On 1 Jul, 23:34, Tom Barry wrote:
John B wrote:
Ish. On AC, absolutely right; on DC, you need banks of resistors as
well because putting it back to the grid if there isn't a conveniently
placed train to take it is Too Bloody Hard. However, given the traffic
density on LUL, most of the time there'll be someone accelerating
while you're breaking so it should work out OK...

[hangs head in shame at 'braking' typo]

Yes, that's what I was getting at. In mitigation, I hadn't read Cap'n
Deltic's latest screed in MR at that point, which suggests that
receptivity in DC networks is higher than previously thought, so you
might not need as much resistance capacity and associated cooling around
the place.

My last bout of crazy-travelling-about-the-place ended just before the
latest edition came out, so I've only seen the preview email. Bring on
the next train voyage...

What's the effect of the suggested upping of the voltage to 750v DC on
some lines? Being a concrete'n'steel type engineer (manqué) I don't
understand electricity as well as I should.

transmission losses [hence, here, heat gains] = current^2 / resistance

Since resistance is ~constant and power delivered to the train =
current * voltage, increasing the voltage from 630V to 750V reduces
the current required to provide the same power by 16%, which reduces
transmission losses by 29%.

This ignores the effect of almost everything real-world (especially
the fact that you're dealing with two rails at +420 and -210 rather
than a single rail at +630), but you get the idea.

--
John Band
john at johnband dot org
www.johnband.org