Voltage Stabilisers for XLPE Cable Insulation

Doctoral thesis, 2008

The demand for energy in the Western world has istorically increased steadily and a steady increase is forecasted for the next decades. This steady increase together with the huge merging markets in India and China are setting new standards for the energy transmission. The distribution grid will grow to support more and bigger consumers and to facilitate that an increase in transmission voltage is needed. To withstand an increased transmission voltage the radius of the cable insulation are usually increased,which has drawbacks such as shorter cable length per reel, which in turn leading to more joints. This is both time consuming and decreases the reliability since joints are weak points in the insulation system. This thesis is aimed at introducing an active species, a voltage stabiliser, in cable insulation i.e. polyethylene to improve its resistance to a degradation mechanism called electrical treeing. This is a fast degradation mechanism which has a fast lapse from initiation to material breakdown when comparing with most other degradation mechanisms present in cable insulation. If this is the limiting mechanism for the breakdown of cable isolation an increase of field strength over the insulation would be possible. The effect of voltage stabilisers have been investigated and patented, however the mechanism behind voltage stabilisation has not yet been fully understood, nor has voltage stabilisers had a commercial significance. The voltage stabilisers presented in this study have all been synthesised in our laboratory. The selection of what voltage stabiliser to be synthesised has been facilitated by estimating their molecular properties by means of quantum mechanical calculations on a large number of potential molecules. The selection criteria has followed what Ashcraft et al. suggested for a series of alternating aromatics in the late 70:s, that the efficiency is dependent of the ionisation potential of the voltage stabiliser. The quantum mechanical calculations have been seen to correlate well to experimental data both in the literature and measurements on the actual voltage stabilisers in our own lab. The assessment of the electrical treeing was done by developing a new method more suited for the assessment of lab scaled synthesised voltage stabilisers. This new method is utilising far less material than conventional and have shown to generate easily interpreted data mainly because of its visual analysis capabilities. This thesis has shown that it is possible to increase the electrical treeing resistance in cross-linked polyethylene used for high voltage cables with up to 50% with such small additions as 0.4 %wt.