HomeElectrificationOvercoming the clearance issue

Overcoming the clearance issue

Whilst third rail and other low-level current collection systems add a lot of equipment to a railway at ground level, the general approach to electrification across the world has involved the adoption of overhead contact systems. In the UK, the early schemes tended to consist of installations at relatively low voltages and the clearance from surrounding structures was not a significant challenge.

However, technical advances arising from the need for higher speeds and heavier loads led the industry towards high-voltage overhead line equipment (OLE) which demanded much greater clearances. The initial higher-voltage installations accepted what would nowadays be quite conservative clearances for live equipment and consequently the early stages of the West Coast Main Line electrification involved a considerably quantity of overbridge reconstructions. The media was enlivened by images of exploding bridges whilst clever solutions for prefabricated new arches were introduced.

British Railways – as it then was – realised that the case for further electrification was at risk from the emerging costs and engaged in much significant development effort. This culminated in modified design protocols for the extension of the early work to complete the Euston-Birmingham route.

Cardiff Intersection Bridge carries the Cardiff- Merthyr line over the South Wales Main Line. PHOTO: NETWORK RAIL

Lower voltage

The early clearances were foreshadowed at a conference held in 1960 by the Institution of Mechanical Engineers and permission was obtained from the Chief Inspecting Officer for Railways to experiment with a view to determining minimum safe clearance dimensions. As presented in the follow-up electrification conference in 1966, revised electrical clearances were approved by the Minister of Transport in August 1962. This allowed the London Midland Region electrification to be completed at 25kV throughout and advantage was gained from the acceptance of the new dimensions for all domestic schemes.

In a precursor to current times, incisive questions were posed about the cost of electrification and, as well as installation and equipment costs, the need to create clearance was examined closely. One solution had been to employ a lower voltage and, in place of the standard nominal 25kV, equipment was installed to provide traction power at 6.25kV – an approach employed on the Great Eastern Lines and in Scotland for the Glasgow suburban projects.

An overview of the railway layout. PHOTO: GOOGLE EARTH/LANDSAT/COPERNICUS

The London, Tilbury and Southend Lines were also thus equipped; the 6.25kV section was from Fenchurch Street to beyond Barking, with changeovers there on both the Upminster and Tilbury lines, together with a section between Chalkwell and Shoeburyness. The remainder was at 25kV and the 6.25kV sections were converted to 25kV in the early 1980s.

However, this apparently attractive project introduced unwanted complications, particularly the application of dual voltage capability to rolling stock and the requirements for changeover equipment.

The search therefore continued for the ability to install overhead line equipment with lower clearances to fixed infrastructure. Electrification of the East Coast Main Line emerged with costs acceptable to government and, for a while, electrification rolled forward; however, it slowed considerably during the era of privatisation and the splitting of British Rail into infrastructure and other portions.

The view west towards Cardiff Central. PHOTO: ANDROMEDA

Eventually, under influences from the need to consider the environment more fully, electrification began to restart, albeit under close cost examination. As the challenges to the cost of electrification grew and the go-ahead was given for proposals on the Great Western and Midland main lines, as well as schemes such as ‘the electric spine’, the search for solutions to reducing clearance were pursued with greater vigour. As has been described elsewhere, electrification costs came under even more scrutiny and the situation became more serious with much proposed work being cancelled.

Bridging the gap

However, the industry was determined to prove that costs could be controlled and an opportunity to show how arose within the Great Western Electrification Programme (GWEP). Recently projects have shown that civils work – especially bridge reconstructions – can make up around a third of the cost.

One example is the Cardiff Intersection Bridge, a substantial skewed crossing of the Cardiff and Merthyr line with the South Wales Main Line. Initial plans to reconstruct the bridge were estimated to be around £40 million. Alternative proposals involved lowering the track, rebuilding a culvert and pumping water from the canal around Cardiff city centre, which was estimated to cost around £20 million. Both solutions would be very disruptive to passengers and the costs threatened the viability of continuing the electrification west into Cardiff Central Station, only 400 metres away.

The challenge was met by a group consisting of Andromeda Engineering of Speke (Liverpool), GLS Coatings, Great Western Railway, Network Rail (Infrastructure Projects, Safety Technical & Engineering (STE) and Wales & Western Route), Pace Networks, Siemens Mobility and the University of Southampton (UoS) High Voltage Laboratory.

Andromeda Engineering, GWEP’s OLE designers for Cardiff, and Network Rail (Wales & Western), contacted Network Rail (STE) to propose an alternative design – initially in the form of a proof of concept – to avoid further descoping of the programme. A steering group was formed to bring in all the decision-makers and focus on achieving an engineering solution.

STE identified modern equipment from Europe that could assist with the challenge. This included:

Bonomi insulated bridge arms, supplied by Pace Networks,

Siemens 25kV surge arrestors, complete with Siemens Arrestor Condition Monitor (ACM)

an electrically-insulating coating from GLS Coatings (GLS 100R) that Network Rail had previously used for signalling power supplies.

