DIfference in electrical clearances required for 3kV DC versus 1.5kV DC Overhead systems ?

[AHBD]

Is the difference in the railway electrical clearances required for 3kV DC versus 1.5kV DC Overhead systems significant enough to rule it out in early 1900s when Britain planned for a move from coal based steam locomotives to mainly coal based electricity driving 1.5kv overhead electric locomotives?

I had presumed not but my searches have found no answer so I wondered if anyone knew?

 

[edwin_m]

It was said that conversion of the GE lines from 1.5kV to 6.25kV AC (the voltage used closer to Liverpool Street) didn't involve any change to clearances. So I doubt 3kV would have been different either.

 

[stuu]

Is the difference in the railway electrical clearances required for 3kV DC versus 1.5kV DC Overhead systems significant enough to rule it out in early 1900s when Britain planned for a move from coal based steam locomotives to mainly coal based electricity driving 1.5kv overhead electric locomotives?

I had presumed not but my searches have found no answer so I wondered if anyone knew?

The difference in electrical clearance will be tiny, the mechanical clearance (movement of the wires) is far larger than the electrical clearance so there would be no or very little practical difference.

 

[ac6000cw]

(I know I triggered the original question about this) Out of interest, this is the original 1921 'Electrification of Railways Advisory Commitee' report - https://www.railwaysarchive.co.uk/documents/MoT_Elec1920.pdf

A couple of quotes from it regarding voltages ('pressures') and OHLE clearances:



Note (ii) (c) allows for the use of higher voltages provided they are multiples of the standard (1500V) voltage.

Clearances for 1500V OHLE:

 

[RichardAsh1981]

It's important to understand that electrical clearances (certainly out in the real world on the railway) are not precise figures. They are the result of compromise and engineering judgement, i.e. they are to some extent subjective, and certainly subject to change as external (not electrical) factors change over time. Clearances have been reduced very substantially over time as a result of a combination of:

• Better insulator technologies, so that supporting insulators are not the limiting factor.
• Better OHL design so that the wires move less (electrical clearance with the wire in the worst case position demand the mechanical movement is added on).
• Better voltage regulation, so that the highest voltage (which requires the biggest clearances) is nearer the nominal voltage.
• Better circuit breaker and protection relay technology, so that when a flashover does occur, it gets cleared faster (less damage) and the line re-energised faster (less service disruption). This makes more flashovers tolerable in operation, so smaller clearances are acceptable.
• Increased experience of operation, so confidence that clearances work increases - having to go back and obtain more clearance is very expensive, so no-one wants to take risks with designs. On the other hand, compromises that are marginal according to the rules but work fine in practice lead to pressure to change the rules (downwards).

This is the main reason why GE lines were able to convert from 1.5kV DC (pre-war scheme) to 6.25kV AC (1960s) - in the mean time the acceptable clearances had shrunk, so 6.25 kV AC would now "fit" in the pre-war 1.5kV DC clearances. Of course the 6.25 kV AC schemes have all now been increased to run at 25kV AC, using the "Special Reduced" clearances developed by BR in the 1980s in a lot of cases to avoid actually moving things.
NR has made further progress with this in using surge arrestors at bridge sites to allow tight clearances through bridges, because this brings down the peak surge voltage (surge arrestors work best nearby, so ones on the line at bridges are more effective than ones back at the feeder station - but more things to wear out, fail and need changing!

Back with the original question about why not 3KV in the Weir report, I think the answer is mostly not clearances but the on-train equipment. A 3kV DC motor was essentially impossible (due to insulation technology) issues at the time, 3kV traction had to have series pairs of 1.5kV motors, which made the control system harder, and the whole thing worked less well. The Weir report is essentially a judgement that, in the state of the art at the time and given the UK's geography (mostly quite short lines, so lots of feeder stations were possible), 1.5kV was a more sensible choice than going up to 3kV or down to ~750V.

Parts of mainland Europe with longer routes made (for a while) the 4-fold reduction in feeder stations for 3kV DC attractive enough to outweigh the motor issues - until post-war AC electrification at 50/60Hz became possible, because mobile rectifiers were developed. AC systems can use very high line voltages, then step down to nice sensible voltages (usually still around 1000V) for the motors using a transformer, but also needing a rectifier to convert AC into DC for the motors.

 

[Taunton]

Back with the original question about why not 3KV in the Weir report, I think the answer is mostly not clearances but the on-train equipment. A 3kV DC motor was essentially impossible (due to insulation technology) issues at the time, 3kV traction had to have series pairs of 1.5kV motors, which made the control system harder, and the whole thing worked less well. The Weir report is essentially a judgement that, in the state of the art at the time and given the UK's geography (mostly quite short lines, so lots of feeder stations were possible), 1.5kV was a more sensible choice than going up to 3kV or down to ~750V.

