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#135027 - 12/18/02 02:34 PM Electric trains  
C-H  Offline
Joined: Sep 2002
Posts: 1,497
Stockholm, Sweden

How does a 2 x 25 kV AC system work? I have figured out that it's a center tapped system of some sort. Are there two pantographs and catenaries?

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#135028 - 12/18/02 04:37 PM Re: Electric trains  
Hutch  Offline
Joined: May 2002
Posts: 381
South Oxfordshire, UK
A two 25kV system was featured in one of the issues of Railway Magazine earlier this year. I can't find my copy right now but I'll keep looking. If I recall correctly the system is used by the French on their high speed lines and will be used on the Channel Tunnel Rail Link in south eastern England. It is more a method of power distribution (rather than traction), cutting down on substations and voltage drops. The locomotives still use a single phase of 25kV 50Hz thus it is available for all types of existing stock.

When I find the article, I'll post an extract of the details here.

The Sishen iron ore line that runs from the interior of South Africa's Northern Cape to Saldana Bay uses an ultra-high voltage (for trains) of 50kV. This is to compensate for the almost complete lack of any infrastructure along it's route.

[This message has been edited by Hutch (edited 12-18-2002).]

#135029 - 12/20/02 05:26 PM Re: Electric trains  
Hutch  Offline
Joined: May 2002
Posts: 381
South Oxfordshire, UK

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).]

#135030 - 12/21/02 10:29 AM Re: Electric trains  
C-H  Offline
Joined: Sep 2002
Posts: 1,497
Stockholm, Sweden
Thank Hutch! It was exactly what I wanted to know. Very informative article. I found a number of references to this system on the net, but no clues as to how it worked.

I didn't realise this was such a new system. It seems much more economical than the present Swedish 15 kV, 16 2/3 Hz. I wonder if "we" are going to switch?

#135031 - 12/21/02 12:22 PM Re: Electric trains  
C-H  Offline
Joined: Sep 2002
Posts: 1,497
Stockholm, Sweden
Railway voltages of Europe


Austria		15 kV (16 2/3 Hz)
		3 kV (DC)
		6.5 kV (25 Hz)

Belgium		25 kV (50 Hz)
		1.5 kV (DC)
		15 kV (16 2/3 Hz)

Bulgaria		25 kV (50 Hz)

Czech Rep.		25 kV (50 Hz)
		3 kV DC

Denmark		25 kV (50 Hz)
		1.5 kV (DC)

Estonia		3 kV (DC)

Finland		25 kV (50 Hz)

France		25 kV (50 Hz)
		15 kV, (16 2/3 Hz)
		3 kV (DC)
		1.5 kV (DC)
		0.75 kV (DC)

Germany		25 kV (50 Hz)
		15 kV (16 2/3 Hz)
		0.8 kV (DC)

Greece		25 kV (50 Hz)

Hungary		25 kV (50 Hz)

Ireland		1.5 kV (DC)

Italy		25 kV (50 Hz) ?
		3 kV (DC)

Latvia		3 kV (DC)

Lithuania		25 kV (50 Hz)

Macedonia		25 kV (50 Hz)

Netherlands	1.5 kV (DC)

Norway		15 kV (16 2/3 Hz)

Poland		3 kV (DC)
		0.6 kV (DC)

Portugal		25 kV (50 Hz)
		1.5 kV (DC)

Romania		25 kV (50 Hz)

Russia		25 kV (50 Hz)
		3 kV (DC)

Slovakia		25 kV (50 Hz)
		3 kV (DC)
		1.5 kV (DC)

Slovenia		25 kV (50 Hz)
		15 kV (16 2/3 Hz)
		3 kV (DC)

Spain		25 kV (50 Hz)
		3 kV (DC)
		1.5 kV (DC)

Sweden		15 kV (16 2/3 Hz)

Switzerland	15 kV (16 2/3 Hz)

Turkey		25 kV (50 Hz)

Ukraine		25 kV (50 Hz)
		3 kV (DC)

UK		25 kV (50 Hz)
		0.75 kV (DC)
		0.63 kV (DC)
		660/750 V (DC) (Third rail)

[This message has been edited by C-H (edited 12-21-2002).]

#135032 - 12/21/02 12:36 PM Re: Electric trains  
Texas_Ranger  Offline
Joined: Dec 2001
Posts: 2,402
Vienna, Austria
Austrian trams and subways use 650V DC, older cars have DC motors, newer ones convert the power to 3ph AC first.

#135033 - 12/21/02 03:39 PM Re: Electric trains  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
The U.K. also had some 3kV DC years ago. Although that went out of use, didn't somebody in here (Hutch?) mention that it had been re-introduced in some of the new city "Metro" systems?

The old "tram" (street-car) and "trolley-bus" systems here ran on a nominal 500V DC in most cities. I assume that the famous trams in Blackpool still use that.

#135034 - 12/21/02 05:27 PM Re: Electric trains  
Hutch  Offline
Joined: May 2002
Posts: 381
South Oxfordshire, UK
1500V DC overhead is used on the Tyne and Wear Metro.

1200V DC shielded third rail was used on the Manchester to Bury route. This now forms part the Manchester Metrolink which is energised at 750V DC overhead.

Traditional tramways including Blackpool use 550V DC.

I can't recall any 3kV systems past or present in the UK - can easily be wrong though. [Linked Image]

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