A classical coastal pollution event is characterized by strong wind from the sea followed by some kind of wetting. However, in the case of a silicone rubber insulator, high humidity alone is usually not sufficient to lead to high current. Therefore additional wetting in the form of light rain is usually needed. This conclusion is supported by recording a typical pollution event on a silicone rubber insulator.
Sensitivity of MVA can be very high. For example, during the pre-pollution phase of the pollution event at one Test Station, salt is transported inland by strong winds. The most dangerous wind direction (associated with the four most severe pollution events and leading to the highest currents) has an angle of between 0 and 20° to the shore. This is due to a wall of sand that protects the site from the perpendicular (90°) direction. The description of the pollution event at the Kelso Test Station on South Africa’s Indian Ocean coast is slightly different. During the pre-pollution phase at Kelso, salt comes in not only from the sea but also from the land since wind direction is almost parallel to the coast.
Another practical application of leakage current data would be to convert it into pollution flashover performance curves. This approach was tested by applying STRI’s IST program (discussed in Part 1 of this article) to 10 uniquely different insulators at Eskom’s Koeberg Insulator Pollution Test Station (KIPTS) on the Atlantic coast near Cape Town. Results proved quite reasonable in predicting outage rate compared to actual experience, i.e. number of blown fuses), (see Fig. 2). Results of such field station testing can be used practically well beyond simply assigning a ‘pass/fail’ grade to each insulator tested. For example, they can help estimate the pollution flashover performance of a complete overhead line located in the same area and equipped with these same insulators (geometry and material).
These days, however, many power supply companies are looking for simple and robust systems for pollution monitoring and advance warning in place of complicated ‘research-type’ systems for measuring leakage current. This is because uprating/upgrading existing overhead lines requires accurate evaluation of pollution levels. Indeed, attempts to use the systems described above for this purpose have not always been successful because they are expensive and need maintenance at least once a year. Moreover, there were communication problems in many applications. Therefore, single-channel systems (for peak current only) applicable to both overhead lines and substations are now most in demand and under active development. The goal is to have relatively inexpensive and reliable devices cover large areas and allow data collection and transfer via the Internet.
Specific Techniques Applying to Composite Insulators
Specific diagnostic techniques for composite insulators (along with practical examples from field and laboratory investigation) include:
• Hydrophobicity measurement;
• Hydrophobicity transfer (localized ESDD);
• Rapid flashover procedure to obtain dielectric strength of naturally polluted insulators.
Hydrophobicity measurements are performed according to IEC TS 62073 and comparatively simple.
2. Hydrophobicity Transfer (localized ESDD)
Hydrophobicity transfer (HT) is a measure of a material’s ability to recover hydrophobicity. This process is due to diffusion of low molecular weight species (LMW) in the silicone rubber bulk material through the pollution layer to the surface. These then encapsulate any pollution particles, including salt. Even if the polluted surface appears hydrophilic, part of the pollution layer is penetrated by LMW silicones and effective resistance increases. HT is defined as:
where ASDD is Apparent Salt Deposit Density (or localized ESDD). Both ESDD and ASDD are measured with a small cell filled with deionized water where the bottom is the surface of the polluted insulator. ASDD is measured initially as current through the cell when the encapsulated pollution has not yet dissolved. After 5 minutes or when current has stabilized, the bottom surface of the cell is scraped with a glass rod to set any encapsulated pollution free. This is then the measure of ESDD. This parameter provides a good indication of the ability of a composite insulator to recover under different environments. A compilation of HT values and their spread for different AC and DC silicone insulators is presented in Fig. 3 (high HT is considered > 0.5).