The Evolution of GIS Cable Terminations: From Fluid-Filled to Dry-Type Innovation
2026-07-16 15:56The connection between high-voltage power cables and Gas-Insulated Switchgear (GIS) has undergone a remarkable transformation over the past five decades. What began as cumbersome, oil-filled installations requiring extensive on-site work has evolved into compact, dry-type, plug-in systems that are faster, safer, and more reliable than ever. This article traces the evolution of GIS cable terminations, from their fluid-filled origins to the cutting-edge dry technologies of today.
1. The Early Days: Fluid-Filled Terminations
Before compact dry-type plug-in terminations were developed, transformers and switchgear connections were made with air-insulated or oil-filled terminations. In the late 1970s, the first accessories were realized with cable compartments that contained oil-filled GIS or transformer terminations with pre-molded stress cones.
Traditional fluid-filled terminations were composed of a supporting insulator (porcelain or composite) filled with oil or SF₆, with a pre-molded rubber stress cone slipped onto the prepared cable. The volume between the cable with its stress cone and the supporting insulator was filled with dielectric fluid such as oil or SF₆. This design had a very long and good track record but also carried significant drawbacks.
2. The Challenges of Fluid-Filled Designs
While fluid-filled terminations were effective, they presented several persistent challenges:
Complex On-Site Installation
The GIS cable compartment had to be opened on site, and the high-voltage cable installer mounted the epoxy insulator and inserted the cable with the mounted stress cone and connector. Afterwards, the insulator was evacuated and filled with oil. For transformer connections, the procedure was even more demanding—the transformer oil needed to be pumped out of the cable compartment and re-filled again.
Leakage Risks
The presence of fluid required very careful sealing of the termination to avoid any leak that could lead to an electrical breakdown. The most critical point was the interface between the cable and the termination base plate, where the sealing system had to fit onto various cable sizes. This potential risk of leakage required periodic inspection to check oil level or gas pressure.
Safety Hazards
In the case of an internal arc, fluid-filled terminations—especially oil-filled ones—could produce severe damage to surrounding equipment and pose risks to personnel. The sealing of the system had to be perfectly made to avoid any leakage and ensure good performance throughout the entire product lifetime.
Space Requirements
Despite these challenges, the big advantage of these internal connections was the insulated and protected installation that could be situated in buildings rather than outdoors—though this came with an associated huge demand for space.
3. The Shift to XLPE and Solid Insulation
A major turning point came with the evolution of cable insulation materials. Over the last decades, the dominating cable insulation material shifted from impregnated paper and oil to cross-linked polyethylene (XLPE). In addition, the insulation material of cable accessories changed to solid materials such as insulation elastomers like silicone rubber (SIR).
This transition fundamentally altered the design possibilities for cable terminations. With polymer-insulated cables, the driving factors for using oil no longer existed. The industry began to explore fluid-free alternatives that could eliminate the risks associated with liquid insulation.
4. The Rise of Dry-Type Terminations
Fluid-free sealing ends, also called dry terminations, were introduced more recently in high-voltage systems but are now widely used for the termination of extruded cables.
Key Advantages of Dry Technology:
Lighter weight and more compact design
Dry sealing ends have been used for many years for GIS terminations up to the extra high voltage level (550 kV), and such designs are now commonly accepted and tend to be more and more the standard for such applications. For indoor applications, such as GIS or transformer connections, dry systems are now becoming the standard as their advantages clearly outweigh the traditional fluid-filled system.
5. The Plug-In Revolution
One of the most significant innovations in GIS cable termination technology has been the development of dry plug-in termination systems. Compact, dry plug-in type termination systems have been available since 1966 for connecting transformers, joint boxes, and gas-insulated switchgear.
What Makes Plug-In Design Revolutionary:
Factory Pre-Assembly
The epoxy insulator (bushing) is pre-installed by the GIS manufacturer at the factory. This ensures perfect installation and eliminates the risk of contamination in the GIS compartment.
Simplified On-Site Installation
The equipment bushing is pre-installed in the factory according to IEC standards for dry-type systems. On-site, the cable installer simply prepares the cable end and plugs in the "plug-in part," which contains the stress cone and conductor connector.
