The effects of moisture content, frequency and temperature on the life of miniature XLPE cables

Abstract

The main points of the study reported in this paper are as follows: (i) The amount of moisture in XLPE does not affect the short term ac breakdown strength, provided the insulation is not subjected to electrical stress prior to the breakdown test. (ii) Preimmersing XLPE insulation in tap water prior to electrical aging at room temperature results in a reduction of more than 50% in the breakdown voltage. The preimmersion increases the moisture content, particularly liquid water at room temperature, resulting in increased degradation. (iii) Preimmersion does not have any effect on the breakdown voltage of cables electrically aged at 70°C or 90°C. (iv) For some aging conditions, the short term breakdown voltages of aged cables give no indication of the number of and times to breakdown of the cables during the electrical aging. (v) Direct voltage is quite sensitive to degradation of XLPE by the combined action of moisture and electric stress. Care must be exercised when dc testing aged cables in service. (vi) Complete drying of cables aged in water restores the breakdown voltage to its original value. The improvement is, however, only temporary and the breakdown voltage decreases at a faster rate when aging is continued. (vii) Temperature cycling of the insulation causes small changes in the short term ac breakdown voltage. However, the number of breakdowns occurring during the initial aging (1000 h) is much larger for the temperature cycled cable, indicating that temperature cycling could be an important variable in an accelerated aging test. Whether there is an optimum temperature change to produce maximum degradation still remains to be determined.