Medium voltage DC for affordable electrification

[squizzler]

I wonder if 12kV or similar medium voltage dc might be considered as a replacement for the 15 kV 16 2⁄3 Hz used in much of Central Europe and Scandinavia? The idea behind that choice of current is that it works in dc motors, a consideration which subsequent advances made irrelevant. In practise this means that these networks are lumbered with a 'jack of both trades' current, a rubbish form of ac which requires transformers to be bigger than with 50hz and an ersatz dc in the traction motors of increasingly rare older stock.

 

[apk55]

I would not like to design equipment to work on 12KV DC.
To pack control equipment it in a compact under floor equipment cases in a EMU would be probably be impossible because of creapage and clearance distances involved. It is difficult enough with 3KV.
With HV AC equipment to only part that is subjected to line voltage is the transformer a compact and reliable component immersed in an oil bath for insulation and cooling. The rest of the power circuit is at lower voltage typically 750 to 1500V.

 

[Jan]

The transformer size difference between 16.7 and 50 Hz is indeed a thing, but note that as far as substation spacing is concerned, 25 kV 50 Hz interestingly enough isn't really that advantageous when compared to 15 kV 16.7 Hz: What you gain in increased efficiency through the higher voltage you lose again through higher inductive reactance losses because of the increased frequency and also inefficiencies because adjacent substations have to be separated by neutral sections, instead of being able to feed the OHLE sections together.

 

[edwin_m]

The transformer size difference between 16.7 and 50 Hz is indeed a thing, but note that as far as substation spacing is concerned, 25 kV 50 Hz interestingly enough isn't really that advantageous when compared to 15 kV 16.7 Hz: What you gain in increased efficiency through the higher voltage you lose again through higher inductive reactance losses because of the increased frequency and also inefficiencies because adjacent substations have to be separated by neutral sections, instead of being able to feed the OHLE sections together.

I think that means you're saying the 16.7Hz network is fully synchronised (at least within each country). The frequency difference means they must use some form of solid-state conversion (or obsolete rotary converters) where they take power from the 50Hz grid rather than dedicated power stations. The 25kV 50Hz system just needs a transformer, but if necessary it could adopt similar converters to get round the phase imbalance issue and take its grid supply at a lower voltage - or even be synchronised right across the network to eliminate neutral sections.

 

[AM9]

This thread seems to have drifted away from one of the prime reasons suggested by the OP that our constrained structure gauge would benefit from a lower voltage. In theory that is true but in practice, there would be very little difference between the number of overhead clearances that needed significant civils. As I indicated inpost #17, most of the the clearance stated in the TSI, even with the RSSB easements allowed when accompanied with risk assessments, are dominated by the dynamic nature of catenary supported conductor wires.
The electrical dry-air flashover distance is approximately 1mm per kV, so for a 29kV ac maximum abnormal voltage, the flashover distance would be 41mm, compared to 3mm (3kV) or to 12mm (12kV) for a unique DC system. I doubt that an additional 38mm breathing space would significantly reduce the number of structures requiring modification. Indeed, the higher current of a lower voltage system would require a much higher current capability at each feed point (about eight times for a 3kV system), and that would require more substantial safety earthing on overhead structures to prevent them rising dangerously above earth. Furthermore, a DC supply also requires greater arc snubbing.

 

[squizzler]

As I indicated inpost #17, most of the the clearance stated in the TSI, even with the RSSB easements allowed when accompanied with risk assessments, are dominated by the dynamic nature of catenary supported conductor wires.

I understood that it was due to an attempt to harmonise railway standards with broader EU laws thad define clearances for electricity at high voltage. Specifically the distance that must be allowed between the live parts and people, hence all the talk about pantograph horns and selfie sticks on the platform. The adoption of these regulations explains the GWML cost explosion.

The electrical dry-air flashover distance is approximately 1mm per kV, so for a 29kV ac maximum abnormal voltage, the flashover distance would be 41mm, compared to 3mm (3kV) or to 12mm (12kV) for a unique DC system. I doubt that an additional 38mm breathing space would significantly reduce the number of structures requiring modification.

Assuming that the vast increase in clearance is the result of a generous safety factor being applied to the flashover distances you identify, the reduction of safe clearance to 1/13th (3kV) or 1/4th (12kV) of that needed by 25kV would be most welcome.

 

[AM9]

... Assuming that the vast increase in clearance is the result of a generous safety factor being applied to the flashover distances you identify, the reduction of safe clearance to 1/13th (3kV) or 1/4th (12kV) of that needed by 25kV would be most welcome.

