25kv Return Current.

[Westinghouse]

Does the return current on the 25kv Overhead System return via both running rails or just one ? I feel I should know this, but I don't !
Thanks.

 

[QueensCurve]

I don't quite understand how this works, but the "booster transformers" encourage the current to travel in the return wire on the side of the masts rather than in the running rails.

 

[snowball]

I don't fully understand it either, but booster transformers have been removed from some lines in recent years.

 

[ABB125]

Regarding booster transformers, my understanding is that they would better be described as discouraging the return current from travelling through (or inducing current in) signalling cables. I believe this is done by passing the traction current through one side of the transformer, and the return conductor through the other side. The traction current passing through effectively sucks the return current through the other side. (Very technical description! )

 

[Elecman]

Regarding booster transformers, my understanding is that they would better be described as discouraging the return current from travelling through (or inducing current in) signalling cables. I believe this is done by passing the traction current through one side of the transformer, and the return conductor through the other side. The traction current passing through effectively sucks the return current through the other side. (Very technical description! )

In layman terms that is correct, in general traction current normally returns through 1 rail

 

[jfowkes]

FYI everyone, Overhead Line Electrification for Railways by Garry Keenor is an excellent resource for how OLE works in both theory and practise.

 

[McRhu]

The current returns through the return conductor wire as per above. At intervals along the track you'll see a red-sheathed cable leading from a rail up to the mast-mounted return conductor wire, taking the return current. The booster transformers maximise the efficiency of the return.

 

[snowball]

The booster transformers maximise the efficiency of the return.

Then why have booster transformers been omitted from some recent electrification schemes, and actually been removed in some places where they already existed on already electrified lines?

(I'm referring to lines not equipped with an autotransformer system.)

 

[McRhu]

Then why have booster transformers been omitted from some recent electrification schemes, and actually been removed in some places where they already existed on already electrified lines?

(I'm referring to lines not equipped with an autotransformer system.)

I've no idea to be honest; obviously there's a better way of doing things now, or at least in certain circumstances they're not deemed necessary. One for the OLE engineers to answer.

 

[ijmad]

Regarding booster transformers, my understanding is that they would better be described as discouraging the return current from travelling through (or inducing current in) signalling cables. I believe this is done by passing the traction current through one side of the transformer, and the return conductor through the other side. The traction current passing through effectively sucks the return current through the other side. (Very technical description! )

This is a good way of explaining it.

To put it in slightly more technical terms, the induced current in the secondary winding of the booster transformer creates a potential difference that allows the return current from the rails to flow through the secondary winding into the return conductor. The return conductor is a low impedance path (impedance being a more complete description of resistance to current flow in an AC circuit, covering reactance as well as material resistance), so the current has a tendency to flow that way instead of flowing in the return rails, earth, or other cables such as signalling cables.

This only applies to AC systems, i.e. 25kV AC OHLE. It would have no use on a 750V DC third rail system, although a return conductor may still be provided to deal with points, rail gaps, corroded rails, or other issues that might interrupt the return path, and since the wires are likely to be thick copper rather than steel they would still likely attract most of the current flow due to lower material resistance.

An easy way to think about this is to imagine them as a sort of lightning rod, diverting electrical flow away from sensitive things.

 

[edwin_m]

This is a good way of explaining it.

To put it in slightly more technical terms, the induced current in the secondary winding of the booster transformer creates a potential difference that allows the return current from the rails to flow through the secondary winding into the return conductor. The return conductor is a low impedance path (impedance being a more complete description of resistance to current flow in an AC circuit, covering reactance as well as material resistance), so the current has a tendency to flow that way instead of flowing in the return rails, earth, or other cables such as signalling cables.

Correct. Just to add that most transformers have different numbers of coils in the primary and secondary. Ignoring the small losses in the transformer, the ratio of the voltages is the same as the ratio of the coils and the ratio of the currents is the inverse of this ratio. The booster transformer has the same number of primary and secondary coils, so forces the same current through the secondary (the return conductor) as is flowing in the primary (the catenary).

Many years ago I was involved in testing a navigation systems on trains, that was designed for road vehicles and used triangulation of long wave radio signals. It didn't work at all on electrified line, but it was noticeable that the signal to noise ratio was even worse on a section not fitted with return conductors and booster transformers. Having the return current in the return conductor close to the catenary induces less interference than if it is further away in the rails.

 

[ijmad]

Correct. Just to add that most transformers have different numbers of coils in the primary and secondary. Ignoring the small losses in the transformer, the ratio of the voltages is the same as the ratio of the coils and the ratio of the currents is the inverse of this ratio. The booster transformer has the same number of primary and secondary coils, so forces the same current through the secondary (the return conductor) as is flowing in the primary (the catenary).

Many years ago I was involved in testing a navigation systems on trains, that was designed for road vehicles and used triangulation of long wave radio signals. It didn't work at all on electrified line, but it was noticeable that the signal to noise ratio was even worse on a section not fitted with return conductors and booster transformers. Having the return current in the return conductor close to the catenary induces less interference than if it is further away in the rails.

