Pollution flashover voltage of an insulator is influenced by many factors, including type of pollution, wetting conditions (rain or fog), insulator profile, insulator material and voltage applied, among others. These factors can make correct dimensioning of creepage distance a complex task for utility engineers.
However, several Chinese as well as international specifications or standards have been prepared based on years of collecting and analyzing operating experience. For most of these standards, the basic principle for establishing the dimensioning of an insulator's creepage distance has been the same and consisted of a series of distinct steps: (1) using a porcelain reference insulator to determine pollution severity for a given site or transmission line, (2) comparing that insulator's experience or pollution withstand voltage level under the pollution severity established, and (3) determining the insulator's required creepage for any given profile. Traditionally, porcelain and glass insulators have been specified according to this procedure.
In regard to silicone rubber insulators, however, the situation is different and determining ideal creepage distance is not quite so clear. Silicone insulators are known for their superior pollution performance and huge quantities of these have been in use for from 10 to 20 or more years. Yet there is still no clear methodology to determine suitable creepage distance for them based on pollution conditions. As a result, the creepage of most silicone insulators still depends on the pollution severity collected on a porcelain insulator's surface. This seems to make little sense since it's obvious that a silicone insulator's creepage should ideally be based on the expected pollution severity collected on its surface--and not porcelain's.
Form the late 1980s to the beginning of the 1990s, silicone rubber insulators began their first trial use throughout China. The only available method for selecting creepage distance at the time was simply adopting that of porcelain based on how porcelain insulators behaved under the applicable pollution levels.
Now, silicone insulators have been in widespread use on Chinese power systems for some time. Both 500kV HVAC and ±500kV HVDC silicone insulators are already being used in this country for almost a decade. And, most recently, silicone insulators will account for about two-thirds of all suspension insulators for China's first 1000kV AC line--now under construction and planned to be in operation by the end of this year.
The process for determining the creepage of these silicone insulators was first of all to determine the required creepage distance for porcelain according to the pollution map; and secondly converting this into the required creepage for silicone insulators based simply on taking some proportion of the level for porcelain.
This proportion has not been a strict value but rather has varied anywhere from 75% to 85% in most cases and up to 90% or higher in other cases. For example, for medium pollution areas of the UHVAC project, the proportion selected was about 85% to 90%. In the case of the project's heavy pollution areas, this proportion was lowered to 75% because insulator section length must be well controlled under such conditions. The required creepage distance for silicone bushings and post insulators have been similarly determined.
In this sense, the creepage distance for the porcelain insulator has become a sort of transitional parameter linking the behavior of porcelain at a given pollution severity and the creepage distance required of a silicone insulator. By most logic, this seems unreasonable. Of course, the advantage of this method is that it's simple and makes use of all the experience acquired using porcelain. But its disadvantage is also obvious--namely that converting creepage distance between the two types of insulators according to some pre-selected proportion seems quite arbitrary. Moreover, since there is no clear rule to standardize this proportion, it can fluctuate across a relatively broad range, i.e. From 75% to 90% or higher. Therefore, it would seem appropriate to look for a procedure to determine a silicone insulator's creepage based solely on the expected pollution severity affecting the silicone insulator itself.
One possible way to do is: (1) determine the silicone insulator's pollution severity. (2) collect data on its experience of withstand voltage under that pollution level, and (3) determine its ideal creepage distance. The problem, however, is that the first step in this process is very difficult. Pollution severity is classically described only by the pollution on a porcelain insulator according to all previous and present standards, e.g. IEC 815-1986, the new revised IEC 60815-1 and other national standards. It is almost impossible to plot a new pollution map based on pollution collected only on silicone insulators. And it's equally unlikely to imagine utilities being in a position to plot two pollution maps simultaneously.
Therefore, a second possible way would be as follows: (1) determine the porcelain insulator's pollution severity, (2) calculate the silicone insulator's pollution severity accordingly, and (3) determine the silicone insulator's creepage distance based on its operation experience and pollution withstand voltage level.
The most important step in this procedure is of course the second one, i.e. calculating the silicone insulator's pollution severity based on the pollution affecting the porcelain insulator,. The pollution accumulation ratio between silicone rubber and porcelain insulators could for example be obtained by measuring the pollution on both types of insulator in the same or in similar locations. This should not be too difficult since both types of insulator now have plenty of operational experience to draw on.
Clearly, it seems a more reasonable methodology to transition from knowing the pollution affecting a porcelain insulator to selecting the ideal creepage distance for composite insulator installed in that service location. In any case, it's obviously superior to the present arbitrary method of creepage distance conversion.
