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Distributed rectification for third rail

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HSTEd

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Well, for a long time now I have been talking about how advances in the design and performance of power electronics would effect the economics of third rail electrification and now it is time for me to finally write up one of my proposals.

This is actually a rather simpler proposal than I originally had in mind because I realised that the alternative to providing a fancy booster electronics to draw return current out of the rail and thus increase the effective supply voltage was rather unnecessary when we can simply provide more substations.

The normal cricitism of third rail technologies is that they normally require substations at relatively close spacings of only a few kilometres to keep supply currents and voltage drops within reason.
This tends to increase the capital cost of the installation relative to spacings of 50km or more that can be obtained in 25kV systems.
There are two approaches to try and keep the voltages under control - one is to attempt to reduce the loop impedance of the circuit by paralleling cables with the running rails (far more effective than doing so with the conductor rail as the running rails have a far higher starting impedance).
The second approach is to simply move the substations closer together.

Traditional 1MW+ substations are simple voltage source systems that require HV type supplies in the rectifier hut with all the relevant grounding and other infrastructure that comes with that. Indeed Network Rail operates a large network of 33kV feeder cables specifically to provide supply to its traction system.
Portland Streetcar in the United States pioneered the concept of massing small substations with a power of ~300kW that could be supplied by the low voltage network. This drastically reduced the size and complexity of the substations - and effectively their cost.

This approach is the one that I intend to apply in this example.
For the purposes of compiling estimates I have assumed that the specimen train to be used is a Class 319 with a maximum traction load of approximately 1000kW delivered. These trains are available in substantial quantity and would likely be the workhorse of any near-term third rail extension - should one be authorised.

The third rail standards document used in this example is a copy of Railway Group Standard GM/RT1001 Issue 1 'Classic 750V d.c third rail Electrification sSystem and T&RS parameters to ensure Interworking'. Although this document dates from the mid-1995 and has obviously been withdrawn it was the copy I have to hand and is thus instructive in outline, if not in detail.
As this standard dates to after the introduction of the Class 319 it seems highly likely that the train will function properly if the supply is kept within the parameters laid out within it.

Example Line - modelling assumptions
The system is modeled as twin-track plain line without junctions, however the running rails and conductor rails are all paralled together respectively at intervals of 100m or so, allowing the cross bond resistance to be neglected and both conductor rails and running rails to be modelled as entirely shorted together. This is estimated based on the Delta Rail report DeltaRail*ES*2010*003 - 'Low Cost Electrification for Branch Lines' to cost roughly £5k/single track km. Or £10km/route-km.
The system utilises high conductivity 18kg/m aluminium-stainless steel conductor rails and twin-rail traction current return, the cost of instalaltion of which varies between ~£50k/track-km (in the case of utilising existing sleepers modified for conductor rails and insulated chairs) and ~£500k/track-km (in the case of total resleepering and ballast renewal).

That takes our track preparation costs to somewhere in vicinity of £110k-£1m/route-km depending on if the existing sleepers and ground conductivity conditions produce acceptable stray-current performance. It is clear that the size of this cost in the specific case to be considered will be the making or breaking of the business case.
All trains on the route are considered to consist of a single Class 319 unit with 1000kW of installed traction power. This represents a significant increase in comparison with two or three vehicle diesels, many with shorter vehicles, that are often utilised on many of the exemplar routes in the North West of England and elsewhere.
Due to the lightly trafficked nature of the route regeneration is assumed to be disabled, as permitted by the Group Standard above.

As all conductor and running rails are paralleled at regular intervals the overral route loop-resistance between a substation and train-load can be modelled as approximately 13 milli-ohm/km, as described in the Railway Group Standard referenced above.

Substations are placed at 1000m intervals and consist of 300kW DC-power variable-voltage substations, but it is instructive to describe the operating mode of this substation design here.
It will operate in a constant voltage mode at an output voltage of approximately 850V until total substation DC-power reaches approximately 300kW, and this time the output voltage of the substation will drop as the current increases such that the substation does not overload and continues to output 300kW.

Worst case load model

As the trains consist of 1000kW-max point loads the worst case scenario that can be envisaged is that of two units accelerating away from a single station in opposite directions. As this is only likely to occur in a station, which would be the likely location of a substation, it will be assumed that the 2000kW total load is positioned at a substation.
It is further assumed, for the purpose of iteration, that the voltage at the pick up shoes of the units is 825Vdc [it is much simpler to calculate this and determine if your substation voltage is sufficient than the reverse].
In order to provide 2000kW at 825Vdc it is necessary for the traction systems on the two units to require a total of 2424A.
As the local substation is not anywhere near capable of producing this the substation output voltage will sag in a controlled manner [to 825V in this example] and the substation will produce 364A. This will cause approximately 2060A to be drawn to the units through the conductor/return rails from the directions of the adjacent substations.
Assuming the substations are evenly spaced and have similar loop impedances 1030A will be drawn from both directions.
[This case is also useful for determining the end-of-line condition with a single set, as a train is highly unlikely to be accelerating at full power into the buffer stops!]

