Determination of diffusion constants for peroxide by-products formed during the crosslinking of polyethylene

Today the preferred insulation material for cables rated from 1 kV to 500 kV is crosslinked polyethylene (XLPE). After crosslinking the peroxide by-products are contained within the XLPE matrix. For all power cables there is a need to remove these by-products or to reduce their concentrations, especially for high voltage (HV) and extra high voltage (EHV) cables. One aspect to ensure a long term performance of the cable when put into operation is to secure a proper degassing. It is commonly known that methane, one of the by-products, should be reduced to a very low level for several reasons. Besides deforming the shielding of the cable during the operation the presence of methane can also create a hazardous event during the jointing process as methane is a flammable gas and can create a highly explosive environment. 200 ppm in the cable is regarded as an acceptable level for a safe operation of the cable but still there is a risk due to a possible accumulation in the joints during operation. Here certain producers recommend degassing down to methane levels of 30 ppm. Studies have been carried out to define the field methods detecting methane at these very low levels, but are until now not too successful. The time to reach a proper degassing, e.g. the time needed for a certain reduction of the by-product level, is a very important time factor during the production of high and extra high voltage cables. Also recent modifications in the medium voltage cable construction emphasizes the importance of degassing as well. However the time to degas still depends on a lot of factors like construction of the conductor, use of swelling tapes etc. To have access to simulation tools around the degassing process would facilitate the degassing operation for the cable producer. In this study the diffusion coefficients have been determined for the major peroxide by-products formed from the crosslinking process.