Post by newshound Post by SH Post by newshound Post by SH Post by newshound Post by Tim+ Post by SH Post by Brian Gaff (Sofa)
Well, I've often wondered how come some of those cross channel cables have
not developed faults before now, however will anyone look at the eggs in one
basket issues of telephones, power etc, which are being made at the moment?
Of course not till it bites us all in the bum.
An interesting point about these interconnecotrs is that they are DC.
SO whats the energy losses like rectifying AC current to DC and again
when its converted back to AC?
WWhy do this double conversion in the first place?
Would it not be easier to synchronise the two countries AC frequencies
and phase, and also less lossy?
It’s to do with power losses due to cable capacitance over long distances.
“Most electrical power transmission systems use alternating current (AC),
because transformers can easily change voltages as needed. High-voltage
direct current transmission requires a converter at each end of a direct
current line to interface to an alternating current grid. A system using
submarine power cables may be less costly overall if using high-voltage
direct current transmission, especially on a long link where the
capacitance of the cable would require too much additional charging
current. The inner and outer conductors of a cable form the plates of a
capacitor, and if the cable is long (on the order of tens of kilometres),
the current that flows through this capacitance may be significant compared
to the load current. This would require larger, therefore more costly,
conductors for a given quantity of usable power to be transmitted.”
And of course the rectification and inversion is all done in solid
state these days, so the conversion losses are relatively low, and
big transformers are pretty efficient.
https://en.wikipedia.org/wiki/IFA-2 says its a cable operating at
320 kV and power of 1,000 MW.
SO from P=IV, I = P /V whihc gives a current of 3,125 Amps.
if we assume silicon diodes are used with a junction voltage of 0.6
V and that we assume that a full wave bridge rectifier is used.
lets assume 3,125 amps is flowing through that diode. The power loss
across that diode which is lost to heat is going to be 1,875 Watts
So in a full wave bridge rectifier of 4 diodes, thats 7.5 kW of heat
to dissipate..... :-)
And you don't call that efficient?
Don;t forget there's actually 3 phases so the heat loss is triple that
at 22.5 kW as then 3 sets of bridge rectifiers are needed.
OK that is 99.9978% efficient then.
There are more than silicon diodes available. Active rectification
allows taking advantage of the ON resistance of a controlled
switch, instead of the passive response curve of a silicon diode.
Even Schottky diodes for bridge rectifiers aren't all
that good, once the current flow increases in magnitude.
A Schottky might manage 0.2 to 0.3 volts of Vf at small
currents. But once you raise the current high enough,
they're about as wasteful as regular diodes. I have some
very nice 2 amp Schottky diodes here, they look positively heroic
at low current, but up around 0.5 amp, they're not so good.
In your IBM PC compatible, the ATX power supply sometimes
has rectifier components that are bolted to heatsinks
inside, to dissipate the heat from the method.
A heatsink to dissipate 22.5kW, would be impressively large.
That's more than enough heat, to heat a home.
And if you build a "hall" around such a device, the side
effects of fire can be pretty messy. A substation here which
is housed in a "barn", when something in there caught fire,
they were doing maintenance and repair, while wearing air packs.
Not a very convenient way to work. We've had fires in some
underground facilities (a popular method for housing power
components in another city), and when you see pictures
of what the working conditions are like, after a fire in
one of those, it's like a dark, sooty, hell-hole. You can't
bring enough auxiliary lighting into one of those, to see
what you're doing, as everything is dirty afterwards.
A "hall" around one of these really large devices, is
likely an essential part of it. But if there's a failure
in there, it's not going to be the most pleasant working