It is 1000m to the next station, generating a loop impedance to the next station of approximately 13 milli-ohm.
That means that 13.4V will drop between the central substation and the first one working outwards in each direction [as the system is symmetric]. This means the voltage seen by this substation is 839Vdc
As the substation can only provide 358A at that voltage the remaining 672A will flow from further along the conductor rail.
It is 1000m to the second substation out, at 13 milli-ohm impedance a further 8.74V will drop. This means the second substation sees a rail voltage of 848Vdc. At this voltage level the substation is capable of providing 354A - as such the remaining 318A will flow along the conductor rail from the third and final substation.

The last 13 milli-ohm 1000m section sees a voltage drop of 4.13V drop, taking the voltage drop at the final rectifier to ~853Vdc, with the substation being capable of providing the entire remaining current requirement. THis requires the substation to produce roughly 272kW of DC-power.
853Vdc is well within the rating of the RGS standard, indeed an 855Vdc substation voltage would still provide substantial opportunity for regeneration if it was enabled on route as the maximum permanent voltage in the standard is 900Vdc.

In this worst-case situation there are 7 substations involved in supplying the 2000kW of traction power to the two units in question. 5 operate at their full rated 300kW power whilst the remaining two produce 272kW. That translates to a total DC power of 2044kW.
~600V chopper rectifiers in industry have been demonstrated to have efficiencies of at least 97.5% - which will be covered later.
This means that something approaching ~2100kW of AC power is required to produce the 2000kW of traction power. An overall efficiency of ~95% in this very severe scenario. THis is comparable to AC systems and demonstrates the advantages of providing current to a train from close by - even if the entire train supply cannot be handled in such a manner.

Provision of electricity supplies
Clearly this approach is radically different to that which is normally pursued in the creation of such substations.
Because of the physically greater number of substations required for this technique, one per route-km, it is necessary to reduce the cost of each one significantly.
One way of reducing costs, as mentioned previously, is to take a supply from the electricity utility at Low Voltage. This is beneficial as an electricity utility normally has lower costs associated with maintenance of HV equipment and transformers due to not requiring rail qualified staff and due to economies of scale that come with operating virtually all the substations in a given area.
The examples from the Electricity Northwest document 'Statement of Methodology and Connection Charges (5 December 2016)' were used to formulate approximate values for the cost of providing multiple ~350kVA LV supplies at positions scattered along the railway. These are not exact by any means but merely attempt to provide a conservative estimate of the costs. AS these improvements are required to supply the customer (the railway) with a minimum service, the railway would be required to pay for the required works, either by the utility or by another authorised contractor.

AS the railway will pass through rural areas in most cases providing a HV supply from which to draw a supply may prove challenging, therefore it is assumed that an 11kV HV feeder has been provided parallel to the railway, as this is unlikely to be required along the entire route it is assumed that this value includes necessary off-route connections. An estimate drawn from information in the document suggests that a cost of roughly £100k/km would be suitable for an overhead line system running adjacent to the railway.
The provision of 350kVA LV compact substations and metering equipment, as well as terminals for the short LV service cable to the substation can be estimated based on the stated values at approximately ~£40k per example.
This would suggest that costs relating to the provision of an electricity supply is roughly ~£140k/km.
As the Point of Connection for each substation would be at 400V the railway would not be liable for network reinforcement costs above the 11kV level, as stated in the methodology provided by Electricity North West.

Substation design

The substations would be of a chopper-rectifier design with an input 12-pulse low voltage transformer.
These systems function as transformer-isolated diode rectifiers with what amounts to a buck-converter on the output of the diode rectifier. This allows them to maintain an excellent input power factor whilst providing a highly controllable output, voltage and current of the supply output can be monitored at all times and the operation of the chopper-converter can be controlled to keep them within the required values.

They are capable of operating at at least 97.6% in the 600Vdc+ range utilising existing silicon IGBTs and it is highly likely that higher efficiencies could be obtained by utilising newly available Silicon Carbide diodes and MOSFETs in the output stage (1700V SiC MOSFETs are available and are suitable for this application).
Additionally the higher junction temperature tolerable with silicon carbide devices and their inherently higher thermal conductivity will permit more compact substations to be designed which will further tend to reduce costs.

Example costs for a 300kW-output rectifier, based on the experience of Portland Streetcar and in the opinion of Delta-Rail, seems likely to be in the range £300k per example. That translates to £300k/route-km. However it should be noted that as numerous identical unit substations would be acquired under any reasonably sized scheme it is highly likely that costs could be reduced drastically.
Additionally due to the low input voltage it seems highly likely that significant economies could be made by adopting a medium-frequency transformer topology with the attendant reduction in the costs of substation magnetics.

