[time-nuts] Features of a Precision Clock?

Dean Weiten dmw at weiten.com
Sat Oct 7 16:53:30 UTC 2006


Hi there,

Having worked with the folks who operate the power utilities (I designed
protective relaying and recorder electronics for several years), I can
advise that they do take the long-term accuracy of their power
seriously.  However, the short-term is not a big concern, and in fact,
they cannot control it all that well.

It turns out that power flow on an AC line requires a phase difference
between end points (as opposed to a DC system where it is resistance
that counts).  The resistance of the line is not important.  This is
because power transmission lines are almost pure inductive reactance -
in power systems terms, the line angle (impedance angle) is generally
near to 90 degrees.

Systems are connected at multiple points, like a mesh of rubber bands
connecting weights and support points.  Some of these points are heavier
(down) or pull stronger (up), some have stronger bands, some have very
weak bands.

When the load changes, or when a line opens or closes, the phase angles
of the power through all these interconnected ties will shift to
establish a new equilibrium.  In so doing, your power will advance or
retard somewhat.  If you have a clock running on the phase of AC power,
your clock will gain or lose a bit of time.  It is unclear whether you
will ever be corrected - the new equilibrium might just be a fact of life.

The prime movers of the systems (generators) are almost all physical
moving devices, like hydro-electric (water dams) or thermal (coal,
natural gas, or nuclear powered turbines).  When they are loaded down,
they slow down - and when less loaded, they speed up.  This isn't as bad
as it sounds - the rest of the system rolls along at the "system
frequency", and the generator's slight frequency change actually becomes
a phase change, which, as per above, changes its power output.  Then the
generator gets back into sync, but with a phase angle different than before.

As you can imagine, it is a challenge to maintain tight control of the
phase, with all the changing conditions on the power grid.  In the case
of our utility (Manitoba Hydro), they keep power system clocks at the
big "24 by 7" staffed power stations and in the main control room, and
will, under their rules of operation, tweak things slightly over time. 
I am not certain of the rules of operation, or of the way they tweak
things (generator bias?), but could find out from friends and
colleagues, if you wish.

Here in Manitoba, we are blessed to have much of our power supplied from
the hydro-electric generators in the north, through a DC link.  It turns
out that this is economical above a certain distance and power level -
related partly to the "skin effect" (yes it becomes important, even at
60 Hz).  At the south end of the link, we have a DC-to-AC inverter
system (huge - pretty impressive), fibre optic fired thyristors
(equivalent to triacs?  SCRs?).  We can change the firing angle on a
cycle-by-cycle basis, adjusting the power flow in and out, and exerting
extraordinary control of the system phase.  We use it to stabilize the
system more than for power frequency correction, but I assume that this
could be done too, just unsure of the algorithm.

Of course, the system is a lot more complex than I describe it here,
with phase shifting transformers, tap changers, and more modern
back-to-back DC links, wind generation at the distribution (lower
voltage) level, etc.  More complex than I understand, to be sure.  But
those are the basics.

Regards,


Dean Weiten,
Elecsys Solutions,
Winnipeg, Manitoba.






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