Lifetime extension of medium voltage cables

In many Austrian distribution grids PE-insulated medium voltage cables installed in the 1980’s are still in service. Failures caused by water-trees in the insulation are well known and reported. For grid operators this raises the question of whether to replace the cables to maintain the reliability of the grid.
Another possibility instead of replacing the cables is to extend the service life of PE-insulated cables using a treatment to refit the insulation. This treatment uses a silicon based fluid which is pressed into the insulation of the cable to inactivate existing water trees. Initially cables were treated using a fluid containing phenylmethyl-dimethyloxysilane components. These components often caused accelerated corrosion of aluminium conductors because of chemical reactions in the past. Due to this fact a couple of years ago the fluid was improved using dialkoxysilane to prevent corrosion. The effectiveness of this improved method for life time extension of medium voltage cables has been investigated in a cooperation of the Austrian Netz Burgenland Strom GmbH, the German UtilX Europe GmbH and the Institute of High Voltage Engineering and System Performance, Graz University of Technology. Sections of 20 kV cables were used for investigating this insulation refit. An impulse voltage step test has been carried out on these and on refitted samples to evaluate the effect of the treatment.