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, while guidance for their selection, including specification of housing material, is available for AC (e.g. IEC TR 62039, IEC 61109, IEC 62231), no similar standard or technical specification is yet available for DC. At the same time, relevant field experience with them is still relatively limited (e.g. only some 20,000 units have been installed in recent years on DC lines worldwide). As such, it is probably too soon to draw any general conclusions about their performance, even if preliminary indications would suggest it has been mostly 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, which aims to define the creepage distance requirements for composite insulators in the case of DC. My view, however, is that 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 too 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. Quoting from a recent column by Prof. Liang Xidong at INMR’s web site: “As of now, there is still no internationally-accepted quantitative test methodology to classify good versus poor hydrophobicity transfer, even though it’s 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).
This was illustrated in my most recent column (Q4, 2013) that referred to line insulators with relatively small diameters (about 100 mm on average) and relatively low creepage factors (around 3.3). 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.
More specific indications about the minimal requirements for DC composite insulators must appear as soon as possible in order to permit broader and more cost-effective application.