Numerical simulations performed by CIGRE joint working group B4C4B1.73 have indicated
that in HVDC power transmission networks pole-to-ground failures can lead to temporary
overvoltages in the network which are not covered by the current extruded HVDC cable
type tests. These temporary overvoltages feature considerably slower rise time and fall
time compared to switching impulses. [1], [2]
In the DeMoKab project, of which the thesis is part of, the goal is to examine how the
insulation materials used today in high-voltage direct current (HVDC) cables withstand
temporary overvoltages that can arise in HVDC transmission systems. The thesis focuses
on extruded HVDC cables which are getting more popular than MI cables due to technical
reasons leading to lower costs. In the experimental part of the DeMoKab project the
maximum withstand field of 0.3- and 0.5-mm thick extruded insulation samples subjected
to TOVs of different rise and fall times is measured. The 234 samples tested contain
samples with small contamination particles inserted to replicate defects in the insulation.
There were no significant differences in dielectric strength between the different types of
TOVs, although the rise time ranged from 0.25 ms to 10 ms. [2]
The aim of the research in the thesis is to model these physical tests numerically in
COMSOL. The numerical model predicts the effect of TOVs on the temperature distribution
and electric field distribution around defects, and takes into account the space charge
accumulation in the XLPE insulation.
The literature study includes an overview of the different components and their features in
extruded HVDC cables. More specifically, the main focus is on the characteristics,
morphology and manufacturing process of XLPE. The breakdown, degradation and ageing
mechanism are discussed to explain the effect of space charges and contaminants inherent
to extruded HVDC cable insulation systems. At the end of the literature review, the origin
and characteristics of TOVs are discussed. This background is essential to obtain insight
and to improve understanding of the core part of this thesis, the numerical modelling of
TOVs in extruded HVDC cables. Finally, a summary of the key results of this thesis is
provided to the reader and further possible research is outlined.