Electrical question specific to Amtrak locos

[Crescent ATN & TCL]

I'm currently an ELectrical Engineering student and I can't find an answer to this one anywhere.

What is the return path for current in Amtrak's ELectric locomotives?

In resdidental/commercial/industrial wiring current always returns to the service entrance of the system. Then depending on the system in place its either grounded or connected back to the transformer or transmission lines.

The only two scenarios I can find for rail is grounding the return current to the rail or returning it to the catenary through a separate neutral conductor on the pantograph. If it returns through the rails it could cause signal and crossing gate issues along with a risk of shock to anyone on or near the rail when a locomotive is operating.

 

[BCL]

I'm pretty sure the rails are considered ground. This is typically the case with overhead electric street cars with a single line. Electric buses with overhead lines can't do that and thus require two overhead lines, which makes things tricky.

The other option is a third rail. I thought that the P32AC could do that when exiting NY Penn Station, but switch to diesel sometime after exiting the tunnel.

 

[BCL]

http://en.wikipedia.org/wiki/Amtrak%27s_60_Hz_traction_power_system

At each substation, utility supplied single phase power is transformed and injected into the two electrical sections terminating at that substation. There are eight electrical sections in the system, two for each substation. The substations drive the contact and feed wires in a split phase arrangement so that each wire is at 25 kV with respect to the grounded running rails with 50 kV between them.

At periodically spaced Paralleling Stations within each electrical section the tracks' catenary wires are connected together to one side of an autotransformer and the feeder wires are connected together to the other side of the autotransformer. The autotransformer center-tap is connected to the grounded running rails that return the current from the locomotives. The paralleling stations thus reduce voltage drops by feeding a locomotive from both directions along its contact wire and spreading the load across all the contact and feed wires of a multitrack system. The split-phase arrangement also gains the increased efficiency of operating at 50 kV while the highest voltage with respect to ground remains only 25 kV. (The same split-phase method is used in North American homes to supply high power loads such as air conditioners with the efficiency of a 240 V supply while retaining the safety advantages of a 120 V supply.)

 

[ehbowen]

George Harris could probably give the expert opinion; he's a professional engineer who has worked in the railroad design and construction field for a number of years. But from what I've picked up: BCL is right, the rails are indeed the return conductor. Because they need to carry such a heavy current they either need to be butt-welded in a continuous string (the preferred modern solution) or else have to have heavy bonding cables around any joints which can't be welded.

The old Pennsylvania Railroad portion of the Northeast Corridor was designed and built as single-phase 25 Hz power; that being a frequency which was optimized for commutator-driven motors. Now that Amtrak must buy power from commercial utilities there was about thirty years back a strong push to switch to three-phase 60 Hz service, with unpowered phase breaks between the phases. However, that would have entailed redoing the entire single-phase distribution and catenary system and the money just wasn't there. But since then the cost of industrial-capacity solid state 25 Hz inverters has come down so much that the benefit of switching to 60 Hz is marginal at best. It looks as if it will stay at 25 Hz for the foreseeable future.

 

[Train2104]

If I remember, the signals, crossings, and other indication circuitry runs at a different frequency, allowing it through the rail at the same time as the negative return.

Third rail equipment uses the rail for negative return as well.

Not sure as to why there isn't an electrical hazard posed by the rail, that's a good question.

 

[Karl1459]

My observations of the "light rail" system in Portland is that yes the rail is used as a return, but there are also continuous buried copper "ground bars" which are periodically bonded to the rails.

 

[PRR 60]

The return current comes through whatever is available. Primarily the rail: but also the catenary static wires, shielded communications cables, water pipes and gas pipes in nearby streets: whatever. The current wants to get back, and gets there any way it can.

Amtrak installed an aerial cable to help with return current issues after finding that changing old shielded copper S&C cables to fiber optics caused return current issues. That had decent success, but not total success. It's amazing how far you can be off Amtrak and pick up stray currents in all kinds of longitudinal conductive facilities with the dead give away signature of 25Hz.

 

[neroden]

But since then the cost of industrial-capacity solid state 25 Hz inverters has come down so much that the benefit of switching to 60 Hz is marginal at best. It looks as if it will stay at 25 Hz for the foreseeable future.

The cost is in having both 25 Hz. and 60 Hz. transformers / rectifiers on every locomotive and every MU ordered for the forseeable future. As far as I know those haven't gotten any cheaper. Correct me if I'm wrong.
I'd think the value of switching to 60Hz would still be quite large, even if it consisted of retaining the existing synchronized single-phase distribution system and building phase converters to supply it.

 

[Crescent ATN & TCL]

The return current comes through whatever is available. Primarily the rail: but also the catenary static wires, shielded communications cables, water pipes and gas pipes in nearby streets: whatever. The current wants to get back, and gets there any way it can.

Amtrak installed an aerial cable to help with return current issues after finding that changing old shielded copper S&C cables to fiber optics caused return current issues. That had decent success, but not total success. It's amazing how far you can be off Amtrak and pick up stray currents in all kinds of longitudinal conductive facilities with the dead give away signature of 25Hz.

That means it would be possible to be shocked by rail on the NEC, assuming you provide a better connection to the earth than the normal path.

