Gas-insulated switchgear assemblies for DC application (DC GIS) provide a compact technical solution with a high functional density, optimised for projects with limited space as in offshore HVDC converter plat-forms, onshore HVDC converter stations and transition stations between different transmission media. Compared with technically equivalent air-insulated switchgear, DC GIS require up to 95% less space. Hence, the size of an HVDC offshore converter platform can be reduced by up to 10% and the footprint of a transition station, e.g. for long DC cable lines, by up to 90%.
The technology type of DC GIS is defined by the gas mixture used to insulate the assembly. Two types of DC GIS exist:
› SF6 DC GIS
› SF6 free DC GIS (e.g. Fluorntiril-based, clean air based)
› Encapsulation
› Insulating gas (commonly SF6)
› Cast-resin insulators
› Disconnector and earthing switches
› Interface modules: gas-to-air bushing, cable connection module, DC GIL connection module
› Current measuring device (e.g. Zero Flux)
› Voltage measuring device (e.g. RC divider)
› Surge arrestor
Space-saving installation DC GIS help to significantly reduce the size of HVDC transition and converter stations for on- and offshore applications. Compared with techni-cally equivalent air-insulated switchgear, DC GIS require up to 95% less space for a typical bay layout
Climatic resistance DC GIS operate safely and reliably onshore and offshore in the temperature range of - 30 °C to +50 °C and with modifications even under severe conditions such as polluted and aggressive atmosphere.
Containerised arrangements available DC GIS can be commissioned in prefabricated container modules to reduce environmental impact as well as local erection and commissioning efforts.
High safety DC GIS are fire resistant and do not contain flammable material. They are sealed for lifetime and essentially involve no ageing of insulating materials, resulting in low maintenance and low lifecycle costs. In the event of internal arcs, no fragmentation or burn-through of the enclosure occurs, but decomposition products of SF6 are toxic.
For EHV applications:
2020: TRL 6 (SF6) [ 1 ], TRL 4 – 5 (SF6 free)
2025: TRL 7 (SF6) [ 2 ]
2030: TRL 9 (SF6)
For lower DC voltages of 250 kV, TRL 9 is reached in the Japanese DC GIS installation of Kii Channel Link, which has been in operation since 2001 [ 3 ].
Current fields of research: Demonstration of the technology’s long-term applicability, alternative gases to SF6, partial discharge measurement.
Innovation priority to increase overall TRL: Investigation of long-term effects (gaining operating experience), insulating gas (substitutes for SF6).
The first commercial DC GIS has been in operation since 2001 in Japan with ± 250 kV in ± 500 kV design. In 2019, a new installation in Japan is going into operation with a rated voltage of ± 250 kV. The first offshore installation with ± 320 kV rated voltage is expected to be in operation in 2023. The development of DC GIS up to ± 550 kV is completed.
Rated voltage range: ±250 kV to ±550 kV
Rated current range: up to 5 kA
Rate of failure: Since DC GIS are based on well-proven AC GIS technology, rate of failure can be derived from relevant Cigré surveys. In TB 513, failure rate is given at 0.88 per 100 years.
Expected lifetime: 50 years, derived from AC GIS.
Description
Technology of DC GIS was chosen, due to the heavy salt contamination in the coastal area. Furthermore the space for the installation was greatly reduced.
Design
Disconnector and bus bar, particle traps.
Results
Reliable and space-saving station design.
Description
Within the EU project PROMOTioN the installation of a 320 kV HVDC GIS was completed and commissioned in order to perform a long-term test to demonstrate that the technology of HVDC GIS is ready for application. The test is performed in the DNV GL’s KEMA High Voltage DC Laboratory near Arnhem.
Design
±320 kV DC GIS.
Results
The long-term test over one year is expected to demonstrate that the technology is ready for real world application in order to achieve cost savings due to compact design in HVDC switchyard.
Description
DolWin6 grid connection with HVDC technology. Space-saving DC GIS in DC switchyard for reducing platform size and weight.
Design
±320 kV DC GIS.
Results
Reduction of size and weight of the platform by 10% due to installation of space-saving DC GIS.
Description
30 MW and 150 kV Gotland HVDC line featured the first DC GIS with mixed voltage stress (AC+DC).
Design
DC busbar with superimposed DC voltage of ± 150 kV.
Results
The DC GIS has allowed better use of available space due to its compact size and high reliability.
[ 1 ] Prototype installation test within EU project PROMOTioN [Link]
[ 2 ] Offshore DC GIS for project DolWin6 [Link]
[ 3 ] Kii Channel HVDC Link with DC GIS [Link]
[ 4 ] R. Alvinsson, E. Borg, A. Hjortsberg, T. Höglund, and S. Hörnfeldt, GIS for HVDC Converter Stations, CIGRE 1986, Report, 14.02.