Progress Standardizing Recommended Creepage in DC

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Demand for integrating renewables into energy supply has made HVDC transmission a far more interesting option. This is triggered by a number of factors, e.g. HVDC systems may prove less expensive in terms of required equipment investment (converter stations, overhead lines, etc.), electrical losses are lower and right-of-ways are narrower for any given energy transport scenario.

Comparison of line cost, line loss and power flow for different transmission line scenarios. Progress in Standardizing Recommended Creepage in DC creepage Progress Standardizing Recommended Creepage in DC tpc02 1024x662
Comparison of line cost, line loss and power flow for different transmission line scenarios (CIGRE Paris 2014, Paper B2-104).
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Outdoor insulation must withstand all voltage and environmental stresses. Pollution performance is particularly important in insulation coordination design and in fact becomes the determining factor in DC. Conventional glass and porcelain insulators used to be the only options. Moreover, experience and research provided good understanding of flashover mechanisms based on past models such as developed by Obenaus, Rizk and others. Insulation design could be therefore adapted to perform under many service situations. However, a number of in-situ conditions, e.g. high pollution severity and/or low rainfall, have caused unstable performance in deserts, in tunnels and in coastal areas.
With development of non-ceramic materials, the concept of a composite/polymeric insulator was introduced. This allowed improved performance due to different insulator geometry (i.e. smaller diameters) and surface behaviour (i.e. hydrophobicity) under pollution. Part of the learning curve has been that causes and mechanisms of failure for this technology have been different to conventional insulators. Also, past analysis of service experience with different materials and designs remains valid today. For example, CIGRE has published several documents covering behaviour under pollution to assist progress in standardization, 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, from 2000, accumulated information on 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 the IEC TC 36 WG that 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 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. Due to the comparative lack of experience in DC versus AC applications, this document summarized recommendations for ceramic, glass and polymeric insulators.
Regarding content of Technical Brochure 518, CIGRE WG C4.303 reviewed and analysed available practice and experience for up to 50 years of service. The Guide has become the ‘heart’ of IEC 80615-4 and an important tool to select outdoor insulation under present HVDC system requirements, environmental conditions and the latest insulator technologies. While for HVAC systems switching and lightning performance are dominant factors with main impact on overall length of insulation, for HVDC length 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 is 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 destructive and the thermally stimulated movement of a DC power arc can render creepage distance ineffective.
Long-term experience in DC has shown that, assuming housing materials have durable hydrophobic behaviour, pollution-stimulated flashover is unlikely. Nevertheless, should there be a temporary loss of hydrophobicity, the thermal effects 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 able to quantify retention and transfer of hydrophobicity – has shown that modern HTV formulations of silicone rubber (i.e. those with high ATH content for superior erosion resistance) have excellent hydrophobic properties and are therefore a suitable choice for DC applications.
Dr. Frank Schmuck