420kV SF6-free GIS Substation Design, pioneering in new SF6-free Dry type Cable technology interface testing method standardization, implementation and recommendation for substation design

This paper addresses the design and implementation of a 420 kV SF₆-free GIS substation and introduces a pioneering approach for standardizing interface testing methods for dry-type cable terminations. In addition to the insulation challenges posed by SF₆ alternatives, substation layout optimization plays a key role in reducing footprint and project costs.

To achieve this, the design integrates external cable boxes for dry-type plug-in terminations, eliminating the need for large underground cable basements typically required for bending radii.

This solution significantly reduces building size, construction time, cost and complexity, while improving accessibility for maintenance and replacement during the substation’s lifetime.

Interfaces between GIS and XLPE cables represent one of the most sensitive points in the insulation chain, often being the source of failures when transitioning between technologies. To mitigate this risk, a dedicated qualification program was developed, exceeding conventional

IEC requirements to ensure long-term reliability and operational safety for the Norwegian TSO.

The methodology combines dielectric withstand tests, partial discharge monitoring, mechanical and thermal compatibility checks and environmental stress evaluations under conditions representative of real GIS installations. Unlike traditional type tests, which often rely on simplified back-to-back configurations, the proposed approach replicates actual electric stress distribution at the GIS–cable interface.

This program was implemented in collaboration with the Norwegian transmission system operator (TSO), GIS manufacturer, and cable supplier, ensuring accurate validation of the interface under high-pressure conditions required by alternative gas mixtures.

Results from pilot applications confirm the technical feasibility of SF₆-free GIS designs with dry-type terminations, even under demanding mechanical and thermal constraints.

The paper also highlights discrepancies between existing IEC standards for cables and switchgear, proposing harmonized requirements for future standardization. Finally, practical recommendations are provided for substation design, qualification procedures, and strategies to support the industry’s transition toward sustainable, SF₆-free high-voltage equipment.