On 12/4/12 4:28 PM, Bill Hawkins wrote:
Sorry about this, Tom, but there's some misinformation here.
I wasn't reading this until I saw your posting.
-----Original Message-----
From: Jim Lux
Sent: Tuesday, December 04, 2012 7:58 AM
On 12/3/12 9:59 PM, gary wrote:
I was meditating a bit on the power grid synchronization. If all the
sites but one are in sync, then the generator whose sync is being hacked
will have a hard time trying to feed the grid while being out of phase.
This should be detectable electronically in the generator interface. If
the timing is moved slowly, the the "conflict" would build slowly as well.
The problem is that how would you distinguish this from normal load
dispatch for the generator. That's how you set the power flow: you
adjust the phase of your generator to slightly leading the grid, and
power flows from generator to grid.
----- End Original Message
A. You cannot have one generator feeding the grid while being out of phase.
See any text on coupled (bussed) rotating synchronous machines.
Yes, if the buss is infinitely stiff and infinitesimally short.
If the transmission line is 1000 km long with significant inductance and
capacitance, then the phase of a generator at one end and the phase of a
generator at the other end (or somewhere in between) will be quite
different. The AC intertie in California that runs up the central
valley, for instance, has transients (e.g. instantaneous phase
variations) that take hours to die out. One of the big advantages of DC
links is that they don't have this problem.. one end acts as a constant
current source/sink, the other as a constant voltage source/sink.(DC
links have other problems, but they're still easier than stabilizing a
1500 km long ac transmission line with sources and loads all along it)
It's fascinating to look at the phase/frequency plots during the
northeast blackout a few years ago.
There's even an IEEE Standard (1344) on timing for power lines. IEEE
1344 gives GPS timing as 200ns. that standard calls out a signal at
1pps with 1E-7 stability and 1us max variation from UTC.
Appendix B of that doc says that you need to be able to measure phase to
0.1 degree for state estimation, stability monitoring, control, and
relaying.
B. There is no phase adjustment for the generator. Phase angle with respect
to the grid is determined by whether you are giving or taking power with
respect to the grid. To increase your phase lead on the grid, you apply more
steam to the turbine.
yes.. I was sloppy.. but the point is that the phase of the power coming
out of the generator slightly leads that of the "sink" (e.g. current
flows FROM generator to grid).
Time has nothing to do with it, except at the central dispatching office
for a generating region. If time on the grid, measured in generated cycles,
lags the number of 60 (or 50) Hz cycles since midnight (or some reference)
then the dispatcher calls for or remotely commands more power to be
generated at some stations until the lost cycles are made up. The entire
distribution network stays in synch, as it must.
The way they measure relative phase (and frequency) is by comparison
with a good clock. There's a nifty website out there that shows the
instantaneous and integrated phase difference between various places on
the pacific coast, so you can see power flow changing.
Hope that clears things up.
Now, if you are talking about the huge power inverter at the junction of
a DC transmission tie line and an AC network, then yes, you can adjust the
phase angle to control power flow - but this is not controlled by a clock.
Bill Hawkins
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