Other Costs
Delta-Rail's low cost electrification branch study suggested that other costs could become important, such as the cost of signalling immunisation and the provision of a SCADA system to allow the control of the electrification scheme and the like. It was suggested that immunisation of signalling would cost roughly ~£20k/route-km, but this is obviously heavily dependent upon the signalling system in use at the time. It also seems likely that the eventual arrival of ERTMS-Regional would permit the route to operate without the track circuits that can make signalling systems on newly electrified routes so difficult to maintain and efficienctly operate.

Additionally a price of £100k/route-km was suggested for a SCADA cable along the route that could connect all the substations to a control centre to enable the system to be operated effectively and to enable inter-tripping and other such techniques to be used to improve safety. However with the arrival of the safety grade GSM-R network on the Railway it is possible that this cable could be deleted in favour of simply providing an inexpensive GSM-R data base station at each substation. However this option would be subject to further development and it seems likely the first scheme would utilise a conventional control system.

This suggests other track-related costs to be between £0-£120k/km depending on the conditions prevailing on the route at the beginning of the project.
It is also suggested that ~£2-3m be allocated for project costs, largely indepedently of the size of the scheme proposed (within reason obviously!).

Summary of costs:
Track and conductor rail costs: £110-£1010k / route-km
Provision of electricity supply: £140k / route-km
Substation costs: £300k / route-km
Other costs: £0-120k / route-km + £2-3m total.

Overall costs: £2-3m + (£550-1570k / route km.)
Compared to something like £4m/route-km for a 25kV scheme at the present time, based on examples of schemes being turned out today.
It should be noted that even in the worst case listed above the trains have an enormous amount of margin before they violate the terms of the third rail standard, and should they remain more than ~7-8km apart they should not interact with each other through the traction system.
This means that the system can support a twin track route with several four-car trains per hour in each direction without much difficulty. Even conservatively with 10km average spacings between following trains a line with a ~40km/hr average speed could maintain 4 trains per hour without any driver constraints at all.
This makes it easily capable of handling all likely traffic loads on many secondary routes near existing northern third rail territory.
It is also startling how much difference the existing track condition makes - although it should be made clear that total resleepering would delay the next required resleepering operation in the future - which reduces the railway's future costs and as such some portion of the cost of that operation should be defrayed by the normal railway operational budget as opposed to the electrification budget.

It also seems likely that any future resleepering operations on routes near third rail territory could be a golden opportunity to electrify the routes in question, taking advantage of advances in the technologies mentioned above.
Just my two cents - which comes from reading far too much about power electronics in industry in recent years. And a love for messing around with a pad of paper, a calculator and an engineering problem.
 
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GRALISTAIR

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One for the sticky on electrification- I will update when I get a chance. A well detailed post/thread with a lot of thought and work put into it. A valuable resource imho
 

Elecman

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Just a quick point, if the DNO are going to supply an 11Kv overhead route along the length of the route along with 500 Amp off takes every 1km or so then they will probably determine the whole system and off takes as Customer not Network as drawing large non linear loads would have a serious impact on thier other connected systems. They are far more likely to insist on a 33 or 66 KV feeder system.
 

SpacePhoenix

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Summary of costs:
Track and conductor rail costs: £110-£1010k / route-km
Provision of electricity supply: £140k / route-km
Substation costs: £300k / route-km
Other costs: £0-120k / route-km + £2-3m total.

Overall costs: £2-3m + £550-1570k / route km.
Compared to something like £4m/route-km for a 25kV scheme at the present time,

For that approx £1m/route km difference it's probably better to go for overhead, as you don't have the performance restrictions of 3rd rail or the inherent dangers of 3rd rail to anyone trackside
 

AM9

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... It is also startling how much difference the existing track condition makes - although it should be made clear that total resleepering would delay the next required resleepering operation in the future - which reduces the railway's future costs and as such some portion of the cost of that operation should be defrayed by the normal railway operational budget as opposed to the electrification budget.

It also seems likely that any future resleepering operations on routes near third rail territory could be a golden opportunity to electrify the routes in question, taking advantage of advances in the technologies mentioned above.
Just my two cents - which comes from reading far too much about power electronics in industry in recent years. And a love for messing around with a pad of paper, a calculator and an engineering problem.

If the need to replace sleepers is going to be "defrayed by the normal railway operational budget as opposed to the electrification budget" then in the event of OLE scheme costing, so should the need to replace/refurbish overhead structures such as overbridges, footbridges and tunnels, particularly if the route has sub-standard clearances for (non-OLE powered) freight or modern passenger rolling stock.
 
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HSTEd

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Just a quick point, if the DNO are going to supply an 11Kv overhead route along the length of the route along with 500 Amp off takes every 1km or so then they will probably determine the whole system and off takes as Customer not Network as drawing large non linear loads would have a serious impact on thier other connected systems. They are far more likely to insist on a 33 or 66 KV feeder system.

