Thameslink Programme power enhancement

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London’s rail network is currently undergoing a significant transformation with major projects such as the £5.5 billion Thameslink Programme. This will offer increased capacity and improved journey times on one of the UK’s busiest routes – the north-south line across the Thames connecting Bedford to the north, on the Midland Main Line, with Brighton on the south coast – terminus of the Brighton Main Line. To achieve this and reduce overcrowding, the programme must ultimately thread 24 12-car trains an hour through central London, rather than the current hourly throughput of eight 8-car trains.

Co-located teams

Balfour Beatty Rail and Balfour Beatty Engineering Services, working together in an internal joint venture arrangement, were awarded a framework contract by Network Rail in early 2008 to undertake elements of the power upgrade design, supply and installation works for the Thameslink Programme’s inner and outer areas. Delivering such works whilst causing minimum disruption to the travelling public is the type of challenge that these world-leading contractors excel at.

The Balfour Beatty team managing the framework is co-located with Network Rail’s team at James Forbes House in Southwark, South London. This approach has been instrumental in driving efficient delivery and aligning the project focus, objectives and achievements.

The capacity of the existing Midland Main Line electrification system had to be enhanced over the 20km section between Kentish Town and Borehamwood to provide sufficient traction power for the planned throughput of trains. Prior to the project, it comprised a classic booster transformer system in which power was supplied from the Grahame Park and Borehamwood feeder stations, with a track sectioning cabin located at Kentish Town. Traction power was distributed using the 25kV catenary system with traction return occurring in the conventional way through the rails and via the return conductor cable.

thameslink programme cross track
The cross-track feesding arrangements

The new scheme

The scheme specified by Network Rail requires new traction power supply sites, equipped with auto-transformers, at Kentish Town, Grahame Park and Borehamwood. The traction power will be distributed via a 2 x 25kV system, using an auto-transformer feeding cable. Such a system was first introduced in North America and re-established in Japan for the San Yo line in 1972. Nowadays, it is common in most high performance railway systems.

At its heart is an auto-transformer with a centre tap connected to the traction return rails via duplicate red bonds. The system’s other distinguishing feature is the return feeding line, known as the ‘auto-transformer feeder’. The voltages between the auto-transformer feeder and the rails and between the overhead catenary equipment are both 25kV. A potential difference of 50kV exists between the auto-transformer feeder and catenary equipment so the power is transmitted at 50kV between the substation and the auto-transformer preceding the section in which the traction unit is drawing power. This has the advantage of increasing the required feeding distance.

In addition, the low currents involved with this method of power transmission result in lower voltage drops in the catenary system. In the section between the substation and auto-transformer, the current flowing in the rails is low because of the almost 180° phase shift in potential in the catenary (positive 25kV) and auto-transformer feeder (negative 25kV) through which the current flows are of an equal magnitude. As a consequence, improved telecom interference suppression is inherently provided. Between the two auto-transformers, the traction units are fed from both ends, the rails serving as return conductors in the customary manner.

Another significant advantage of an auto-transformer system is that standard 25kV traction units can be used. Electrical clearances apply for a 25kV system rather than a 50kV system, however additional electrical clearance is needed between the auto-transformer feeder and catenary wires as these specific features have a 50kV potential difference.

Auto-transformer sites

The Network Rail remit required a new Sectioning Auto-Transformer Site (SATS) to be constructed at Kentish Town to replace the existing track sectioning cabin. The SATS is located alongside it and supports the new 2 x 25kV auto-transformer distribution system north to Borehamwood. South of Kentish Town, the classic 25kV configuration is retained but fed from the new SATS. Adjacent to the existing Grahame Park feeding station will be a Midpoint Auto-Transformer Site (MPATS).

The new system’s incoming power supply from the distribution network comes via a new Auto-Transformer Feeder Station (ATFS) at Borehamwood, replacing and located alongside the existing classic 25kV feeder station. It will supply power south to Kentish Town via the 2 x 25kV distribution systems whilst, northwards, the classic 25kV booster system is retained, fed from the ATFS.

thameslink programme steel pile
A tubular steel pile is driven for an auto-transformer feeder mast

Conversion challenge

The challenge for the Balfour Beatty and Network Rail engineering and construction teams was to devise a methodology that would allow the existing traction power system to be converted to a high-performance 2 x 25kV auto-transformer system with minimal disruption to one of the UK’s busiest main lines.