Reduced Installation Time
Assembly times are considerably reduced compared with conventional terminations, as the system totally dispenses with liquid insulating materials. The plug-in capability enables the cable to be quickly and easily disconnected from the system component in the case of a fault.
Compact Design
Plug-in systems offer a considerably reduced installation length of 50% compared with the conventional design. As these systems work on solid insulating materials, any desired spatial arrangement can be realized—horizontal, vertical, and even angled arrangements from top or bottom are possible.
High Reliability
The use of pre-fabricated and tested components provides a high level of safety and reliability, while assembly errors are minimised. More than 2,000 of these plug-in systems are in use worldwide in cable networks up to 145 kV.
6. Inner Cone vs. Outer Cone: Two Interface Standards
As dry plug-in technology matured, two different general designs emerged: the "inner-cone" and "outer-cone" systems. The traditional fluid-filled sealing systems have been eliminated and replaced by a rubber/epoxy interface under mechanical pressure.
Inner Cone System:
The stress cone fits inside the epoxy insulator, creating a compact, shielded interface. This design is widely used in GIS applications and is often standardized for medium-voltage ranges.
Outer Cone System:
The stress cone fits outside the epoxy insulator, offering different mechanical and installation characteristics. Both systems have their proponents, and manufacturers continue to develop variants for different voltage classes and applications.
7. The Role of Standards: From IEC 60859 to IEC 62271-209
The evolution of GIS cable terminations has been accompanied by the development of international standards that define interfaces, dimensions, and testing requirements.
IEC 60859 (1986): The earliest version of this standard was issued in 1986. It covered cable connections for gas-insulated metal-enclosed switchgear where the cable terminations were fluid-filled.
IEC TS 60859 (1999): This version expanded to cover both fluid-filled and dry-type terminations.
IEC 62271-209 (2007, 2019): The current standard covers the connection assembly of fluid-filled and extruded cables to GIS, where the cable terminations are fluid-filled or dry-type. It specifies the interfaces between the termination and the switchgear, ensuring that components from different manufacturers are interchangeable.
CIGRE TB 784 (2019): This technical brochure provides standard design guidance for a common, dry-type plug-in interface for GIS and power cables up to 145 kV.
8. Today's Mature Dry Technology
Dry designs are today mature and dominating for cable terminations for connection to GIS and transformers up to 550 kV. For cable terminations connecting to GIS and transformers, the overwhelming market is now dry technology.
The standard plug-in termination design has been extended to voltage levels up to 245 kV and cable cross-sections up to 1600 mm². More recently, the plug-in technology has been introduced for EHV outdoor terminations up to rated voltages of 550 kV.
Key Benefits of Modern Dry-Type GIS Terminations:
No liquids—eliminating leakage risks and periodic inspections
Factory pre-assembly—ensuring quality and reducing contamination risk
9. The Next Frontier: SF₆-Free and Sustainable Solutions
The evolution of GIS cable terminations continues with a new focus on environmental sustainability. SF₆ gas, while an excellent insulator, is a potent greenhouse gas. The industry is now developing SF₆-free GIS designs with dry-type terminations.
Recent Developments:
Nexans achieved the world's first electrical type test for 525 kV DC SF₆-free cable terminations in 2023.
By eliminating the use of SF₆ gas, cable terminations can reduce their potential greenhouse gas emissions by 99% or more.
420 kV SF₆-free GIS substations with dry-type plug-in terminations are now being designed and implemented.
Manufacturers are developing terminations compatible with alternative gases such as fluoronitrile-based mixtures and clean air with vacuum interruption.
The journey of GIS cable terminations reflects the broader evolution of power engineering—from the fluid-filled, labour-intensive designs of the 1970s to today's dry, plug-in systems that are factory-assembled, field-installed in hours, and designed for decades of maintenance-free service. The technology has been driven by a consistent set of goals: eliminating leakage risks, reducing installation time, saving space, and improving reliability.
As the industry moves toward SF₆-free solutions and even higher voltage levels, the evolution continues. The dry plug-in termination concept, once revolutionary, has become the new standard—a testament to the power of innovation in making our electrical infrastructure safer, more reliable, and more sustainable. From the first plug-in systems in 1966 to today's 550 kV dry terminations, GIS cable accessories have come a long way—and the journey is far from over.