[Lowfer] Power Outages

Mon, 18 Aug 2003 17:20:49 -0400

Hi Pete,

I'm employed at a coal fired power plant in Somerset, MA. Our facility has
(1) 100 MW steam powered generator and (1) 22.5 MW jet driven unit for black
start capability.

To help answer some of your questions without going into a lot of detail...

Generators are taken off line or "tripped" by the opening of the main
breaker commonly referred to an OCB (Oil Circuit Breaker) connecting the
generator to the power grid. All in all, think of it as a large knife switch
that is thrown. This typically happens with a sudden or emergency shut down
followed by the closing of the main steam stop valve feeding steam to the
turbine from the boiler. Overall, plants like to discourage this type of
event because it is a sudden and abrupt event, but in the case with the
cascading effect of the blackout most plants were probably caught in this.
Plants also trip due to phase changes detected on the grid itself, such as
the line phase or load swings. The plant I work for is designed to trip if
the line phase goes above or below a couple of Hz.

A controlled trip is usually performed by slowly phasing the generator off
the grid (hence, reducing MW output while maintaining 60 Hz line frequency),
followed by reducing fuel to the boiler producing steam. Once load has been
reduced down to a couple of megawatts, the main OCB is opened and the unit
is tripped by initiating the MFT known as the Master Fuel Trip. This
shutdown ceases all fuel to the boiler, putting the fire out, and closing
the main steam stop valve to the turbine. Once this occurs, the
generator/turbine is allowed to freewheel from 3600 RPM down to a few RPM
and the unit is placed on turning gear. Turning gear is performed to allow
the unit to rotate and cool down uniformly due to high turbine temperatures
near 1000 degrees. Last to mention and prior to the unit trip is that
auxiliary systems such as pumps, control systems, computers and lighting
still require power to function. Breakers are switched over so that the
power grid can back feed the plant and no interruption of power occurs
during shutdown.

Typically, power generators are 3-phase AC and output 14-15KV. The generator
field or stationary windings are excited with an external DC voltage supply
known as an exciter. This voltage of approximately 375 volts is fed through
commutator or slip rings via carbon brushes mounted on the generator shaft.
This is the typical case with our plant generator, Unit 6, which is a 1959
General Electric 120 MW hydrogen cooled generator. Following the main
generator output leads, 14KV is fed parallel to two (2) separate
transformers. One being a 14KV-115KV step up transformer which feeds through
the main OCB followed onto the power grid for transmission. Note that larger
power plants step up power to 345KV and higher dependent upon transmission
requirements. The second transformer is a step down transformer that reduces
the generator output voltage to 2300 volts needed for plant station service.
This power supplies all pumps, fans, auxiliary systems and lighting
necessary for the plant to operate.

FYI, if we did not have any phone or radio comm in our control room, we
would have never known of the blackout. No bumps or brown outs were
encountered on our end.

Denis - W1WV
Swansea, MA

-----Original Message-----
From:	[email protected] [mailto:[email protected]] On
Behalf Of Peter Barick
Sent:	Monday, August 18, 2003 3:14 PM
To:	[email protected]
Subject:	Re: [Lowfer] Power Outages

>>> [email protected] 08/15/03 09:24PM >>>
>> Okay it's not you. What about Mitch?

>Bit of an update - Lost power here ( in my neighborhood ) for 11
hours.

>MP update - back on the air   137.780  today ( Friday 9PM EDT ).
>Mitch
----------------------------------
Heh-heh, good that's important. Yah, thought about Mitch Thurs., the
last day I was Web connected, he being right on top of the "Lake Erie
Grid." That's the first I heard of that artifact too.

Being near the Chicagoland area and nearer to one of the power nukes I
am aware of the power distribution fields that emanate west and head
east into a network, but didn't know where that extended: to Ohio? PA?
NY? Didn't know and, like most, didn't care before 8-14. Now,
post-Thursday, it's the talk: The Power Grid! Ipso facto, guess the IL
one extents to Indiana and foregoes that Erie "round-about." Nice
concept when it works ... hmm. I have a few questions from the past few
days of news accounts and some reporting gaps. Maybe some on the list
are power industry savvy and could answer.

1) How is a power generator taken off (or put back on) the power grid?
I think some envision a large knife-switch being operated, which I
reject, but somewhere contacts have to be broken, no? (Ha, maybe this is
where the concept of electric welding began.)

2) Do all plants generate 3-phase? What are they timed to?

3) What controls the period, currently 60cps? Is it the armature speed
at some integral of 60?

4) Can a generating plant have it's load removed (say by some event
outside the plant, unplanned) without consequence to the generator, ie,
one moment pushing mega electrons, the next not?

5) Knowing of this recent "cascading effect" of the generators going
down, as on Thursday, is that a very robust system or an engineering
crap shoot? If the latter, how'd they get away with dumping it on the
public and their national security?

Enuf, any care to shed some "light" on this socio-engineering issue?

Peter
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