High Voltage Direct Current electrification

 
  Myrtone Chief Commissioner

Location: North Carlton, Melbourne, Victoria
Sometimes, when railway electrification is brought up in discussions, the advantages of 25kv standard frequency (50Hz) is mentioned.
Currently the highest DC voltage in use is 3kV, which traditionally required pairs of 1,500 volt motors permanently connected in series. I don't know if that still applies to newer 3kV rolling stock, or whether that instead has something called a DC-to-DC converter.
Traditionally, DC systems required the line voltage to be the same as the voltage of the traction motors, except for the 3kV DC systems where it was double, motors permanently connected in series. For now, bear in mind that 3kV DC is both the weighted average (R.M.S) and the peak voltage.

D.C systems take three-phase power from the national grid, convert it to six phase at a lower voltage, and then rectify it, all phases loaded evenly and power delivery is constant.

A.C allows a higher voltage than the traction motors, 25 kV root mean squared in case of standard frequency electrification, with a transformer on-board stepping down the voltage, which is then rectified. But single wire overhead with rail return means it's only single phase, each section of a 25kV A.C network is typically fed with one of the three phases of the national network, with phase breaks between different sections. Additionally, being fed straight off the national grid means that the frequency is the same as the national grid, 50Hz in this country, meaning that transformers on board need to be quite large. And after rectification, being single phase at standard frequency, the ripple filters also need to be quite large.

Let me introduce something called a DC-DC converter. Just as a transformer changes A.C voltage, a DC-DC converter does the same with DC voltage. Like transformers, they can also provide dielectric isolation. Any transformer is designed to change the voltage of A.C at some given frequency, and when used with A.C at a frequency any less than that, it becomes more like an inductor, and most definitely an inductor on D.C.
The first basic principle of a DC-DC converter is that switching a D.C supply off an on repeatedly, a transformer can be made to work without an external A.C source. The faster the switching, the greater the transformer's power rating can be in relation to size and weight. The primary coil of a transformer might be integrated into a tank circuit with the power switched on and off repeatedly, connections to the tank circuit being reversed between pulses of current.
The second principle is reversing the connections from the secondary coil every time the secondary current changes direction, see here.

It is now technically possible to use DC-DC converters instead of classic transformers to step down the voltage, this meaning that new D.C electrification could also have a line voltage higher than what's used internally. This would combine the advantages of high voltage A.C (such as greater distance between substations) with the constant power delivery and even phase loading of D.C electrification.
On-board conversion of D.C would not be limited to one phase, so power delivery can be kept constant, and at a frequency higher than the mains, say 400Hz, so a smaller and lighter transformer could be used, and the higher frequency and phase order also makes the rectified current easier to filter.

In fact, a 30kV DC system could use smaller insulators than are needed with 25kV A.C, the peak voltage of the latter, rounded to the nearest volt is 35,355.

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  YM-Mundrabilla Minister for Railways

Location: Mundrabilla but I'd rather be in Narvik
Think I'll stick with steam - 'lecky' is a bit complicated for me.............................Smile
  Pressman Spirit of the Vine

Location: Wherever the Tin Chook or Qantas takes me
And where did you get your Electrical Engineering degree again?
  LancedDendrite Chief Commissioner

Location: Gheringhap Loop Autonomous Zone
Nurse!
  ed31880 Train Controller

Ouch. My head hurts.
  Valvegear Oliver Bullied, CME

Location: Norda Fittazroy
I'm with YM-Mundrabilla on this. I'm perfectly happy to shovel coal and keep a careful eye on the gauge glass.
  dthead Site Admin

Location: Melbourne, Australia
And where did you get your Electrical Engineering degree again?
Pressman
@Mytone, Seriously, where did the information come from?  It would be prudent to publish some links.
  Boss Chief Commissioner

Location: Caulfield Line
Try googling dc-dc converters

http://https://en.wikipedia.org/wiki/DC-to-DC_converter
  georges Train Controller
  RTT_Rules Oliver Bullied, CME

Location: Dubai UAE
25kVAC is currently the world standard for new networks and lines where O/H supply is available, the argument should probably end there.

Qld and Sth Africa run 15,000t coal trains with 3 locos totaling about 15MW with more than one train on the tracks at a time. 15,000kW at 25kVA = 600amps, the slightly higher voltage while DC offers little reduction in cabling costs.

