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From: squishy@casbah.acns.nwu.edu (Shishin Yamada)
Newsgroups: sci.electronics
Subject: Re: Power Plants
Message-ID: <2573@casbah.acns.nwu.edu>
Date: 13 Jan 91 22:57:00 GMT
References: <3122@naucse.cse.nau.edu> <10012@as0c.sei.cmu.edu>
Organization: Northwestern University
Lines: 31

In article <10012@as0c.sei.cmu.edu> rsd@sei.cmu.edu (Richard S D'Ippolito) writes:
>In article <3122@naucse.cse.nau.edu> Robert Wier writes:
>
> > I was suprised to learn when the plant was shut down that they didn't
> > disconnect from the grid (for example, to clean the pipes and
> > turbine valves).  They just let the grid power the generators as 
> > large electric motors.  Said it was a whole lot easier than trying
> > to bring them up from a dead stop and re-synch them to the grid
> > (which had to be done by hand).  
>
>
>There's another reason that _very_ large horizontally-mounted generators are
>never stopped -- if they sat there for a long time, the weight would cause a
>permanent flat spot in the shaft and bearings!
>
This is probably one of the reasons. The other most likely is that they are
using these generators like large synchonous motors. They are then used to
store power and correct for minor phase differences, much like a large
capacitor (or inductor in other cases as seen by the network). They can
then be used to stabilize the power grid. Systems like this were built
since the 1920s and are called something like synchronous stabilizers.
Nowdays, switching capacitor banks are used as they are cheaper and easier
to control. At least this is what we just learned in EECS C61 - Power
Transmission and Control class.
----------------------------------------------------------
Shishin Yamada - Northwestern University
Electrcial Engineering Class of 1991
----------------------------------------------------------
PS: It seems the old stabilizers were large motors often without loads
attached to their shafts. By their shear shaft inertia, they would _help_
correct the phase back to normal.