Development of Test Methods for a Combined DC Power and Fiber Optic Deep Water Subsea Cable - Effect of DC Pre-stress on AC Withstand Strength
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- Institutt for elkraftteknikk 
Repeatered fiber optic submarine cables require a very high reliability. The cables operate at a voltage of 10 kV DC to supply electrical power to the repeaters that are placed every 50-70 km along the cable to amplify the optical signal. To ensure that the cable's polyethylene insulation system can operate without faults for 25 years, electrical test methods that take relevant stresses, defects and service conditions into consideration are needed. The ROC-2 cable is currently type-tested with 100 kV DC for 9 days under the assumption that an inverse power law relation holds for the insulation's electrical lifetime. Inverse power law relations are known experimentally to hold under AC voltage, but may not hold under DC. Oxidized particles, metal inclusions and protrusions on the conductor are unavoidable in polymeric cables. An infrared spectroscopic analysis of 15 pellets with contaminants found in 23 kg of cable insulation raw material with a pellet scanner indicated the presence of oxidized material. Electrical degradation often occurs around such insulation irregularities, and they are thus a limiting factor in the insulation lifetime. The long-term effect on the cable insulation of a DC field corresponding to an applied voltage of 100 kV DC over the cable was studied using Rogowski test objects with and without insulation irregularities in form of metal inclusions and oxidized particles. The test objects were tested to breakdown with ramped 50 Hz AC voltage after different lengths of DC pre-stress (0, 9, 35 and 163 days). Some objects were breakdown tested with a ramped 0.1 Hz (VLF) before and after 35 days of DC pre-stress. In addition to this a few test objects were tested for partial discharge activity (PD) before and after 35 days of DC pre-stress. No significant deterioration of the insulation was observed after the DC pre-stress. The AC breakdown strength of test objects without particles and test objects with oxidized particles seemed to increase after 163 days of DC pre-stress, but this is probably a result of manufacturing variations and statistical errors due to the small amount of test objects. The average VLF breakdown strength was at least 2-3 times as high as the AC breakdown strength. PD inception in the insulation occurred at a lower field after 35 days of DC pre-stress, but the inception values are low enough to suspect surface discharges due to the conducting paint's poor adhesion to the insulation rather than internal discharges. The results indicate that a long-term high DC field does not accelerate any of the tested breakdown processes in the cable insulation. The current type test's ability to detect the types of irregularities that have been found in the raw material and those that have previously been found in manufactured cable lengths is therefore questionable.