Posted by Iain McClatchie on December 29, 2005, 7:29 pm
Conversion is done with SCRs (silicon controlled rectifier). Nowadays
these are actually controlled by signals on fiber-optic cables, to
avoid problems with sending signals across voltage domains.
It used to be done with mercury arc valves.
Conversion efficiency is about 99%, so you lose 2% going to DC and
back. The benefits are substantial enough that there are a fair
number of back-to-back HVDC installations. That is, the transmission
distance is basically zero.
HVDC does not change the optimal RMS current in a wire with a
particular cost and conductivity. But at the same maximum voltage
and same RMS current, HVDC transmits sqrt(2) times the power as
HVAC -- 41% more.
So, that example cable can transmit 245 MW using 500 kV HVDC. It can
go 2271 miles and lose 5%. However, if you lose 2% in the two
conversions and keep total loss to 5%, it doesn't go quite as far:
If you wanted to pump electricity from Florida to New York (2500
miles), you'd lose 5.5% in line losses in a 500 kV HVDC system. The
line losses and wire cost alone would add a half cent per kW-hr to the
cost of the electricity shipped.
Posted by Derek Broughton on December 30, 2005, 1:13 am
Jeff Thies wrote:
Well, the major transmission line running from Churchill Falls on the
Quebec/Labrador border to New York state is HVDC. It seemed worthwhile to
Posted by daestrom on December 29, 2005, 9:22 pm
An important point that has been overlooked is that AC lines cannot carry
power over those kinds of distances in one continuous line. While you are
correct about the resistance losses not being excessive, the *reactance* of
even just 60 hz poses quite a problem. Voltage limits on the receiving end
prevent carrying AC power such distances in a dedicated line. The impedance
of your typical HVAC line is a couple of orders of magnitude higher than the
the resistance alone.
You won't find any AC lines that cover such distances because the voltage at
the receiving end would not be stable enough to carry the amount of power
you suggest. But this is *exactly* the kind of problem that HVDC solves.
Not only does HVDC carry somewhat more power for a given conductor cost, it
doesn't suffer from the reactance issues.
So when the distance the power must be transferred exceeds a certain point,
the additional cost of the converter/inverter units at each end of an HVDC
line can be justified over that of multiple stop HVAC lines with
substations. But there aren't that many places where the power is needed
that far away with no intervening loads/generation. Most parts of the
country, the 'grid' of generation/loads is tied with AC lines because of the
cheaper capital costs of transformers versus converters/inverters.
HVDC also has benefits where the two ends of the line are not part of the
same grid (as in the Tx to La tie).
Posted by Me on December 29, 2005, 7:05 pm
That is true, but that is not what the OP is talking about. If you look
again, you will notice that there are three or six groups of cables on
each tower. Each group is made up of three cables, spaced about 10 to
12 inces apart. The groups are the PHASE Groups either 3, 2 groups of
3, or 6 Phase Circuits, The three cable groups are all carrying the
same Phase for that group, and by stringing three cables in a group
they get about 2.8 times the current carrying capacity per group over
what a single cable can carry. Cables are usually Steel Core, which
supplies the mechanical strength, with either Aluminum or Copper outer
jackets, that supply the current carrying capabilities.
Posted by daestrom on December 29, 2005, 9:30 pm
There are a couple of reasons why many high powered transmission lines are
composed of multiple conductors. First, as you surmised, you need at least
three conductors to transmit three phase power.
But many towers will have three phases on each side. Strictly speaking,
this is *not* 'six-phase' or '18 phase'. They are multiple parallel
circuits. A common arrangement is to have three 'groups' of conductors
suspended from one side of a tower, and three more 'groups' suspended from
the other side. Quite often, the three phases on one side will meet up with
and join (through appropriate circuit breakers), the three phases on the
other side at the substation. These parallel circuits provide more capacity
and flexibility in that a fault on one line will not completely interrupt
the power flow.
Now, as to why a single phase of the line is actually a 'group' of
conductors and not just one large conductor, there are a couple of factors.
One is as R. H. Allen has explained, larger conductors don't necessarily
carry larger amounts of power. But another reason is that suspending
several small conductors in a close-nit arrangement (such 'groups' usually
have spacers to hold their inter-spacing) can reduce the maximum field
strength in the air immediately surrounding the conductors. This reduces
the corona losses by making the overall 'apparent size' of the conductor
larger than the simple addition of their individual cross sections.