Water absorption effects on prebreakdown phenomena under divergent electrical field conditions

In the characterisation of cable insulation compounds the presence of water is generally considered in the field of water treeing phenomena. Nevertheless humidity in the insulation can have an effect also on other degradation processes which are generally studied under dry conditions such as the prebreakdown phenomena under divergent field (electrical treeing tests). The paper highlights that the water content strongly affects the pre-breakdown phenomena both as concerns the tree growth rate and the relevant Partial Discharges (PD) activity. Experiments have been performed on ethylen-vynilacetate copolymer (EVA) both applying different constant AC voltage levels and applying increasing voltage amplitude. A point-plane electrode geometry was adopted and specimens with and without an artificial void (controlled geometry) set in front of the needle point. Specimens had been conditioned by means of a storage in an oven where saturated humidity ambient was present. Different conditioning times were adopted. Tree inception voltage (TIV), breakdown voltage (BDV), tree length vs. the applied voltage or vs. the aging time, time to breakdown and the patterns relevant to PD which act inside the tree branches, are the measured outputs of the experiments