The US is hooked up into a single power grid so that power can be shared
from one region to another over long distances.  In order to do this easily,
it is necessary for all the generators not only to be producing the same
frequency, but the same phase as well.

AC motors and generators (which are pretty nearly the same thing) are a lot
like stepping motors we are familiar with.  For a given phase of the power,
their rotors want to be in a corresponding physical position.

If you tie two generators together that are not in phase, the one that's
ahead will supply current to the one that's behind, which will act as a
motor trying to catch up (which will also load down the one that's ahead,
slowing it down).  To do this, they will exchange current -- a lot of
current, because they will want to do this as nearly instantaneously as they
can.

So the load is getting heavy, and someone goes to switch on another
generator to handle the load, but it's out of phase.  Humongous relays clang
closed, and huge currents rush about as rotating machines as big as houses
jump and buck and make the ground shake, and the lights dim.

Actually what happens is circuit breakers take both generators off line
before something breaks, and now the grid really is overloaded...

Power companies pay a whole lot of attention to frequency and phase.  They
coordinate this, and keep the whole grid locked to very precise frequency
standards.  Sometimes when the load is heavy, they do get behind a few
cycles, but later on when the load lightens, they speed up.  If you observe
an electric wall clock carefully and compare it to something like WWV, you
may see it get a few seconds behind on a hot summer afternoon, but it will
be caught up by the next morning.

So the grid, at least in the US has great long-term stability, but only
pretty good short-term.

> -----Original Message-----
> From: Tony Nixon [mailto:Tony.Nixon@ENG.MONASH.EDU.AU]
> Sent: Thursday, February 22, 2001 3:50 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [OT]: Re: [PIC] Timer Tribulations
>
>
> Vasile Surducan wrote:
> >
> > On Wed, 21 Feb 2001, Tony Nixon wrote:
> >
> > > You will find that the clock will drift one way or the
> other depending
> > > on temperature, so using the mains as a frequency source
> is much more
> > > accurate.
> >
> >   You are certainly joking ! Or in your country mains
> frequency drift is
> > under 0.5% ...
> >
> >   Vasile
> >
>
>
> Perhaps a bit of AC math about power consumption in a system
> as large as
> a state grid may reveal why power companies try to keep the
> frequency as
> stable as possible.
>
> I'm definitely no expert here, but I'll bet it is definitely in the
> power company's best interest.
>
> Upteen million clocks around the world wouldn't be using the principle
> if it was unreliable.
>
>
> --
> Best regards
>
> Tony
>
> mICro's
> http://www.picnpoke.com
> mailto:sales@picnpoke.com
>
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