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Re: Cascading Failures Could Crash the Global Internet

  • From: N. Richard Solis
  • Date: Thu Feb 06 19:15:53 2003

Vadim Antonov wrote:



On Thu, 6 Feb 2003, N. Richard Solis wrote:

> The main cause of AC disruption is a power plant getting out of phase
> with the rest of the power plants on the grid.

This is typically a result of sudden load change (loss of transmission
line, short, etc) changing the electromagnetic drag in generators, and,
therefore, the speed of rotation of turbines.

> When that happens, the plant "trips" of goes off-line to protect the
> entire grid.

Some difference in phase is tolerable, the resulting cross-currents
generate heat in the trasmission lines and transformers.

It is not sufficient to disconnect a generator from the grid. Since water
gates or steam supply can not be closed off fast, the unloaded turbine
would accelerate to the point of very violent self-destruction.  So the
generators are connected to the resistive load to dump the energy there.
Those resistors are huge, and go red-hot in seconds.  If a gate or valve
gets stuck, they melt down, with the resulting explosion of the turbine.

> You lose some generating capacity but you dont fry everything on the
> network either.

Well... not that simple.  A plant going off-line causes sudden load
redistribution in the network, potentially causing overload and phase
shifting in other plants, etc.  A cascading failure, in other words.
Yeah yeah yeah.  I know that everything isn't simple.  I actually worked at a power plant so none of this is new to me.  Can cascading failures occur?  Yes.  Witness the Great Blackout in NYC.  My point was that there are places where the electrical network is designed to "blow the bolts" to TRY and protect everything.  Does it work?  Most of the time, yes.  All of the time? NO.

It is a complicated problem but you'd be suprised at how fast things can happen when you HAVE to keep the system running.  There is a tremendous amount of skill concentrated in that field and they do a good job of keeping everything running well.  How many turbine overspeed events do you get notified about?  Those guys can do a rapid shutdown of a plant VERY quickly.  Turning it back on though is a whole different matter.  We needed to have one station operating so that we could actually get the big one going.  Then we'd take the small one offline and bring it back up quickly to handle specific load peaks.

The loss of a single transmission line isn't going to cause a whole station to trip.  If you're losing a bunch though, you've probably got lots of other problems to worry about.


--vadim