HomeElectrificationThe changing face of electrification

The changing face of electrification

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The removal of diesel-powered trains from the national rail network by 2040 (2035 in Scotland) is key to achieving the UK Government’s net-zero-carbon target by 2050. 

To meet its sector targets, the rail industry must continue to find sustainable and cost-efficient solutions. Electrification is one of the key drivers, along with hydrogen and battery train technology, which will enable a broad range of benefits to be realised: improving people’s health, creating better places to live and travel in, and driving clean economic growth.

A changing market

Over the last decade, electricity distribution has undergone significant changes. Not only has there been a trend towards the decentralisation of power generation, but the use of renewable energy has also increased significantly. Renewables suppliers are now producing more than 20 per cent of the UK’s electricity, a share which is forecast to increase to 50 per cent by 2025.

Rail electrification has also seen some major changes, with benefits coming from both product and process innovation. One example of this is the use of static frequency converter (SFC) technology, which addresses the issue of phase imbalance. Whereas the UK electricity network operates on a three-phase balanced system, railway power supplies have traditionally been connected directly using just two phases of the network. This causes a phase imbalance, restricting the amount of power that can be obtained from the network operator’s electrical supply. The issue is exacerbated by the shift to renewable power generation, which is less resilient to phase imbalance than coal-fired generation.

The use of static frequency convertors eliminates phase imbalance and provides more flexibility for supply to the railway, making connections to the network possible at various voltage levels, as well as supporting the shift to renewable energy supply. 

Early contractor involvement

With an ever-growing demand for a greener, smarter, more reliable railway, the industry is also continually challenged to make the optimal use of the available investment. One approach that is increasingly being used to help meet this challenge is the adoption of early contractor involvement (ECI). By involving the technology supplier at an early stage, a much greater degree of operational flexibility can be achieved, and programme disruptions minimised. This means that potential risks and challenges can be identified and enables strategies to be developed to ensure that projects meet the requirements of their stakeholders.

For example, when engaged at the ECI stage, Siemens Mobility typically deploys its Sitras Sidytrac simulation software, together with calculations from its Sicat IT tools. This provides the programme team with key insights into the various system interactions and the power quality that is required, both to allow for voltage fluctuations and also to avoid operational disruptions. The software package allows for comprehensive studies of the electrical network to be undertaken, including electromagnetic capability (EMC) and power modelling.

By engaging the electrification team at the initial feasibility stage, system support can also be delivered consistently throughout the life of the project, so reducing overall costs and programme length. Through early involvement, Siemens Mobility has demonstrated substantial project cost savings and has been able to bring forward large programmes by several years, supporting business cases and enabling projects to secure funding.

From the early stages of a project, it is also beneficial to look at the railway as a complete ‘end-to-end’ system, with a single supplier, taking a holistic view, able to design and deliver the most cost-effective and efficient solution. Conversely, when each individual discipline is procured separately, there is a risk that the solution becomes over-engineered, with each supplier focusing purely on its own, relatively narrow area.

By involving technology providers for the whole route, rather than small sections of it, ECI programmes enable the optimum solution to be developed and assessed. Early modelling at GRIP (Governance for Railway Investment Projects) stages 3 and 4 also enables the electrification design to be shaped to meet both the strategic and the business case objectives. GRIP 3 is the stage at which the preferred option is identified, with stage 4 covering its subsequent development.

As an example, on one project for which the client had initially considered an auto transformer solution, by looking at a ‘whole system’ design, an alternative technology was ultimately selected. This delivered cost savings of around 60 per cent and a reduction in the programme length of around two years.

The integration of the strategy and technology required to control and power trains can also be better identified at an early stage of a programme. Then, passenger flow, train location, power loading and efficient infrastructure placement can all be modelled to inform the electrification system design. This also allows new technologies to be considered that may have otherwise been passed over. For example, systems such as air insulated switchgear, static frequency convertors, efficient overhead line solutions and surge arrestors can all contribute to a more efficient traction solution and to an overall decarbonisation programme.

