Space charge development and breakdown in XLPE under DC field

The correlation between space charge accumulation and insulation failure has been investigated in four, polyethylene-based materials subjected to a DC field of 50 kV/mm. Two of the materials contained tree-retardant additives and all four materials were aged for extended periods up to 19,000 h. The specimens were removed periodically from the external field and subjected to space charge density measurements under no-voltage conditions using the thermal step (TS) technique. Not a single parameter that would consistently correlate with times to insulation failure could be derived from the space charge measurements. However, the space charge measurements provided valuable insight into a possible link between the dynamics of space charge development and the time of DC aging. In particular, a consistent surge in the amount of accumulated space charge shortly before insulation breakdown was observed in practically all samples. Also, a consistent relationship between time to breakdown and the position of the internal space charge density peaks in the sample was observed. Samples that had space charge density peaks further apart consistently survived longer than specimens for which the peaks were closer together. These phenomena, if confirmed, could be used to screen new materials for DC voltage applications.