Making sure

The solution identified the benefits of utilising the latest insulation coating technology to provide an electrically-insulating layer to the bridge to support reduced electrical clearance. This was the first proposal of its kind for new electrification. Surge arresters were included in the design to ensure potential transient over-voltages at the bridge could be safely controlled without the need for additional clearance.

The trial of a coated steel plate at Paisley, Scotland.
The trial of a coated steel plate at Paisley, Scotland.

Suppliers agreed to submit their equipment to high-voltage electrical testing devised by Network Rail (STE) and the University of Southampton.

The Siemens surge arrestor is a recent development. When installed at bridges or tunnels, they significantly reduce the impact of over-voltages, for example from a lightning strike. They were recently installed on the Danish railways to reduce static electrical clearances from 270mm to 150mm.

The testing at the UoS High Voltage Laboratory included combinations of components from different suppliers to determine the optimal arrangement. Each was tested in dry and wet conditions, with a pollution mix added to contaminate all insulating surfaces in order to make the tests as realistic as possible.

They delivered much better results than expected. When all the equipment was used together as a system, a minimum clearance of just 20mm was proven as sufficient to avoid flashovers in wet and polluted conditions. However, even this was insufficient to energise the OLE under the bridge. It was proposed to install the contact wire at a reduced height.

Standard deviation

Andromeda confirmed in their proof of concept design that if a case could be made to deviate from the RSSB Group Standard minimum OLE wire height requirement, a design could be developed which interfaced with the railway in the vicinity of the bridge. Additional high-voltage testing was developed by Network Rail (STE), UoS and GWR involving the minimum electrical clearance required between the OLE and various train roof gauges. These tests confirmed that the surge arrestor connected to the OLE provided an additional benefit and only 70mm was needed to avoid flashovers between the OLE and top of rail vehicles.

To gain confidence in the design, Network Rail (Scotland) agreed to carry out an in-service trial which included lowering a steel plate coated in GLS100R closer to the OLE in stages. This simulated a metallic bridge and proved the concept on an operational railway. It provided Network Rail (Wales & Western) with the confidence to proceed and agree for the bridge to be coated. GWR provided further assistance by working with their suppliers to confirm that pantographs would be able to operate satisfactorily under the lower wire and supported Network Rail with the necessary deviation to RSSB.

A clearance of 20mm was proven to be sufficient to avoid flashovers. PHOTO: ANDROMEDA

Working together

Adopting a collaborative approach to utilising innovation, managing risk and decision making was vital in securing support from a wide range of stakeholders for the unconventional proposal. With acceptance of the solution, the key to the success of the project was commitment by Andromeda Engineering to positive stakeholder management, ensuring everyone would be satisfied with each stage of the works.

The assembly of a Strategy Steering Group and Delivery Working Group ensured a top-down approach would be adopted to facilitate collaboration. The groups met regularly to discuss the key areas, making sure that clear and concise deliverables for implementation emerged. The approach also realised significant efficiencies and demonstrated what can be achieved when industry partners come together with a common goal.

Estimates suggest that the approach led to considerable savings, as well as preventing significant disruption to the railway and derisking the programme. The total cost of development and installation was less than £1 million, equating to a 95% cost efficiency. The OLE was energised under the bridge in December 2019. So far, there have been no flashovers and the design has worked perfectly. The route is still clear for all standard rail vehicle gauges despite the reduced wire height.

Development and installation cost less than £1 million. PHOTO: ANDROMEDA

The significance of the project was acknowledged by Andromeda Engineering becoming winners of the Railway Industry Innovation Award in 2018 and finalists in the ‘Driving Efficiencies’ category at the Rail Partnership Awards in the same year.

The concept of Voltage Controlled Clearances (VCC) has been adopted by future electrification projects, including TransPennine and in Scotland. Collectively, it is estimated that VCC could save over £100 million. It will therefore play a pivotal role in making electrification more efficient and help to decarbonise the railway.

Peter Stanton BSc CEng FIMechE FIET FPWIhttp://therailengineer.com

SPECIALIST AREAS
Electrification, traction power supplies and distribution networks


Peter Stanton undertook, between 1968 and 1972, a ‘thin sandwich’ degree course at City University, London, sponsored by British Railways Midlands Region and with practical training at Crewe and Willesden.

In 1980, following a spell as Area Maintenance Engineer at King’s Cross, Peter took on the interesting and challenging role of being the Personal Assistant to the British Railways Board Member for Engineering. As such, he was project manager for several major inter-regional inter-functional schemes.

Under Railtrack, Peter became Engineering Manager for Infrastructure Contracts, based in Birmingham, and then Electrification and Plant specialist for the West Coast Route Modernisation under Network Rail.

Since 2007, as an independent consultant, he has worked on the national electrification programme, Dubai Metro Red Line, Network Rail Crossrail, and Great Western Electrification. He sits on the Railway Technical Advisory panel of the IET and the Conference and Seminars Committee of the Railway Division of the IMechE.

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