AC supply sub stations are less complex than DC units and hence cheaper and can be spaced further apart than lower voltage systems. 30000VDC doesn't offer any benefits against this and now you need a rectifier to boot.

Even skinny gauge Qld has no issue in fitting 4000kW of transformer and electrical equipment.
  Myrtone Chief Commissioner

Location: North Carlton, Melbourne, Victoria
25kVAC is currently the world standard for new networks and lines where O/H supply is available, the argument should probably end there.
RTT_Rules
I'm not sure what you mean by the part after 'and' but 25kV 50 or 60Hz is only the new standard for non-metro heavy rail electrification, it is not used on metro rail that is separate from mainline railways, even the few with overhead wires. Also, the standards for new electrified networks have changed over time. When our suburban rail network and Sydney suburban were electrified, the most common standard for new electrified networks was 15,000 volts D.C, I think. Some larger networks had 3kV D.C or even a higher voltage A.C at lower than standard frequency.
This changed in the 1950s when 25kV standard frequency was introduced, I think in France.

You have made the point elsewhere about technology changing over time and the need to keep up with the times, why not with electrification standards?

Qld and Sth Africa run 15,000t coal trains with 3 locos totaling about 15MW with more than one train on the tracks at a time. 15,000kW at 25kVA = 600amps, the slightly higher voltage while DC offers little reduction in cabling costs.
RTT_Rules
As noted above, 25kvAC is 25kV R.M.S. 600amps is also 600amps R.M.S, the peak current being slightly more than 848.5 amps.
That same wattage at 30kvDC would be 500amps, both the R.M.S and peak current in this case.

AC supply sub stations are less complex than DC units and hence cheaper and can be spaced further apart than lower voltage systems. 30000VDC doesn't offer any benefits against this and now you need a rectifier to boot.
RTT_Rules
Even with High voltage A.C, you need a rectifier onboard, and quite a large transformer. 30kV D.C does offer benefits as mentioned above. With A.C, as noted, there must be different sections on different phases, with phase breaks between them. D.C does not need this, and it delivers constant power - no pulsation.

Even skinny gauge Qld has no issue in fitting 4000kW of transformer and electrical equipment.
RTT_Rules
Remember that commercial jet planes are larger than every carriage on a train, and they have A.C electrical systems with a utility frequency of 400Hz, apparently to minimise the size of much less powerful transformers.

Or many modern household electronic appliances actually used mains voltage rectifiers, filters and DC-DC converters like the ones mentioned above, rather than mains fed transformers. If modern mains powered electronics can have it, what about modern high voltage rolling stock. If you can use DC-DC converters instead of classic transformers to step down the voltage, then the electrification can be D.C rather than A.C.
  TheMeddlingMonk Deputy Commissioner

Location: The Time Vortex near Melbourne, Australia
Currently the highest DC voltage in use is 3kV, which traditionally required pairs of 1,500 volt motors permanently connected in series. I don't know if that still applies to newer 3kV rolling stock, or whether that instead has something called a DC-to-DC converter.

Traditionally, DC systems required the line voltage to be the same as the voltage of the traction motors, except for the 3kV DC systems where it was double, motors permanently connected in series. For now, bear in mind that 3kV DC is both the weighted average (R.M.S) and the peak voltage.
Myrtone

In terms of Melbourne's suburban rollingstock, only the Comeng units have DC traction motors. They are a pain to service and, as a passenger, you can feel the series resistances being switched in and out to change the voltage to the motors as the train takes off (this is why they initially jolt).

The newer X'trapolis and Siemens trains have AC traction motors. These use an inverter to convert the DC from the overhead into variable-frequency AC - much easier to service and much smoother control.

The preference is to go towards AC overhead (not that that's going to happen in Melbourne anytime soon). My understanding is that it is safer and easier to isolate than DC. Infrastructure is cheaper (as RTT_Rules mentioned, substations are simpler and can be further apart). As a result, rollingstock will be fitted with a rectifier, then an inverter (the advantage being that this allows the variable frequency, whereas AC overhead direct to AC traction motors does not).
  Myrtone Chief Commissioner

Location: North Carlton, Melbourne, Victoria
In terms of Melbourne's suburban rollingstock, only the Comeng units have DC traction motors. They are a pain to service and, as a passenger, you can feel the series resistances being switched in and out to change the voltage to the motors as the train takes off (this is why they initially jolt).
TheMeddlingMonk
They are the last, actually. Many other electric trains elsewhere dating from around the same time as the Comeng units also have D.C motors, but chopper controlled.

