Effect of cross-linking agent on space charge accumulation in XLPE under DC electric stress

Cross-linked polyethylene (XLPE) is extensively used for AC high voltage power cable for many years. Because of the risk of space charge under DC electric field in unmodified XLPE, this material is seldom used today for DC applications. Improvement of the cross-linking process could be an alternative way to prevent premature failure in high voltage direct current cables. In this work, the effect of two different peroxides (used as cross-linking agents) on space charge accumulation in XLPE for DC power cables has been investigated using the pulsed electro-acoustic method (PEA). Press-molded XLPE samples were degassed at 50°C for 3 days to remove cross-linking by-products and gold electrodes were deposited onto the specimens by sputtering. The samples have been successively stressed at different levels of electric field from 10 to 40 kV/mm followed by a polarity reversal at -40 kV/mm. The electrodes are grounded after each polarization procedure. Both XLPEs show almost the same tendency, especially for field lower than 30 kV/mm. Also, the so-called "mirror image effect" has been observed during the polarity reversal for both materials.