The advantage of chopper-rectifiers is that they have near unity power factor [values of 0.95 or more typical]. So they are less damaging to a network than a conventional thyristor rectifier.

If we were to move to more advanced switch mode topologies power factors four-nines are probably achievable.
The capacity of an 11kV feeder is several megawatts and they would be tied in to all the HV feeders that cross the route as a matter of course.
And the costs of improving distribution services are actually rather cheap compared to the rests of the scheme - distribution capacity as cheap.
 

HSTEd

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For that approx £1m/route km difference it's probably better to go for overhead, as you don't have the performance restrictions of 3rd rail or the inherent dangers of 3rd rail to anyone trackside

Even the top end of the number I've given you is £2.5m/route-km less than the typical 25kV value. It is less than half what the supposed price of the 25kV scheme is.
If the need to replace sleepers is going to be "defrayed by the normal railway operational budget as opposed to the electrification budget" then in the event of OLE scheme costing, so should the need to replace/refurbish overhead structures such as overbridges, footbridges and tunnels, particularly if the route has sub-standard clearances for (non-OLE powered) freight or modern passenger rolling stock.

Perhaps they should - but you would only be able to defray numbers similar to the value of such benefits to the railway.
As most bridges, footbridges and tunnels have very long operational lives so the value of pushing back reconstruction is tiny.

Additionally there is very little value on improving clearances since the value of the extra traffic it allows is negligible.
It should also be noted that you have quoted me without the previous several worlds which make it clear that I do not expect the whole resleepering cost to be defrayed like that or even a large portion of it.
 
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MarkyT

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I wouldn't get too hung up on signalling. Whatever modern solution was employed it would not employ track circuits, which have always been the biggest challenge in connection with electrification due to their electrical commonality with the traction return path. For each of the sort of '10 minute headway blocks' of around 10km you're aiming this proposal at you would have needed up to 10 individual track circuit sections per single line using traditional 1960s/70s style colour light signalling, and at each of these TC boundaries there would be equipment housings, power supplies, communications, and either insulated rail joints (for jointed TCs) or impedance bonds (for jointless TCs). The capital and maintenance implications of traditional TCB in this form was one of the factors why AB (Absolute Block, with little intermediate equipment for long blocks) has hung on for so long on many secondary lines, despite its comparatively high operational staffing costs. Modern equipment is designed to be intrinsically immune to electrification, whether AC or DC, and today axle counters would be used by default for train detection. That technology is particularly suitable for the long blocks of secondary and rural routes with (like AB) very little intermediate equipment required between the signals or block markers, only inductive wheel sensors at the block extremities (i.e. equipment every 10km rather than every 1km). Your SCADA cable for the substations might also carry signalling and voice communications to cut costs, or at least the power cable route might be shared (unlike traditional Southern Region practice).

Your approach to power distribution should be suitable for lower voltage OHLE, probably far more acceptable from a safety perspective while avoiding the worst of the structure clearance issues being faced by those engineering 25kV schemes currently.
 

najaB

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Well, for a long time now I have been talking about how advances in the design and performance of power electronics would effect the economics of third rail electrification and now it is time for me to finally write up one of my proposals.
It's an interesting idea (which I *think* I understand fully), and definitely something that's worth investigation. The only immediate drawback is that there's a lot more 'stuff' involved which means a lot places that things could go bang.
 

Elecman

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If you are only going to have LV connections then your business case needs to allow for ongoing standing charges costs of over £1/KVA/ month for EACH of your point of connection and the fact that LV electric KWHr costs are about 150% higher along with the higher TRIAD/ DUOS charges than HV bulk supply costs. I doubt your scheme would pass go over a whole life costing business case.
 

HSTEd

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If you are only going to have LV connections then your business case needs to allow for ongoing standing charges costs of over £1/KVA/ month for EACH of your point of connection and the fact that LV electric KWHr costs are about 150% higher along with the higher TRIAD/ DUOS charges than HV bulk supply costs. I doubt your scheme would pass go over a whole life costing business case.

Even at domestic/light commercial electricity rates (suitable for LV connections) it is still far cheaper than diesel.
And standing charges of £1/kVA-month is rather high really for 350kVA supplies. I have seen examples on the internet where the standing charge is far far far less than that, indeed it is often projected that supply standing charges are only ~£20-30/month for large supplies.

Additionally considering NR apparently already pays 8p/kWh, 150% higher would make them pay something like 20p/kWh. Which is utterly ridiculous.

And whole life costings are notoriously unreliable because they are so dependant on your applied discount rates.
 
