The Importance of Scale in Testing for Electrical Tree Growth

The importance of electrical trees and interfacial electrical track growth as aging processes in high voltage polymeric insulation systems is well established. It is argued that the majority of testing using needle-plane electrode systems with small insulation gaps is not representative of insulation in high voltage service conditions. In particular, the relationship between partial discharge magnitude, tree length and sample scale is considered. Track growth has been considered in laboratory samples with a 90 mm gap between electrodes. This arrangement allows the channels to grow independent of their proximity to the electrodes. The applied voltage and sample thickness, as well as the electrical field, are critical parameters to be considered. The growth of channels here was voltage limited. Smaller and lower voltage tests cannot replicate this behavior because in small samples tree channels are always in close proximity to, and dominated by, the electrode geometry.