Progress Standardizing Recommended Creepage in DC

Today's demand for integrating renewables into energy supply has made HVDC transmission a much more interesting option than in the past. This is triggered by factors such as that HVDC systems may be less expensive in terms of required equipment investment (converter stations, overhead lines, etc.), that electrical losses are lower and that right-of-ways are narrower for any given energy transport scenario.

In service, outdoor insulation must withstand all voltage and environmental stresses. Pollution performance is particularly important in insulation coordination design and becomes the determining factor in DC. Conventional glass and porcelain insulators used to be the only options and experience combined with research provided a good understanding of flashover mechanisms using models developed by Obenaus, Rizk and others. On this basis, insulation design could be adapted to perform well in many situations. However, a number of in-situ conditions, e.g. high pollution severity and/or low rainfall caused unstable performance in deserts, tunnels and coastal areas. With the development of non-ceramic materials, the concept of a composite/polymeric insulator was introduced and this allowed improved performance due to different insulator geometry (smaller diameters) and surface behaviour (hydrophobicity) under pollution. It has been part of the learning curve that the causes and mechanisms of failure have been different to conventional insulators and past analyses of service experience with different materials and designs remains valid today. For example, CIGRE published several documents covering behaviour under pollution to assist standardization work by IEC TC 36 WG 11, including:

• CIGRE TF 33.04.01: Polluted Insulators: A Review Of Current Knowledge. Technical Brochure 158, 2000,

• CIGRE WG C4.303: Outdoor Insulation in Polluted Conditions: Guidelines for Selection and Dimensioning – Part 1: General Principles and the AC Case. Technical Brochure 361, 2008,

• CIGRE WG C4.303: Outdoor Insulation in Polluted Conditions: Guidelines for Selection and Dimensioning – Part 2: The DC Case. Technical Brochure 518, 2012.

The first document, from 2000, accumulated information about the performance of glass, porcelain and polymeric insulators. Based on this, CIGRE SC C4 was able to provide more specific guidelines for selecting and dimensioning outdoor insulation given the variety of different housing materials, insulator types and applications. Two additional documents were published covering AC (2008) and DC (2012) cases of outdoor insulation. A main element in these guides was performance-based methodology that considered field as well as laboratory experience. During compilation of these documents, close liaison was established with IEC TC 36 WG 11, which has been responsible for rewriting and updating IEC 60815, “Selection and dimensioning of high-voltage insulators for polluted conditions”, first published in 1986. The following IEC documents were subsequently published:

• IEC/TS 60815-1 Ed. 1: 2008: Selection and dimensioning of high-voltage insulators intended for use in polluted conditions – Part 1: Definitions, information and general principles,

• IEC/TS 60815-2 Ed. 1: 2008: Selection and dimensioning of high-voltage insulators intended for use in polluted conditions – Part 2: Ceramic and glass insulators for a.c. systems,

• IEC/TS 60815-3 Ed. 1: 2008: Selection and dimensioning of high-voltage insulators intended for use in polluted conditions – Part 3: Polymer insulators for a.c. systems.

With the availability of Technical Brochure 518 as a guide to DC pollution, the work of IEC/TS 60815-4 Ed. 1.0 titled “Selection and dimensioning of high-voltage insulators intended for use in polluted conditions – Part 4: Insulators for d.c. systems” continued and resulted in a Community Draft for voting. Due to the comparative lack of experience in DC versus AC applications, this document summarized recommendations for ceramic, glass and polymeric insulators.

Regarding the content of Technical Brochure 518, CIGRE WG C4.303 reviewed and analysed available practice and experience for up to 50 years’ service. The Guide can be seen as the ‘heart’ of IEC 80615-4 and has become an important tool for selecting outdoor insulation under present HVDC system requirements, environmental conditions and the latest insulator technologies. While in HVAC systems, switching and lightning performance are the dominant factors with main impact on overall length of insulation, length in HVDC is governed mainly by demand for creepage. This is due to a stable electrostatic field along the length of the insulator that, in conjunction with prevailing winds, lead to continuous build-up of pollutants on the surface. These typically range from 1 to 4 or more times more severe than on comparable HVAC insulation in the same service environment. The situation only made worse by the fact that leakage current in the pollution layer does not experience natural current zero stages. As a result, dry band arcing can prove quite destructive and the thermally stimulated movement of a DC power arc can render creepage distance ineffective.

Long-term experience in DC has shown that, given housing materials that have durable hydrophobic behaviour, pollution-stimulated flashover is unlikely. Nevertheless, if there is temporary loss of hydrophobicity, the thermal effect of dry band discharges can cause more severe damage than for the equivalent AC case (see also Technical Brochure 611, published in 2015 and titled “Feasibility Study for a DC Tracking & Erosion Test”). Ongoing research – especially for a test procedure to quantify retention and transfer of hydrophobicity – has shown that modern HTV formulations of silicone rubber (i.e. with high ATH content for superior erosion resistance) have excellent hydrophobic properties and are therefore also an excellent choice for DC applications.

Dr. Frank Schmuck

www.inmr.com/progress-standardizing-recommended-creepage-in-dc