Effect of moisture and ionic contaminations on DC insulation for subsea applications

DC electrical insulation is critical for the reliable operation of subsea DC electrical systems. Contamination due to exposure to seawater may significantly affect the reliability of DC insulation. This paper reports the impact of seawater-related contamination on DC-related properties of epoxy insulation. Diffusion coefficients and saturation levels of seawater in the insulation were measured through water uptake experiments. DC conductivity and breakdown strength are significantly affected by seawater contamination. For the seawater-saturated samples, conduction current increases with time under sufficiently high DC field. Whereas the short-term DC breakdown strength is 23 kV/mm, a seawater-saturated sample breaks down after 130 min under only 12 kV/mm. Space charge measurement reveals different charge evolution behaviors under different electric field levels. Under average field of 12 kV/mm, much higher local field of 27 kV/mm exists near the positive electrode. A possible explanation is the interplay between heterocharge-induced field enhancement around the electrodes and the promotion of charge injection due to the enhanced field.