HSTEd
Veteran Member
- Joined
- 14 Jul 2011
- Messages
- 16,726
I am often talking about the advantages of third rail versus 25kV in certain low-power and low-speed scenarios. Where the low capital cost overwhelms the operational savings of 25kV.
Now, I want to talk about the other extreme.
Extremely high service density routes, especially new construction high speed rail alignments and potential future heavy freight routes (if such things are ever built, especially using high performance FMUs).
For example:
Take a HS2-type high speed rail alignment operating at 320kph and 18tph.
That means on average there is a train every ~20km on each line. So 10km in total.
A 400m double decker train could be drawing 11MWe average (assuming 0.033kWh/seat-km), and far more than that at peak.
That means on average you will be drawing 1.1MWe per route-km on average. Even with auto-transformer feeding the net current will be 22A/km. This very quickly becomes problematic if you attempt to use substation spacings of 60km or more as is standard practice for 25kV systems. You will end up drawing something like 1320A average at each feeding station.
The resistance of a typical overhead wiring cable is something like 0.15ohm/km.
And at these currents you start to get serious voltage drops on order of 200V/km on each of the conductors (both feed and return). Which doesn't sound like a lot but it could be dropping over a kilovolt (on each side) between the substation and the first auto transformer feed point. And the average ~400 extra amps flowing between the train and its nearest ATFs will add even more to that. It is going to get bad in the middle of the section in voltage terms.
We have reached the point where 25kV ATF electrification starts to lose its very high efficiency capability. You have two options - you can reduce the substation spacings and pay for additional grid access points which are very expensive and have a price that scales highly non-linearly with kilowattage..... or we can go to a higher voltage.
An obvious choice would be 50kV with ATF feed. I don't think such a system has ever been tried operationally but 50kV with booster transformers or rail return is used in South Africa and the US, and has been used in Canada in the past.
This cuts your currents by half to something a little more reasonable with only 660A or so at each feeding station. And cuts your resistive losses (which are starting to become unreasonable) by an even greater factor.
You could even be able to stand shifting your substations further apart, or your autotransformers further apart, so even though each substation might become more expensive (although I don't think swapping a 400kV/25-0-25kV transformer out for a 400kV/50-0-50kV and some 50kV circuit breakers in place of 25kV ones will break the bank. Those are still fairly mild by transmission voltage standards) you could have less of them.
In summary - 50kV will start to overwhelm 25kV's advantages in terms of lower capital cost when we reach very high traffic densities. This critical density is approached in the example of HS2 should it reduce full capacity.
However the clearance issues that have surfaced with the loss of the 25kV derogation make the situation a bit more interesting. It seems unlikely that this derogation can be revived in the current political climate (someone will get themselves killed and people will scream about profit before people or something if the standard is reduced) however there is no extant European 50kV standard.
Given how conservative the 25kV standard now is, it is not entirely clear that a reasonably defined, and it would be defined by British engineers if we move first, 50kV standard would have enormously larger clearances than the new 25kV one does.
So its not even sure how much more a 50kV standard line would cost than a 25kV standard line - in the case of new construction the difference is surely negligible but a detailed study would have to be done in the case of electrification of existing lines.
And this is before we consider absurdities like 700m long double stack Freight multiple units moving at 90mph to kill road freight by moving goods cheaper and faster with fewer staff.
EIther way - I am not just a third rail fan - I believe in the best tool for the best job taking a whole problem approach. Especially now the problems that early dual voltage systems (25/6.25kV as an example) have had in the past seem to be behind us.
Now, I want to talk about the other extreme.
Extremely high service density routes, especially new construction high speed rail alignments and potential future heavy freight routes (if such things are ever built, especially using high performance FMUs).
For example:
Take a HS2-type high speed rail alignment operating at 320kph and 18tph.
That means on average there is a train every ~20km on each line. So 10km in total.
A 400m double decker train could be drawing 11MWe average (assuming 0.033kWh/seat-km), and far more than that at peak.
That means on average you will be drawing 1.1MWe per route-km on average. Even with auto-transformer feeding the net current will be 22A/km. This very quickly becomes problematic if you attempt to use substation spacings of 60km or more as is standard practice for 25kV systems. You will end up drawing something like 1320A average at each feeding station.
The resistance of a typical overhead wiring cable is something like 0.15ohm/km.
And at these currents you start to get serious voltage drops on order of 200V/km on each of the conductors (both feed and return). Which doesn't sound like a lot but it could be dropping over a kilovolt (on each side) between the substation and the first auto transformer feed point. And the average ~400 extra amps flowing between the train and its nearest ATFs will add even more to that. It is going to get bad in the middle of the section in voltage terms.
We have reached the point where 25kV ATF electrification starts to lose its very high efficiency capability. You have two options - you can reduce the substation spacings and pay for additional grid access points which are very expensive and have a price that scales highly non-linearly with kilowattage..... or we can go to a higher voltage.
An obvious choice would be 50kV with ATF feed. I don't think such a system has ever been tried operationally but 50kV with booster transformers or rail return is used in South Africa and the US, and has been used in Canada in the past.
This cuts your currents by half to something a little more reasonable with only 660A or so at each feeding station. And cuts your resistive losses (which are starting to become unreasonable) by an even greater factor.
You could even be able to stand shifting your substations further apart, or your autotransformers further apart, so even though each substation might become more expensive (although I don't think swapping a 400kV/25-0-25kV transformer out for a 400kV/50-0-50kV and some 50kV circuit breakers in place of 25kV ones will break the bank. Those are still fairly mild by transmission voltage standards) you could have less of them.
In summary - 50kV will start to overwhelm 25kV's advantages in terms of lower capital cost when we reach very high traffic densities. This critical density is approached in the example of HS2 should it reduce full capacity.
However the clearance issues that have surfaced with the loss of the 25kV derogation make the situation a bit more interesting. It seems unlikely that this derogation can be revived in the current political climate (someone will get themselves killed and people will scream about profit before people or something if the standard is reduced) however there is no extant European 50kV standard.
Given how conservative the 25kV standard now is, it is not entirely clear that a reasonably defined, and it would be defined by British engineers if we move first, 50kV standard would have enormously larger clearances than the new 25kV one does.
So its not even sure how much more a 50kV standard line would cost than a 25kV standard line - in the case of new construction the difference is surely negligible but a detailed study would have to be done in the case of electrification of existing lines.
And this is before we consider absurdities like 700m long double stack Freight multiple units moving at 90mph to kill road freight by moving goods cheaper and faster with fewer staff.
EIther way - I am not just a third rail fan - I believe in the best tool for the best job taking a whole problem approach. Especially now the problems that early dual voltage systems (25/6.25kV as an example) have had in the past seem to be behind us.
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