The world’s first DC lines were equipped with basically the same ceramic insulators used for AC. However, experience showed that, after only several years of service, there were problems with unacceptably high rates of puncture of porcelain and shattering of glass discs. In 1992, IEC issued 61235 ed. 1 “Insulators for overhead lines with a nominal voltage above 1000 V – Ceramic or glass insulator units for d.c. systems – Definitions, test methods and acceptance criteria”. Afterwards, high resistivity insulators were developed to conform to the new standard and there was a dramatic improvement in performance. Looking at composite insulators, guidance for their selection including specification of housing material has long been available for AC (e.g. IEC TR 62039, IEC 61109, IEC 62231). But this is not the case for DC and relevant field experience is still relatively limited. As such, it is too soon to draw general conclusions about their performance although indications suggest it has been positive. It should also be noted that the majority of these insulators have been installed in China and therefore have had to meet local Technical Specification DL/T 810-2002 for ±500 kV DC long rod composite insulators. This specification derives largely from IEC 61235 and, among other things, prescribes specific tests, including:
• Verifying minimum required volume resistivity;
• Verifying impact of ion migration on mechanical performance;
• Tracking and erosion tests with specific DC requirements;
• 1000 hour tracking and erosion tests, but with DC parameters;
• Artificial pollution tests specifically for DC using the solid layer method;
• Tests to assess hydrophobicity transfer and recovery, within specific limits.
The IEC has been working on IEC TS 60815 to define creepage distance requirements for composite insulators in the case of DC. However in addition to offering guidance for selecting insulators from the standpoint of pollution, other basic guidance must also be given. For example, standards (or specifications or indications) about minimal hydrophobicity requirements are especially important since they form the starting point for selecting insulators from the creepage point of view. The present definition of hydrophobicity transfer material (HTM) and non-HTM materials, as in IEC TS 60815, is mostly qualitative. Indeed, hydrophobicity transfer and recovery time (and thus pollution performance in service) can vary dramatically for different housing materials used in such insulators, e.g. according to relative quantities and grades of silicone and fillers used – even if all are intrinsically HTM. There is still no internationally accepted quantitative test methodology to classify good versus poor hydrophobicity transfer, even though it is obvious that this can mean substantially different service performance under pollution.
Requirements in regard to hydrophobicity are particularly important when it comes to DC. While in IEC TC 60815 Part 3, the USCD requirements for composite insulators are nearly the same as for ceramic insulators, there is a basic assumption that composite insulators could be used at far lower USCDs. However, this can only be the case if strict requirements are placed on hydrophobicity transfer and recovery (to some extent able to evaluated by measuring surface wettability, WC). Such requirements become all the more evident if data for relatively small line insulators is extrapolated to large substation insulators with CFs up to 4.5 and diameters up to 1200 mm, as required for UHV and as shown schematically in Figs. 1 and 2.
There must be more specific indications about minimal requirements for DC composite insulators in order to permit broader, more cost-effective application.