I don't have a figure for total dynamic clearance of 3kV, but what we are talking about is a reduction of less than 50mm in a total of 200mm or more. My point is that most of the clearance mandated is to accommodate movement of components within the high voltage envelope in use. That means that even if the voltage was 230v, the clearance would still be 150mm -so that's not really going to liberate space under many bridges that isn't already there. As an example, the GEML was electrified in 1949 by BR in accordance with the LNER plans in the late 1930s. The GE was not known for its generous structure gauge so apart from the Ilford flyover which was built with OLE clearance in mind, some of the overbridges required special measures to squeeze the wires into the available space. Take Ilford station bridge for instance, it was necessary to lower the tracks through the station meaning that to this day there are loading gauge issues there with the high platforms.
Around 1960, the system was extended into Essex using the new 1955 standard (25kV ac) to which the original 1500VDC was to be converted as well. Owing to the limited clearances, the famous 6.25kV compromise was used from Liverpool St to Shenfield and Southend, notably using the same wiring, mounting and insulators. In the '70s, the voltage was raised to 25kV but interestingly there were virtually no structural modifications to bridges, most of the changes being confined to mounting hardwareand insulator replacement. In the last 10 years, virtually all of the original OLE has been udated with F&F components similar to the GWML scheme, all virtually within the envelope that was provided for the 1949 1500VDC wiring. So that demonstrates that virtually the same requirements for clearance of 1500VDC are usable with 25kV, mainly because the dynamic clearance dominates the dimension requirements, the flashover distance being almost a minor issue.

 

[edwin_m]

This thread seems to have drifted away from one of the prime reasons suggested by the OP that our constrained structure gauge would benefit from a lower voltage. In theory that is true but in practice, there would be very little difference between the number of overhead clearances that needed significant civils. As I indicated inpost #17, most of the the clearance stated in the TSI, even with the RSSB easements allowed when accompanied with risk assessments, are dominated by the dynamic nature of catenary supported conductor wires.
The electrical dry-air flashover distance is approximately 1mm per kV, so for a 29kV ac maximum abnormal voltage, the flashover distance would be 41mm, compared to 3mm (3kV) or to 12mm (12kV) for a unique DC system. I doubt that an additional 38mm breathing space would significantly reduce the number of structures requiring modification. Indeed, the higher current of a lower voltage system would require a much higher current capability at each feed point (about eight times for a 3kV system), and that would require more substantial safety earthing on overhead structures to prevent them rising dangerously above earth. Furthermore, a DC supply also requires greater arc snubbing.

The higher current requires a thicker wire, which would also need a bit more space. Are the snubbers to do with lightning strike - which would presumably require the same mitigation regardless of line voltage?

 

[squizzler]

So that demonstrates that virtually the same requirements for clearance of 1500VDC are usable with 25kV, mainly because the dynamic clearance dominates the dimension requirements, the flashover distance being almost a minor issue.

I still think you choose not to acknowledge the impact of new electrical safety regs in all this.

The Series one on the GWEP is the butt of jokes for its over-engineered design. The movement of the contact wire and other live parts should be less than that of legacy systems, yet it is designed to greater clearances than previous projects. This difference is entirely due to the more stringent regulations.

The Great Eastern system you referred to presumably has grandfather rights and could be modified without re-doing all the bridges, just like we continue to improve the third rail system within its existing routes but can add no new ones.

 

[Greybeard33]

I think that means you're saying the 16.7Hz network is fully synchronised (at least within each country). The frequency difference means they must use some form of solid-state conversion (or obsolete rotary converters) where they take power from the 50Hz grid rather than dedicated power stations. The 25kV 50Hz system just needs a transformer, but if necessary it could adopt similar converters to get round the phase imbalance issue and take its grid supply at a lower voltage - or even be synchronised right across the network to eliminate neutral sections.

In fact the European 16.7Hz network mostly uses rotary converters where power is drawn from the 50Hz grid. Much of the network is supplied from a dedicated 16.7Hz grid with single phase transmission lines. This has its own railway-only power stations that generate power directly at 16.7Hz, as well as large converter stations.

Rotary converters use mechanical inertia to smooth out the pulsating power demand from the single phase load and so present a smooth, balanced load to the 3-phase 50Hz grid. This can be more economical than the large smoothing capacitors needed in a solid state converter. The 1995 frequency change from 16_2⁄3Hz to 16.7Hz was because asynchronous rotary converters are used to avoid synchronising the two grids, and these machines do not like the output frequency being an exact sub-multiple of the 50Hz supply frequency for long periods.

https://en.m.wikipedia.org/wiki/15_kV_AC_railway_electrification gives more detail.

Solid state 3-phase to DC converters do not need large smoothing capacitors, unlike 3-phase to single phase of whichever frequency, and so are much more cost-effective than rotary AC to DC converters.

 

[AM9]

In a totally different environment I've come across 400Hz three phase generators that when presented with a 'lumpy' load, tended to have chunks pulled out of their slip rings. Maybe the input side motor, (which I would imagine would be a highly-tuned induction motor), would suffer similar damage as the loading would produce a continuous ripple in the magnetic field at the same angles.