The newer X'trapolis and Siemens trains have AC traction motors. These use an inverter to convert the DC from the overhead into variable-frequency AC - much easier to service and much smoother control.
TheMeddlingMonk
Actually, judging from the electronic sounds they make, they have choppers as well as inverters, varying both the frequency and voltage.

The preference is to go towards AC overhead (not that that's going to happen in Melbourne anytime soon). My understanding is that it is safer and easier to isolate than DC. Infrastructure is cheaper (as RTT_Rules mentioned, substations are simpler and can be further apart). As a result, rollingstock will be fitted with a rectifier, then an inverter (the advantage being that this allows the variable frequency, whereas AC overhead direct to AC traction motors does not).
TheMeddlingMonk
The preference on mainline type heavy rail has been to go towards A.C overhead at a higher voltage that any D.C system. A.C electrification is only used where the line voltage is higher that what the trains use internally, so that a classic transformer can step down the voltage on-board.

This transformer is mains fed, and at standard frequency, and given its power rating, ought to be quite large. With only single wire overhead, the A.C supply is only single phase, so the rectification filters are also quite large.

High voltage D.C electrification would use a fixed frequency inverter to convert the high voltage supply into polyphase at a frequency higher than the mains, maybe 400Hz, step it down with a transformer, then rectify the secondary current, which being polyphase and at a higher than standard frequency, would be extremely easy to filter.
  LancedDendrite Chief Commissioner

Location: Gheringhap Loop Autonomous Zone
I don't know what rubbish they're teaching kids in engineering classes these days, but here's one thing I learned:

It's about solving your client's problem, not imaginary ones.

Let me explain this with a parable:

The boss tells me to start preparing a Request for Tender for electrifying an interurban railway line that has no connections with any existing electrified rail network. In my infinite wisdom (and because it's 4PM on a Friday), I decide to let someone else have some fun by getting a pair of Cadet Engineers to look at the problem whilst I deal with the other half a dozen RFTs that I also have to help out with. I give them until the end of next week to get back to me.

So I'm having morning tea on Friday when the cadets come over to me with their ideas. Our young cadet Myrtone is so excited that he looks like he's about to drop a fusible plug, so I let him go first:

"I've got this great proposal for you, boss! Advances in power electronics mean that we can build the most efficient, economical electric railway in the world! I think we should use 30kV DC as the basis for the RFT. Because we can now use compact SiC-based rectifiers, we could use pole-mounted DC traction substations so the infrastructure footprint is really small! Trains with AC overhead power supply and AC traction motors have an inefficient 2-step power conversion process (AC -> DC -> weird traction AC), but using solid-state DC-DC converters and hybrid Medium-Voltage DC circuit breakers for onboard protection means that we can eliminate the rectification step! Look, by getting rid of AC we don't have any reactive power to worry about and the whole thing's so efficient because steady-state peak current draw is the same as RMS current draw! AND THERE'S NO PHASE BREAKS!"

I decide to cut him off before he starts talking about the set of steak-knives he's going to throw into the deal and listen to the other cadet, Dave:

"Because we're starting from scratch and have a reasonably long railway line, we should base the RFT around a 25kVAC scheme. Every multi-national rail electrification vendor on the planet has a 25kVAC product that they can base their bid around, so we should be able to get a really good deal on just about every major procurement contract."

I thank both cadets for the excellent effort they both put in.

I then hand Myrtone a glossy brochure for post-graduate EE courses at my old Uni and politely suggest that he might find working in a lab more comfortable.

(Postscript: 3 weeks later I receive a letter from HR advising me that a cadet has made a formal complaint alleging that giving them career advice was in fact 'workplace bullying' and that I'm going to have to submit to them a stat-dec about it or else I will be immediately stood down)
  drichardson Beginner

Location: Frankfurt, Germany
Modern train traction systems work with the following basic architecture:

(1) Internal DC bus
(2) An input package that either rectifies the AC input to the DC voltage required by the bus or does a DC-DC converter to get the required voltage (or is a diesel generator with a rectifier producing the DC directly)
(3) Inverters to produce the required phases, frequency and voltage for the traction motors

The DC bus is optimized for power transmission in a confined space and a short distance, which means low voltage and high current (lower voltage means smaller insulators and less arcing). The overhead, in comparison, must be optimized for long distances, which means high voltage and low current (low current means low energy loss during transmission). So, changing the overhead to HVDC doesn't actually avoid the need to have (2) in a locomotive which means that there isn't much incentive to change to it.

