DC breakdown voltage tests may not be a good indicator of long-term ageing behaviour: A study of silica — XLPE nanocomposites

Short-term DC progressive-stress tests on 200 micron thick specimens of a commercial XLPE and its silica nanocomposite intended for DC use have shown that the DC nanocomposite has a considerably higher breakdown strength than the AC XLPE. However, constant-stress tests have shown that both the nanocomposite and the unfilled XLPE have similar lifetimes at medium fields of 125–175 kV/mm. There is also evidence from the Weibull shape parameters that the nanocomposite may be progressively deteriorating at these fields, whereas the unfilled XLPE may show little sign of aging. On the basis of the inverse power law, it is possible that the unfilled materials may survive longer at lower fields: median life exponents of n = 11.3 were found for the nano-composite and n = 14.0 for the XLPE. These values are consistent with estimates from step tests on the same materials that have been recently published. The paper raises questions regarding the long-term DC performance of these nanocomposites, and whether it is reasonable, at least for DC conditions and perhaps for nanocomposites, to judge long-term performance from shortterm testing.