PES design for CVL electrification - a low cost and low carbon option

01 March 2021
Image of overhead lines with clouds in the background
Amey Consulting found a novel way to cut the high civil engineering costs of electrifying the Core Valley Lines network

Amey is carrying out the electrification of the Core Valley Lines (CVL) as part of its overarching contract with Transport for Wales to transform the Wales & Borders rail network. The electrification is a major part of the new infrastructure for the project, requiring a smart approach to keep within budget. Amey Consulting proposed a ‘build clever’ innovative solution to the problem of bridges too low to accommodate live overhead lines.

The infrastructure engineering works needed to allow for the installation of the Overhead Line Electrification (OLE) to the core valley lines are significant. For example, the CVL network has 55 existing bridges, on the routes, lacking the height for live overhead lines to pass safely through. To meet one of the projects five core requirements, Environmental - zero carbon, demolishing the bridges and replacing them, or the option of possibly lowering the track, would not be the lowest carbon option, extend the programme and increase the cost significantly for the modifications or replacement of the existing bridges.

Amey Consulting’s task was to find a lower carbon option that would provide efficient delivery with a lower cost.

The solution applied to the OLE on the routes was for discontinuous electrification and the use of permanently earthed section (PES) for bridges where clearance was out of tolerance. PES’s are where the section of overhead line passing under a bridge or similar structure is kept permanently earthed. The application of PES’s on UK infrastructure has been used in specific circumstances, where electric trains would ‘coast’ through the earthed section before reverting back to overhead power. Eliminating the current in the PES section means that a structure too low to provide the necessary safety clearance can be retained without any modification.

Amey’s OLE design principles for the CVL design were to apply the PES approach, on a much wider scale, with 60 PES sections designed to be installed over a total of 24km track. This was made possible by the hybrid nature of the new light rail vehicles planned for the CVL network. They are either tri-mode or bi-mode, meaning they can switch easily between electric and battery power. As the train approaches the earthed section, a beacon sends a signal to the onboard power system to stop drawing current from the overhead line and switch to battery. Once through the bridge, the command is reversed and the train reverts to overhead power.

Amey’s signalling, electrification and power (E&P) teams developed an innovative power modelling approach to determine the level and to optimise the state of charge in battery for optimum operation window to meet the timetable of four trains per hour. They also collaborated with the rolling stock team to ensure seamless operation with the trains. 

The key advantage of this proposal is the relatively low initial cost compared to the traditional alternative of replacing a bridge. For use of a PES it requires is extra cabling and a pair of insulators at each bridge. If the bridge is rebuilt to the correct height in future, the earthed line can simply be repowered.

Amey’s OLE design team developed and commissioned high-quality bespoke insulators to ensure resilience and reliability, since the insulators are essential to the smooth operation of the OLE for the routes. These insulators were developed in collaboration with the track and infrastructure teams to ensure the maintenance needs were met.

The design has been approved in principle, and the next stage is to test it on a non-operational piece of infrastructure to ensure it works as expected. This approach for the use of PES’s on the CVL scheme will play a significant part in making the electrification of the project feasible and affordable.