Detection of Degradation in XLPE Power Cable Insulation Caused by Water Migration

Polymeric cable insulation used in wet environments is susceptible to an insulation degradation mechanism which results in the formation of water trees.

Normally the susceptibility of insulation to such deradation is assessed by ageing some cable samples several metres in length, and then subjecting them to a breakdown test.

The two methods most widely used in Europe for full-size cables are the German Standard DIN 0273 and The International Union of Procedures of Electrical Energy (UNIPEDE) method.

Both tests require the ageing of a considerable amount of laboratory resources.

This report describes the development and use of a small test cell about one metre long to age XLPE-insulated cable cores in a wet environment, under varying conditions of electric stress and temperature, using both tap water and a sodium chloride solution.

These short lengths of cores are then used to provide samples for subsequent breakdown testing using an assessment technique based on measuring the electrical strength of tape veneered from the cable wall. This is a material assessment technique already developed by ERA which reduces the length of cable required and, thereby, reduces the amount of laboratory space needed to set out the cables under test.

Useful conclusions are drawn, and breakdown data from aged tapes is provided in the appendix.