Annika Smedberg

The role and measurement of DC conductivity for HVDC cable insulation materials

This paper discusses the importance of DC conductivity for insulation materials to be used in extruded HVDC cables. It is one of the key electrical properties, especially for cables operating at very high voltage and stress levels when heating from dielectric losses generated by the leakage current might potentially lead to thermal runaway. Therefore, in material development and quality control it is vital to ensure that the DC conductivity measurements are carried out in an accurate and reproducible way. DC conductivity measurements have been done on XLPE pressmoulded plaques under different temperature and electric field conditions. The test set-up has been designed to limit the diffusion of peroxide decomposition products during the 23 hours of measurement time that are needed to reach a quasi-steadystate. This includes the use of thick plaques, in this case with a thickness of 1 mm, as such samples are expected to retain the decomposition products better than the significantly thinner plaques/films that are commonly discussed in the scientific literature. Full-sized crosslinked HVDC cables will always contain peroxide decomposition products giving an increase of the DC conductivity. For this reason the most relevant results from plaque measurements will be obtained from samples containing a controlled level of peroxide decomposition products. The conclusion is that care needs to be taken when the temperature and stress dependence parameters are used as a cable construction design tool and it is important that the data used are generated on samples as representative as possible, for example with a controlled level of peroxide decomposition products.

What is crosslinked polyethylene

The generally accepted reaction scheme for peroxide initiated crosslinking of polyethylene looks relatively simple. However, a deeper look into this subject reveals a far more complex and fascinating area. The presence of vinyl groups in the LDPE material proved to have a significant effect on the gel formation and on the network densities reached. The vinyl groups were found to be rapidly consumed via a so-called polymerisation reaction. Deeper studies of the network revealed that 2/3 of the network points present constituted of partially to fully trapped entanglements which means that only 1/3 are of chemical nature. It is therefore important to characterise the network by different techniques.

The Influence of Network Density and Physics of Crosslinked Polyethylene on Water Treeing

Three low density polyethylene (LDPE) grades with varying vinyl content were crosslinked using different mechanisms. One target was to identify whether there were any differences in water treeing properties for the three LDPE grades that had similar network density, although the network was obtained in different ways. The second target was to observe the behaviour of water treeing in a crosslinked LDPE with increasing amounts of dicumyl peroxide. It was found that the way the network was formed is not important for the water treeing properties. The water trees grew with increasing dicumyl peroxide content, possibly depending on the simultaneous decrease in crystallinity.

The effect of different type of crosslinks on electrical properties in crosslinked polyethylene

In this study LDPE has been crosslinked below and above Tm with the use of different techniques. Two of the materials were crosslinked with dicumyl peroxide (DCP), and one material was crosslinked with silane crosslinking, where silane groups are converted to silanol that form crosslinks via a condensation reaction in the presence of a catalyst. A major difference between the crosslinking methods is that peroxide crosslinking takes place in the melt, whereas silane crosslinking take place in the solid state. It has been found that degree of crosslinking and morphology is of importance for a number of electrical degradation properties, i.e. water treeing and electrical treeing.

Structural effects on treeing phenomena in XLPE

Two low density polyethylene (LDPE) grades with low (material A) and high vinyl content (material B), respectively, were crosslinked to varying network densities. The crosslinks present in material B mainly consist of reacted vinyl groups, in contrast to the ordinary combination crosslinks present in material A. The crosslinked network was analyzed by gel content, swelling and differential scanning calorimetry (DSC) measurements. The morphology was observed with a scanning electron microscope (SEM) and the water tree growth was analyzed by means of a water needle test. It was found that the water tree length was enhanced at increasing crosslinking degree until a certain network density was reached, where the water tree length decreased again. This change was found to correspond to a change in morphology. No significant differences could be observed between the two materials, consisting of the two types of crosslinks.

Influence of sample preparation on space charge behavior of crosslinked polyethylene for HVDC cables

The aim of this study is to demonstrate how the space charge behavior can be dependent on the choice of sample preparation and electrode configuration. The space charge behavior of crosslinked polyethylene (XLPE) has been investigated by measuring with the pulsed electro acoustic method on plaque samples from both pellets and pre-extruded tape and tested with three different electrode configurations. The result revealed that different types of plaque preparation had a clear impact on the space charge data at elevated test temperature, whereas the effect from electrode configuration could be seen even at room temperature.