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From: karn@petrus.UUCP (Phil R. Karn)
Newsgroups: net.analog
Subject: Re: Selling Energy to the Utilities
Message-ID: <482@petrus.UUCP>
Date: Mon, 26-Aug-85 17:45:05 EDT
Article-I.D.: petrus.482
Posted: Mon Aug 26 17:45:05 1985
Date-Received: Wed, 28-Aug-85 20:31:39 EDT
References: <288@ihnet.UUCP>
Distribution: net
Organization: Bell Communications Research, Inc
Lines: 36

To feed power into the utility grid, your generator must be synchronous
with the grid's frequency, but leading slightly in phase. You control
how much power you put into the network by changing the phase difference.
If you match it exactly, no power will flow at all, and if you lag the
grid's phase, you will draw power from it instead.

Here's a mechanical analog that might help to visualize this. Imagine your
AC wall outlet as a steel shaft rotating at 3600 RPM (60 revolutions/sec).
No matter what you connect to this shaft, you cannot change the speed at
which it turns (unless you exceed its breaking strength, at which it stops!)
but you can still take power out or put it in.

If you connect a gasoline engine to the wall shaft through a stiff rubber
coupling, it will turn even without fuel except that you'll be drawing power
from the outside to do this, and the rubber coupling will twist such that
the engine shaft phase lags the "line" phase. Now feed some gas into the
engine and open the throttle slightly. If you give it exactly enough gas to
overcome engine friction while operating at 3600 RPM, the rubber coupling
will not be twisted in either direction and no power will flow across it.

If you now open the throttle further, the engine will attempt to drive the
outside shaft. The engine will now lead the drive shaft's phase, twisting
the rubber coupling in the other direction.  The amount of this phase
difference is proportional to the amount of power being fed into the outside
network. Notice that during all of this the engine has been running at
constant speed; only its phase relationship with the outside has changed.

This model works well as long as your generator is tiny in comparison with
the others feeding the grid (valid for home windmills and the like).
However, I suspect that things get tricky when dealing with the output of a
GW nuclear plant, and I understand that sudden and large power flows can
occur when things like blackouts and plant trips occur. One of the
advantages of DC power transmission is supposed to be greatly increased
control over power flows.

Phil