Cavities in mass-impregnated HVDC subsea cables studied by AC partial discharge measurements

Partial discharge measurements have been used for studying shrinkage voids in the
insulation of mass-impregnated high voltage DC subsea power cables. Three 4.5-m long
cable samples were subjected to ac partial discharge measurements at different
ambient temperatures, both under isothermal conditions and after load current turnoffs.
Two distinctly different phase resolved partial discharge patterns were observed,
suggesting that two different "types" of cavities were created. The type causing by far
the most powerful discharges appeared a few hours into the cooling period after load
turn-off, which coincides in time with when load cycling breakdowns during type
testing usually occur. Current loading has a significant effect on the cavities, as it leaves
them – at least temporary – with a different dielectric strength afterwards. The
mechanisms behind this change are not identified, but the associated time constants are
of the order of days and weeks. Consequently, accurately determining the limitations of
such cables with regard to their ability to withstand polarity reversals and rapid load
changes may be more complicated than previously assumed.