Are nano-composites really better DC insulators? A study using silica nanoparticles in XLPE

A DC step test and a number of DC life endurance tests have been performed at a temperature of 20°C on 200 μm thick samples of a commercial (AC grade) XLPE and its nanocomposite which is intended for DC use. It was found that the breakdown strength of the nano-composite on the DC step test was considerably larger than that of the unfilled XLPE. However, the endurance tests showed that the difference between the characteristic lifetime of the nano-composite and its unfilled base polymer reduced as the applied field was reduced and the lifetimes became essentially the same when the applied field was 130 kV/mm, which is the lowest for which we have data at present. The life line was analyzed as an inverse power law and gave median life exponents of n = 10.76 for the nano-composite and n = 13.58 for the XLPE. These values are consistent with estimates from step tests on the same materials that have been recently published, and imply that the nano-composite will perform worse than the AC grade XLPE as a DC insulator if the inverse power law continues to be obeyed down to service fields.