External insulation is one of the critical design aspects for HVDC equipment at converter stations. This is especially the case for UHVDC systems. The basic design principal for such a system with energy transmission capability up to 10 GW is to achieve superior reliability. This INMR article, contributed by Dong Wu of ABB HVDC, offers an overview of these issues based on R&D findings as well as design and operating experience of existing UHVDC systems. The issues addressed cover ambient conditions, type of insulators, dimensioning of air clearances, corona and electric field at converter stations.
External insulation is a critical design aspect for HVDC equipment at converter stations and especially so for UHVDC systems. Although voltage level is a main concern, with increasing voltage and size of equipment, other constraints such as mechanical strength and thermal effects also become more significant in design. Some, such as the effect of multiple gaps, become critical for UHVDC. Moreover, even design practice for personnel safety used at lower voltage levels may need to be re-considered. External insulation is a wide area that includes insulation design for air clearance, corona shielding and insulators for both indoor and outdoor atmospheric conditions. This report provides a general review of these and related issues.
Critical Issues re External Insulation
a) Site Conditions
Site pollution severity of a planned converter station could be estimated by different methods. Since there are already many HVDC systems in operation under different site conditions, one effective way is to study how pollution has affected these systems. Another is to set-up a test station near the location of the planed converter station. For UHVDC, it is even more important to get an accurate estimation of site pollution severity since decisions made based on this can significantly impact reliability as well as cost of a project. Other conditions that impact insulation design are wind speed and seismic requirements as these parameters will determine the mechanical requirements for equipment and become a constraint in insulation design.
b) Indoor Conditions
At converter stations, some equipment, such as converter valves, are generally located indoors. Other equipment is often located outdoors but can also be installed indoors i.e., inside a so-called indoor DC yard. Although the indoor condition is less complicated than that of outdoors, it is still influenced by the ambient air with different temperature, humidity and different levels of dust accumulation. These ambient parameters therefore need to be defined and controlled.
c) Selection Between Indoor & Outdoor DC Yards
For HVDC systems, an indoor DC yard is sometimes built for extreme site pollution severity or for stringent requirements of compact design. For UHVDC systems, the length of equipment has arrived at a value where mechanical integrity is critical. Considering the large amount of energy transmitted, an indoor solution may therefore become attractive. Still, such a decision requires a thorough study and comparison of different alternatives.
Type of Insulators
a) Surface Materials
For simplicity and mainly from a pollution performance point of view, IEC has divided the surface characteristics of insulators into HTM (hydrophobic property transferable material) and non-HTM. HTM includes mainly insulators with a silicone rubber surface. Non-HTM includes those insulators with surfaces that cannot maintain or recover their hydrophobic properties after being covered with pollution. Based on today’s technical developments, almost all insulators used in UHVDC systems have an HTM surface. For information related to the advantages and disadvantages of silicone rubber insulators made using different types of materials or with different types of manufacturing techniques, this is often too generalized and could be misleading. Any such discussions should therefore be based on a comprehensive study of operating experience. It is also inaccurate to generalize the hydrophobic property of an insulator by using only HC (hydrophobicity class) levels. HC measurement is a simple and quick method to get a rough estimate of the status of a surface. However, reports from on-site HC measurements have shown that HC levels are in most cases different at different points along an insulator and also can change quickly with ambient conditions. Expected pollution performance of an insulator cannot be obtained based on just a few HC measurements.