Yeah, very interesting! I am not a railway engineer myself but have some background in electrical engineering, and I can see the benefits of controlling EMF leakage by having current flow in a well insulated return conductor instead of in an unshielded third rail. Could play havoc on anything nearby or indeed on a train as you observed.

 

[edwin_m]

Yeah, very interesting! I am not a railway engineer myself but have some background in electrical engineering, and I can see the benefits of controlling EMF leakage by having current flow in a well insulated return conductor instead of in an unshielded third rail. Could play havoc on anything nearby or indeed on a train as you observed.

That's a slightly different issue. Stray current leaking out of the rails on DC systems causes big problems with corrosion of underground services, but being DC it can't be controlled with booster transformers. Attempts are therefore made to isolate the rails on DC systems from earth, though this can never be anywhere as good as an insulated return conductor.

On AC systems the opposite applies - as the current isn't all in one direction electrolytic corrosion is less of a problem, and attempts are made to keep the rails near earth potential. This is the reason earthing and bonding in dual-electrified areas is very complicated.

 

[thecrofter]

Then why have booster transformers been omitted from some recent electrification schemes, and actually been removed in some places where they already existed on already electrified lines?

(I'm referring to lines not equipped with an autotransformer system.)

Booster Transformers and their corresponding lineside equipment were / are expensive to procure, install and maintain. Their purpose was to reduce lineside interference from the 25kV currents into copper signalling cables. Recent electrification schemes have benefitted from innovations such as fibre optic cables (for signalling and telecoms circuits) and now with the use of Return Screening Conductor (RSC), booster systems are becoming a thing of the past.

 

[ac6000cw]

A diagram about booster transformer systems from Garry Keenor's (free, Creative Commons licensed) book mentioned in post #6. The red arrows indicate traction current flow - the upper winding of the booster transformer will offer a lower impedance path (induced by the current flow in the lower winding) for the return current than the rails do, so most of it will flow that way.

If you want a water flow analogy, the booster transformer is a bit like having two waterwheels on a common shaft, one driven by water flow down the hill into a pool, the other pumping it from the pool to somewhere else. The amount that is pumped is directly related to the downward flow.

 

[snowball]

In areas with AC electrification, and axle counters rather than track circuits, are both rails of a track bonded together?

 

[MarkyT]

In areas with AC electrification, and axle counters rather than track circuits, are both rails of a track bonded together?

Yes they are. Many plain line parts of the railway also use jointless double rail track circuits, which have a matched frequency transmitter and receiver at a section's extremities. Different frequencies are used for adjoining sections with a 'tuned zone' at the boundary to clearly define it and prevent neighbouring sections interfering with one another. In jointless areas, traction current returns through both rails and is bonded across to other tracks and return conductors via impedance bonds, bulky items that sit in the 4 foot containing electrical wizardry connected across the rails forming a filter to only pass traction frequency current to or from either rail through its centre 'tap' without interfering with the track circuits. Elsewhere single rail track circuits are used, with one rail declared the signalling rail and the other the traction return rail. The traction rail can be parallel bonded while the signalling rail must be insulated from other return paths and serially bonded (so as to detect any discontinuity in the circuit). Similar techniques apply in DC territory, but different impedance bonds must be used, neither rail must ever be earthed (unless it's a dual electrified area!), and although modern jointless TCs are usually certified for both AC and DC use (with differing application restrictions and configuration), the choice of single rail track circuit is defined quite tightly by the electrification type. Simple DC track circuit types cannot be used in DC electrified areas, and 50 Hz (or any significant harmonic of 50Hz) TCs can't be used in AC electrified areas. Fortunately there are some TC types that can work with both systems or dual electrified areas would have been impossible before axle counters. Dual immune TCs must be AC or use some other changing waveform technique to work with DC traction and must have frequencies selected carefully to avoid 50Hz and its harmonics to work with AC traction. Where a double rail return system joins a single rail system there must be an impedance bond and insulated block joints. Engineers liked jointless track circuits because they avoided countless troublesome block joints on long plain line sections. They were the perfect partner for long welded rail and started to be used widely soon after LW rail became popular from the late 1960s. Using both rails for return also lowers the return resistance which is always beneficial. Unfortunately, double rail jointless arrangements cannot be used through junctions; single rail configurations must be used instead. Axle counters are best for traction return in AC and DC as all IBJs are avoided, on plain line and through junctions, and traction bonding can be placed almost anywhere relative to train detection sections. Impedance bonds are also avoided.

This diagram from Mr Keenor's book illustrates:

 

[Westinghouse]

Many thanks to all who have joined up the dots for me on this subject. Much appreciated ! Garry Keenor's book is a mine of information indeed.

 

[Smoggy]

On schemes such as East Coast PSU where Boosters were removed, RSC was installed and the bonding was done in a 'mesh' system.