Water Diffusion in High Voltage Subsea Cable Outer Sheath Materials

Water treeing is considered to be the main mechanism limiting the service time for polymeric cables with wet design. Water trees do not grow at a low relative humidity (RH) and therefore the RH in the insulation should be kept as low as possible for as long as possible. A system of polymeric outer sheaths can be designed to slow down the transport of water into the high voltage insulation system. In order to numerically calculate this effect the materials must be characterised. However, the diffusion properties can be altered by changing the manufacturing conditions. This has been examined in this paper, as well as the effect of elevated hydrostatic pressure on the water diffusion. One thermoplastic polyurethane (TPU) compound and one carbon black filled polypropylene (PP) were moulded and cooled at two different cooling rates, and at two different moulding pressures. Water absorption measurements were performed both at atmospheric pressure and at a hydrostatic pressure of 100 bar. The resulting morphology was studied by DSC and SEM analysis. For the TPU the cooling rate was found to have only a small effect on the water diffusion. The activation energies for the calculated permeability were about 2.5 kJ/mol for the slowly cooled and 5.9 kJ/mol for the rapidly cooled samples. The measured permeability gave similar results, indicating that Henry´s law is approximately valid for this material. The water absorption rate in the carbon black filled PP-compound was significantly higher for the slowly cooled samples, even though these had an apparent higher crystallinity. Permeability measurements also confirmed this as the activation energy was found to be 25.2 kJ/mol for the slowly cooled, and 12.4 kJ/mol for the rapidly cooled sample. The water absorption rate also increased strongly at 100 bars for this material. This can be explained by enhanced water migration into micro-voids and cracks at elevated hydrostatic pressure. SEM analysis revealed the presence of such cr- acks in the interior of the samples, and this was particularly clear for the slowly cooled samples.