I found the article in the March 2002 copy of The Railway Magazine pp. 59-61 which I will quote for the purposes of private study (but which if they object, I will remove)
BACK in 1955, at the time of the BR Modernisation Plan, it was decided to adopt the 25kV ac overhead system for the future electrification of all future main line routes apart from the Southern Region's third-rail dc system. On the Channel Tunnel Rail Link, however, a significant variant is being adopted.
On conventional ac-electrified lines, power is tapped off the National Grid at convenient sites, usually about 30 miles apart. Transformers reduce this to 25kV and feed the operating voltage into the overhead system above the tracks. The other side of the circuit is connected to a 'return conductor', which is usually carried on small insulators fixed to the lineside masts. This is at comparatively low voltage, as it is effectively earthed at each feeder station, and there are only the voltage-variations to consider over the average 15-mile stretch between the mid-section neutral section and that point.
Trains draw power from the overhead line and return it to the rails, which are not too well insulated from the ground below them, so if there were to be a major flow of current from them into the soil, this could leak into any underground cables nearby, causing interference with telecommunications circuits. To prevent this, the rails are, at intervals of two miles or so, connected to the return conductor. At similar spacings, a 1:1 booster transformer is mounted on one of the lineside masts and this is arranged to 'pull' the return flow of current out of the rails and into the return conductor.
Systems like this have done yeoman service for nearly half a century, but as lines become busier, and more powerful trains are introduced for high-speed travel. such a system starts to approach its technical limitations. This is because the voltage drop in the wires increases with the amount of current being used, significantly reducing the power available for trains half way between feeder stations. To overcome this, the booster transformer system would have to require more feeder stations, and there may not be any suitable intermediate points where the Grid can be tapped for more of these.
The French Paris-Sud-Est TGV line hit this problem as its electrical loading progressively increased. By mid-1995, it had a power-demand factor of 1.4, compared with 0.5 on Britain's busiest overhead electrified route, the West Coast Main Line.
To overcome this, the SNCF introduced a system using auto transformers, usually referred to as the '2 x 25kV' or '25kV-0- 25kV' arrangement. The CTRL power-demand factor is expected to be about 1.4 and it was thus decided to use this French feeder arrangement for it as well. In this system, the Grid transformer at each feeder station has two end-on 25kV secondary windings, the mid-point of them being connected to the rails and to earth. One of the ends feeds the overhead track wires (OIH), and the other the return feeder, which is suspended from full-size 25kV insulators, normally hung, high up, on the line side masts. This plays a much more significant role than the return conductor on the earlier systems, as it carries current at 25kV, its polarity being opposite to that in the track wires.
At intervals, 1:1 auto transformers are provided, with the two sides linking the rail to the overhead and to the return feeder. This arrangement continues to supply power to the trains at 25kV; but the overall voltage supply along the line is effectively twice this, markedly reducing the proportionate voltage drop. Feeder stations - and connections to the Grid - can similarly be further apart.
When both CTRL sections are opened, only three feeder stations will be needed. These will be located at Barking (12.1miles from St Pancras), Singlewell (27 miles) and Sellindge (61.1miles).
A classic 25kV booster transformer system supplying the power loads needed from three feeder stations would not have been practical for such a busy high-speed line with the availability of Grid connections in the area traversed. (The alternative would have required more high- voltage power lines to be built across Kent, which would have attracted further environmental opposition).
As a result, Eurostars operating into St Pancras will be able to run on 25kV supplies all the way from Paris and Brussels, but those heading for Waterloo international will continue to switch to third-rail power at a change-over point on the Fawkham link.
Hope that helps.
Edited for paragraph clarification
[This message has been edited by Hutch (edited 12-20-2002).]