If trains were powered by an external electric grid, and these same trains has the capability to generate electricty as a part of the braking process, would certain RRs that have lots of grades in their network benefit from this? As a train is going ‘down hill’ it generates power that is fed back into the grid, and this power then supplements the power required for a train going uphill. For some RRs this could provide decent savings for their energy needs. Yes maybe no?
http://www.scn.org/cedar_butte/milw-elec.html
An excerpt from the page about the Milwaukee road.
Perhaps the coolest aspect of the electrification system adopted by the Milwaukee was regeneration. When trains went down hill the electric motors were used as generators. This both slowed the train down (with substantial savings in the cost of replacing brake-shoes), and returned power to the system to help ascending trains, reducing the overall power needs by about twelve percent. Heavy capital costs, but significant operational savings (despite padding of costs); the net benefit was later found to amount to a return on investment of nine percent annually.
The Milwaukee estimated that they recovered ~15% of the electricity used to move trains uphill. Not a huge savings, but nothing to sneeze at either. As you hinted, this was especially welcome when a descending train was meeting an ascending train.
Regeneration was also used by the N&W, Virginian and Great Northern, though the last set of locomotives for the Virginian did not support regeneration. The cars used on BART and most light rail lines are capable of regenerative braking - the limitation is in the power supply.
Regenerative braking can work well, but it depends upon another train being under load at the same time, since storage of such amounts of energy is highly problematic.
I think subways already use this too, as they operate at a high enough density that there’s almost always trains accelerating and decelerating at the same time.
If you’re thinking of this as a reason for more electrification of North American freight railroads, don’t hold your breath. It’s one small advantage compared to a host of large dreawbacks.
If there is not an ascending train to use the power, the power is supplied back to the power company supply and is metered for credit. On the Milwaukee Road, up to 50% of an ascending train’s power needs could be supplied by an equivalent train regenerating on the downgrade.
Your idea is what is done.
Adrianspeeder
I was under the impression that heavy railway electrification was not supplied by the regular distribution grid, but by dedicated sources. If that’s not the case then yes, energy could be returned to the grid, assuming the railway operates on AC, preferably at 60 Hz. If it’s AC of another frequency, or DC, conversion losses would come into the equation.
Yes…as long as there were ascending and descending trains operating concurrently.
My point is that regenerated energy has to have somewhere to go - it cannot be stored with existing technologies.
Well, maybe it’s picking a nit, but “Not Exactly”. I would, however, agree that it can’t be stored direcetly as electricity…(yet)
There is a Nuke plant in Virginia I took some students to a few years ago(1991). Instead of building a Nuke plant big enough to supply the peak demand, they built a damn and a lake. During non-peak electrical use, instead of operating at a non-economical low output level, they generate extra power and pump water from the river up to the lake. during Peak electrical useage, they use the water for Hydro generation. Certainly there are losses involved, but they are storing the excess electrical energy for use later…with “just a few” conversion losses…
I believe the plant was North Anna, but the current web site doesent mention the “offline storage”. Perhaps it turned out not to be economical…but it was in fact physically possible!
Current electrical storage research is focusing on very large capacitors which may add to the practicality of electric railroading.
dd
capacitors (and batteries) would work for storing DC, but you would need a “Tuned Tank” resonant circuit, with a capacitor and a coil, to store AC. I love to see that sucker! But dont take any wrist watches or pacemakers near it…the magnetic field generated by a coil that big would be something to behold…[8D]
It is supplied by the grid.
Until recently though it was difficult to get a clean enough waveform that the electricity companies would accept, traction circuits produce a lot of dirty harmonics. Power electronics changed all that, fortunately.
On Milwaukee, the power was in effect stored.
Milwaukee Road’s early advertising about “white coal” referred to the use of water power exclusively at that time to supply Milwaukee’s DC system. Milwaukee itself had surveyed and purchased several of the early hydroelectric sites, and participated in their development in order to supply the railway company. The 'battery" was the reservoir storage. If the MILW regenerated, the power company could use that power to supply other users rather than “use down” the water storage capacity to supply those users. In essence, regeneration “saved” battery storage capacity.
MILW generated clean waveforms to the power companies by reversing the stationary Motor Generator sets at the Substations, producing 2300 v AC current from the 600 rpm synchronous AC motors, and stepping up to 110 kV AC through transformers. Most of the losses on regeneration are line losses – AC, DC doesn’t matter. The conversion losses, AC to different AC, or DC to AC were small by comparison.
A slight quibble on the wording (your explanation is mostly dead on) - the M-G sets were not reversed when the direction of power flow changed, the only thing that reversed was AC machine was leading the power grid in regeneration rather than lagging as in motoring (this can be seen by putting a AC-line synch’ed stroboscope on the shaft of the M-G set). Put it simply, motoring occurred when the DC bus voltage was less than the no-load voltage of the DC generators and regeneration occurred when the DC bus voltage was higher than the no load voltage of the DC generators.
The GN single phase electrics and the VGN EL-2’s had M-G sets in the cab, so regeneration worked in a manner very similar to the Milwaukee’s system.
It isn’t that difficult to set up a solid state “rectifier” substation to accept regenerated power - “dual converters” have been around for decades - and they were one of the subjects in a power electronics class I took slightly over thirty
The MILW system used a motor generator system in the substation which had a large AC motor with a DC generator on each end of the shaft. When a train needed power the AC motor turned the tDC generators. When a train was in regeneration then the locomotive traction motors provided power to turn the DC generators in the substations into motors which turned the AC motor into a generator which provided synchronous AC to the grid… There was a combination of manual and automatic control used on the system which was designed near the turn of the centuries as well as a few system upgrades after.
