On 6/24/11 6:47 PM, Hal Murray wrote:
I don't see how they connect grid that aren't totally in sync.
Each grid is in sync with itself. In the US, there are several grids. They
aren't connected except maybe by DC lines.
all of the US is interconnected, except for Texas.
That said there's not much interchange across the intermountain West
(e.g. not much power is run from the Pacific coast across Utah and
Colorado). This is why the Pacific coast will have less variation than
the east coast.
As others have pointed out, power flows are done by changing the
relative phase of the two zones. You can either keep the power flows
carefully "net zero" or you can keep the frequency the same, but not
both. So what they do is try to have times when they don't need to flow
power, so they can "open the switch" on the intertie, change the
frequency for a while to get back to where they think they should be,
then carefully resynchronize and reclose the switch.
There are remarkably few DC interties to help. the big Pacific DC
intertie is done not so much to flow power both ways (it's almost
entirely southbound), but to make it even feasible to send power several
thousand km. The alternate (AC) path down through the middle of
California is a apparently a bear to keep stable, and transients can
take tens of hours to die out.
After all, that transmission line is starting to be a significant
fraction of a wavelength at 60 Hz. (lambda in free space is 5000km..
prop velocity on a power line is quite a bit slower)
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