On 2/9/17 4:03 PM, Poul-Henning Kamp wrote:
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In message <63beea7a-f9fc-6e1d-b855-2c7056de3...@earthlink.net>, jimlux writes:
I think also of the issues from distributed generation - consider a
rooftop solar installation with 20 or so MicroInverters, all "slaved" to
the line. Just from manufacturing variations, I suspect each
microinverter is a little bit different than the others.
Surprising there is almost no variation, because it hurts badly on
both your nameplate efficiency and thermal design.
I was thinking about phase stability and "matching" to the grid.. each
microinverter (in a short time sense) might have a different phase
relationship (which turns into power factor), essentially introducing
some "noise" into the system.
But shorts are another matter: They are so much easier to deal
with when you have regular zero-crossings a hundred times a second:
HVDC shorts are explosions which doesn't end until you've used up
all the energy.
HV AC lines have exactly the same problem, the switches carry enough
energy that "quenching" the arc is by no means assured through the zero
crossing. If nothing else, inductance in the system (e.g. transmission
lines) insures that the voltage zero isn't at the same time as the
current zero.
There was a fascinating article about this in Scientific American back
in the 70s (Jan 1971, there was a picture of an airblast circuit breaker
on the cover). There's also a good description in IEEE Proceedings in
the article on Vacuum switching
http://ieeexplore.ieee.org/document/1451144/
Aug 73
The pacific dc intertie is a lot more than 10m apart, so it's probably
lower, but still.. 14 uF @ 1MV is a bunch o'Joules. (about 14 Mj)
Fortunately, there's a fairly large series L also to slow down the
transient.
Yes, it slows down the front flank, on the other hand, once you get
the arc going, with DC that L really keeps it going for a long time.
But 14 MJ isn't all that much energy in practical terms.. about a half
liter of gasoline (most fuels are around 50MJ/kg), or 3kg of high explosive.
yeah, 3kg of HE does a fair amount of damage, but it is localized (in a
10s of meters sense).
When the distribution transformer near my house blew up, I suspect that
a lot more energy was dissipated.. it was a 100kVA unit in a underground
vault, and it blew the 2x2m concrete vault lid 10s of meters away.
In reality, there's probably more stored energy in the L of the
transmission line than in the C. A quick back of hte envelope: 1
uH/meter is 1mH/km, so we're looking at 3000A in 1.3H. Oh, that's just
about the same as the energy in the C. about 12 MJ.
The difference in practice is that a HVAC short leaves a burn-mark
on the metal parts, HVDC solidly welds them together if it doesn't
outright melt them.
For big lines, the melting is "explosive" and doesn't leave much solid
metal behind.
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