mailing list archives
RE: East Coast outage?
From: "Eric Germann" <ekgermann () CCTEC COM>
Date: Fri, 15 Aug 2003 23:38:11 -0400
Load management is actually fairly common here in Ohio in the cooperative
electric utilities. Residential users get rebates on heat pumps and water
heaters in exchange for allowing the utility to install RF controlled
interrupting switches on them. Summer ironically isn't the problem for
them, its winter when they want to do peak demand management so as not to
ratchet into a higher wholesale demand rate class.
My guess is when it shakes out, the failure will be traced to a rather large
unit or interconnect tripping offline. Since the load is relatively
constant if you look at the time in a short enough period, and you lose a
couple hundred MVA of feed onto the grid, the other generation on the grid
is going to attempt to absorb it. It works just like a drill, in reverse.
If you put a sanding wheel onto a drill and press it into wood, it will drag
the drill down. Opposite for generation. Steam is driving the turbine,
which is producing power. Throw more load on instantaneously, the rotor
will slow down. Now the units can absorb slight variations in load, but
500MVA falling off quickly cannot be instantaneously absorbed. So, the
rotor slows down. As it slows down, the frequency drops. When the
frequency gets low enough (and we're talking fractions of a Hz), protective
relaying kicks in and opens the breaker between the unit and the grid. This
compounds the effect, because the 500MVA loss may cause another 100MVA in
units to trip off relatively close. Now the grid has 600MVA to absorb and
that loads more units down, which drift farther down and they trip, which
adds another X MVA to the load and it justs keeps going. Same thing can
happen in reverse to when the load is suddenly removed and the unit overruns
This effect was observed a couple of times for a muni electric I used to
work with. They had a tie line to a IOU and when it opened in the summer
becuase of lightning, overload, etc, it would trip all their units off line
because the tie was carrying inbound on the order of 40% of their load.
Interestingly, it had effects on the IOU also, since the muni was consuming
watts, but supplying VAR's, trying to help maintain power factor on the IOU
system. Units can only produce so many MVA's. MVA = sqrt(MW ** 2 + MVAR **
2). As reactive loads go up (like AC units in the summer), MVAR's go up.
According to the formula, MW production goes down since the unit can only
produce so many MVA's (its a nice right triangle, MVA is the hypotenuse, MW
is the horizontal and MVAR is the vertical and power factor is the cosine of
the angle. With a purely resistive load like a light bulb, PF = 1 since
there are no VAR flows there [cos 0 = 1]). They do cheat sometimes and use
capacitors or synchronous condensors/reactors (an overexcited motor which
looks like a variable capacitor, kind of cool) to try and equal out the
power factor. The bite is, Joe Consumer doesn't pay for VAR's, he pays for
Watts. But the transmission and distribution system has to account for and
carry the VAR flows also. And if you size the lines and forget the VAR
flows, in the summer, things can go boom.
Everyone whines because of the "antiquated" system. The system worked like
it should. It may suck to be without power for 48 hours, but try 18 months
if the unit came apart. You don't go to Ace Hardware and buy a new 50MVA
steam driven unit. And the nukes tripping off was probably more an artifact
of frequency instability on the grid than a problem with the nukes
themselves. Coal, gas or nuke, you still have to maintain frequency. As an
old EE prof of mine said, the system will seek stability. Seeking may be
nice like flow re-distribution, or it may be ugly like the rotor and frame
separating. Either way, it ends up stable (albeit maybe in the field next
to the plant) ...
From: owner-nanog () merit edu [mailto:owner-nanog () merit edu]On Behalf Of
Iljitsch van Beijnum
Sent: Friday, August 15, 2003 6:25 PM
To: alex () yuriev com
Cc: nanog list
Subject: Re: East Coast outage?
On vrijdag, aug 15, 2003, at 23:58 Europe/Amsterdam, alex () yuriev com
Amount of energy generated must be balanced with the amount of energy
at any time. Otherwise Bad Things (tm) will happen. The shutown of the
grid is a very good thing compared to what it would have been had it
It seems to me that the power guys are still living somewhere in the
last century. Is it really impossible to absorb power spikes? We can go
from utility to battery or the other way around in milliseconds, so it
should be possible to activate something that can absorb a short spike
much the same way. Balancing intermediate-term generation/usage
mismatches should be possible by simply communicating with users. There
is lots of stuff out there that switches on and off periodically (all
kinds of cooling systems, battery charging, lights), so let it switch
on or off for a few minutes when the power network needs it to.
I think the idea that the power should be always present and always
reliable is actually harmful, as it doesn't provide for any "congestion
contnrol" by bringing the users into the loop.