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Date: Tue, 13 Aug 1996 18:27:03 -0400
From: "Robert I. Eachus" <>
Subject: Re: Computers causing power outages (Hughett, RISKS-18.32)

> ... Not something that I want to hook my house up to.

Sorry about that, but your house is hooked up to it.  Philadelphia Electric
used to be pretty good about reserve capacity. (I haven't lived there for
years, so I don't currently know.)  However, what do you think caused the
Great Northeast Power Blackout?  What was obviously the real cause of the
recent western blackouts?  The interconnect grids are great if demand stays
within bounds, but when heavily stressed they become amplifiers.

In the recent past politicians and financial analysts alike have been
encouraging utilities to reduce their on-line excess capacity, and to shut
down plants when not required for the current demand level.  But excess
capacity on-line is necessary to have a stable interconnect system.  The
difference in stability between 10 plants at running at of 81% capacity and
9 plants at 90% is dramatic.  (If one facility has a glitch the network goes
wild.  It has just been thrown from stable operation into instantaneous
overload, and at the best of times it will take the network hundreds of
milliseconds--a dozen or so cycles--to stabilize.  If transients cause
generation facilities or transmission lines kick out before substations, the
system is gone before any humans can react.)

The Northeast Blackout was caused by a coil failing in a relay in Niagara
Falls.  The relay had a backup coil which was energized immediately, and the
main contacts never completely opened.  In other words, the failover met
specification.  By the time the (amplified) voltage transient reached New
York City, the result was spectacular, did damage in the tens to hundreds of
millions of dollars, and killed a number of people.  Some of you just saw a
similar demonstration.  Trying to figure out whether a fire in Idaho or a
breaker tripped in California started the electric dominos toppling is
detail.  The important lesson is that if you run a grid too near the edge it
amplifies fluctuations and imbalances.  One glitch and we all reap the

Robert I. Eachus

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