Insulators

Hydrophobicity is the most recognized property of silicone rubber materials used for outdoor insulation and key to providing their added value of enhanced pollution performance. A great deal of research has therefore been devoted to investigating and better understanding this property that is expected to endure over the entire service life of an insulator. CIGRE Technical Brochure 442 describes a variety of test methods to quantify retention of hydrophobicity under simultaneous electrical and electrolytic stress and also hydrophobicity recovery and transfer.

Retention of Hydrophobicity

The basic idea of the Dynamic Drop Test considered the following requirements:

• relatively short duration
• simple geometry of test specimen
• high reproducibility
• low scatter of results

The test employs the same basic principle as the Inclined Plane Test (IEC 60587) however has different electrode geometries and an electrolyte, without wetting agent, that is applied to the top surface of the plate-shaped specimen (see Fig. 1). What is then evaluated is how much time is needed to achieve a continuous path of water between the electrodes. Depending on test voltage applied (typically between 4 and 5 kV), a leakage current of 2 mA demonstrates loss of hydrophobicity.

The Technical Brochure also provides results from Dynamic Drop Tests conducted in the past using an international Round Robin methodology. These results confirmed that the test has high reproducibility among the participating laboratories but criteria or material classes cannot be defined since it is better to have comparative results versus other families of polymeric materials.

Quantification of Hydrophobic Properties
Fig. 1: Dynamic Drop Test set-up.
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Hydrophobicity Recovery

An investigation was performed to determine whether this test can also be used to evaluate recovery of hydrophobicity after stress treatments such as corona or UV. Plate-shaped specimens that lost hydrophobicity were then investigated after a certain rest period. Generally, hydrophobicity recovered and the continuous water path soon changed to one of down-rolling droplets.

Hydrophobicity Transfer

The Technical Brochure also describes application of a defined artificial pollution layer and a methodology to evaluate droplets formed after transfer of hydrophobicity to this layer. Test reproducibility was confirmed by means of Round Robin testing. Additional internal investigations were carried out as well to ascertain whether the principle of the Dynamic Drop Test could also be used to quantify hydrophobicity transfer. Based on work performed in the past by CIGRE WG C4.03.03 (dealing with investigation of pollution layers and application methods to improve the reproducibility of pollution tests for polymeric insulators), some of these artificial pollution layers were applied to test specimens, which were then covered and stored under well-defined conditions. After a certain transfer time, the polluted surface indeed showed the expected effect of hydrophobicity transfer (see Fig. 2). More work is needed to develop a way to ensure reproducible application of the pollution layer, especially considering that materials might have different initial hydrophobicity levels.

Quantification of Hydrophobic Properties
Fig. 2: Formation of droplets on artificially polluted surface.
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Summary

CIGRE TB 442 describes test methods to better quantify hydrophobicity. This is important since this much-valued property of an insulator can be quite different in terms of retention, recovery and transfer to the pollution layer. Care has been taken to develop test methods which are relatively easy and can be performed without need for sophisticated equipment. Reproducibility of these test methods was then confirmed by international Round Robin tests.

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