OHLE clearances, and 25kv v 1500v dc.

[Elecman]

There are no 11kv DNO feeders that I am aware of the smallest I've seen is 33kv

 

[HSTEd]

There are no 11kv DNO feeders that I am aware of the smallest I've seen is 33kv

Chris Gibb's report [Page 64/163] mentions that an 11kV supply was used for the East Grinstead electrification project in 1987.

 

[3141]

Probably steered deliberately by some clever "Sir Humphrey" in the Treasury or Department for Roads who probably had a very good idea of the eventual fallout in the hated rail industry ...

You might like to imagine that, but if it was true it would mean that Sir Humphrey "in the Treasury or Department for Roads" was remarkably clued up on railway matters and understood the situation better than Network Rail.

 

[Searchlight]

For all the discussion about "efficency", DC losses, etc. It seems to be overlooked that Diesel-Electric Traction, Battery Traction, Fuel Cells, etc.......Are far MORE inefficient than 3Kv DC
Electrification. At 6KV DC the losses are equal to 25Kv AC. So, 3Kv is only marginally less efficient than "25Kv AC. "25 Kv needs Expensive 400Kv substations Grid-fed Single-Phase. DC does not,
and can be fed from 11Kv or 33Kv local feeders. Substations every 10 miles........That doesn t seem excessive to me. Present 25Kv knitting would be OK for 3Kv DC, and no need for Neutral
Feeders or Boosters with DC. Should every house have a 25Kv sub in the front garden? MAD....Yet every train has to carry its sub around.....Some 3 of them! Most modern 3-phase traction
packages have a "DC LInk" of 3Kv.......Why not cut out the 25Kv, and feed direct?

Think about it.

 

[billio]

So back to the Dick, Kerr experimental line of 1913, though that used 3.5kV ?.
http://www.lyrs.org.uk/images/uploa...n_-_Traction_car_No_3501,_Trailer_No_3601.jpg

 

[edwin_m]

Electrification. At 6KV DC the losses are equal to 25Kv AC. So,

Source? With the feeder stations at the same spacing the I2R losses would be about nine times higher, and the discrepancy increases if the 25kV is autotransformer fed. For the same reason regenerative braking becomes more efficient too.

"25 Kv needs Expensive 400Kv substations Grid-fed Single-Phase. DC does not,
and can be fed from 11Kv or 33Kv local feeders. .......Why not cut out the 25Kv, and feed direct?

Quite simply because the higher the voltage the lower the I2R losses. The longer the distance and the less intensive the service the higher the most economic voltage, even if that results in each train having to carry a transformer. Hence why trams have 750V, the few electrified heavy haul railways tend to go for 50kV, but practically every mixed traffic railway that has electrified post-war and hasn't been constrained by an existing system (and some that have been) has gone for 25kV. Power electronics has made some things possible, including potentially balancingg phase loads on the Grid at 25kV feeders, but it hasn't changed the laws of physics.

The 400kV feeder stations are much further apart, for example there is one near Newton le Willows and the next one east will be beyond Stalybridge (not connected directly to OLE due to electrification delays, but designed to be in future). I imagine it would be possible to feed 25kV from local feeders too - the power demand per length of line would be similar, lower actually because 25kV is more efficient - but it isn't done because one big feeder is more economic than many small ones.

 

[Jonny]

The ECML seems to be fed from 132kV lines (into 25kV AC), at least over the ECM5 section (York to Newcastle).

 

[GRALISTAIR]

I will one of these days do a full calculation and post on this web site. How someone can say 3kV losses are the same as 25kV bewilders me - to deliver power to a modern power hungry train with air con etc.

 

[snowball]

The ECML seems to be fed from 132kV lines (into 25kV AC), at least over the ECM5 section (York to Newcastle).

I have seen it suggested that the power supply on the northern reaches of the ECML is about at its limits and will need to be beefed up if extra electric trains are to be run.

An upgrade to the power supply on the southern ECML between London and Bawtry has just been completed.