Parts of mainland Europe with longer routes made (for a while) the 4-fold reduction in feeder stations for 3kV DC attractive enough to outweigh the motor issues - until post-war AC electrification at 50/60Hz became possible, because mobile rectifiers were developed. AC systems can use very high line voltages, then step down to nice sensible voltages (usually still around 1000V) for the motors using a transformer, but also needing a rectifier to convert AC into DC for the motors.

3kV DC became perfectly practical as the standard in Belgium, Italy, the Soviet Union, and a number of others. I've always felt that those countries that standardised in the 1920s went for 1.5kV, and those a bit later went for 3kV as equipment developed. Unlike in Britain, nobody has changed these over since, so there can be no real issues.

I also feel that Weir, reporting in 1931, was swayed by his own Scottish heavy engineering companies having established contacts with 1.5kV equipment manufacturers. Likewise the Merz & McLellan detailed report to the GWR in 1938 for electrification of Taunton to Penzance ignored Weir's "standard", and proposed 3kV, which M&M just happened to have different professional contacts with ...

AC is fine, and 25kV is commonly quoted as reducing the number of substations, conveniently omitting that you then need substations on each individual train instead, in fact three of them in a 12-car formation. The GE 1.5kV DC system was changed over in 1960 to 25kV, justified by a reduced number of substations, when in fact they then bought 400 transformer/rectifier substations to fit under their emus.

 

[edwin_m]

3kV DC became perfectly practical as the standard in Belgium, Italy, the Soviet Union, and a number of others. I've always felt that those countries that standardised in the 1920s went for 1.5kV, and those a bit later went for 3kV as equipment developed. Unlike in Britain, nobody has changed these over since, so there can be no real issues.

I also feel that Weir, reporting in 1931, was swayed by his own Scottish heavy engineering companies having established contacts with 1.5kV equipment manufacturers. Likewise the Merz & McLellan detailed report to the GWR in 1938 for electrification of Taunton to Penzance ignored Weir's "standard", and proposed 3kV, which M&M just happened to have different professional contacts with ...

AC is fine, and 25kV is commonly quoted as reducing the number of substations, conveniently omitting that you then need substations on each individual train instead, in fact three of them in a 12-car formation. The GE 1.5kV DC system was changed over in 1960 to 25kV, justified by a reduced number of substations, when in fact they then bought 400 transformer/rectifier substations to fit under their emus.

However, many countries such as France and Russia have adopted 25kV for new schemes, and there have been a few conversions. Agreed each train needs its own transformer, but DC systems have much higher resistance losses so greater operating costs.

 

[AM9]

3kV DC became perfectly practical as the standard in Belgium, Italy, the Soviet Union, and a number of others. I've always felt that those countries that standardised in the 1920s went for 1.5kV, and those a bit later went for 3kV as equipment developed. Unlike in Britain, nobody has changed these over since, so there can be no real issues.

I also feel that Weir, reporting in 1931, was swayed by his own Scottish heavy engineering companies having established contacts with 1.5kV equipment manufacturers. Likewise the Merz & McLellan detailed report to the GWR in 1938 for electrification of Taunton to Penzance ignored Weir's "standard", and proposed 3kV, which M&M just happened to have different professional contacts with ...

AC is fine, and 25kV is commonly quoted as reducing the number of substations, conveniently omitting that you then need substations on each individual train instead, in fact three of them in a 12-car formation. The GE 1.5kV DC system was changed over in 1960 to 25kV, justified by a reduced number of substations, when in fact they then bought 400 transformer/rectifier substations to fit under their emus.

But those transformer-rectifier sets were tailored to the specific needs of the stock in which they were fitted. There has been continuous development in transformer and rectified design throughout the 20th century so 'the carrying around substations' expression has rapidly become an irrelevant handle. Today, there.s not that much difference in the weight of a train fitted with modern low iron cored transformer and a cooled solid state rectifier pack, when added to an EMU compared with a DC only set, ignoring the fact that DC only trains for OLE would also have the weight of pantographs. The remailing difference is mostly negated by the much simpler arrangements for regen. back to the grid.
Then there's the reliability, the GE, (along with the NW LMR and Glasgow) had to deal with faulty switchgear controls overloading transformers causing them to explode. Modern electronic control systems look after the hardware much better, (maybe too well in the case of the class 700 shutdowns in 2019 - now fixed with a software update).

 

[WAO]

The pity is that 25kV was chosen being the French system (they do use some odd Voltages, as you might expect), rather than the standard UK intermediate voltage of 33kV. The FS transformers would therefore have been "off the peg" rather than needing different turns ratios.

