two-system operation at
Birkenwerder (next to Berlin)
The first electric locos started running on the main lines to Schönefeld Airport station on the outskirts of Berlin in the spring of 1983. This involved the electrification of the tracks with two different systems:
both third-rail DC and overhead AC. Subsequently, similar arragements were made at other places in the Berlin area where the main-line electrification co-exisits with Berlin's suburban S-Bahn network. In particular, this requires the return current of the 750 V DC power to be kept separate from that of the 15 kV AC system. The corresponding technical arrangements have been defined in a Work Standard of the German State Railway (Reichsbahn). Three cases must be considered:
1. The normal case of parallel electrical operation. This is achieved by means of physical separation of the two return current paths.
2. The special case where the electrical power supply to a particular track is switched between the two systems
3. Another special case where the return current paths for both power supplies are electrically coupled together. Electrically-powered trains suitable for either system can use the track at any time.
At the time of writing, there is only one example of the last-mentioned special case, in the station at Birkenwerder near Berlin. It is planned to introduce electric operation of main-line trains here in the autumn of 1983.
In this station the tracks of the Berlin S-Bahn line to Oranienburg with 750 VDC electric power also form part of the hitherto un-electrified Birkenerder - Hohen Neuendorf West - Falkenhagen main line. Practical constraints require the stations' only platform to be used by both electrically-powered S-Bahn services and the rakes of coaches hauled by diesel locos that run as (classified as S-Bahn, too) to Falkenhagen or Albrechtshof. A glance at the timetable shows that the shuttle service to and from Falkenhagen (formerly consisting of a set of four double-deck coaches and a DR type 110 loco) runs every hour. As a rule this train arrives five minutes before the electrical S-Bahn train calls at the same platform, and leaves again four minutes after the electrical S-Bahn train has departed.
To make room for the DC S-Bahn train, in the meanwhile the diesel-powered shuttle has run into a reversing siding to the north of the platform.
Moreover, four pairs of passenger trains run daily between Henningsdorf and Oranienburg, halting at Berkenwerder en route:
Despite the enormous effort made to achieve a low-restance path for the return current though the track network of the Berlin S-Bahn - you need only recall the copper bonds used to bridge the joints between rails - a similarly high part of the current from the DC line flows through the AC line's power circuit (running rails - motive power - cables - substation).
The basis of this coupled operation is shown in the diagram of tracks 6, 8, 16, 14 and 4 below. The power supply to the main-line track is fed through twin impedance bonds and pairs of insulating fishplates provide separation. No technical changes are made to the equipment for the DC system. The overhead catenary is fed through a 1:1 transformer rated at 1250 kVA. The transformers for this special arrangement were produced by modifying transformers originally intended for [Schuppenspannungsprülanlagen] operation. The installation has two transformers, one of which is in reserve for the other. This equipment then belonged to the future switching centre at Hohen Neuendorf. The power to the area equipped for both electrification systems was fed by an open-air line.
If the track network and overhead catenary of this area was not electrically separated from the adjacent sections of line with AC electrification, the DC return current would cause unwanted pre-magnetisation of the transformer in the AC motive power.
Furthermore, it was found in practice that this phenomenon had an impact on the performance of motive powoer operating with 15 kV (16 2/3 Hz). As well as the galvanic separation of the overhead catenary and group of tracks 4,6,8, 14 and 16 from the rest of the AC electrified network by means of impedance units and insulating fishplates, a transformer is therefore needed. It connects the catenary above the tracks of this group to the rest of the network electrified with AC, and thus to the nearest feeder station, without allowing current from the DC system to pass. In other words, it acts as a barrier to DC power.
|by Detlef Scheibe, taken from "Modelleisenbahner, 2/1983"
translation by Alan Reekie