When the rail engineer reports on a significant project, it is usual for the precise location to be given. Then readers who know the area can visualise the work being undertaken, and imagine the challenges that the project engineers face.

However, it is an indication of the significance of a recent electrical substation renewal, which UK Power Networks Services undertook as part of SSR2 (Sub-Surface Railway 2), that London Underground Limited (LUL) has requested the precise location not be disclosed.

What can be revealed is that it is central, that it is surrounded by listed buildings, and that access is limited. It is also a major thoroughfare in its own right, so the entire operation affected not only LUL’s passengers, but the London public at large.

More trains – more power

This renewal forms part of a programme of 14 substation replacements on London’s sub-surface lines. Its purpose is to increase passenger capacity by allowing more modern trains to be run. Eventually it’s expected that the planned 191 new Bombardier S Stock trains will allow a 65 per cent capacity increase on the Circle and Hammersmith & City lines, 24 per cent on the District line and 27 per cent on the Metropolitan line.

In electrical engineering terms this means that the new substations and associated transformers not only need greater capacity, 2.5MVA rather than the existing 1.5MVA, but that they are significantly larger and heavier. In the constricted environment of a subsurface railway that is a challenge, and one compounded by the structural confusion left by over a century of building works and improvements.

Yet, at this location, those years of previous building work would prove to be an unexpected boon rather than the expected hindrance.

Re-using the unused

Beneath a public park adjacent to the station, a previous generation of engineers had constructed a subterranean power station. Unused and empty, it had long been derelict, was partly flooded and, though unsuitable without a great deal of work, was the ideal location for a new substation.

This technique of reclaiming existing infrastructure, of bringing old facilities back into use, has been common throughout UK Power Networks Services’ involvement with SSR2. This is one of the largest ever upgrades planned on London Underground, and one of the most challenging ever undertaken in a major capital city where space is at a premium.

Bringing redundant assets back into use, therefore, makes practical as well as commercial sense, and goes a long way towards ensuring that the public are inconvenienced as little as possible.


Construction work began with the demolition of various structures; a long abandoned overhead crane, brick walls of varying vintages, and walkways that had become dangerously unstable. Above ground, tree surgeons were called in to clear branches and allow access, whilst hazardous material was carefully identified and safely removed.

Over the following months, two new concrete floors, new access stairs and a two-hour fire separation wall were built. This new wall, which now divides the underground area, left around 30% of the original floor space for future station use with the remainder being the new substation site.

To make matters even more complex, all personnel, materials and equipment could only reach the site via one, small access shaft that originally formed part of the ventilation system. And, of course, all building debris had to come out the same way.

Throughout these early construction stages, one of the overriding concerns was to keep inconvenience to the general public to a minimum. In fact, few passengers, or pedestrians, or even people simply enjoying sunny days in the park, would have been aware of the major construction work taking place beneath their feet.

The big day comes

Finally, on the last weekend of October, and after months of work and planning, everything was ready for the sub-station equipment delivery.

This was substantial and weighty equipment. The three, 2,500kW KNAN transformer rectifier units had already been substantially modified but they still weighed 12,500kg each and they only just fitted between the roof beams of the old power station. And there was also nothing lightweight about the eight-panel 11kV switchboard, the seven-panel DC positive and negative switchboard, the battery chargers (complete with batteries and RTU) and the marshalling cabinet that came with them.

Even though the new equipment was delivered in its component parts for final assembly on-site, it was clear that the ventilation shaft was no longer sufficient, so a larger main ventilation shaft was opened for the first time. A section of the park was fenced off and footpaths and grassed areas were covered by protective roadways.

At 6am on Saturday morning a 150-ton crane began to be assembled. Permission had already been granted for it to be located within the park, and for it to be driven over the Underground Lines below. This was soon followed by three low-loaders carrying the transformer-rectifiers, and the complex lifting operation began.

Work continued throughout the day, and by nightfall the first stage of the operation was complete with each one of the transformer-rectifiers having been safely lowered out of sight ready for assembly. Next day the process began again, the low-loaders returned carrying the switchboards and remaining equipment, the main ventilation shaft was reopened, and installation resumed. By 8pm on Sunday evening the lifting operation was complete, and on Monday morning, when the protective roadways were removed, it’s unlikely that any passing commuter could have guessed at the scale of the operation that had taken place. The whole process had taken less than one weekend.

Out of sight – out of mind

It’s in the nature of sub-surface railways, and indeed of electrical engineering in general, that the work, though complex, is hidden. For instance, completing SSR2 will require 12km of 11kV cabling and over 20km of fibre optics for the advanced signalling systems, yet the travelling public will see nothing of this. That’s why it’s doubly important that disruption is kept to a minimum. Quite understandably, there can be little expectation of sympathy when the reasons are concealed deep below ground.

Steve Howes, UK Power Networks Services’ SSR2 Project Manager explained: “That SSR2 is challenging goes without saying, but we know we’ll be judged on more than our technical abilities alone. The travelling public doesn’t want to be impressed by how we’ve designed a piece of electrical infrastructure or how we’ve incorporated the latest technology, they just want their journey to be as reliable and predictable as possible.

“So working unusual hours in unexpected conditions is the norm rather than the exception for us. I think we only consider our work to be truly successful if it delivers the results that LUL desire without the public realising how that happened. In fact, if we’ve done our job correctly, they shouldn’t be able to tell we’ve been at work at all.”

UK Power Networks Services work on SSR2 continues, and with the scheme not due for completion until 2013, many more projects of comparable scale and complexity are planned. But given equal innovation and teamwork there’s every chance that they’ll be completed, just like the new substation, where most Londoners will not see a thing.