Surface tracking

Surface tracking is the formation of a permanent conducting path across an insulator surface. Usually the conduction path results from degradation of the insulation. For tracking to occur the insulation must be a carbon based compound.

Particularly in industrial areas and where high voltage plant is situated outside, insulators become contaminated with pollution and dirt from the atmosphere. Where substations are situated near the sea, salt very quickly covers the plant. In the presence of moisture, these contaminating layers give rise to leakage current over the insulator surface. This leakage current heats the surface and through evaporation causes interruption in the moisture film. Large potential differences can then occur over the gaps in the moisture film and small sparks can bridge the gaps. Heat from the sparks causes carbonisation of the insulation and leads to the formation of carbon tracks on the surface. As the carbon deposits are themselves conducting, the likelihood of leakage current leading to high potentials and further sparks increases as the tracks become longer. As a result, the rate of growth of carbon tracks trends to increase with time and as tracks become longer and more established.

At some point the insulator will no longer be able to withstand the electrical stress and flashover will occur. This most likely causes severe carbonisation of the path between high voltage and earth such that an arc is maintained until interrupted by the circuit protection.

Tracking as a phenomenon is a major cause of failures in insulating systems. The rate of tracking depends on the structure of the polymers and can be reduced by adding appropriate fillers to the polymer which limit carbonisation.

Surface tracking can be very easily detected using ultrasonics. Handheld instruments or permanent monitors will pick up surface tracking at an early stage and as defects usually take many months before flashover occurs, maintenance can be carried out before catastrophic failure.

Author: Dr Colin Smith, Managing Director, IPEC Ltd

September 2023