Abstract
Installation of high voltage (66 kV) rather than medium voltage (36 kV) inter-array cables for bottom
fixed and future floating wind farms reduces the number of cable strings entering a platform, reduces
system losses, reduces the overall cable length and the number of substations. All together this will
result in a significant overall cost benefit. These developments in offshore renewables as such have
called for new cost-effective and reliable high voltage subsea cable designs. The new designs include
so-called wet designs allowing water molecules to enter the cable core during service. Time to
failure (TTF) testing can provide data to empirically model the service lifetime of the cables at a lower
service stress. For high voltage cables, the needed electrical stress during TTF testing can be very
high, and there are several challenges then to overcome when performing such testing in seawater.
This includes challenges e.g. related to the test-set-up to avoid premature breakdowns at the
terminations, or to avoid anomalies at the water surface due to non-radial temperature and diffusion
gradients, or in the insulation due to much higher electrical stress in the insulation during testing than
in service. This paper presents initial results to facilitate a TTF test set-up and program to model high
voltage cable cores soaked in salt water and subjected to a very high electrical stress and temperature.
A TTF test of model HV subsea cables using medium voltage XLPE cores was undertaken at a high
stress of 30 kV/mm at the conductor screen. From this test it is shown that the modified termination
system had a high breakdown stress and inception voltage for partial discharges which was necessary
to withstand the high voltage stress of about 100 kV for a long time during the TTF test. The
temperature calculations have shown that the anomalous wet ageing conditions occurring close to the
hot water-air interface must be avoided as it could cause non-steady anomalous wet ageing strongly
accelerating the water tree growth. The heated air system included in the set-up resulted in no anomaly
temperature conditions nor any excess ageing phenomena at this interface. All breakdowns occurred in
the active/wet sections of the cable, and the mean time to failure (Weibull eta) at 30 kV/mm was about
116 days. Short vented water trees appeared both at the conductor and insulation screen with about the
same length and appearance despite the higher electrical field at the conductor screen. This indicate
that the electrical field at the conductor screen did not accelerate the length of the vented water trees.
However, a significant higher density of bow-tie trees was detected close to the conductor screen than
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close to the insulation screen. Future work includes TTF testing at other field stresses, where also a
sudden death approach to limit the test time will be considered.