The approach taken has been to build the new supply sites alongside the present facilities, maintaining the latter on line until the former are fully constructed and commissioned. In the first instance, the supplies are commissioned as a classic 25kV system before being commissioned in full 2 x 25kV auto-transformer mode. It was not possible to do this in one stage as the new sites are entered into service before the auto-transformer feeder installation is complete.

A modular ‘plug and play’ construction approach has been taken, simplifying the process and minimising the overall build programme. Switchgear is delivered in self-contained modular cabins, housing all necessary lighting, power and control interfaces – all pre-tested – so they can then be directly mounted onto simple concrete slab bases. A SCADA system is also provided, consisting of a Remote Terminal Unit (RTU) sited within each cabin to provide alarms, controls and equipment monitoring via a comms link to the York Electrical Control Room (ECR). Balfour Beatty Rail’s project team was also responsible for modifying the displays there to incorporate the changes resulting from these electrification enhancements.

Installing the Centipede

The distribution system comprises two new 37/3.78 bare-stranded aluminium Centipede auto-transformer feeding cables mounted on a 25kV insulator arrangement attached to the overhead line support structures. Each cable is installed over the open route sections of the 20km site – typically one per cess, with a ground clearance of 5.2 metres where possible. However there are a number of locations where, due to infrastructure constraints, a suitable clearance could not be achieved. Here, a 25kV insulated 400mm2 cable was laid in a trackside or wall-mounted trough. The Belsize and Elstree tunnels were dealt with in this manner.

As the system had to be maintained in service during the conversion, a Return Screen Conductor (RSC) was installed to suppress telecom system interference for the period between the return conductor being removed and the installation/commissioning of the auto-transformer feeder.

In some instances, the existing overhead line stanchions were either in the wrong location for the new cable’s routing or could not withstand the increased loading generated by its larger size and associated fixings. At these locations, a new structure was installed. Generally these were constructed from a 203 universal column galvanised to provide corrosion protection and founded on either a concrete side-bearing foundation or 610mm diameter steel tubular pile. New structures and cross-track feeder configurations were installed at the SATS, MPATS and ATFS sites to support the new feeding and sectioning arrangements.

In total more than 110 new structures are being installed as part of this scheme together with around 40km of return screen conductor and Centipede auto-transformer feeder cable.

Project timeline

Work on the project started in mid-2008 with Balfour Beatty Rail’s Electrification Engineering Services Group, based in Liverpool, being commissioned to produce the outline design for the scheme to satisfy GRIP stage 4. A detailed design for GRIP stage 5 was then developed. Construction began early in 2009. This is presently going well with almost all the support structures installed, the return screen conductor fully in place and the return conductor and booster transformer decommissioning progressing to plan. More than 2km of the route has been wired with the auto-transformer feeder.

All three traction power supply sites are substantially complete, with the Kentish Town facility commissioned and in service in classic 25kV mode. Work is on target for full commissioning in 2 x 25kV auto-transformer mode in May 2011. The work at these sites has been carried out in green zones, helping to reduce costs as non-railway plant and labour could be used. Lineside activities have been substantially undertaken within ‘Rules of the Route’ possessions and in a 48-hour, all-lines-blocked blockade over Christmas 2009. The Balfour Beatty Rail team is now focussed on this Christmas’ blockade.

The Knotweed quandary

One of the significant environmental challenges was the elimination of Japanese Knotweed from the Kentish Town site. Here, the team engaged Elcot Environmental, a specialist contractor that uses a combination of controlled excavation and herbicide treatment. This method was adopted as the conventional technique – an excavation 3m deep, extending 7m beyond the furthest Knotweed – would have lead to the track being undermined.

Pesticide treatment of the affected area is undertaken before the excavation works. Wherever possible, digging takes place from inside the contaminated areas to its visible edges. When it appears to be clear, skilled personnel then carry out a careful search of the surface. If no rhizomes are found, three more exploratory excavations and inspections take place. If the third check still finds no rhizome, the soil is deemed clear.

Where the Knotweed was still present but soil removal could have undermined the track, the stems were injected with Picloram to stop further growth. Picloram’s use on this site was permitted as it is not within a groundwater protection zone. The site was then monitored for any regrowth, with pesticide treatment applied if needed.

Keynote project

The project has an excellent safety record with a zero Accident Frequency Rate (AFR) to date. This stands testament to the excellent safety procedures and the benefits accrued from Balfour Beatty’s ‘Zero Harm’ and Network Rail’s ‘Safety 365’ campaigns.

On so many levels this is a keynote project for both organisations. Balfour Beatty Rail and its joint venture partner Balfour Beatty Engineering Services are on target to ensure its delivery on time, to a high standard and with an outstanding health, safety and environmental record.

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