Cascading Failures Could Crash the Global Internet

Thu Feb 6 23:25:43 UTC 2003

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.

--vadim