Artificial pollution test on insulators are intended to simulate, in a controlled laboratory setting, the process of natural pollution deposition and wetting of insulator that can lead to flashover. Ideally, the results should demonstrate exactly what would take place under expected service conditions.

Repeatability is, of course, a key requirement in such tests and has one of two possible meanings. It can refer to using the same test method with the same specimen in the same laboratory and with results achieved during different time periods always being basically the same. Or, it can also mean using the same test method with the same specimen conducted in different laboratories, again with the results always being essentially equivalent.

A variety of laboratory pollution tests have been developed over the years, of which the solid layer and salt fog are by far the most common. The solid layer method is well suited to simulate how industrial and inland pollution affects insulators, while the salt fog test better reflects the impact of a marine environment. In the case of China, the solid layer method is the more common since this is the type of contamination problem of greatest concern to local power supply companies. Diatomite or kaolin are typically used to simulate the impact of inert pollutants while steam, cold fog or a mixture of the two simulate natural wettings.

It is important to note that existing artificial pollution test methods were initially created for porcelain and glass insulators. Indeed, the solid layer method has historically worked very well for these in terms of equivalence to the real environment as well as both simplicity and repeatability. However, silicone insulators have now become the most common technology being applied to Chinese overhead networks.

While artificial pollution tests on silicone insulators were initially basically the same as used for porcelain and glass, it quickly became apparent that their intrinsic hydrophobicity and hydrophobicity transfer property greatly affects their pollution behavior. Several factors are noteworthy in this regard:

1. Due to surface hydrophobicity, it is more difficult to apply pollution on composite insulators and it also takes longer for them to reach the fully wetted condition.

2. The difference in flashover voltage (all things being equal) of either diatomite or kaolin to simulate inert pollutants on porcelain and glass insulator is small. However, in the case of silicone insulators, there is a significant difference, with the flashover voltage using diatomite much higher than for kaolin.

3. Drying time after application of the pollution layer has little impact on flashover of porcelain or glass insulators but a major impact in the case of silicone insulators, especially when diatomite is used to simulate inert pollution. The longer the drying time, the higher will be flashover voltage

4. In the case of composite insulators, ambient temperature of drying after application of the pollution layer also has a significant impact on flashover voltage. For example, tests demonstrated that, with the same specimen, same laboratory and same test method, the flashover voltage obtained in winter is significantly lower than that obtained in summer.

Point 1, above, has been already been well recognized by the international standards and testing community. Adequate measures have therefore been taken to overcome the problem of applying solid pollution to a silicone insulator to overcome the problem of applying solid pollution to a silicone insulator by pre-treating the test specimen with dry powder. Similarly, when it comes to silicone insulators, the time needed for the withstand test in the fog chamber has been increased. However, the factors mentioned in points 2 to 4 still have not been fully addressed.

In terms of the difference in the relative impact of diatomite and kaolin on flashover voltage of a silicone insulator, this is due mainly to the different hydrophobicity transfer properties of these two materials, i.e. Their different ability to permit the movement through them of low molecular weight (LMW) silicone species from the insulator's bulk material.

We conducted tests on a variety of inert materials (e.g. Diatomite, kaolin, coal ash, cement, aluminum powder, silica, pollution from steel plants and from the sea) and found that the hydrophobicity transfer property of silicone through kaolin is the worst while that through diatomite is the best- reflecting the two extremes among all the materials tested. Mixing diatomite and kaolin together in a certain proportion therefore will help bring silicone's hydrophobicity transfer property ot a level similar to that through natural inert pollutants.

As indicated, drying time also affects hydrophobicity transfer results. Diatomite, for example, is especially sensitive in this respect and shows significant changes during the initial hours of transfer. It is important to keep in mind that hydrophobicity trnasfer results correlate closely to withstand performance of an insulator- the longer the transfer time, the higher will be its flashover voltage.

Ambient temperature during drying of the pollution layer can also affect the hydrophobicity transfer results. Diatomite, for example, is especially sensitive in this respect and shows significant changes during the initial hours of transfer. It is important to keep in mind that hydrophobicity transfer results correlate closely to withstand performance of an insulator- the longer the transfer time, the higher will be its flashover voltage.

It's clear then that when planning any artificial pollution test on a silicone insulator, the type of inert materials used, the amount of drying time allowed and the ambient temperature during pollution deposition can all affect test results. Presently, artificial pollution test methods on silicone rubber insulators are under discussion in China and elsewhere and it is expected that specific guidelines concerning the above issues will be included in any future test method standard.

Specifically, based on our experience over many tests, it is preferable that kaolin always be used for means of the solid layer method. Because silicone's hydrophobicity transfer property through kaolin is not especially good, this can better simulate the most severe type of service conditions, such as when there is a temporary reduction or even loss of hydrophobicity. In addition, the hydrophobicity transfer property through kaolin is not so sensitive to ambient temperature and transfer time. Therefore, a range can be specified within which, flashover voltage will not vary too much. This will help make the artificial pollution test better in terms of equivalency, simplicity and repeatability.