Passenger Door Open

[d7666]

Feb 25, 2020 7:57:46 GMT philthetube said:

Power supplies on the met certainly had to be upgraded when the S Stock was introduced,

Yes ... but unless someone knows otherwise, not necessarily for overall power.

With DC motor trains, you have a very high current draw at starting, that rapidly drops off. With AC motor trains you have a more or less constant current draw at all train speeds. The old DC traction distribution systems were sized to allow for big brief draw from DC motor trains, followed by much longer intervals of low draw. The key driver is the heating effect on trackside cabling and equipment which is proportional to I^2R, i.e. to the square of the current. The cable will heat a lot very quickly with a DC motor train pulling away, but then cool off, until the next one. Cabling with AC motor trains runs at a continual I^2R load, not nearly as high as the DC motor peak load, but significantly higher than the low loads. The extra transformers and substations put in are to mitigate most of this - and add resilience - rather than an overall increase in power drawn or energy taken. Hope that clear, it's not easy to summarise it all in one paragraph.

[Tom]

Feb 27, 2020 15:49:40 GMT d7666 said:

Feb 25, 2020 7:57:46 GMT philthetube said:

Power supplies on the met certainly had to be upgraded when the S Stock was introduced,

Yes ... but unless someone knows otherwise, not necessarily for overall power.

It was for overall power consumption - more auxiliaries than traction. Similar issues occured with the introduction of 1992 and 1995 stocks (and probably 1996 as well).

Prior to S stock we had traction control fitted to sidings at Harrow and Rayners Lane. An A stock train could happily sit there with its MAs, Compressors etc. running, but would trip the traction control breaker out if it began to motor against the outlet signal. An S stock would cause the breakers to trip just from the current drawn by the auxililaries.

I'm hoping someone like t697 will be along soon and be able to give a fuller explanation than I... I'm just a lowly signal engineer.

[t697]

OK, so yes the S stock does in principle use more power overall than an A stock train. An S8 train weighs about 40 tonnes more than an A stock for the various reasons already elaborated in this thread, but part of this is so as to provide higher performance right through the accelerating range, but not a higher top speed. How much of that available performance actually gets used and hence how much power drawn under the new signalling and timetables remains to be seen. The trains do have regenerative braking on all cars so you get a fair bit back when there are plenty of trains running to use that regenerated power.
Regenerative braking efficiency was also improved as a result of the upgrade because the low loss conductor rail is low loss irrespective of whether power is coming from the sub-station or another train in regen braking. Regen braking is best when the power goes to other trains with only minimal amounts lost in uselessly heating the conductor rails.

Taking the special example of Harrow Siding, the S stock is able to motor at low speeds using a lower current than an A stock because the AC inverter is efficient and not wasting power in starting resistors. Same is true of DC chopper schemes. Therefore the traction control fuse wouldn't trip and prevent a train being motored in the siding while the signal was against it. Conversely, the Heating or Air Conditioning peak load could trip the fuse. The HVAC is of course thermostatically controlled but the peak HVAC load together with other auxiliary loads could be present for example if the doors had been open for a while in the platform for detrainment just before the train moved to the siding.

It was convenient to get rid of the life expired traction control set up when S stock was introduced. I seem to recall it was found that the main contactor may have been reclaimed from Q stock... I seem to recall the trainstop was located or relocated close to the train berthed position so if it SPAD'd the shunt signal to exit, it would stop before it derailed on the catch points.

[christopher125]

Feb 25, 2020 18:53:52 GMT croxleyn said:

For those interested in Power Generation, visit Gridwatch. Shows daily, weekly &c power usage in the UK by energy type, and what we get from/supply to European mainland.

What amazes me is how much electricity is consumed in the small hours.

If the Frequency is high, that means too much electricity is being generated, and vice-versa.

I'd recommend using electricinsights.co.uk - not only is it more user friendly IMO but more representative too, as it includes estimates of non-metered renewables which Gridwatch just measures as a fall in demand IIRC.

[d7666]

Thanks for that t697.

The info in my posting was based on what Railtrack started and Networkrail finished in the Southern electric zone when the assorted Desiros Electrostars Coradias etc started. There was very little in the way of power capacity increase, just supply enhancement to handle higher continuous I^2R. The big exception was some routes to and in Kent some time before to handle 373s and 92s. There was some smaller capacity addition on the Brighton main line and in New Forest but not significantly so. At least that's how I understood it all at the time, there was quite an account of it at the time on the old usenet uk.railway forum.