Abstract
Thermal expansion and contraction of the impregnation
oil (the “mass”) in mass-impregnated non-draining
HVDC cables cause the internal pressure to vary greatly
during load changes. Pressure drops after load turn-off
is a concern since formation of shrinkage cavities may
reduce the dielectric strength. To study the pressure dynamics
of the insulation system, short cable samples
were equipped with sensors measuring the pressure at
the innermost and outermost insulation layers, and with
strain gauges on the lead sheath and steel bands. The
cable temperature and internal pressure were stepped up
and down, while recording pressures and strains. It was
found that the internal pressure is not only determined
by thermal contraction/expansion of the insulation interacting
with elastic compressive forces generated by
the steel bands. Plastic deformations of the sheaths surrounding
the insulation also contribute significantly. The
plastic deformation progresses slowly (over weeks and
months). Consequently, when considering the dynamic
behavior of the insulation system of such cables, and
when assessing what operating conditions the cable can
safely handle, this phenomenon must be included in the
evaluations. It is not sufficient to look at the loading history solely from temperature and space charge redistribution perspectives. © 2021, CIGRE. All rights reserved.
oil (the “mass”) in mass-impregnated non-draining
HVDC cables cause the internal pressure to vary greatly
during load changes. Pressure drops after load turn-off
is a concern since formation of shrinkage cavities may
reduce the dielectric strength. To study the pressure dynamics
of the insulation system, short cable samples
were equipped with sensors measuring the pressure at
the innermost and outermost insulation layers, and with
strain gauges on the lead sheath and steel bands. The
cable temperature and internal pressure were stepped up
and down, while recording pressures and strains. It was
found that the internal pressure is not only determined
by thermal contraction/expansion of the insulation interacting
with elastic compressive forces generated by
the steel bands. Plastic deformations of the sheaths surrounding
the insulation also contribute significantly. The
plastic deformation progresses slowly (over weeks and
months). Consequently, when considering the dynamic
behavior of the insulation system of such cables, and
when assessing what operating conditions the cable can
safely handle, this phenomenon must be included in the
evaluations. It is not sufficient to look at the loading history solely from temperature and space charge redistribution perspectives. © 2021, CIGRE. All rights reserved.