As the largest addition to the south east rail network in 50 years, the engineering challenges presented by Crossrail are well known. But while the major projects are what make the news, there is a host of detailed, day-to-day work taking place behind the scenes which is every bit as vital to its successes.

UK Power Networks Services (UKPN Services) is already designing and installing the power infrastructure that will allow London’s Liverpool Street Station to be remodelled. Eventually Liverpool Street Crossrail will serve the City of London and provide interchanges with London Underground’s Northern, Central, Metropolitan, Circle and Hammersmith & City Lines. However, all the time that work is in progress, the existing station has to continue to provide a near-seamless service to London’s travellers.

At Liverpool Street station, Vinci Construction UK has been awarded a contract to clear the way for the new Broadgate ticket hall, which includes moving an entire underground traction substation – one that currently provides traction power to tracks that serve thousands of passengers daily. UKPN Services has been brought in by VINCI Construction UK to carry out specialist high voltage power installation works.

Preparing the way

Crossrail’s new station will be built beneath London Underground Limited’s (LUL) Liverpool Street and Moorgate stations, with a new ticket hall to be constructed at each end providing step-free access to the platforms. Unfortunately, the existing traction substation in Liverpool Street currently occupies part of the space needed for that ticket hall. This meant a suitable site for a replacement substation was needed, and a disused platform within the station footprint was identified as the best location.

A quandary

Moving an entire substation located on the surface is a complex process, but in this case, because both the old and new sites are below ground level with limited access, the task is more complex still. To complicate matters even further, the new substation requires a new 55-metre cable access tunnel to be constructed – and that tunnel will slice neatly through a disused escalator shaft that currently carries sixteen 750V DC cables and several low voltage (LV) cables from the existing substation.

At first sight it’s almost Catch-22. The new power cable tunnel can’t be excavated as that would disrupt traction power causing the unthinkable – closure of underground lines. Yet the new substation can’t function without the new cable tunnel. It has to be completed to allow Crossrail to be built.

A two-step process

The answer, when it came, sounded straightforward. Temporarily re-route the existing cables away from the route of the new cable tunnel without decommissioning the old substation. Construct the new cable tunnel, build the new substation, commission it and the new cables, then finally decommission the old substation and the temporary cable route.

Trouble is, this all has to take place underground, in Central London and most of the work has to be carried out during night-time engineering slots rarely lasting more than two and a half hours. It was going to take a great deal of planning.

Building a temporary cable route

Sometimes, the fact that the London Underground has been remodelled so many times has hidden advantages. In this case it meant that a disused escalator shaft parallel to the original could be employed as a route for the temporary DC traction power cables. Even so, 935sq mm copper track cables are heavy and inflexible, and access was so limited that everything had to be done by hand. It would take a team of up to 14 workers 10 days just to manoeuvre and fix the cables into place before any jointing work could start.

The cables first had to be manhandled down a vent shaft, then installed on a cable management system fixed to the shaft walls and ceiling. Unlike a conventional stairwell, escalator shafts are surprisingly vertical, so much of the work also had to be carried out in a tight, hot and dusty environment while wearing cumbersome harnesses. Yet it’s unlikely that the thousands of city workers who passed overhead were even aware of the changes taking place beneath their feet.

Planning the new substation

With the cable diversion work nearing completion, detailed planning could begin on the new substation. Due for completion and commissioning in early 2013, it is sited in a now redundant platform, located within a cutting, with access via a nondescript set of doors in a quiet London cul-de-sac. As it is in a cutting, the substation has been arranged over three floors, with the highest at street level and lowest at the original platform. It also means that construction will have to take place next to live rail lines on an extremely busy commuter route.

The street level floor will contain three 2.5MW, 11kV AC/630 and 750V DC transformer rectifiers and two 1.5MVA, 11kV/433v auxiliary transformers. Beneath this will be a switch room containing a 10 circuit high voltage (HV) switchboard, a 400V three-phase and neutral low voltage AC switchboard and a mezzanine level containing positive and negative DC switchboards. The final platform level will be used as a cable basement.

Finally a SCADA system will be installed with a human machine interface (HMI) terminal in a nearby mess-room so the entire installation can be constantly monitored.

Working with multiple partners

On any undertaking of this complexity, it is inevitable that many interested parties will be involved, and that an ability to work effectively with them all will be key to the project’s ultimate success. In this instance not only are LUL, Crossrail, VINCI Construction UK, local authorities and local commercial residents involved but, while the final substation and cabling will be owned by LUL, its maintenance and operation will be the responsibility of another division of UKPN, UKPN Powerlink. The continuity of service this offers is one example of how UKPN is able to effectively manage rail power infrastructure, from initial design, all the way through to operation and maintenance. Indeed, in this case UKPN Services were also able to provide specialist design assurance services as required by LUL, Powerlink and Crossrail.

Final Switchover

It will be 2013 before the new substation is finally commissioned, little more than a year since the original contract was awarded. By that time the new substation will have been built and installed, temporary cabling will have been run and removed and everything will be ready for the final switchover. Aside from hoardings visible in London streets the travelling public should notice nothing.

Yet without this work, and the close cooperation of all the parties involved, the Crossrail project simply cannot progress as planned.

Tight spaces and tighter deadlines

Working in tight spaces and to tight deadlines has to be expected when designing and building traction substations underground in congested city centre locations. It’s a demonstration of how much expertise UKPN Services have built up that they can take on and deliver such projects with confidence. Gary Kelly, Senior Project Manager explains; “UKPN Services is well known for its rail ability – we’ve worked on power transmission projects such as Sub-Surface Railway Package 2, Victoria Line Upgrade, Thameslink, High Speed 1 and Network Rail Traction Substation Renewals. Where it really comes into its own, though, is when we’re faced with projects such as Crossrail at Liverpool Street and where the challenges are a little out of the ordinary. It’s not simply that we might have come across a similar project before; it’s the fact that we can draw on expertise from so many areas within the organisation, and know precisely what our clients and their customers expect of us.”