Arresters with advanced cooling performance for protection of valves in HVDC converters

摘要

Arresters used in HVDC applications are specifically dimensioned based on standard arrester types. Arresters usually consist of an active part and a housing, where the active part is made of metal oxide (MO) resistors connected in series and in parallel, and the housing in most cases to date is a polymer housing. Protective level and design of an arrester for the different applications have to be determined such that after an operation with rated energy stress the arrester will stay thermally stable at continuous operating voltage. Particularly for the valve arresters in a line commutated converter (LCC), their protective level decides about the number of thyristors to be connected in series. Reduced protective levels will result in a lower number of thyristors and reduced costs, accordingly. Depending on the valve configuration in some cases (with additional measures) a landslide reduction of costs is possible. Prerequisite for an optimization of thyristor valve parameters (number of series connected thyristor levels, snubber circuit parameters) and the valve arrester is a detailed investigation of the relevant operating conditions. This has been achieved with improved digital simulation for evaluation of the energy dissipated in the valve arrester. Beneath the properties of the MO resistors like non-linear voltage-current characteristic and energy handling capability, the overall thermal behavior of the arrester is of main importance. Since lower protective levels will result in increased heating under continuous operating conditions, improved cooling is a main concern for the valve arresters. A simple approach is, for instance, replacing the hollow core insulator of a tube design arrester with a cage design housing where silicone rubber is directly attached to the MO resistors. The best solution in terms of cooling would be an arrester without any housing, which can only be used in a controlled indoor environment, though. Such non-housed valve arresters have been in service for many years. To even further improve cooling of the MO resistors, active cooling elements - metal parts with cooling fins - have been developed. To fulfill the creepage distance requirement of 14 mm/kV, silicone rubber sheds are pushed directly over the MO resistors. Geometry of the cooling parts has been optimized by thermal simulations of a complete arrester consisting of MO resistors, polymer sheds and metallic cooling elements. For verification of its improved cooling behavior, a number of tests were performed on real size, completely assembled single- and multi-column arresters. To find the limits of thermal stability and the lowest possible protective level, the test voltage was varied around the assumed continuous operating voltage. The test procedure comprised test voltage application for several hours up to steady state temperature conditions or thermal runaway, respectively. During some tests, the temperature was further increased by a certain amount of energy input up to the maximum specified temperature where thermal stability is guaranteed, to demonstrate cooling at the defined continuous operating voltage. These tests were carried out at ambient temperatures of 20°C and 60°C, the latter taking into account the highest possible temperature in a valve hall. Finally, a complete type test according to the new standard IEC 60099-9, published in June 2014, was successfully carried out using the developed parameters. Some observations on the applicability of the new standard are reported.