WHAT is believed to be the world's first prototype test installation to use unconverted ac power from the public network to supply metro trains is being developed in the German capital. It could be the start of a revolution for the universal application of ac power through the third rail or catenary that would drive down the cost of both infrastructure and rolling stock significantly.

The process between the public power supply and the traction motor of a metro train goes from ac to dc current and then back to ac again, a situation described as "absolutely senseless and really expensive" by Mr Thomas Necker, a member of the board of directors of Berlin Transport Authority (BVG), which operates the 150km, 750V dc U-Bahn as well as 28 light rail routes and 164 bus routes.

The vast majority of the world's metros operate from a dc power supply converted from the ac public supply. Historically traction motors were also dc and some still are in older vehicles, but ac asynchronous motors are the modern norm, making the standard 600-750V dc traction power supply an anachronism. Various combinations of converters (to change ac current to dc or vice versa), inverters (dc to ac converters using, for example, transistors or thyristors), and transformers (to change the voltage) are needed at lineside and on trains to handle the complexities.

For example, a transformer reduces the voltage from the 10-30kV supply from the power generator, and an expensive ac-dc converter installation outputs 750-950V dc for the third rail. On board the train, newer vehicles need a dc-ac converter to feed the asynchronous motors, while old cars with dc motors require special control equipment.

Under the all-ac system, the transformer reduces the 10-30kV supply to about 1000V, which is then fed via an ac/ac converter to the ac motor. Old trains migrating to lines with the new system would require an onboard ac-dc converter for the dc motors.

According to Necker, himself an electrical engineer, "There are absolutely no technical or electro-technical reasons not to feed the line with alternating current, though there are one or two issues concerning the handling of regenerative braking energy to the network."

Participation

BVG will invite the railway supply industry and the power supply company in Berlin to participate in the project, and plans to test the concept from 2006 on a section of line currently designated U55. It will run about 2km from Berlin's new main station, Lehrter, to Pariser Platz, in front of the Brandenburg Gate, with an intermediate station at the German parliament, the Reichstag. It will operate initially as a shuttle service using two trains on one track only.

Line U55 will eventually become part of Line U5 following a 3tml phase two project to connect Pariser Platz with Alexanderplatz, the present terminus of Line U5 in the eastern part of the city centre.

Necker's intention is to test the system for about two years. If it is successful, the extension to Alexanderplatz, on which work will start by 2010 at the latest, will be similarly equipped. "Then, for the next 15 to 20 years we shall need to develop a migration strategy for the whole of the Berlin U-Bahn to go from dc to ac."

The tunnel section between Lehrter station and Pariser Platz, including Lehrter and Reichstag stations, was built under a previous plan which stalled for lack of finance. That is why the U55 will be ready for service in 2006 when it is expected to carry about 6400 passengers each working day. Berlin still carries some of the historical baggage from the days when it was a divided city and the main significance of the U5 extension is that it will connect the eastern districts and suburbs as far out as Honow to the historic centre and government district of Berlin. It will provide the same benefit for people using Lehrter station as well as offering direct transfers to U6 and S-Bahn Lines 1 and 2.

Work on the station in Pariser Platz began in the summer. Cut-and-cover construction was abandoned in favour of tunnelling in order to minimise nuisance to local residents. The area has a very high water table and one of a number of technical challenges is that construction must table place within the water table.

The first task is the demolition of the subway in the adjacent Unter Den Linden S-Bahn station (see diagram on page 24). This will be followed by the sinking of subterranean "curtains" that will form the exterior walls of the excavation pit. They must be able to withstand the pressure of the ground water. A concrete roof will be poured, leaving an opening for an air pressure lock, through which men and materials will be brought in and out. Excavation will be carried out under steadily increasing pressure from compressed air. This will lower the ground water within the work site.

Construction of the final station cross-section will be carried out in stages. In the first phase, micro tunnels will be drilled along the entire length of the station, around the future cross-section up to the terminal subterranean curtain. Icing pipes will be installed in these tunnels to create a thick water and sand mantle to keep out the ground water. Tunnelling excavation, protected by the icing, will then be carried out and the ground will immediately be reinforced by spray-on concrete.