 

[AM9]

I still think you choose not to acknowledge the impact of new electrical safety regs in all this.

This is very much in line with the 'new electrical safety regs', by which I think you mean the RSSB's implementation of the EU TSIs, - see below.

The Series one on the GWEP is the butt of jokes for its over-engineered design. The movement of the contact wire and other live parts should be less than that of legacy systems, yet it is designed to greater clearances than previous projects. This difference is entirely due to the more stringent regulations.

Are these 'jokes' coming those who will be responsible for dealing withfailures duribng the service life of the installation or are they just irrelevant chit-chat?
The F&F derived Series One is designed for 140mph running under with single pantographs and 125mph with two. Once ETCS is fully deployed on the GWML, this installation will be tested. Far better than the flaky 'save money at all cost' systems of MKIIIa & b headspans like the ECML and the MML, which have cost millions in both maintenance and consequential damages in recent years. With the railway making more intensive use of infrastructure, and the commercial cost of it failing in such service, only a fool would say lets do a cheap job, and maybe with a run of good luck it won't fail that often.
The F&F system installed onthe GEML uses some components common to the Series 1, mainly to reduce the footprint of the live components. That is a sensible move that doesn't require any more headroom under structures.

The Great Eastern system you referred to presumably has grandfather rights and could be modified without re-doing all the bridges, just like we continue to improve the third rail system within its existing routes but can add no new ones.

The Reduced static clearance for 25kV conductors is 269 to 200mm and the special reduced clearance is 199 to 150mm. In my view, the '70s conversion of the GEML from 6.25kV to 25kV would comply with this now, so using the latest requirements (Oct 2001), the GE system is compliant. The reduced standards are not 'grandfather rights', they are only applicable if case-specific risk assessments are undertaken (of course with an acceptable finding of the assessment). That would also mean that the majority of infrastructure in the UK would need no more civils whether wired with 25kV or 3kV.

 

[Greybeard33]

In a totally different environment I've come across 400Hz three phase generators that when presented with a 'lumpy' load, tended to have chunks pulled out of their slip rings. Maybe the input side motor, (which I would imagine would be a highly-tuned induction motor), would suffer similar damage as the loading would produce a continuous ripple in the magnetic field at the same angles.

Yes, as I understand it the rotary converters utilise 3-phase slip ring induction motors. According to http://www.bahnstrom.de/bahnstromsysteme/home.htm, the issue was that, at zero slip frequency (synchronous operation) the resultant unbalanced DC field current in one rotor phase caused slip ring and brush damage, and overheating. At 16.7Hz output, the field current slowly rotates around the three phases.

In a non-rail field I encountered a 400Hz single phase generator that had an unexpectedly high failure rate, due to the cyclic vibration causing bearing wear.

Single phase AC is not kind to large electrical machines, but the worldwide rail industry has acquired plenty of experience in mitigation measures, at both 16.7Hz and 50Hz.

 

[squizzler]

In fact the European 16.7Hz network mostly uses rotary converters where power is drawn from the 50Hz grid. Much of the network is supplied from a dedicated 16.7Hz grid with single phase transmission lines. This has its own railway-only power stations that generate power directly at 16.7Hz, as well as large converter stations.

I reacquainted myself with the 15kV system from the Wiki link you provided. The choice of a hokey current which necessitates an entire parallel grid system with its own power stations, such is the awkwardness of deriving it from the mains. And, according to the article, requires the transformer to be three times as massive. If it were not true it would be difficult to make such a thing up.

Actually I think it barely qualifies as AC, its more like a sort of DC that is unable to choose what polarity it wants to be.

 

[squizzler]

One other thing about dc electrification that might not appear in a business case is the export potential. SNCF are in a good position because they already sold the 25kV system globally, but English Electric originally sold the South Africans their 50kV AC system. A DC system of 12, 24 or even 48kV should be ideal for when the USA and Canada need to electrify their transcontinental routes. There are bound to be markets for that technology outside North America too.

 

[AM9]

I reacquainted myself with the 15kV system from the Wiki link you provided. The choice of a hokey current which necessitates an entire parallel grid system with its own power stations, such is the awkwardness of deriving it from the mains. And, according to the article, requires the transformer to be three times as massive. If it were not true it would be difficult to make such a thing up. ...

ISTR that when the system was first launched in the early years of the last century, available induction motors (which were essentially DC designs), were unable to operate at industrial ac supply frequencies, and the options for rectifying an ac supply to create DC* just wasn't viable for on-train use. It would have involved motor-generator sets that would have brought an unacceptable weight and size penalty. The practice of converting to an ac frequency that was within the range of the motors was taken. This meant that static rotary converters could be sited trackside and the penalty was just a very inefficient (but just tolerable) transformer on board.
If you take the date into consideration, I wouldn't call that "difficult to make such a thing up".
* There were no semiconductor rectifiers of sufficient voltage and current ratings until the 1970s so the only non-rotating device available was the mercury-arc rectifier, which was highly unsuitable for use in a railway vehicle.

