Reverse current (railway)

As a back flow is defined as the sum of the electric currents that between orbital energy consumers and traction power supply source on the rails and flows and then, if necessary, on additional return lines. The supply source, forward and return conductors and consumers form an electrical operating circuit .

Electrical operating circuit

Railway energy consumers

Railway energy consumers include:

• Traction vehicles with the traction motors for the actual train conveyance ( traction ) as well as with z. B. Lighting, on-board electronics, compressors , train radio ;
• in the car z. B. lighting, air conditioning, door control, train heating, on-board bistro, TV on board, 50 Hz electrical system and the like. Ä .;
• electric point heaters ;
• Train preheating systems .

Food sources

Supply sources are the substations that transfer the energy from the public or traction current network to the supply and contact line and where the return currents converge again in the return rails and are conducted to the second pole.

Return conductor

Return conductor at 15 kV ~ traction current system

Return conductors lead the return current back to the supply source. Return conductors are first and foremost the rails of the tracks . However, since the tracks are connected directly and / or inductively to the ground and the supply sources are also grounded , part of the return current also flows through the ground. There are also isolated sections in which the return line has to take place via special cables or rails for various reasons . Each supply source supplies a supply district with electrical energy. In each food district there are i. d. Usually several trains, locomotives or stationary consumers. This means that all related return currents arrive at the location of the supply source. The return conductor connections at the location of the supply source must be dimensioned accordingly.

Current direction

The traction power supply of electrified railways works according to the two-wire principle, in which there is only one forward and one return conductor for the electrical current. The direction of the current is defined according to the provision of energy: The consumers are fed from the supply source with current to the consumers. After the energy has been converted on site, the electricity flows back to the supply source.

• Go conductor are the supply and overhead wires.
  • Feed lines are cables to the contact line sections.
  • Contact lines are
    • Overhead lines (for long-distance, regional, works or mine railways),
    • Conductor rails (on S, U or mine railways).
• Return conductors are the rails, the ground, special cables or return current rails.

This directional observation is used for both direct current and alternating current railways, regardless of the physical polarity of the electrical voltage and the physical current flow direction .

Related topics

Risks from reverse current

Galvanically connected networks

Reverse current always affects the protective earthing of systems in the area of ​​electrical railways and vice versa. Appropriate measures should therefore be taken to prevent parts of a reverse current from flowing through earthed systems.

Likewise, it should be ruled out that portions of reverse currents flow via the train busbar of trains . There are special specifications for this in the project planning or planning guidelines. An exception to this rule is, for example, the RENFE series 730 ("Talgo 250 Hybrid"), in which the train busbar is designed twice: When the generator car is in operation (for non-electrified routes), the return current is then routed via one of the two train busbars, and not over the tracks.

At construction stages

During construction work on tracks (e.g. changing rails, separating or inserting rails), it should be noted that if the "return conductor rail" is interrupted, the return current takes a different route. So that this does not flow over the construction machines or people, the rail break point must be bridged beforehand with sufficiently dimensioned return cables.

When working on the contact line, it must be switched off and grounded beforehand. The protective earthing is done by connecting the contact line to the rail at least in front of and behind the work site. In this way, the return current is divided between the previous rail and, in parallel, via the earthing and short-circuit devices on the contact line. This must be taken into account if the contact line is to be cut. As described above for tracks, the intended interruption point in the contact line must be bridged beforehand with adequately dimensioned return cables.

Short circuit current

In the event of a short circuit in the traction power network, currents that are many times higher and thus also reverse currents occur. If the short-circuit is not switched off in good time by the safety devices in the substations , these high reverse currents can endanger people, vehicles and systems in the return current path and lead to the destruction of traction current systems. Missing protective earths and return conductors (e.g. due to incorrectly executed construction stages, destruction or theft) even constitute an immediate, fatal danger for people.

Stray current

The current emerging from the rails into the ground is known as stray current, previously also known as erratic or vagabond current. It flows in the ground and - if present - also over metal parts lying in the ground ( pipes , earthing systems, etc.). This leads to corrosion damage and thermal influences on these systems . The extent and speed of corrosion is different for direct current and alternating current, corrosion damage is much more pronounced with direct current.

Meshing of the return line

In areas with multiple tracks (e.g. in train stations ), the return current is distributed differently depending on the contact resistance between the rails and the ground. This could lead to different electrical potentials of the individual rails or tracks. In contrast, as many rail and track connectors as possible are attached. This serves both to balance the electrical voltage between the tracks and to increase the total cross-section of the return line, which leads to a reduction in the electrical resistance of the return line and thus to a reduction in electrical energy losses in the operating circuit.

The possible number of rail and track connectors is limited by the need to set up isolated track sections for the control and safety technology .

Meeting of direct and alternating current railways

Change of electricity system for Karlsruhe tram

At the intersections of direct and alternating current railways, there may be a mutual disadvantageous influence from reverse currents. Interfaces can be:

• System separation points of international railway lines with different traction current systems ; for example, the German, Austrian and Swiss railway networks have alternating current, the Dutch, Belgian, Italian, Polish and Czech direct current;
• Crossing or parallel routing of long-distance trains (alternating current) and S-Bahn or U-Bahn, mine or trams (direct current);

Influencing one network with return currents from the other network is undesirable in both directions:

• On the one hand, high reverse currents and short-circuit currents from the rail network threaten the mostly low-energy and low-voltage direct current systems.
• On the other hand, the direct currents saturate power transformers and motors in the railway power network.
• In both cases, uncontrolled return currents lead to uncontrolled potentials in the affected network. This could lead to inadmissible exceeding of the contact voltage and thus to health or life endangering of people who come into contact with the parts of the system in one or the other network.

To avoid irregular conditions, the track networks of direct and alternating current railways in Germany are consistently galvanically separated. In the track connections between them, double insulating joints , so-called "locking joints ", are installed. If track sections have to be electrified with AC and DC voltage at the same time, then it is necessary to maintain the electrical isolation to feed the AC voltage via an isolating transformer. One example of this are the platform and sweeping tracks in Birkenwerder station (b Berlin) . Another possibility are switchable contact line sections. It is used in particular in system switching stations between DC and AC voltage networks such as Aachen Hauptbahnhof , but then the simultaneous operation of DC and AC voltage vehicles is not possible.

The problem is handled differently in Switzerland, where in many places the earth systems (EW and rail earth) are connected by suitable measures without disturbances occurring in the respective networks (EW or rail). The prerequisite, however, is that the railway network is equipped with a return cable parallel to the contact line.

literature

• Peter Schmid (Red.): Energy supply for electric railways . Ed .: Institute for electrical systems. VEB Verlag Technik, Berlin 1975, DNB  200306839 .
• Helmut Bendel (Ed.): Electric traction vehicles . 1st edition. transpress VEB publishing house for traffic, Berlin 1981, DNB  820088102 .
• Friedrich Kießling , Rainer Puschmann, Axel Schmieder, Peter Schmidt: Contact lines for electric railways - planning, calculation, execution . 2nd Edition. BG Teubner, Stuttgart 1998, ISBN 3-519-16177-X .
• František Jansa: Elektrická Trakcia . 1st edition. tape 1 . Alfa vydavatelstvo technickej ekonimickej literature, Bratislava 1976, OCLC 5171513 (Czech).