Recommendations for dielectric testing of HVDC gas insulated cable connection assemblies
Résumé
This paper provides recommendations for the dielectric testing of HVDC GIS cable connection assemblies by comparing the dielectric strength applied as a result of cable system and GIS tests. Multi-physics simulations were performed by OEMs for their designs according to a common modelling and simulation guideline to determine the dielectric strength. A selected set of test scenarios representing the worst-case dielectric strength for the tests described in the cable system and GIS technical brochures each, was considered for simulation and subsequently compared. An open-source HVDC GIS cable connection assembly model was used to determine the uncertainty range by studying the sensitivity of the dielectric strength to various factors such as ambient temperature, test equipment tolerances, the duration and magnitude of DC-prestress, geometries, and material characteristics. The simulation results indicated that the GIS test requirements result in slightly higher stresses than cable system test requirements, but well within the uncertainty range. It was noted that the DC pre-stress duration required by the cable system standards is potentially insufficient to reach the required steady state conditions for impulse testing. Based on multi-physics finite element simulation results, the paper concludes that the HVDC GIS and HVDC cable system dielectric testing requirements are dielectrically equivalent. As a result, HVDC GIS cable connection assemblies can for the purpose of dielectric testing be considered to be part of the cable system, and qualified according to the cable system dielectric test requirements. It is recommended to use at least 10 hours of DC pre-stress for the superimposed impulse voltage tests, in line with the CIGRE TB 852 [1]. The required duration of pre-stress is very design specific and the cable connection assembly vendor should provide evidence that the 10h duration of pre-stress is sufficient, and if not, provide a suitable longer required pre-stress duration time. Determination of the required pre-stress duration is similar to procedures used in determining the required pre-stress for HVDC GIS insulators, which are described in CIGRE TB 842 section 7.4.7 [2]. Gas insulated switchgear and cable connection assembly designs are vendor specific and cannot be readily exchanged with designs from other vendors without a careful consideration of the design (geometries, materials and surface finishes, operating pressures and limits of supply). In case of substantial differences, requalification of the new combination gas insulated switchgear and cable connection assembly designs may be necessary. The paper is the result of the work of CIGRE joint working group B1/B3/D1.79 “Dielectric testing recommendations for gas-insulated HVDC cable connection assemblies”. It is a summary of the technical brochure which will be published in 2024. The working group was made up of 19 members representing HVDC cable system, accessory and GIS manufacturers, TSOs, developers, testing laboratories and consultants from Asia, Europe and North America
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