 

[edwin_m]

One other thing about dc electrification that might not appear in a business case is the export potential. SNCF are in a good position because they already sold the 25kV system globally, but English Electric originally sold the South Africans their 50kV AC system. A DC system of 12, 24 or even 48kV should be ideal for when the USA and Canada need to electrify their transcontinental routes. There are bound to be markets for that technology outside North America too.

Why would they want a different system? They have no DC to be compatible with, and 25kV kit is readily availalble and indeed adopted by most of the few recent and current electrification schemes in North America. The autotransformer system is equivalent to 50kV in terms of losses but maintains 25kV at the pantograph, which we have already established is not significantly different from lower-voltage systems in terms of clearance.

ISTR that when the system was first launched in the early years of the last century, available induction motors (which were essentially DC designs), were unable to operate at industrial ac supply frequencies, and the options for rectifying an ac supply to create DC* just wasn't viable for on-train use. It would have involved motor-generator sets that would have brought an unacceptable weight and size penalty. The practice of converting to an ac frequency that was within the range of the motors was taken. This meant that static rotary converters could be sited trackside and the penalty was just a very inefficient (but just tolerable) transformer on board.
If you take the date into consideration, I wouldn't call that "difficult to make such a thing up".
* There were no semiconductor rectifiers of sufficient voltage and current ratings until the 1970s so the only non-rotating device available was the mercury-arc rectifier, which was highly unsuitable for use in a railway vehicle.

I agree, but I'm pretty sure solid state rectifers were a bit earlier than the 70s. According to Wikipedia Class 85 built from 1961 onwards had germanium rectifiers, later replaced (date not given) by silicon.

 

[squizzler]

Why would they want a different system?

I would argue there is not enough electrification to be committed yet to any particular system when it comes to wiring the transcontinentals and other 'class ones'.

They have no DC to be compatible with, and 25kV kit is readily availalble and indeed adopted by most of the few recent and current electrification schemes in North America.

Actually I think you will find lots of dc in the New York area - check out the GE P32 AC DM loco which has shoe gear for 750v dc. I might however agree the USA's days as a technological innovator are behind it, which is a great opportunity for EU countries such as ourselves to sell them stuff.

 

[AM9]

Why would they want a different system? They have no DC to be compatible with, and 25kV kit is readily availalble and indeed adopted by most of the few recent and current electrification schemes in North America. The autotransformer system is equivalent to 50kV in terms of losses but maintains 25kV at the pantograph, which we have already established is not significantly different from lower-voltage systems in terms of clearance.


I agree, but I'm pretty sure solid state rectifers were a bit earlier than the 70s. According to Wikipedia Class 85 built from 1961 onwards had germanium rectifiers, later replaced (date not given) by silicon.

Yes there were germanium rectifiers available but they were expensive, of limited voltage/current and under operational conditions, liable to thermal runaway/breakdown. The silicon rectifiers were much more robust, despite having a higher dissipation owing to a higher forward voltage drop. They also rapidly became affordable as commodity devices so were accepted for general use.

 

[etr221]

I would argue there is not enough electrification to be committed yet to any particular system when it comes to wiring the transcontinentals and other 'class ones'.

Actually I think you will find lots of dc in the New York area - check out the GE P32 AC DM loco which has shoe gear for 750v dc. I might however agree the USA's days as a technological innovator are behind it, which is a great opportunity for EU countries such as ourselves to sell them stuff.

The surviving 'main line' electrification in the USA is the 'North East Corridor' (and branches), originally electrified from New York to Washington by the Pennsylvania RR, and from NY to New Haven by the New Haven, both at 11kV 25Hz;now extended to Boston at by Amtrak at 25kV 60Hz (American industrial frequency). It is all now owned by Amtrak and state commuter rail authorities, and the 25Hz lines are being converted to 60Hz at either 12.5kV or 25kV - I don't know how far/fast this is proceeding. Out of New York there is suburban electrification 3rd rail c750v dc, from Penn Station onto Long Island; and from Grand Central Terminal on the ex NYC lines north. As both the NY terminals (and approach lines) are underground, there has long been electric working out from them - initially with loco changes, more recently with diesel-electric-electric locos, able to use electric only in third rail areas. But all this is now passenger only - electric freight operation ceased a while back; and other 'steam railroad' electrification schemes ended.

The most recent electrification proposals I have seen have been 60Hz ac at either 25 or 50kV - either for high speed rail or freight - but an issue over there, beyond the economics, is the need for clearances for 'double stack' container operation.