 

[PRR 60]

But since then the cost of industrial-capacity solid state 25 Hz inverters has come down so much that the benefit of switching to 60 Hz is marginal at best. It looks as if it will stay at 25 Hz for the foreseeable future.

The cost is in having both 25 Hz. and 60 Hz. transformers / rectifiers on every locomotive and every MU ordered for the forseeable future. As far as I know those haven't gotten any cheaper. Correct me if I'm wrong.
I'd think the value of switching to 60Hz would still be quite large, even if it consisted of retaining the existing synchronized single-phase distribution system and building phase converters to supply it.

They do not have two transformers/rectifiers on every locomotive. They have one 25Hz transformer. That works for both 25Hz and 60Hz. The only additional cost is the slightly higher cost of a 25Hz transformer due to a heavier core. The rectifier does not care. It converts AC to DC and the AC can be whatever. I know that you do not believe that (we've been through this before), but facts are facts, and your belief is not correct.

 

[PRR 60]

The return current comes through whatever is available. Primarily the rail: but also the catenary static wires, shielded communications cables, water pipes and gas pipes in nearby streets: whatever. The current wants to get back, and gets there any way it can.

Amtrak installed an aerial cable to help with return current issues after finding that changing old shielded copper S&C cables to fiber optics caused return current issues. That had decent success, but not total success. It's amazing how far you can be off Amtrak and pick up stray currents in all kinds of longitudinal conductive facilities with the dead give away signature of 25Hz.

That means it would be possible to be shocked by rail on the NEC, assuming you provide a better connection to the earth than the normal path.

It is possible. For that, and lots of other reasons, touching a rail on an electrified railroad is not a good idea. Track crews treat rail as if it is energized. They ground it and often use rubber gloves to work it. Having said that, the electrical design must meet standards for maximum rail voltage. There are standards for normal operation and maximum fault (short circuit) conditions. That is designed to protect track crews and the public from lethal shock.

One correction from above. The rails are not electrically continuous. They have insulated breaks every mile or so for signaling. In electrified territory, the breaks have a device called an impedance bond. Simplistically, an impedance bond is like a selective electrical valve. It blocks the 91-2/3Hz signal power from passing, but allows the 25Hz and 60Hz power to flow to ground and/or through. This is one of the methods used to keep rail voltage within standards while maintaining the integrity of the signal track circuits.

 

[jis]

But since then the cost of industrial-capacity solid state 25 Hz inverters has come down so much that the benefit of switching to 60 Hz is marginal at best. It looks as if it will stay at 25 Hz for the foreseeable future.

The cost is in having both 25 Hz. and 60 Hz. transformers / rectifiers on every locomotive and every MU ordered for the forseeable future. As far as I know those haven't gotten any cheaper. Correct me if I'm wrong.
I'd think the value of switching to 60Hz would still be quite large, even if it consisted of retaining the existing synchronized single-phase distribution system and building phase converters to supply it.

They do not have two transformers/rectifiers on every locomotive. They have one 25Hz transformer. That works for both 25Hz and 60Hz. The only additional cost is the slightly higher cost of a 25Hz transformer due to a heavier core. The rectifier does not care. It converts AC to DC and the AC can be whatever. I know that you do not believe that (we've been through this before), but facts are facts, and your belief is not correct.

Also higher weight of the bigger core. The other slight difference to accommodate 25 Hz would be somewhat differently rated smoothing capacitors I'd imagine. But that is getting into the weeds.
But I agree facts are what they are. The misinformed are free to continue to believe otherwise or have other unfounded opinions.

 

[Crescent ATN & TCL]

The return current comes through whatever is available. Primarily the rail: but also the catenary static wires, shielded communications cables, water pipes and gas pipes in nearby streets: whatever. The current wants to get back, and gets there any way it can.

Amtrak installed an aerial cable to help with return current issues after finding that changing old shielded copper S&C cables to fiber optics caused return current issues. That had decent success, but not total success. It's amazing how far you can be off Amtrak and pick up stray currents in all kinds of longitudinal conductive facilities with the dead give away signature of 25Hz.

That means it would be possible to be shocked by rail on the NEC, assuming you provide a better connection to the earth than the normal path.

It is possible. For that, and lots of other reasons, touching a rail on an electrified railroad is not a good idea. Track crews treat rail as if it is energized. They ground it and often use rubber gloves to work it. Having said that, the electrical design must meet standards for maximum rail voltage. There are standards for normal operation and maximum fault (short circuit) conditions. That is designed to protect track crews and the public from lethal shock.

One correction from above. The rails are not electrically continuous. They have insulated breaks every mile or so for signaling. In electrified territory, the breaks have a device called an impedance bond. Simplistically, an impedance bond is like a selective electrical valve. It blocks the 91-2/3Hz signal power from passing, but allows the 25Hz and 60Hz power to flow to ground and/or through. This is one of the methods used to keep rail voltage within standards while maintaining the integrity of the signal track circuits.

Now that I think about it it is also feasible for a diesel to use the rail as a return path. Contrary to popular belief you don't need a complete circuit for current to flow, just a potential difference in charge. For example you could take a voltmeter and a wire and form a connection from your breaker box to the earth and have 120volts across the meter and current flowing into the earth.

 

[Crescent ATN & TCL]

I was half asleep when I made that post, I corrected it.