Nasadowsk wrote:
Hey, maybe this will push Amtrak to *finally* dump the antique 25hz infestructure around NYP for 12kv 60hz. Then there's no big cables everywhere cause you can get power for the substation *anywhere*
David Benton replied:
25 khz cables would be smaller than 12 khz ones , 1/2 the size for the same voltage drop .
Just to clear up some electrical terminology; the voltages are in kV
(k=kilo or thousand; V=volts) and the line frequencies are Hz.
But the idea is correct; you can either transmit twice as much power down the same size cable by doubling the voltage from 12.5kV to 25kV; or at 25kV you could half the cable size of what you would need at 12.5kV to transmit the same amount of power, while maintaining the same losses.
While the catenary system voltage is around 12.5kV, the main distribution system voltage between substations is approximately 138kV. If you look at the usual "H" tower structure spanning the rails supporting the caternary... each pair of cables supported from large insulators hanging on a crossarm attached to one of the vertical support columns above the catenary is one 138Kv circuit. There are usually 2 or 4 sets, and they are redundant circuits. If there are cables arranged in groups of three, they are 3 phase circuits for a commercial utility who has "rented" the space on the columns along the right-of-way, and they are not Amtrak power related.
The frequency is a separate problem. The original PRR electrification was and still is 25Hz through NYP. New Haven-Boston is 60Hz, as are electrified portions of NJT that were changed over from the original Edison-era 3kV DC.
I know that some of the NJT equipment can traverse the phase breaks between different system voltages or frequencies "on the fly", but I believe there is still a significant amount of equipment that is fixed as to which voltage system it can operate on. Also, whether there is any equipment left that can only run on 25Hz and not 60, I just do not know. The same applies to Amtrak. The Acela equipment will run on either, but I am not sure about all the rest of the Amtrak electric equipment.
I do not think it is just a matter of switching from the PRR-era 25Hz system to commercial 60Hz power. Interestingly enough, if the locomotives did not care about the frequency, you could switch the entire NEC 25Hz electrical system to 60Hz
tonight... The transformers don't care; in fact, increasing their frequency from 25Hz to 60Hz would increase their efficiency; there would be less internal losses and they would run cooler at 60Hz! (Don't try this in
reverse; running a 60Hz transformer at 25Hz will probably destroy it due to greater internal losses and heating...)
I believe that one possible problem is that some signal equipment, mostly track circuit relays, cab signal track code relays and anything else related to the actual track circuits would have to be changed out when moving traction power from 25Hz to 60Hz. This is because the signal power is another
different frequency, which can not be a harmonic (or an even multiple) of traction power. This is so that the signal relays can ignore stray rail currents from 25Hz traction power or an accidental short from a commercial 60Hz circuit, yet still respond properly to signal rail currents of their unique, unrelated frequency. At one time, that signal power frequency was some odd number just above 90Hz. So changing out the signal system may cost an order of magnitude more than just connecting a local substation to commercial 60Hz power.
Another problem is the 60Hz commercial power system itself... The existing PRR era system is a unique, 25Hz single-phase power distribution system. My understanding is that there are very few, perhaps only two or three; "sources" of single-phase 25Hz power to the NEC. They were most likely originally coal or oil fired power plants with single phase generators (rarely seen then, let alone today) or were motor-generator stations operating on commercial 3 phase power and producing single phase power; built exclusively for the PRR. Some of the motor-generator "frequency converters" I believe have been replaced with modern electronic equivalents. (Does anyone know any present details concerning the sources of NEC power and how it is derived?)
In any event, it is a rather simple single-phase distribution system with just a few power plants directly tied together through the 138kV distribution system.
Commercial power today is 3 phase, and although everyone talks about the system being a "grid" that ties everything tightly together, that really is not exactly the case. Some of those connections are not as "tight" as you would think; and as such, there are indeed fluctuations in voltage and frequency between utilities throughout the US. If all of the former 25Hz NEC substations that formerly received power from a single transmission system were to be connected to commercial utilities receiving their power from a myriad of different utilities, grids and circuits; we could be creating more problems than we have solved. (Go to
http://synchrophasor.selinc.com/live-sy ... asors.html to see real-time changing differences in frequency and phase between utilities)
You can end up with unwanted circulating currents travelling over the catenary between substations connected to different sources; phase breaks would probably have to be installed in every catenary mid-way between every substation. Also, protective relaying (utility talk for the very specialized protection of substations, power plants and transmission lines from faults; their equal to a circuit breaker in your house, except much more sophisticated) becomes much more complex and makes it much more difficult to identify a fault or short circuit and isolate only the smallest section of the circuit. Example: A train on track 3 near Princeton Junction gets a pantograph entangled with the catenary causing a short circuit to the catenary. Do you want the electrical system to: A) Shut down everything between Trenton and New Brunswick; stopping every train between those points? B) Shut down just the substation at Nassau in Princeton Junction; resulting in extreme low voltage in the Princeton Junction area and forcing trains to move through the affected area one every 15 minutes or so? or C) Kill only the track 3 catenary segment, only through Princeton Junction and let trains on tracks 1,2 & 4 run? I'll take "C", which I believe is the way the original system was designed and still works today.
Can the 25Hz NEC be upgraded? Sure! Should it be? Sure! But it is not just a matter of changing some parts and pieces to change frequency and making sure the insulators and clearances are good to raise the voltage to 25Kv. What about overhead tunnel clearances in the East and North River tunnels? Wasn't there a niche or recess built into the top to accomodate the catenary and insulators for the 11kV? What happens at 25kV?
The new electrification from New Haven to Boston is 25kV, 60Hz and is fed from commercial utility power. I have not seen a "one line diagram" as to how it was done, but it appears that they are taking utility power at 3 phase and converting it to single phase for traction use. Siemens supplied the equipment, and they offer only vague information on their web site. They refer to supplying "substations, switching stations and paralleling stations", and I couldn't infer much about them or how it all worked from a recent Acela trip to Boston... we passed the substations way too quickly! (Does anyone have further information on that 60Hz system?) Not having seen a schematic, I cannot be sure; but I do not think that the Boston system is as simple or "bullet-proof" for today as what the PRR designed in the 30's... I guess we'll find out around 2070 or so, but I won't be here then...
So I would be careful before describing the existing system as "antique". "Old" it is. "In need of repair or replacement" it is. I have very recently been involved in the modification and design of high voltage substation switching and control schemes, and it amazes me how many of the designs and techniques are 40-50 or more years old. We may implement the designs today with electronics and microprocessors that were done with vacuum tubes and with mechanical relays before that, but the principles do not change. As I get older and am now able to locate historical information, specifically about the PRR and signaling and power systems, I am absolutely amazed at the sophistication of these systems that were built in the 30's.
So my plea to Amtrak engineering: When you fix/replace the NEC electrical distribution, don't toss out everything that was done in the 30's. The laws of physics have not changed (...at least as regards electrons in electricity!) and there is very good reason that it still works after 70 years: Keep it simple, build in multiple redundancy, oversize everything and build it to last.
