Overhead Catenary Cables to Charge Electric Lorries.

wallywombat:

Dumb question here - with a trolley bus (which doesn't have rails), how is the return path for the current provided?

Easy - two overhead wires.  But that means a simple pantograph won't work.  You need two spring loaded arms, each hooked onto one wire.

Dumb question here - with a trolley bus (which doesn't have rails), how is the return path for the current provided?

And do not make the vehicle class 1 !

Indeed!

Taking the  commutating pantograph and DC bus idea further, and making those pairs of diodes totem poles of high power FETs, you can then also push power back into the line or take it out, as suits for example during braking.

Now I like that idea!

So if we wanted  to use railway like voltages, do 25KV FETs that can switch hundreds of amps exist?

My original (trolley bus) very rough reckoning figured that a 400V 3-phase system might do (part of the attraction being that using 'off the shelf' DNO 11/0.4kV transformers would be much more economic than any specials (e.g. the old 650V d.c. system) and LV overheads would be much more acceptable over public highways than HV ones). To match the output of a conventional diesel bus engine we should need less than 100A/phase - which I reckoned a pantograph should cope with. Indeed if there was a small battery on-board, the overhead line system need not ever have to supply the peak load - but just a little over the average - which of course would be significantly lower - making the numbers even more palatable.  Trucks of course are larger/heavier than buses - but not hugely so - a factor of two or three perhaps rather than tens, so perhaps a similar approach might still be workable.


  - Andy.

It is not clear why railway lines are empty most of the time

I suspect a lot of the problem is the signalling system - mostly still using the Victorian principle of dividing the track into "sections" each of which is guarded by a signal and has to accommodate the stopping distance of the fastest train (and smooth steel wheels on smooth steel track mean they can't exactly stop on a sixpence). So at most one train per section and not in the least flexible if you have a mix of fast and slow traffic. It costs a fortune to adjust anything as it means not only moving the signals, cables and updating the control system but all the track based train detection systems too, not to mention an incredible amount of testing.


The powers that be would love to run more trains - both passenger and freight - but are up against track capacity issues all over the place. That's why HS2 is so popular with the planners around here - they're not in the least bothered about people getting to London 30 minutes quicker - what they want the fast passenger trains off the existing rails so they can use them for a larger number of slower services - both local passenger and long-distance freight.


   - Andy.

And do not make the vehicle class 1 !


Taking the  commutating pantograph and DC bus idea further, and making those pairs of diodes totem poles of high power FETs, you can then also push power back into the line or take it out, as suits for example during braking.


Now we have a HVDC reserve, either side of which is at our overhead line voltage depending on instantaneous phase of each cycle.


We could play a trick, and supply power to the overhead lines in a similar way - i.e. excite each one with an AC plus a DC such that the most negative part of the cycle is near ground - then the negative side of our DC bus, will always be at or near that potential.


So if we wanted  to use railway like voltages, do 25KV FETs that can switch hundreds of amps exist? - well not as discrete components, yet, the upper bound is a few KV, and you need a factor of 2 or 3 for transients, but techniques do exist for stacking such devices. to make high voltage modules examples . 

The next step in any real design then next act is to transform down to a voltage and current more suited to batteries and traction motors, probably the low to mid hundreds of volts . It is the leakage of that transformer from HV to LV through inter-winding capacitance that would need great thought in the choice of switching frequency and core and insulation construction. None of it is impossible with existing parts, but it is certainly in the realms of  bosky powertronics, and not without risks.

How would you deal with leakage/unbalance currents to avoid the vehicle metalwork rising above normal earth potential especially if one pantograph lost contact with it's wire? 'Traditional' trolley buses operate at 600-750V DC apparently without problems. Would higher voltages and AC cause bigger problems?

The gap between the wires must be greater than the width of the pantograph which restricts the width of the pick up head and hence the allowable amount of sideways deviation. The trolley bus type solution is only suitable for relatively low speeds.

I did a bit of a 'thought experiment' along these lines a while ago - it was actually thinking about the possibility of re-introducing electric trolley buses but given the increase in congestion on modern roads looking for a way of allowing the bus to connect to the overheads automatically when it was 'in lane' but still being free to move out of lane (and run from small batteries) whenever the need arose. One idea to come out of that was of a 'segmented pantograph' (think something lile the commutator on a universal motor, but perhaps herring-bone shaped to allow smooth transitions of the overhead wire between segments) and two or even three a.c. overhead lines. The lines need only be a few segments apart to prevent shorts (so preventing direct wire-to-wire shorts due to wind movement would more likely be the limit). Each segment would then be connected via a pair of rectifiers (big diodes) - like the first half of a bridge rectifier - to provide unsmoothed d.c. to the vehicle. An automated system would then look to lower the pantograph smartly whenever it was drifting out of lane, and re-connect automatically when the bus was steady under the wires again. By providing a 3-phase supply (or balanced single phase) and by disconnecting the vehicle from the pantograph contacts when anything other than all phases were present, leakage or capacitive coupling currents should mostly cancel out.


  - Andy.

Thinking further about this with reference to Mike's Solar PV and tingles from the flashing post.

How would you deal with leakage/unbalance currents to avoid the vehicle metalwork rising above normal earth potential especially if one pantograph lost contact with it's wire? 'Traditional' trolley buses operate at 600-750V DC apparently without problems. Would higher voltages and AC cause bigger problems? Conductive tyres could deal with small leakage currents but what about actual faults in the system? Conventional electric rail systems always have a good ground reference.


Best regards


Roger

As you said on one of the climate threads sometimes you have to change the structure not just try to do a 'Green' like for like.

I'm inclined to agree, so before messing with roads, perhaps finishing to electrify the railway would be a better start, we only started with the first electric trains in the late 1800s, how hard can it be to just get BrXXX no , no, no, , railway electrification, sorted.   (I think the oldest electric train still running in the UK is the narrow gauge on Brighton seafront dating from 1883)

Seriously the best energy wise would be a joined up system with more rail freight for long haul, except for the last few miles, which can be smaller lorries that could then be electric. It is not clear why railway lines are empty most of the time, and therefore expensive compared to road transport.

The only reason a coach is cheaper than a train, ignoring the suggestions in the 1998 song by Divine Comedy, is that the road is very busy, and in effect more people are paying for it. The rail user it seems, is paying for the whole train.