Not only can this approach help minimise the overall cost of delivery, but, if appropriate, it can also lead to the introduction of output-based specifications, which encourage technology providers to take a more holistic, whole-life approach. 

Similarly, frameworks for traction decarbonisation can also include reward structures for efficiencies and collaboration – recognising the appropriate risk allocation between customer and contractors. Innovative commercial models could then be developed, for example to encourage technology providers to offer licensing or ‘paid on performance’ schemes.

The technology explained

Air-insulated switchgear

The Sitras ASG25 air-insulated switchgear for 25kV AC traction power supplies is suitable for use in single and two-phase AC traction power supply systems and is currently the only containerised air-insulated switchgear solution on the UK market.

Air-insulated switchgear removes the use of sulphur hexafluoride (SF6) gas insulation, and so eliminates the need for any special precautions to be taken during manufacturing, operation or recycling. The containerised unit is manufactured, assembled and tested off-site in the UK and then transported into position ready to be connected to the power distribution system. This means there is a reduction in carbon emissions (compared to transporting equipment from overseas), as well as supporting the creation of jobs and skills within the UK.

Static-frequency converters

Traction requires a single-phase 25kV AC supply, whereas the electricity supply from the grid is three-phase. Traditionally, the railway has taken power from two phases, creating a phase imbalance, an issue which SFCs eliminate by converting three-phase input into the single-phase traction current required.

Surge arresters

When 25kV catenary equipment is to be installed underneath bridges or tunnels, in the past the structure has had to be modified, removed or replaced to provide sufficient electrical clearance. Surge arresters work in circuit with the overhead line system to address this issue, enabling reduced electrical clearances to be applied, so that if over-voltages do occur (for example from a lightning strike), the surge arrester reduces the impact.

Effective ECI

To benefit from ECI, it is important to define exactly what the outputs from this stage of the programme need to be. ECI should clearly add value to either cost or programme certainty, or both. The ECI process and period must also be managed effectively, ensuring sufficient time and effort is set aside to build the team and set common goals, so that all parties are aligned.

The make-up of the ECI team is also important. It should include people who are not only capable of challenging the norm, but also have a true understanding and a passion to achieve the overall programme goals. They must also be willing and empowered to make the best decisions. Similarly, when the client is fully engaged, a true ‘one team’ approach develops, with all members being open and transparent throughout.

To achieve the best output for the programme, the client needs to ensure the ECI process and its team are properly resourced and funded, and ideally are co-located, which will help build its success and encourage the establishment of strong relationships.

Edinburgh Tram approach

As supplier of the SCADA (supervisory control and data acquisition), telecommunications,  electrification, signalling and road traffic systems to the first phase of the Edinburgh Tram programme, Siemens Mobility was brought in right at the start of the second phase of the programme to deliver a 2.9 mile extension to the original network. 

Working closely with the client, to develop the technology solution and business case fully, ensured the programme was within the cost envelope, was achievable and that the relevant risks were carried by the client, the main contractor and the system subcontractor as appropriate.

Initial discussions began around two years before the planned start of the project, enabling the technological solutions to be agreed at an early stage. This in turn helped the appointed main contractor to fully understand the tender and gave the client confidence to start an ECI stage with them. As a result, any misunderstandings or queries were addressed in a collaborative way, building the trust required to establish the optimum cost and programme plan.

Guest Editor: Shaun Cooper is Managing Director – Electrification, Siemens Mobility.


  1. If 25 kVAC electrification now needs substations frequency converters to avoid unbalancing the Grid, is it not perhaps time to think of high-voltage DC electrification? In the USSR there were experiments with using 6 kVDC, and more recently 12 kVDC has been proposed. Industrial-frequency AC electrified networks are extensive and so would not be economic to convert, but systems operating at 1.5 or 3 kVDC might be upgraded and perhaps also AC systems using 16.7 Hz.

    The advantage of 12 kVDC over 25 kVAC with frequency converters would be: simpler substations; lighter electrical equipment on board trains (no transformer) and the possibility of OHLE without copper (since there would be no ‘skin effect’ to worry about, contact wires could be constructed from aluminium with a stainless steel contact face).


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