Evaluation of the retention and recovery of hydrophobicity of insulating materials in high voltage outdoor applications under AC and DC stresses with the Dynamic Drop Test

This paper shows the results of the development of a test procedure to quantify dynamic properties of hydrophobicity, focusing on the retention and the recovery of hydrophobicity. The Dynamic Drop Test (DDT) has proven to be a suitable test method and the investigations are intended to improve the test with respect to repeatability and reproducibility. The results include an overview on the investigations for determining suitable test parameters and test conditions in the DDT at two independent test laboratories. Parameters like the volume conductivity, the flow rate of the electrolyte, test voltage and magnitude, as well as the definition of a physically determined failure criterion are analysed for a variety of polymeric insulating materials used in electrical applications. The recovery of hydrophobicity is evaluated by using the DDT with a so called interval testing. This procedure enables a defined evaluation of the recovery of hydrophobicity compared to the test method previously used. The interval testing covers resting and stressing periods under ac and dc stresses.