Forgive my poor wording.  I should have said "...an immediate TOTAL loss
of load...".  Opening the blow off valves completely is done when the
load disappears and the plant needs to dissipate the enormous amount of
stored energy as fast as possible before it does damage.  A typical
large generation station's net load varies fairly gradually.  Due to the
short term random nature of various small incremental loads being
continuously switched on and off, the generator is not normally hit with
large abrupt load changes.  They can and do occur, however, for instance
when large industrial loads come on and off line, or as a result of
protective relaying opening a transmission line segment in response to a
fault, and when the faulted line and connected load is restored.
 
A sudden 5% load change is fairly abrupt and large.  Time is required
for the generator, as well as all the other interconnected generators,
to respond to the load change.  The load presented to the generator
decreases and the local system frequency will increase.  The generator
control systems will respond by lowering excitation voltage, changing
generator power factor angle, backing off the fuel supply, and in more
drastic cases, venting some steam.
 
If the magnitude and rate of a sudden load increase is too great for a
generator to accommodate it, it can begin to get out of synch with the
system, and relaying will take it off line.  The additional load still
exists and now a source of power has been loss.  This causes an even
greater amount of load to suddenly be presented to the remaining
generators on the system.  Unless intelligent relaying takes off the
loads and stops the dominos, generators will be begin to cascade offline
and you'll end up with a regional blackout.
 
M Peterson
 
 
>Message: 39
>Date: Tue, 31 Jul 2007 22:42:20 -0400
>From: Dr Skip <drskip@gmail.com>
>Subject: Re: [EE] Shutdown the planet for 5min..
>
>Why would there not be a redirection of the steam at its nominal usage
>rate, rather than a total blowoff, until someone determined why things
>went away? You could vent and throttle down, but still keep the
>'boilers' going until someone higher up determined this wasn't a 5
>minute "lights out" prank or such. Seems much cheaper and reasonable
>than blowing it all out as a first response...
>
>-Skip
>
>Mark Peterson wrote:
>> Generating stations can handle an immediate loss of load without
damage,
>> and do so regularly.  Although they can handle it, such events are
>> painstakingly avoided because they are very annoying and very costly.
>> When such a loss happens, they immediately blow off their steam
>> reservoir to prevent the generator rotor from over speeding.  I've
been
>> involved in several unplanned shutdowns of this type at a 600 MW coal
>> fired power plant; two due to operator error, one to a nearby
industrial
>> explosions, and one as the result of a tornado.  The sudden release
of
>> steam generates a roar that will you cause you to shake and/or drop
to
>> the ground.
>>  
>> The operator error caused shutdowns were always a case for extreme
>> chastisement of the offending employee because it would take nearly 8
>> hours to bring the unit back on line with a total restart cost and
>> loss-of-revenue cost of approximately $100,000.
>>  
>> M Peterson

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