<i>If you need to transform 138kV 3 phase to 25kV single phase, you would use a transformer or transformer bank connected in what is known as a "Scott-T" arrangement to do so. Although not frequently done, the transformers are similar and do not represent any "special" technology. This is what I think they did on the New Haven-Boston system, because the utility feeds are definitely 3 phase, the traction single phase, and there is no frequency change or similar equipment involved. </i>
Ahh, ok. I was never really good at this end of electrical, and high power stuff is a field in itself. Now, if they're fed off a Scott T, do they load the phases evenly? This is always the traditional excuse for retaining 25hz - that there'd be uneven loading of the phases which would be bad (though I suspect utilities have to tolerate a bit of it anyway).
3 phase distribution at the track level runs into an obvious problem, if you remember there's a return circuit. Basically, you could only ever have one leg as the return circuit, because it's virtually impossible to do a phase break with the running rail to the tune of a few thousand volts. Easy up above. On the ground? No way.
I suspect, then, that the power generation might actyually have been 3 phase, and they converted it to single phase for distribution and track level transmission.
I'm not sure how New Haven did it, though I do know the generation was 3 phase.
Redundancy? Yes, it's a great thing. I suspect that the PRR system is much more redundant than the New Haven, and this was at least partly why it's hung on.
Comercial frequency at 12kv or 25kv typically is the cheapest system for large scale implementation. Substation distances are far, the wires are lighter than 3kv or 1.5kv, and equipment is 'off the shelf'. I really can't think of any technical reason to retain 25hz. In Europe, where there's 10's of thousands of miles of 16 2/3rds hz power, it's a bit more entrenched - and they may still have 'legacy' equipment there too. But they also have a generation/distribution system at that frequency (because it's still so widely used). In the US, we really have neither, and thus it's better to avoid the stuff altogether.
Plus, as noted, it allows lighter transformers, which with the FRA regulations, is something that's desperately needed. I bet you could shave a few tons off a locomotive's weight if you didn't need 25hz capacity. is there any quick rule of thumb for calculating the iron in a transformer as a function of power requirements and frequency? At least then, a % difference in weight could be calculated.
FWIW, a Metroliner's main tranny was about 5 tons, I think. I have the entire specs here, I'll look them up if anyone's curious.
I know the NYCTA is quickly getting rid of legacy 25hz equipment - substations (rotary converters, all 25hz ones gone as of 2000) and signals, which used it. Mills? I recall seeing a picture of a 5000hp VFD from the 60's. The electronics was the size of a football field (almost). Today, a comperable system is a little bit bigger than a Sub Zero fridge. Oh yeah, and is backed up by computers that could blow away even supercomputers from the 60's. Though the average pocket calculator probbably can today too....
AFAIK, Amtrak and Septa are the last large scale 25hz users in the world.
BTW - that last name is Nasadowski - the 'i' was historically chopped by the various ISPs I had, so I just chop it everywhere these days
Ahh, ok. I was never really good at this end of electrical, and high power stuff is a field in itself. Now, if they're fed off a Scott T, do they load the phases evenly? This is always the traditional excuse for retaining 25hz - that there'd be uneven loading of the phases which would be bad (though I suspect utilities have to tolerate a bit of it anyway).
3 phase distribution at the track level runs into an obvious problem, if you remember there's a return circuit. Basically, you could only ever have one leg as the return circuit, because it's virtually impossible to do a phase break with the running rail to the tune of a few thousand volts. Easy up above. On the ground? No way.
I suspect, then, that the power generation might actyually have been 3 phase, and they converted it to single phase for distribution and track level transmission.
I'm not sure how New Haven did it, though I do know the generation was 3 phase.
Redundancy? Yes, it's a great thing. I suspect that the PRR system is much more redundant than the New Haven, and this was at least partly why it's hung on.
Comercial frequency at 12kv or 25kv typically is the cheapest system for large scale implementation. Substation distances are far, the wires are lighter than 3kv or 1.5kv, and equipment is 'off the shelf'. I really can't think of any technical reason to retain 25hz. In Europe, where there's 10's of thousands of miles of 16 2/3rds hz power, it's a bit more entrenched - and they may still have 'legacy' equipment there too. But they also have a generation/distribution system at that frequency (because it's still so widely used). In the US, we really have neither, and thus it's better to avoid the stuff altogether.
Plus, as noted, it allows lighter transformers, which with the FRA regulations, is something that's desperately needed. I bet you could shave a few tons off a locomotive's weight if you didn't need 25hz capacity. is there any quick rule of thumb for calculating the iron in a transformer as a function of power requirements and frequency? At least then, a % difference in weight could be calculated.
FWIW, a Metroliner's main tranny was about 5 tons, I think. I have the entire specs here, I'll look them up if anyone's curious.
I know the NYCTA is quickly getting rid of legacy 25hz equipment - substations (rotary converters, all 25hz ones gone as of 2000) and signals, which used it. Mills? I recall seeing a picture of a 5000hp VFD from the 60's. The electronics was the size of a football field (almost). Today, a comperable system is a little bit bigger than a Sub Zero fridge. Oh yeah, and is backed up by computers that could blow away even supercomputers from the 60's. Though the average pocket calculator probbably can today too....
AFAIK, Amtrak and Septa are the last large scale 25hz users in the world.
BTW - that last name is Nasadowski - the 'i' was historically chopped by the various ISPs I had, so I just chop it everywhere these days