There is also the commercial factor- as long as both China and the EU chose 25kVAC for their new projects, all of the global suppliers will maintain off-the-shelf products and teams of experienced engineers to work with it. A railway would need a compelling argument to give up this advantage and choose something new.
  Myrtone Chief Commissioner

Location: North Carlton, Melbourne, Victoria
Another cadet comes along says:

"Actually, a 30kV D.C electric train set wouldn't be that much different from the same model train running on 25kV A.C, the input package would be different, but pretty much all else would be the same. The internal DC bus and the traction inverters would be the same, for example. Although there aren't yet any examples, I'm sure it could still be built with off-the-shelf components."
  apw5910 Deputy Commissioner

Location: Location: Location.
(Postscript: 3 weeks later I receive a letter from HR advising me that a cadet has made a formal complaint alleging that giving them career advice was in fact 'workplace bullying' and that I'm going to have to submit to them a stat-dec about it or else I will be immediately stood down)
LancedDendrite
I feel your pain.
  Myrtone Chief Commissioner

Location: North Carlton, Melbourne, Victoria
The cadet continues:

"When France's S.N.C.F adopted 25kV A.C between Aix-les-Bains and La Roche-sur-Foron in southern France in 1953, they were in the same position as we would be adopting 30kV D.C. They were using a standard that no one else had at that time. The standard for new electrification projects has changed over time. If the advent of rectifiers on board trains made standard frequency A.C feasible, where previously either D.C with D.C motors or low frequency A.C with universal motors would have been used, then high voltage inverters on board trains should make way for D.C electrification. Only D.C electrification loads phases evenly, and D.C is the only way to deliver constant power with single wire overhead. And we need DC-DC converters to make it work at higher voltages than 3kV, we didn't have back then, now we do."
  M636C Minister for Railways

25kVAC is currently the world standard for new networks and lines where O/H supply is available, the argument should probably end there.

Qld and Sth Africa run 15,000t coal trains with 3 locos totaling about 15MW with more than one train on the tracks at a time. 15,000kW at 25kVA = 600amps, the slightly higher voltage while DC offers little reduction in cabling costs.

AC supply sub stations are less complex than DC units and hence cheaper and can be spaced further apart than lower voltage systems. 30000VDC doesn't offer any benefits against this and now you need a rectifier to boot.

Even skinny gauge Qld has no issue in fitting 4000kW of transformer and electrical equipment.
RTT_Rules
Umm...

The mighty State of the Art line of 1975 in South Africa (and/or Namibia) was the Sishen Saldanha line electrified at 50kV.

They got the latest technology from the UK at the time.

The locomotives had a dramatically lowered roof line under the huge pantographs (and huge insulators) to keep the sparks separate from the rails.

Photos of this line that I've seen lately have diesels assisting the electrics.

I think at least some of the old Brit locos are in service and there is one more recent 50 kV class with basically the same specs as the 1975 units...

I think something was said about not being able to afford to upgrade the overhead and substations to allow all electric trains of the size required to run and since the demand exceeds the supply, every train uses a combination of electric and diesel power...

This must have something to do with it being a non standard system, since the 25kV elsewhere in Zuid Africa seems to operate normally.

M636C
  Myrtone Chief Commissioner

Location: North Carlton, Melbourne, Victoria
The mighty State of the Art line of 1975 in South Africa (and/or Namibia) was the Sishen Saldanha line electrified at 50kV.
M636C
And this would have been the weighted average, as noted above.

They got the latest technology from the UK at the time.
M636C
And this was in spite of the apartheid. South Africa wasn't able to import oil because of that regime.

The locomotives had a dramatically lowered roof line under the huge pantographs (and huge insulators) to keep the sparks separate from the rails.
M636C
I made a calculation and the peak voltage, to a third of a volt would be a whopping 70,7102/3volts...

This must have something to do with it being a non standard system, since the 25kV elsewhere in Zuid Africa seems to operate normally.
M636C
But a 50kV locomotive can surely have identical traction equipment to a 25kV, only the transformer would be different.

This ABB document describes a High Voltage D.C circuit breaker, and this is for voltages of hundreds of kilovolts.