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Elecman

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You sure of your facts on what NR pays for its LV supplies ? my 150% would lead to 12p/KWHr which is in the ball park not your 20p that you just suggested. You must remember that since the Electricity Supply Industry changed the rules on CT supplies ( under P272 regulations) Shipper now treat all CT metered (supplies of over 100 amp rating) supplies as Max Demand supplies so supply capacity charges each month are set at your declared KVA rating( in your case 350KVA) and they are of the order of £1/KVA .
 

HSTEd

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You sure of your facts on what NR pays for its LV supplies ? my 150% would lead to 12p/KWHr which is in the ball park not your 20p that you just suggested.
There was another thread in this forum [I believe it was the North Downs electrification thread] not long ago where it was discussed how much Network Rail pays for its traction electricity.
The conclusion was something on order of 8p/unit.
You must remember that since the Electricity Supply Industry changed the rules on CT supplies ( under P272 regulations) Shipper now treat all CT metered (supplies of over 100 amp rating) supplies as Max Demand supplies so supply capacity charges each month are set at your declared KVA rating( in your case 350KVA) and they are of the order of £1/KVA .
If they are £1/kVA-month then even at 350kVA it would amount to only £4,200/route-km-yr in charges.
It would take an awful long time before it caused any serious economic concerns as the savings in moving to LV feed are truly enormous.
At 2 trains per hour, 16 hours a day and 350 days a year that translates to 37.5p/train-km.
For Comparison Northern Rail spends ~£8.70/train-km in operating costs (with shorter trains, average of 2.43 vehicles), and most other operators are much higher than that [Merseyrail spends ~£16/train-km].
At 4 trains per hour, the cost drops to ~19p/train-km.

Its not bank breaking
 
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SpacePhoenix

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Is there any examples of the system in use anywhere in the world atm?

How well would it handle say a 12 car desiro (either 1x700 or 3x450)? (Would a 12 car Electrostar draw more or less power than a 12 car desiro?)
 

HSTEd

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Is there any examples of the system in use anywhere in the world atm?
Portland Streetcar use it with short-trams but as far as I know noone does it at the present time.
How well would it handle say a 12 car desiro (either 1x700 or 3x450)? (Would a 12 car Electrostar draw more or less power than a 12 car desiro?)

It would probably cave in if two of them tried to pass each other at full power. It really isn't designed for that kind of thing.
However if you doubled the number of substations and put them every 500m then you might be able to cope with regular 8-10 car trains.
Would increase the cost of the system by something like ~£400k/route-km though.

QUOTE:
If we go to the ONS report 'Gas and electricity prices in the non-domestic sector' - December 2016 edition, we find that a pricing schedule for the all up costs (including DUOS/TUOS and all the other charges) for business electriciy providers broken down by the size of supply.
If we assume 2 trains per hour, 16 hours a day 350 days a year that is 11,200 trains per year. At four-cars each and 2.2kWh/vehicle.km that suggests something approaching ~100MWh/yr per substation. [Although if it was kept below that value we would not actually need half hourly meters on the substations]
Which makes it a 'Small' business by the categories of the ONS statistics. Suggesting we might expect to pay ~12.2p/kWh all up, including standing charges and the CCL.
Now whilst our supply system is atypical it should be noted that we would have a series of supplies that would have a well defined aggregate total demand which means it should be possible to negotiate a deal with the distribution operator with regards to diversity in the 'Maximum Demand'.

For example it might even be possible for the railway to pay the distribution operator to design, build and operate the 11kV feeders as if it were part of its own distribution system but maintain the 'logical' point of supply at the point where the 11kV feeders join the main grid.
This would allow us to retain the cost benefits of the supplies being maintained by the grid operator without getting hit hard by capacity charges that obviously aren't designed for this.
If for example twenty substations were taken to be one larger supply we might expect to pay only ~10p/kWh for our electricity.
 
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HSTEd

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I believe it is certainly worth investigating - and I believe meaningful work could be undertaken for less than a million pounds.
The obvious targets would be to manufacture a sample 300kW compact rectifier substation of the type specified, and then to discuss with distribution operators what arrangements could be reached over the feeder systems required. With reference to a trial route.

But I very much doubt anything would be done and it is obviously far beyond my private resources. Unless I was to crowd fund it but even then I doubt I could reach it. It is not as if the railway industry even has an active research establishment any more.

EDIT:

It seems likely that the cost of the substations could be reduced drastically if we were to accept that the third rail installation was to have multiple substations operating as one isolation area. As we have electronically controlled chopper rectifiers at each and every location it is entirely possible to operate with no isolation at all. We can order a substation to cease current production at any time using the SCADA system, and the ultimate protection would be to open the AC side circuit breaker (which are far cheaper than specialist DC circuit breakers).
It is entirely possible to make do with only make-only contactors at each location, circuit breaking could be achieved by momentarily turning off the DC supply system over the entire isolation area which would necessarily cut current to zero, opening the relevant contactors and then reactivating the system in it's new configuration. This procedure would only require a handful of seconds potentially.
Regeneration, if allowed, might complicate things slightly but it should be noted that the specification states that a train will not regenerate if it is the only source into the rail.