 

[apk55]

A big problem with all DC systems is the return path voltage drop via the rails which must be limited to a few volts for safety (you could get a shock if you touched an object connected to the rails) and corrosion problems in nearby pipes etc. This is probably the dominant factor on substation distance in high voltage DC systems. Running rails are not chosen for their conductivity rather the wearing properties. The only way this could be improved is by putting a heavy low resistance conductor in parallel with the rails.
On AC systems there are some methods of reducing this by booster transformers or +~ 25KV systems and it is made easier because the currents are very much lower

 

[Searchlight]

Source? With the feeder stations at the same spacing the I2R losses would be about nine times higher, and the discrepancy increases if the 25kV is autotransformer fed. For the same reason regenerative braking becomes more efficient too.

Quite simply because the higher the voltage the lower the I2R losses. The longer the distance and the less intensive the service the higher the most economic voltage, even if that results in each train having to carry a transformer. Hence why trams have 750V, the few electrified heavy haul railways tend to go for 50kV, but practically every mixed traffic railway that has electrified post-war and hasn't been constrained by an existing system (and some that have been) has gone for 25kV. Power electronics has made some things possible, including potentially balancingg phase loads on the Grid at 25kV feeders, but it hasn't changed the laws of physics.

The 400kV feeder stations are much further apart, for example there is one near Newton le Willows and the next one east will be beyond Stalybridge (not connected directly to OLE due to electrification delays, but designed to be in future). I imagine it would be possible to feed 25kV from local feeders too - the power demand per length of line would be similar, lower actually because 25kV is more efficient - but it isn't done because one big feeder is more economic than many small ones.

400Kv Feeder Stations may be far apart, but, they are VERY large and expensive. Compared to 33Kv Substations. The Higher the voltage the more expensive the equipment, and the more space it
takes-up.

6Kv DC Re-electrification was seriously considered in Russia in recent times.......And, even went right to government decision-making, where the proposal was rejected. This is where I got the
quote of parity with 25Kv AC. AC has something called IMPEDENCE, which DC does not. In practical terms, this means conductors of AC are not fully utilized (skin effect). This especially applies to steel rails, but manifests in all conductors to some extent. DC only has to deal with RESISTANCE. Yes, I can see the point of 25Kv for High-Speed lines like HS2, but for most local
rail routes in UK, it is turning out to be horrendously expensive, and progress painfully slow, with 60% of the system still un-electrified, and Ministers claiming Diesel is good enough, here in the 21st
century. Yes, you COULD electrify at 60Kv AC, but would it be sensible?.....You COULD build railways to 7ft gauge too? but, didn t someone try that? Whats so magic about 25Kv?

 

[najaB]

AC has something called IMPEDENCE, which DC does not. In practical terms, this means conductors of AC are not fully utilized (skin effect).

The skin effect is pretty much a non-factor at 50Hz.

Whats so magic about 25Kv?

It's high enough that it significantly reduces current requirements, but low enough that insulation and clearance requirements are reasonable.

 

[edwin_m]

400Kv Feeder Stations may be far apart, but, they are VERY large and expensive. Compared to 33Kv Substations. The Higher the voltage the more expensive the equipment, and the more space it
takes-up.

But you need a lot more of them.

6Kv DC Re-electrification was seriously considered in Russia in recent times.......And, even went right to government decision-making, where the proposal was rejected. This is where I got the
quote of parity with 25Kv AC.

Considered and rejected, so Russia decided 25kV was better despite already having DC electrification at a lower voltage which would have made a DC conversion easier.

AC has something called IMPEDENCE, which DC does not. In practical terms, this means conductors of AC are not fully utilized (skin effect). This especially applies to steel rails, but manifests in all conductors to some extent. DC only has to deal with RESISTANCE.

That's why AC wires are the thickness they are, and most of the return current passes through return conductors rather than rails. If you needed less impedance you would use several wires rather than a fatter one.

Yes, I can see the point of 25Kv for High-Speed lines like HS2, but for most local
rail routes in UK, it is turning out to be horrendously expensive, and progress painfully slow, with 60% of the system still un-electrified, and Ministers claiming Diesel is good enough, here in the 21st
century.

I think the problems with AC in the UK are to do with delivery and standards rather than the system itself - BR managed to do it at competitive prices in the past. Who knows how much a new DC scheme would cost on NR if done today? The tram-train is not a good omen!

Yes, you COULD electrify at 60Kv AC, but would it be sensible?.....You COULD build railways to 7ft gauge too? but, didn t someone try that? Whats so magic about 25Kv?

As I've already suggested, pretty much every mixed traffic railway making the decision in modern times has settled on 25kV (little K big V by the way) - even some that had previously adopted a lower voltage.