The term ‘hydrophobicity’ has by now become an all too familiar catchword in the power industry. Yet many users of hydrophobic materials and those that provide hydrophobicity transfer are still not fully aware of its significance on performance of electrical insulation.
In a nutshell, water repellency (or hydrophobicity) of silicone elastomers and silicone coatings prevents water and watery solutions from spreading across a surface and forming a thin conductive layer. Instead, when a sur face becomes exposed to wetting, there is formation of droplets. These droplet layers do not allow resistive current to flow and therefore total insulator leakage is limited to extremely low capacitive currents – even at high field strength. This is easy to measure. However, what is not so simple is evaluating the resulting enhanced insulation performance (i.e. flashover voltage). At the same time, it is worthwhile to learn more about the flashover process itself and about the influence of insulator design.
Since we could not find a hydrophilic and a hydrophobic insulator with exactly the same design, we used glass insulators with and without a polymer-rich silicone coating to compare pollution flashover voltage. By applying the coating, the test laboratory managed to turn these into perfectly hydrophobic composite insulators. Tests were then conducted to evaluate wet insulation performance versus non-coated caps. Insulators used in this research had creepage distance of 445 mm and flashover distance of 250 mm (see Fig. 1).
Both types of insulators were artificially polluted. Those without the silicone coating were dipped into a slurry of water, pyrogenic silica and sodium chloride. Coated units were polluted by spraying a solution of water and sodium chloride that resulted in a uniform surface layer of droplets (as in Fig. 2). Droplet diameters varied from 0.1 to 1.5 mm and had average liquid content of 20 mg/cm2.
To measure wet pollution flashover voltage, polluted insulators were exposed to test voltage within 3 minutes after application of the wet pollution. Test voltage was set to the selected start value and the insulator to be tested was immediately charged by closing the switch. Average pollution flashover voltage value was determined by applying the so-called ‘up down method’, using a powerful 200 kV/2000 kVA transformer. A recovery period of at least 48 h was allowed after each test to guarantee steady hydrophobicity (i.e. HC 1). Each series consisted of at least 10 tests to allow statistical confidence.
As expected, silicone-coated glass insulators showed a higher pollution flashover voltage than non-coated units. The differences were impressive, e.g. 2.3 times at a pollution conductivity of 1.6 mS/cm and more than 3.2 times at conductivity of 10 mS/cm. Clearly, the benefit of a hydrophobic coating increases with higher pollution level (i.e. conductivity of the wet layer).
A high-speed camera offered close-ups of the flashover arc. Flashover of the coated hydrophobic insulator typically occurred without visible pre-arcs along the surface and within only some milliseconds after applying test voltage (as in Fig. 3). This process did not proceed as in conventional pollution flashovers in a film layer, which typically show growing arcs along the surface. Rather, there was a quick streamer discharge. The flashover voltage of the silicone-coated glass insulator was confirmed to be far higher than the maximum expected voltage stress in service (namely about 18 kV) – a technical reserve of several hundred percent. That is the real impact of hydrophobicity!
Dr. Jens Lambrecht