Unfortunately this is not a full-scale test and therefore does not consider the influence of length or of field grading on the termination. Rather, the only result is information about the housing material and quality of design. In any case, information about pollution withstand behaviour is essential, even if the new composite insulators are hydrophobic at the start. Typical requests for testing these types of cable terminations include:
• Tests according to the standards used for porcelain insulators (i.e. IEC 60507 (1991-04) Ed. 2.0, IEC/TS 61245 (1993-10) Ed.1.0) including pre-conditioning for salt fog testing, regardless of their exclusion from the scope of existing standards;
• Tests according to the standards for porcelain insulators but without pre-conditioning for salt fog tests, again even though these are outside of present standards;
• Tests according to whatever specifications are set by the buyer.
From the above, there are obvious problems in the case of composite insulators exposed to the same standard pollution test conditions used for porcelain. For example, where there is salt fog testing, the standard mandates pre-conditioning consisting of 8 flashovers. This, of course, will reduce the hydrophobicity of a composite housing given the impact of electrical discharges. As such, the test does not properly reflect normal behaviour in service but rather only a ‘worst case’ scenario.
The problem with solid layer testing is similar. Since the artificial pollution layer cannot be applied to a silicone rubber surface, it is necessary to ‘overpower’ the hydrophobicity by covering the surface with kaolin powder, done by rubbing the surface with a cotton cloth and the powder or using a brush. Afterwards, the composite insulator surface behaves much like porcelain. Again, such a test does not reflect normal behaviour in service but rather only the worst case.
To arrive at an acceptable solution, given the above, it seems that the best way for test laboratories now is to follow whatever specifications are required by the purchaser of the composite-housed termination. At least this guarantees acceptance of the results by the cable system’s end user. An example of such a specification is UX LK208, used by Italian grid operator, Terna, and which proposes the following procedure for testing terminations equipped with composite insulators:
Salt fog test method with all parameters according IEC 60507 but with important modifications that include: a) Preconditioning with 80% of test voltage over a period of 3 hours without interruption; b)12 subsequent 1-hour withstand tests, at 100% test voltage, with the requirement that at least 8 tests must be passed without flashover or breakdown.
Since test voltage is equal to U0 (phase-to-ground voltage), the termination is not so heavily stressed as by application of repeated flashovers. On the other hand, experience has shown that the insulator of the termination does not remain hydrophobic over the entire test period. As such, this procedure seems to offer a good compromise.
While the pollution withstand performance of MV cable terminations is part of the standard according to IEC 61442, no such test is yet defined for HV terminations. For those terminations with porcelain housings, existing test standards IEC 60507 and IEC 61245 can be applied. Still, care has to be taken to avoid internal dielectric breakdown.
For terminations equipped with composite housings, these test standards are not applicable since composite insulators are excluded from the scope of existing standards. Therefore, existing procedures have to be somehow modified or the tests carried out according to specifications set by the purchaser. Revisions to the pollution test standards are in process but composite insulators are apparently still not included in their scope and will have to await a future new standard.