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Water permeability
The build up of humidity inside
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Water treeing in cables requires three conditions: an electric field, humidity and impurities in the insulation and/or semi-conductive screens. The use in an electricity network and the materials already having been processed leaves the condition of humidity still open. If the relative humidity in a cable becomes at least some 70% then water treeing may start.
Watertight jackets are a relatively expensive means to keep the humidity sufficiently low. More economical methods are polymer sheets with or without swelling material. The swelling material is also used as a drying agent that retards the humidity increase in a cable. The effectiveness of jacket materials to retard the build-up of harmful humidity levels determines how long the possible start of water treeing can be postponed.
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Assessment
KEMA has developed a methodology to assess the permeability of water and the resulting build-up of humidity in cables.
The cable jackets are tested in a tank that allows a specified humidity. The jackets taken as samples must be sealed extremely well. A dry flow of air inside the jackets carries off the humidity that is measured with precise sensors. The long-term behavior of the jackets is determined based on the development of that air flow humidity. This behavior predicts how long the start of water treeing can be postponed by the jackets and additional drying systems like swelling material.
Benefits: - Clear judgement of the jacket with respect to water tightness
- Prediction of the minimum time by which the start of the water tree can be postponed
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