WAO

 

[edwin_m]

The pity is that 25kV was chosen being the French system (they do use some odd Voltages, as you might expect), rather than the standard UK intermediate voltage of 33kV. The FS transformers would therefore have been "off the peg" rather than needing different turns ratios.

WAO

Wouldn't they be off the peg anyway, due to being single phase?

 

[AM9]

The pity is that 25kV was chosen being the French system (they do use some odd Voltages, as you might expect), rather than the standard UK intermediate voltage of 33kV. The FS transformers would therefore have been "off the peg" rather than needing different turns ratios.

WAO

I think the cost of having, unique to UK, on board transformers on every train would cost more than the far fewer linside transformers that would be needed. I doubt that there is a wrehouse somewhere in the UK stuffed full of in-stock transformers, and calculating conductor sizes and turns ratio is not exactly difficult. On another point, 33Kv on OLE would create even bigger issues on clearances, including those on pantograph mountings and of course overbridges.
Anyway, just because our grids are stepped at 400Kv, 275Kv, 132Kv, etc., doesn't mean that other similar countries have the same voltage breaks so the primaries are also unique.

 

[AHBD]

....
Back with the original question about why not 3KV in the Weir report, I think the answer is mostly not clearances but the on-train equipment. A 3kV DC motor was essentially impossible (due to insulation technology) issues at the time, 3kV traction had to have series pairs of 1.5kV motors, ....

But a locomotive or emu motor carriage with two bogies of 2 or more axles each would allow one motor per axle, using the normal 750V motors in series that gives 3Kv, so surely 3kv is just as possible as 1.5kv in motor terms? Or one motor per wheel or as mentioned in this thread

#10

series pairs of motors with two windings two motors could still be wired to use up all the 3kv.

 

[edwin_m]

But a locomotive or emu motor carriage with two bogies of 2 or more axles each would allow one motor per axle, using the normal 750V motors in series that gives 3Kv, so surely 3kv is just as possible as 1.5kv in motor terms? Or one motor per wheel or as mentioned up thread series pairs of motors with two windings two motors could still be wired to use up all the 3kv.

The French, and probably others, were quite keen on monomotor bogies, so there would only have been two on a locomotive.

 

[AHBD]

The French, and probably others, were quite keen on monomotor bogies, so there would only have been two on a locomotive.

But we don't / didn't have to copy the French....

And the two double winding approach linked to woukd still allow such a monomotor approach..

 

[WAO]

The use of 1.5 or 3.0kV supply in today's terms would be facilitated by a modern inverter ac motor control system. This would be very tolerant to the dc Voltage applied.

The clearances for 33kV are similar to 25kV as is the switchgear etc and proportionately fewer FS's would have been needed. Just the turns ratio has to be reduced for 25kV as it's beyond the normal -15% tapping.

WAO

 

[AM9]

But a locomotive or emu motor carriage with two bogies of 2 or more axles each would allow one motor per axle, using the normal 750V motors in series that gives 3Kv, so surely 3kv is just as possible as 1.5kv in motor terms? Or one motor per wheel or as mentioned in this thread

Mk1 EMUs had four 750V DC series wound motors. If all four were stacked in series, any failure (even a low current failure mode) would result in the whole car (and unit) being removed from service, - i.e towed home. In normal use it was possible to shut one or more out of circuit and limp on.
The DC units that were modified to be class 306 units on the Liverpool St-Shenfield services were wired with 2 750V series would motors in series on each of the two motor bogies. This saved on resistive energy loss on starting, as only a single motor current was dissipated in the resistors. When the reverse EMF equalled the 1500VDC, the contactors momentarily shorted the lower motor out and allowed it to be switched to its own resistor set to run up to it'sbalancing speed. This changeover happened at about 20mph, usually whilst the rear cars were still in station.

 

[QueensCurve]

This is the main reason why GE lines were able to convert from 1.5kV DC (pre-war scheme) to 6.25kV AC (1960s) - in the mean time the acceptable clearances had shrunk, so 6.25 kV AC would now "fit" in the pre-war 1.5kV DC clearances. Of course the 6.25 kV AC schemes have all now been increased to run at 25kV AC, using the "Special Reduced" clearances developed by BR in the 1980s in a lot of cases to avoid actually moving things.

The increase from 6.25kV to 25kV famously followed the experiment in a Crewe tunnel which confirmed that with a 50mm air gap 25kV would not flash over, even when subjected to the full blast from a steam locomotive chimney.

