Correlation between space charge development and breakdown in polymeric insulation under dc field

The correlation between space charge accumulation and insulation failure has been investigated in two types of cross-linked polyethylene and ethylenepropylene rubber subjected to a positive DC field of 50 kV/mm for periods up to 3% years. One type of XLPE films was equipped with tree-retardant additives. A buildup of space charge in the films was monitored periodically during aging with Thermal Step Method, which measures space charge distributions under no voltage condition. 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 density peaks that appeared initially near the electrodes moved closer towards one another during aging and the area under the space charge density peaks showed a noticeable increase shortly before insulation breakdown. These observed properties of the material behavior under dc fields could be used to screen new materials for practical applications.