Rethinking line-to-cable transitions

Context

As the electricity grid evolves, transmission system operators increasingly need flexible solutions to connect overhead lines with underground cables. Today, these line-to-cable transitions usually require dedicated ground installations. For a single circuit, such a transition can occupy around 30 x 20 meters, creating challenges in terms of land acquisition, permitting, costs and environmental impact.

This challenge becomes particularly relevant when an existing pylon located inside a substation needs to be converted into a transition pylon. In such cases, the conventional solution may require purchasing additional land outside the substation and building a complete transition area. The Line-to-Cable innovation project explores an alternative: integrating the transition directly into the pylon itself. 

Approach

The concept is straightforward: route the underground cable up the pylon and connect it directly to the overhead line through a dedicated termination. While similar principles exist for lower voltage applications, this approach is not yet standard practice for higher voltage levels within Elia.

To assess the potential of this solution, Elia has launched a feasibility study focused on a 150 kV lattice tower, considering both one-circuit and two-circuit configurations The study builds on existing solutions, supplied input and international practices, including insights from Swissgrid.

This topic is also being addressed at international level. CIGRE has launched Joint Working Group B2/B1.90 on transition facilities between overhead and underground lines, bringing together expertise on both technologies and addressing key aspects, such as layout insulation coordination, mechanical loads and cost drivers. 

Two types of termination technologies are currently under consideration:

• Oil-insulated terminations are well-established and available for all voltage levels, but they bring added complexity when installed at height, including safety and environmental risks. 
• Dry, solid-insulated terminations offer a promising alternative, potentially reducing these risks by eliminating oil from the system.

Results

The first analyses confirm that integrating the transition into the pylon could be technically feasible, but only under specific conditions. The concept offers clear potential benefits: reduced footprint, possible avoidance of land acquisition, shorter negotiation phases before permitting, potential cost reduction, and a path towards standardisation for space-constrained locations.

At the same time, important challenges remain. Pylon-mounted solutions create additional mechanical loads, require careful verification of electrical clearances and may complicate maintenance and repair activities, particularly when equipment is installed 20 to 45 metres above ground. Oil-insulated terminations, in particular, raise  safety and environmental concerns, such as oil leakage, pollution risks and possible damage in case of failure. 

Experience from other transmission system operators, shows that conventional, portal-based transition structures remain the preferred solution in most cases, with pylon-integrated solutions mainly relevant where space is limited.

However, dry terminations could significantly change the outlook for this concept. By removing oil from the termination system, they could reduce key risks related to leakage, pollution and explosive failure, making pylon-integrated transitions safer, cleaner and more attractive from an operational perspective. 
Today, no dry termination has yet been qualified for 150 kV application. If such a solution can be technically validated, it could become a real gamechanger for line-to-cable transitions in constrained.

Next steps

Elia will continue to monitor technological developments, particularly dry terminations and the outputs of the CIGRE Joint Working Group B2/B1.90. 
In parallel, a dedicated feasibility study is being launched to further assess integration into a standard 150 kV lattice, focusing on technical conditions such as mechanical forces, safety distances, insulation and structural requirements

Based on these findings, potential pilot opportunities will be explored, particularly for new projects or locations where conventional solutions are difficult to implement. A comparative business case will also evaluate costs, operational implications and risk profiles against standard approaches.