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Water Treeing in XLPE Insulation at a Combined DC and High Frequency AC Stress


For many subsea HVDC cable systems, the DC
current is transmitted through the HV cable in a voltage source
converter (VSC) - HVDC scheme. Modulated upon the DC voltage
are harmonics, which originate from the switching feature of the
VSC stations. The main purpose of this paper is to examine the
effect of a superpositioned high frequency AC voltage on wet
polymeric XLPE submarine cable systems.
Rogowski type test objects were manufactured with an AC
XLPE insulation and corresponding semi-conductive screens.
Small sodium chloride particles were introduced as initiation sites
for vented water treeing from the lower semi-conductive screen. A
DC stress was applied to the object with a combined stress of a
10% sinusoidal voltage at 5 kHz. As references, parallel test
objects were subjected to a sinusoidal 50 Hz or 5 kHz AC stress
with the same peak magnitude but without the DC stress. The
ageing was characterized by AC breakdown voltage testing, water
tree analysis recording the tree lengths and SEM analysis on
selected samples.
The results show that acceleration of water tree growth by
frequency is strongly dependent on the size of the contamination
at the initiation site. No water trees are observed under high field
DC conditions, even for the samples with sodium chloride
inclusions. The combined DC and high frequency AC stress
generates very long vented water trees. The initiation and growth
rates are mainly determined by the AC part of the stress.
However, the results also indicate that the high DC stress impedes the water tree growth. ©2013 IEEE


Academic chapter/article/Conference paper




  • SINTEF Energy Research / Elkraftteknologi




IEEE conference proceedings


Proceedings of the 31st Electrical Insualtion Conference




494 - 498

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