The functionality and service life of composite insulators is largely dependent on surface and interface ageing of their polymer material. Hydrophobicity and sealing of the rod by the silicone housing must be guaranteed over an insulator’s entire life cycle. But the housing can be exposed to a broad range of different environmental conditions such as wide temperature fluctuations, high UV, pollution, biological attack as well as combinations thereof. If hydrophobicity is lost, leakage currents start to flow through the moistened pollution layer.
In high voltage technology, an electrically sufficient hydrophobicity is assumed at a receding contact angle of approximately 40° (i.e. a hydrophobicity classification of HC2). At lower dynamic contact receding angles, significant current can flow through the pollution layer and occasional initial dry band discharges can ignite. If such discharges occur over a long period, further thermal and chemical damage can occur until such time that the composite insulator degrades and eventually fails.
Key requirements for polymeric insulation materials for outdoor applications are provided in technical specification IEC TR 62039. However, in regard to hydrophobicity there is no detailed specification simply because no test method is available. Long-term governmental-funded research in Germany aimed to overcome this by testing hydrophobicity retention and recovery using the dynamic drop test. Additionally, hydrophobicity transfer has been analysed using the hydrophobicity transfer test.
The impact of hydrophobicity on flashover voltage as a function of pollution layer conductivity is shown schematically in Fig. 1.
The pollution flashover exponent of non-hydrophobic insulators is 0.25 whereas for hydrophobic insulators it is only 0.01 to 0.1. This lower value leads to less reduction in pollution flashover voltage with increasing conductivity of the pollution layer. Due to different stresses and multi-stress situations, the hydrophobicity of a polymeric material can be reduced over time. This leads to a reduction in the flashover performance from the hydrophobic curve to the non-hydrophobic curve as well as significant reduction in flashover performance. Fortunately, especially in the case of silicone rubber material, there is a certain recovery of hydrophobicity, which means insulators in operation can revert to their initial pollution flashover performance. Furthermore, for so-called hydrophobicity transfer materials, reductions in creepage distance factors can be introduced.
Understanding hydrophobicity behaviour and the processes that leads to its reduction and recovery is therefore crucial in order to select the right polymeric material and ensure required operating performance of a composite insulator.
Attend the 2022 INMR WORLD CONGRESS where Professor Stefan Kornhuber of the Zittau/Görlitz University of Applied Sciences will explain and compare existing test methods to evaluate hydrophobicity and provide an outlook to the future of how this property is measured.