The modern AC diesel locomotives from EMD and GE use alternators to generate AC power. Solid state rectifiers then convert the AC power to a pure form of DC which is then fed through another set of solid state devices to provide the AC to power the traction motors. The same system could be used today to convert the train supplied power to what ever choice of electricity the grid would need to accept.
I can just hear the power distribution desk now calling the railroad to ask if they could run some trains downhill since it is about 5:30 p.m. and peak usage is coming on. Perhaps they could even have the railroads stage their coal trains bringing coal to the power plants at the tops of grades until additional capacity is needed for peak demand periods. Now if you can just get permission to build a few long yard tracks at the tops of the hills and figure out how to run the trains up the hill during low demand, keep the crews from running out of time while they are waiting for peak time to come around and hold all up grade intermodal trains while it is a downhill railroad you have a place in railroad management.
Exactly right. Poor wording on my part – the MG sets, the rotating machinery, rotated in only one direction; the power flow “reversed”.
MILW had looked at rectifiers, and several studies recommended their purchase to augment the system. About $325,000 each. Inverters were about the same cost, but MILW already had the rotating machinery which achieved the same result, so inverters were never considered for regeneration.
I was thinking more along the lines of BART and light rail lines with respect to inverters (specifically dual converters). The motor control circuits on the first generation of BART cars were set up to use regenerative braking if there was a load to take the regenerated power, otherwise dynamic braking resistors would kick in when the third rail voltage went above 1100 to 1200V (BART was nominally 1000V). One work-around would be putting in capacitor banks (ultra-caps are technically feasible, but not necessarily economically feasible) or flywheels (Pentadyne has some for data centers that float on ~500V DC busses - wouldn’t be too much of a stretch to bump that up to 600V). Having the energy storage could smooth out the demand a bit.
The M-G sets on the MILW were just fine for regen - can’t ask for a better load than a synchronous motor. There were several places that rectifiers could have been dropped in where the extra power was needed for motoring not regen.
In case anyone is wondering - a Dual Converter is noting more than a pair of three phase bridge rectifiers using thyristors (AKA SCR’s) - where one bridge is hooked up in the same way as straight diodes and the other bridge hooked up in the opposite direction. By adjusting which bank of thyristors is being fired, the dual converter can transfer power from the AC to DC or from DC to AC.
This thread is really interesting because it’s addressing an issue I’ve wondered about for decades.
Look at the MILW motor-generator substation model. I can well understand what’s going on: an AC motor spinning more-or-less at a constant r.p.m. dictated by the frequency of the utility supplied current spins the DC dynamos that produce the trolley wire voltage. If the frequency of the utility supplied current bumps up to 61, 62, or 63 hz., the DC output may rise a fraction of a volt, a couple of volts, or whatever because the dynamos are spinning faster. If the frequency of the utility supplied current drops a little to 59, 58, or 57 hz., the DC output will drop a little. Either way, it’s no big deal with 3500-volt overhead. The train still climbs the mountain with no perceptible change in speed.
But regenerative braking is a whole nother animal. If the kinetic energy of a descending train is being converted to electricity by the locomotive and that action raises the voltage of the trolley wire, when that higher voltage reaches the DC dynamos at a substation, won’t that cause the motor generator set to spin faster? And if the AC portion of the set is now in power generator mode, doesn’t the faster spinning armature introduce spurious frequencies into the commercial power grid, frequencies that could be damaging to other electric motors using the same utility power grid?
I should think that the power being pumped out of a Milwaukee Road substation due to the movement of one or more trains in regenerative mode would have to be perfectly synchonized with the waveform of the commercial power grid supplying that substation. How, with 1916 technology, was that accomplished?
What host of large drawbacks? Alright, given, the capital investments which would be required to convert North American Railroads to electric power would be astronomical. Then there would be maintenance costs. But, living in Germany, I’m spoiled. I don’t know any exact numbers, but the German Railway has invested quite an effort (and lots of capital) in getting as many miles of track under caternary as possible, and, yes, the new generation of electrics all have dynamic/regenerative braking capacity (they are what NJ Transit’s new ALP 46 electrics are based on). Now why would they do something like that if it wasn’t more economical? Your typical freight train over here doesn’t come close to a North American train in length or tonnage, but you just need one loco, and, man, they sure move fast! When you have as many trains moving simultaneously as on Germany’s railroads, it would be foolish not to utilize regenerative braking.
The M-G sets used synchronous motors. As long as torque on the shaft was less than pull-out torque, the motor would be running at exactly the speed set by the line frequency. An AC synchronous motor is not a lot different from an AC generator (alternator) and shaft speed remains locked to line frequency as long as the applied torque is less than pull-out. (Called pull-out because the motor or alternator is pulled out of synchronism).
One thing that does happen is that a regenerating traon can cause the line frequency to rise slightly, which then causes the governors on the power plant generators to back off a bit. Conversely, when a train is heading upgrade, the extra load causes a slight decrease in line frequency which is then compensated by the power plant governors to increase power a bit.
There’s also a bit of phase shift over the power lines as power is being transmitted - you can think of it as being equivalent to a
Bob-F,
When the substation operator knew he had a train about to go into regeneration he would reduce the line voltage so it was below the normal 3300-3500 volts. That allowed a lower voltage in the trolley than what would be coming out of the traction motors so the current had a place with a lower potential to flow to.
The synchronous AC motors/generators in the substations only wanted to run at the frequency of the grid power. They were amazingly self regualting in this matter. If the frequency of the line was 60 cycle then the output from the MGs in the substation would be 60 cycles.
The differences between the various electrification systems are best summarized in the book “When the Steam Railroads Electrified” from Kalmbach. It is long out of print and sometimes expensive on the used market but should be available in most city libraries.