPE cable insulation materials was considered. For this purpose, the long-term polarity reversal experiment was performed using molded electrodes that can exclude the effects of surface discharge and surrounding insulating oil on the specimen for the 3 different kinds of specimens such as AC XLPE, VSC type DC XLPE and LCC type DC XLPE under the electrical and thermal stress. As a result, in the cases of low applied electric field of 3 E 0 (75 kV/mm), all kinds of specimens with different experimental conditions such as electric field magnitude and temperature showed very large lifetime index in the E-N (applied electric field vs. polarity reversal cycle) characteristics. From Weibull statistical analysis, the shape parameter was smaller than 1, which corresponds to the early failure mode or the random failure mode. It was analyzed that the magnitude of the applied electric field and temperature did not give an effect to the polarity reversal lifetime. Through the lightning impulse breakdown analysis for the survived specimens up to 1050 polarity reversal cycles, the impulse strength was decreased by thermal degradation rather than electrical degradation. Among the three types of specimens applied in this study, the scale parameter of the LCC type DC XLPE material was more than 1000 cycles, showing the best polarity reversal characteristics. From the above analysis, it is concluded that the polarity reversal lifetime for DC insulation material can be evaluated through the process and the electrode configuration as-suggested in this study.