NKT HV Cables AB, Sweden
Online published on 3 October, 2018.
Interconnectors and subsea power transmission, off-shore wind farm export cable applications and underground power transmission through populated or sensitive areas are the main applications of extruded DC cable technology. The development and commercial introduction of extruded HVDC cable systems starting at 80 kV level has grown very rapidly to 150 kV followed by 200 kV and 320 kV which is currently the highest voltage level installed. A 400 kV cable system has been contracted.
In order to reach even more efficient power transmission and the HVDC grid vision, next steps in the development for higher voltage levels were taken. Intensive research and development in collaboration with an experienced insulation material supplier, led to the development of the next generation insulation material system based on previous and well proven technology. Thisnovel – or if you like – upgraded material system together with an optimized process, know-how in production and installation of DC cables and advanced field grading technology provided a new platform for reaching far higher voltage levels. In august 2014, this new technology was introduced by launching a complete HVDC cable system for both land and submarine applications at 525 kV[1].
Recently, the highest voltage of 640 kV in extruded DC underground cable systems was introduced. It builds on the same technology platform as the 525 kV system. The insulation material of the new 640 kV cableis exactly the same as already introduced in the 525 kV level. Theinsulation material has excellent electrical, chemical, mechanical, thermal, manufacturing and logistical properties.
The novel insulation material system has an optimal conductivity;type testing the full size 640 kV cable system at 1,85 times the nominal voltage has shown that the risk of thermal runaway and electrical failurehas been totally eliminated due tothe extremely low leakage current of the cable system. The proven robust performance of the cable system during severe testing conditions, enables a reliable power transmission of up to 3GW at service conditions.
Application of non-linear field grading material has been used in many projects and proven to be a reliable and robust method for controlling the electrical stresses in cable accessories. For the 525 kV and also 640 kV terminations and prefabricated HVDC joints, an optimized design with non-linear resistive field grading and geometric stress grading has been utilized. This will make sure that the stress distribution of the component will be adaptively controlled in response to the stress type and load conditions. The gas filled termination with composite insulators, offer an oil-freeand internal-arc resistant solution compared to the porcelain insulators. Extensive testing and simulation have shown the proven performance of the 640 kV cable and accessories during transient and dynamic conditions and under sever stresses.