As High Speed DC circuit breakers in this application can cost ~£30k each, and require an enclosure that can easily cost ~£80k by itself, the savings are clear.
This would convert the system openly into something drastically different from existing applications. But could easily cut the cost of the rectifier stations by ~£100k or more.

Which reduces the price range of the scheme to £2-3m + (£450-1470k / route km.)
 
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AM9

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I believe it is certainly worth investigating - and I believe meaningful work could be undertaken for less than a million pounds.
The obvious targets would be to manufacture a sample 300kW compact rectifier substation of the type specified, and then to discuss with distribution operators what arrangements could be reached over the feeder systems required. With reference to a trial route.

But I very much doubt anything would be done and it is obviously far beyond my private resources. Unless I was to crowd fund it but even then I doubt I could reach it. It is not as if the railway industry even has an active research establishment any more.

EDIT:

It seems likely that the cost of the substations could be reduced drastically if we were to accept that the third rail installation was to have multiple substations operating as one isolation area. As we have electronically controlled chopper rectifiers at each and every location it is entirely possible to operate with no isolation at all. We can order a substation to cease current production at any time using the SCADA system, and the ultimate protection would be to open the AC side circuit breaker (which are far cheaper than specialist DC circuit breakers).
It is entirely possible to make do with only make-only contactors at each location, circuit breaking could be achieved by momentarily turning off the DC supply system over the entire isolation area which would necessarily cut current to zero, opening the relevant contactors and then reactivating the system in it's new configuration. This procedure would only require a handful of seconds potentially.
Regeneration, if allowed, might complicate things slightly but it should be noted that the specification states that a train will not regenerate if it is the only source into the rail.

As High Speed DC circuit breakers in this application can cost ~£30k each, and require an enclosure that can easily cost ~£80k by itself, the savings are clear.
This would convert the system openly into something drastically different from existing applications. But could easily cut the cost of the rectifier stations by ~£100k or more.

Which reduces the price range of the scheme to £2-3m + (£450-1470k / route km.)

So how would current protection work? No single feed would have the capacity to supply a single 1MW load, which means that there would be no single overload sensing point and no effective supervisory control of shutdown. Just expecting multiple contactors to drop out using their return springs would not be considered safe and would not protect their transformer-rectifier sets from reverse feeds under some fault conditions.
The system would also waste regen power as well as power through heavy resistive losses.
Then there is the exposed conductor rail safety issue.
I think that it introduces so many potential issues that wouldn't be mitigated by any cost savings. The risks of electrification are well known and legislation/codes of practice have been hohned over the years to bring the safety, reliability and capital risks down to politically acceptable levels. Any radical new approach to encourage the deployment of an obsolete electrification system that has effectively been ruled out for future projects is likely to get a cool reception. As you acknowledge, there is no appetite for original research and development in the current political or economic climate.
 

najaB

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So how would current protection work?
That's a good point. Along with planned isolations - sounds like it would be a nightmare having to visit multiple substations to ensure the current is turned off at all of them.
 

edwin_m

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How is this different from current Southern electrification, most of which has its own lineside feed at 11kV or 33kV?
 

HSTEd

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So how would current protection work? No single feed would have the capacity to supply a single 1MW load, which means that there would be no single overload sensing point and no effective supervisory control of shutdown.
The substation will not provide a megawatt - the station can only supply 300kW. The output voltage will sag in a controlled manner, and should the voltage at any one substation (which is almost inevitable in a short circuit scenario) drop under 450V (or some other arbitrarily set value) then it will set its chopper duty cycle to zero and effectively cease inputting current.

It will then report this to the control system via the SCADA connection which will then cause all other substations to follow suit.
So the entire system intertrips.

(EDIT: You can also put a hall effect sensor in series with the conductor rail (across a gap for example) and use that to detect currents flowing through the conductor rail if you want, with negligible additional resistance.)

3 Just expecting multiple contactors to drop out using their return springs would not be considered safe and would not protect their transformer-rectifier sets from reverse feeds under some fault conditions.
The system would also waste regen power as well as power through heavy resistive losses.
DC Contactors should never open under load.
Since we have intertripping there is no fault condition that will cause a dangerous backfeed to the rectifier through the chopper array which will inevitably be built of SiC MOSFETs with a reverse voltage of 1700V at the absolute minimum.
Fault conditions would lead to the cessation of all current production and if necessary the opening of the AC circuit breakers.

Regen power is a relatively minor saving, only about 10% of power saving according to Network Rail, and significant regen is still possible even with an 855Vdc substation voltage as you can produce current at 900Vdc at the train conductor shoe.

