Susanne Nilsson

Capacitance measurements and tree length estimation during electrical treeing in sub-picofarad samples

A method for measuring capacitance below 1 pF is presented, which is used to measure dielectric properties during electrical treeing in XLPE samples. The measured capacitance is compared with finite element computations and from these simulations a simple relationship between the capacitance increase and the tree length can be established which corresponds well to the tree length observed by optical microscopy.

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.

Continuous monitoring of dielectric properties of LDPE samples during electrical treeing

A technique capable of utilizing voltage waveforms containing harmonics for measurements of dielectric response over a range of frequencies is presented. It has proved useful in several applications and offers high accuracy and flexibility. This paper presents measurement of the changes imposed by electrical treeing in the dielectric properties of LDPE samples and shows that the capacitance increase monitored during the tree growth corresponds well with the optically measured tree length. There is also a change of dielectric loss associated with partial discharge activity during the tree growth, which could be correlated to different treeing modes. The method provides an attractive alternative to the traditional measurements of tree growth by means of optical observations, as it allows for investigations of nontransparent materials.

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.

Effects of inclusions of oxidized particles in XLPE on treeing phenomena

One issue with the use of crosslinked polyethylene (XLPE) insulation in high voltage power cables is the presence of oxidized particles. Oxidation may occur during production of polyethylene, during the extrusion and curing process used to produce XLPE cables, or in subsequent processes/operation of the cables. Oxidized particles are here referred to as organic contaminants. In the present study artificial organic contaminants were introduced in XLPE samples made for electrical treeing measurements. Instead of using the common needle-needle or needle-plane configuration, a wire-plane configuration was used. Organic contaminants have an increasing conductivity and permittivity as function of increasing degree of oxidation. These properties most probably give rise to local electric field enhancements in the material. The morphology of organic contaminants also differs from virgin XLPE, which probably cause bad adhesion between the XLPE matrix and the contaminants. A combination of these factors was found to affect the electrical performance of the insulation. Several different parameters, such as dry and wet ageing, as well as AC voltage and DC voltage stress, were included in the study.