It opens with the paragraph:

Existing mechanical HVDC breakers are capable of interrupting HVDC currents within several tens of milliseconds, but this is too slow to fulfill the requirements of a reliable HVDC grid. HVDC breakers based on semiconductors can easily overcome the limitations of operating speed, but generate large transfer losses, typically in the range of 30 percent of the losses of a voltage source converter station.

So mechanical H.V.D.C circuit breakers do exist for voltages as high as hundreds of kilovolts, and can interrupt circuits in a fraction of a second.
So if a mechanical D.C circuit breaker can interrupt hundreds of kilovolts in "several tens of milliseconds", could a 30kV D.C circuit breaker of any kind interrupt a 30kV circuit in less than that time.
  M636C Minister for Railways


This must have something to do with it being a non standard system, since the 25kV elsewhere in Zuid Africa seems to operate normally.
But a 50kV locomotive can surely have identical traction equipment to a 25kV, only the transformer would be different.

This ABB document describes a High Voltage D.C circuit breaker, and this is for voltages of hundreds of kilovolts.

It opens with the paragraph:

Existing mechanical HVDC breakers are capable of interrupting HVDC currents within several tens of milliseconds, but this is too slow to fulfill the requirements of a reliable HVDC grid. HVDC breakers based on semiconductors can easily overcome the limitations of operating speed, but generate large transfer losses, typically in the range of 30 percent of the losses of a voltage source converter station.

So mechanical H.V.D.C circuit breakers do exist for voltages as high as hundreds of kilovolts, and can interrupt circuits in a fraction of a second.
So if a mechanical D.C circuit breaker can interrupt hundreds of kilovolts in "several tens of milliseconds", could a 30kV D.C circuit breaker of any kind interrupt a 30kV circuit in less than that time.
Myrtone

I don't think there is a problem with the locomotives and I agree the transformer would be the main change.

The problem is with the supply.

The line runs through desert in the west of Africa.

I think the cost of providing more feeders and substations on a long isolated line is the problem, maybe even finding a couple more power stations in the right area...

It might have been better just to run diesels only, although a couple of 4MW electric locomotives per train will cut the fuel usage. The diesels might just be used to get the train moving and then throttle back to let the electrics do most of the work.

But I don't claim to know what's going on.

It is a non standard system with more difficulties than the 25kV system elsewhere in South Africa.

I think 50kV was selected because it was an isolated line with long distances to civilisation (and power stations).

But it hasn't been a big success technically or operationally...

I've heard that Aurizon run diesel trains on the Blackwater system because the power supply won't meet the maximum demand were all trains to be electric. It is just that they run trains with diesel or electric power, not both.

The electric trains are faster since the locomotives are more powerful. The have started running four diesels on a train although three could haul it, to keep the speeds up on hills, so as not to delay the electric trains.

M636C
  Pressman Spirit of the Vine

Location: Wherever the Tin Chook or Qantas takes me
I do wonder if cadet Myrtone can supply information on the extra size of, and extra space required for arc suppression that a 30kVdc circuit breaker requires of a 25kVac circuit breaker.
I do wonder if he realises the differences in the breaking arc characteristics of a DC Circuit Breaker compared to an AC one?
  LancedDendrite Chief Commissioner

Location: Gheringhap Loop Autonomous Zone
Another cadet comes along says:

"Actually, a 30kV D.C electric train set wouldn't be that much different from the same model train running on 25kV A.C, the input package would be different, but pretty much all else would be the same. The internal DC bus and the traction inverters would be the same, for example. Although there aren't yet any examples, I'm sure it could still be built with off-the-shelf components."
Myrtone
  tazzer96 Deputy Commissioner

DC supply means corrosion in the wires, slightly greater resistance than AC.
  RustyRick Chief Commissioner

Location: South West Vic
I normally just read Myrtone's posts when I need a shot of fiction.

Here's another rabbit hole to go down. Electrolysis. Specifically the effect of stray DC currents on other infrastructure. 1500 VDC is bad enough. Stray currents from 30,000 VDC would be a nightmare to control. Yes, there are ways to mitigate the risk like insulating the rails as per the Box Hill tram extension, but members of the Victorian Electrolysis Committee would not be happy. A change to 25 kVAC would see a dramatic reduction in stray currents. See Victorian Electrolysis Committee Resource Manual on http://www.esv.vic.gov.au.

And - I step back and see what left field ideas are thrown up.

Rick

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