The risks of electrification are well known and legislation/codes of practice have been hohned over the years to bring the safety, reliability and capital risks down to politically acceptable levels.
Which is why the electrification programme is in total meltdown with massive cost overruns, huge postponements and descoping?
The 25kV programme has failed miserably to deliver on its promises of reduced capital costs and it has probably set electrification back 20 years or more already.
Any radical new approach to encourage the deployment of an obsolete electrification system that has effectively been ruled out for future projects is likely to get a cool reception. As you acknowledge, there is no appetite for original research and development in the current political or economic climate.
And so electrification is dead.
That's a good point. Along with planned isolations - sounds like it would be a nightmare having to visit multiple substations to ensure the current is turned off at all of them.
I press a button and all the AC circuit breakers in the isolation area open?
Remote isolation is considered acceptable in grid-scenarios for linemen? If necessary an AC motorised-switch-disconnector could be provided in series with the circuit breakers, which would permit the circuit breaker to be isolated for maintenance without contacting the grid operator.
THere is little benefit in segmenting the supply system on a sub-route basis as this is effectively tramway feed.

If you really wanted manual knife isolation switches could be provided to segment the system to enable you to work on a line section under isolation (you request the current switched off, open the knife switch and then have control switch the current in the remaining area back on). But I don't really see much point in most cases.

There is no point having a power supply between Atherton and Wigan [for example] if you can't run any further - since the service would unworkable anyway.
How is this different from current Southern electrification, most of which has its own lineside feed at 11kV or 33kV?

Adoption of small distributed rectification in place of larger more distantly spread stations.
 
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najaB

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I press a button and all the AC circuit breakers in the isolation area open?
I'd like physical isolation - what's stopping someone inadvertently pressing the button to make it all live again? And it's not like AC where you can just short the live conductor to earth and count on the breakers tripping.
 
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HSTEd

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I'd like physical isolation - what's stopping someone inadvertently pressing the button to make it all live again? And it's not like AC where you can just short the live conductor to earth and count on the breakers tripping.

Then you would disconnect the section from the surrounding track using manual isolators provided and then visit all substations in the isolation area and either lock their AC circuit breakers open or use another manual isolator attached to the substation outlet. This might be a faff if the isolation area is several kilometres long, but if it is only working in the vicinity of a crossover or similar that would probably only be one or two substations.

Whilst DC circuit breakers are expensive manual isolators are not.
So you would have the same sort of segmentation you would have ordinarily - just that it can't break currents and relies on intertripping for protection purposes.
 

AM9

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... DC Contactors should never open under load. ...

You are correct in using the word 'should'.
But, they 'must' be capable of breaking their circuit safely under full load as other fault conditions may place them in the position of last resort. Breaking under load is catered for using contacts in a vacuum or compressed air/gas/oil environment. Omitting such a capability, even to save capital costs would not be allowed.

And so electrification is dead.

I doubt it. Some of the projects with less needs may well be put back, (North Downs seems to have a weak case), but any lines that have either high carbon fuel consumption or need the additional performance brought by electric train operation will inevitably remain near the top of the list waiting for the appropriate political and economic conditions. The rules are already there for them to be 25kV so nothing will change there. Exposed LVDC track will not be permitted as expedience for a lack of necessary budget of political will.
 

HSTEd

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You are correct in using the word 'should'.
But, they 'must' be capable of breaking their circuit safely under full load as other fault conditions may place them in the position of last resort. Breaking under load is catered for using contacts in a vacuum or compressed air/gas/oil environment. Omitting such a capability, even to save capital costs would not be allowed.
Why do you require braking under load though?
DC traction systems do contain switches that aren't circuit breakers - which isolation devices have to be able to break current is decided at the design phase.

In order for a DC circuit breaker to be necessary at the substation you have to assume a failure of the AC side circuit breaker and simultaneous total loss of rectifier control with the MOSFETs failing short, apparently every one of them simultaneously because one failing short will rapidly fail open as it blows apart.
Since you would probably double up the LV AC circuit breakers because they cost a tenth of what a DC circuit breaker costs that possibility is vanishingly small.
Intertripping can also be designed to be fail-open, and even if one substation refuses to acknowledge a fail open or commanded intertrip it will rapidly trip on under-voltage as it tries to sustain a short circuit or other fault alone.
And if a pair of AC circuit breakers and the chopper control have all failed - what makes you think a DC circuit breaker has any chance?

EDIT:
Conventional Southern Region practice has a complex array of DC circuit breakers because they have busbars in the substation supplied by multiple rectifiers and with multiple outputs, potentially to multiple entirely independent routes. Therefore the only way they can provide any semblence of protection is to break the DC current.
However since all our substations are low power rectifiers that support precisely one tramway-fed (so both up and down lines are paralleled at all times) outlet we can protect the system by simply taking out the rectifier power supply on the AC side. Since that has the same operational effect as breaking the current between the rectifier and the track. The rectifier has negligible power storage (thanks to MOSFETs in the chopper having high switching frequency filters are not really required) and as such killing the AC side is functionally the same as killing the DC side.
We can also protect the rectifiers if neccessary by placing SiC diodes in the substation output to protect it against overvoltages - as the voltage drop from ~3.4kV of reverse voltage protection is something like 3V.