 

[AM9]

It's important to understand that electrical clearances (certainly out in the real world on the railway) are not precise figures. They are the result of compromise and engineering judgement, i.e. they are to some extent subjective, and certainly subject to change as external (not electrical) factors change over time. Clearances have been reduced very substantially over time as a result of a combination of:

• Better insulator technologies, so that supporting insulators are not the limiting factor.
• Better OHL design so that the wires move less (electrical clearance with the wire in the worst case position demand the mechanical movement is added on).
• Better voltage regulation, so that the highest voltage (which requires the biggest clearances) is nearer the nominal voltage.
• Better circuit breaker and protection relay technology, so that when a flashover does occur, it gets cleared faster (less damage) and the line re-energised faster (less service disruption). This makes more flashovers tolerable in operation, so smaller clearances are acceptable.
• Increased experience of operation, so confidence that clearances work increases - having to go back and obtain more clearance is very expensive, so no-one wants to take risks with designs. On the other hand, compromises that are marginal according to the rules but work fine in practice lead to pressure to change the rules (downwards).

This is the main reason why GE lines were able to convert from 1.5kV DC (pre-war scheme) to 6.25kV AC (1960s) - in the mean time the acceptable clearances had shrunk, so 6.25 kV AC would now "fit" in the pre-war 1.5kV DC clearances. Of course the 6.25 kV AC schemes have all now been increased to run at 25kV AC, using the "Special Reduced" clearances developed by BR in the 1980s in a lot of cases to avoid actually moving things.

Whereas, on the GEML, - original 1500VDC insulators (single disc type) were pressed into use on 6.25kV, but when it was upped to 25kV twenty years later, they were changed to conventional multi-ribbed insulators.

 

[The exile]

3kV DC became perfectly practical as the standard in Belgium, Italy, the Soviet Union, and a number of others. I've always felt that those countries that standardised in the 1920s went for 1.5kV, and those a bit later went for 3kV as equipment developed. Unlike in Britain, nobody has changed these over since, so there can be no real issues.

IIRC the Czechs have embarked on a (semi-)national conversion of their dc lines.

 

[zwk500]

IIRC the Czechs have embarked on a (semi-)national conversion of their dc lines.

And the Belgians are upgrading some of their lines that interact with France and Lux.

 

[Taunton]

But those transformer-rectifier sets were tailored to the specific needs of the stock in which they were fitted. There has been continuous development in transformer and rectified design throughout the 20th century so 'the carrying around substations' expression has rapidly become an irrelevant handle.

Then there's the reliability, the GE, (along with the NW LMR and Glasgow) had to deal with faulty switchgear controls overloading transformers causing them to explode. Modern electronic control systems look after the hardware much better

One presumes that advances in trainborne transformers have been equalled by comparable advances in lineside ones.

The serious explosions in the early trains were all with the designs/products of one manufacturer on the 6.25/25kV changeover. The approach at the time was that the new technology would be spread around multiple suppliers to give them a base for exports, which was a government priority of the era. Unfortunately a parallel government priority was to merge them all up into larger organisations, and the whole lot ended up in GEC - which progressively shrank and then disappeared.

 

[59CosG95]

IIRC the Czechs have embarked on a (semi-)national conversion of their dc lines.

They are indeed - all of České Dráhy's new RegioPanter EMUs are now dual-voltage (3kV DC & 25kV AC), after the Class 440s were converted to Class 640s.
(4xx EMUs are DC, 5xx EMUs are AC, 6xx EMUs are dual-voltage.)

 

[AM9]

One presumes that advances in trainborne transformers have been equalled by comparable advances in lineside ones.

The serious explosions in the early trains were all with the designs/products of one manufacturer on the 6.25/25kV changeover. The approach at the time was that the new technology would be spread around multiple suppliers to give them a base for exports, which was a government priority of the era. Unfortunately a parallel government priority was to merge them all up into larger organisations, and the whole lot ended up in GEC - which progressively shrank and then disappeared.

Lineside transformers, i.e. large static transformers haven't changed much, - if they're not being carted around, there's no need to go to low-iron cores, but the cost of preparing a site and connection for them is much more expensive.

 

[Taunton]

They are indeed - all of České Dráhy's new RegioPanter EMUs are now dual-voltage (3kV DC & 25kV AC), after the Class 440s were converted to Class 640s.
(4xx EMUs are DC, 5xx EMUs are AC, 6xx EMUs are dual-voltage.)

Seems to be becoming the norm - France is pretty much everything new is dual voltage now. Seems it's more effective now to make the trains dual voltage rather than have a disruptive conversion programme.

 

[AM9]

Seems to be becoming the norm - France is pretty much everything new is dual voltage now. Seems it's more effective now to make the trains dual voltage rather than have a disruptive conversion programme.

No different to the majority of new EMUs here which have traction systems based on a 750V 'ish bus enabling configuration for 3rd rail running.