Space Charge Criteria in the Assessment of Insulation Materials for HVDC

The purpose of this work is to present and to discuss a methodology for the assessment of materials intended to be used as insulation in HVDC cables, particularly as regards the evaluation of the effects of material formulation on space charge accumulation within the insulation. Sample design, testing method and protocol and criteria used for the material evaluation are specifically considered. The criteria being put forward are based on available models of insulation life vs. stress with consideration of polarity reversal effects. It is shown through several examples using low density polyethylene and crosslinked polyethylene materials that criteria such as the Field Enhancement Factor and the space-averaged residual charge density are indeed sensitive to material formulation and especially to the presence of crosslinking by-products. When using a crosslinking process that does not produce residues, the material tends to behave like low density polyethylene regarding space charge features. Finally, the question of the representativeness of tests performed on flat specimens vs. model or full cables is discussed.