I doubt it. Some of the projects with less needs may well be put back, (North Downs seems to have a weak case), but any lines that have either high carbon fuel consumption or need the additional performance brought by electric train operation will inevitably remain near the top of the list waiting for the appropriate political and economic conditions. The rules are already there for them to be 25kV so nothing will change there.
At £2m/track-km all the proposed projects have a weak case.
Even MML doesn't look so hot at that price.
And 'additional performance brought by electric train operation' died with the Voyager.
So LVDC Exposed LVDC track will not be permitted as expedience for a lack of necessary budget of political will.
Why not?
Noone has yet to demonstrate that LVDC is any more dangerous than 25kV in real life and both appear so safe as to make any attempts doomed to fail for lack of data.

EDIT #2:
Further to my statement about cutting down on the circuit breakers - the Delta Rail Low COst Branch Electrification report I mentioned proposed using a 750V (OLE but that is largely unimportant on this point) with only two circuit breakers. Page 52 for the diagram.

I still believe that dropping the circuit breakers for disconnectors is acceptable if provision is made to drop every single DC supply point if a DC side fault occurs.
However this could be operationally problematic but the problem is the DC circuit breakers approach half the price of the entire substation, even at £300k each.
Even such an arrangement would be significantly cheaper than a 'conventional' layout. However I am having significant trouble determining what the price of a standalone DC circuit breaker actually is. They are not a commodity component.

However until I get some better data I am sticking to Delta-Rails suggestion of ~£300k for 300kW.
 
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SpacePhoenix

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Would it handle ok situations like where anything conductive has fallen onto either two adjacent conductor rails or between a conductor rail and a running rail?
 

HSTEd

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Would it handle ok situations like where anything conductive has fallen onto either two adjacent conductor rails
Two adjacent conductor rails should make little difference because they will almost certianly be paralleled together. Whatever it is would just sit there until a shoe knocked it off or similar.
Although if that is a problem a scheme can easily be designed so that adjacent lines do not have adjacent conductor rails - especially in a twin track scheme.
or between a conductor rail and a running rail?

If it's not blown off explosively by the short it would cause a low-voltage warning at the closest substation, which should then cease current input and send an intertrip signal to all the other substations in the isolation area to stop traction current production.
A couple of volts would likely be fed into the rail by the chopper that sent out the low-voltage alarm - just enough to be able to determine if the short has been removed.
 

najaB

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If it's not blown off explosively by the short it would cause a low-voltage warning at the closest substation, which should then cease current input and send an intertrip signal to all the other substations in the isolation area to stop traction current production.
How would it 'know', given that in normal operation the substation will be running flat out to supply traction current (with the shortfall provided by adjacent substations)?
 

HSTEd

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How would it 'know', given that in normal operation the substation will be running flat out to supply traction current (with the shortfall provided by adjacent substations)?

Because it knows the voltage at which it is outputting current.
If the voltage collapses as a result of something shorting out the rail then it can detect a low-voltage warning and conclude something is wrong.
For example the minimum voltage permitted in normal operation on third rail is 450V (I imagine the voltage would be set rather higher than that in this case, but you get the idea).

Even if the short is sufficiently high resistance to avoid detection by a local under-volt - hall effect sensors on the conductor rail jumpers will be able to detect enormously high currents in the rail. And thats assuming the control room doesn't initiate an intertrip on the basis of every single substation in the isolation area all jumping to max current at once.

EDIT:
For example - a dead short has an absolute minimum impedance equal to the loop of rails between it and the supplying substation(s).
If we assume a project where there are 20 substations at intervals in one direction from the fault (as a fault is symmetrical).
The loop resistance to the first substation (assuming all are operational for the moment) is at most 6.5 milliohm.
That means that in order for a dead short to reach 450V at that substation's busbar and avoid an undervolt (I would set higher, perhaps 600V myself) you would have to have a fault current flowing of nearly 70,000A.
Since all substations would be electronically limited to 300kW and to a current proportional to it then it is highly unlikely they would be able to supply such a current without going into undervolt.

In order to avoid going into an undervolt the resistance of the short (whatever it is) would have to be very much greater than the loop resistance of the conductor rails - which would then mean that effectively all the heating would be passing into whatever it is that is bridging the conductor rails.
For example a one ohm resistance across the rails would have a minimum of 300kW being dispersed in it, probably much more.
Whatever it is will be blown clear rather quickly with minimal damage to the conductor rail or rectifier system - which could quite easily keep that up all day;
It is also highly unlikely that such a fault would trip conventional circuit breakers anyway.
 
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rebmcr

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I'm not sure how you would get the current from the insulated conductor rail to the overhead wire?
 
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