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Re: Kopp and SF continue to fight over downtown hsr station
Author: OldPoleBurner
Date: 09-24-2009 - 10:22
Hi Bert,
This is a subtle one, but I'll try to help. Those who are not technically inclined - please bear with us. I'll be happy to answer any technical questions if I can. The issue here, was explained to me by those involved in the planning and engineering of both the Caltrain electrification, and BART's facilities in Santa Clara area.
BART will be on a roughly parallel r.o.w. between a joint Santa Clara Station and downtown San Jose. They will of necessity then, be drawing electricity from the same region of the power grid. And that presents an electrical issue to be solved, not necessarily an incompatibility between the two. But if it is not solved, you could say that the two would be incompatible customers of the power company.
The issue is an electrical subtlety with huge impact on BART's train control system. PG&E tries vary hard to keep the voltage levels equal between each phase on its power grid; But when it doesn't succeed, a full and powerful set of second and thirds harmonics can be induced into the outputs of any multiphase rectification to DC. Most electronic equipment using rectified DC, such as a computer or TV, draw little enough power, that filtering this out is very practical. But not so much, with a heavy DC use such as BART's traction power system.
Thus the BART r.o.w. can be saturated with these electrical harmonics; which can sometimes be so strong as to even drown out your car radio on a nearby road. In fact, such is also the case along power lines where PG&E has not kept the phases balance. They can also drown out the train control communication signals sent along the rails for train detection and cab signaling (ATP/PTS).
Thus, when PG&E delivers power to BART with unbalanced phases, electrical interference is produced, causing the BART system to sometimes believe there is a train where there isn't one; or it prevents the train itself from picking up its commands. In either case, it promptly comes to a stop. While this is safe. it is quite annoying to BART's passengers to be thusly delayed.
While PG&E is usually quite successful in keeping the phases reasonably balanced (they do this for their own reasons as well), it is only possible to do so with relatively stable or at least predictable loads. But the single phase use by an AC traction train nearby, would present a very large unbalanced and unpredictable load to PG&E. And thus it would be difficult or impossible for either PG&E or BART to dynamically compensate. It is this compensation method that is the current stumbling block.
As I report this, I am currently out of that loop, so I am presently unaware if a solution has been found and settled upon, or which particular solutions are being investigated. But I believe it will be solved. It should be noted, that BART's engineering was very proactive in bringing this issue up - and that's good! It will prevent a lot of future pain.
OPB
This is a subtle one, but I'll try to help. Those who are not technically inclined - please bear with us. I'll be happy to answer any technical questions if I can. The issue here, was explained to me by those involved in the planning and engineering of both the Caltrain electrification, and BART's facilities in Santa Clara area.
BART will be on a roughly parallel r.o.w. between a joint Santa Clara Station and downtown San Jose. They will of necessity then, be drawing electricity from the same region of the power grid. And that presents an electrical issue to be solved, not necessarily an incompatibility between the two. But if it is not solved, you could say that the two would be incompatible customers of the power company.
The issue is an electrical subtlety with huge impact on BART's train control system. PG&E tries vary hard to keep the voltage levels equal between each phase on its power grid; But when it doesn't succeed, a full and powerful set of second and thirds harmonics can be induced into the outputs of any multiphase rectification to DC. Most electronic equipment using rectified DC, such as a computer or TV, draw little enough power, that filtering this out is very practical. But not so much, with a heavy DC use such as BART's traction power system.
Thus the BART r.o.w. can be saturated with these electrical harmonics; which can sometimes be so strong as to even drown out your car radio on a nearby road. In fact, such is also the case along power lines where PG&E has not kept the phases balance. They can also drown out the train control communication signals sent along the rails for train detection and cab signaling (ATP/PTS).
Thus, when PG&E delivers power to BART with unbalanced phases, electrical interference is produced, causing the BART system to sometimes believe there is a train where there isn't one; or it prevents the train itself from picking up its commands. In either case, it promptly comes to a stop. While this is safe. it is quite annoying to BART's passengers to be thusly delayed.
While PG&E is usually quite successful in keeping the phases reasonably balanced (they do this for their own reasons as well), it is only possible to do so with relatively stable or at least predictable loads. But the single phase use by an AC traction train nearby, would present a very large unbalanced and unpredictable load to PG&E. And thus it would be difficult or impossible for either PG&E or BART to dynamically compensate. It is this compensation method that is the current stumbling block.
As I report this, I am currently out of that loop, so I am presently unaware if a solution has been found and settled upon, or which particular solutions are being investigated. But I believe it will be solved. It should be noted, that BART's engineering was very proactive in bringing this issue up - and that's good! It will prevent a lot of future pain.
OPB
