iwire — Sub-synchronous resonance occurs when voltage and current fluctuate at well below normal line frequency—60Hz in the US. Imagine a power system as a huge rotating shaft that stretched across a state. Now, for this to work, every generator {for instance, water wheels} are connected by belts to the giant shaft. Also connected to the shaft with belts are grinding stones that crush wheat into flour. One could have pairs of water wheels and stones, but because river water flows 24 hour a day, it would be nice to be able to get your local water power to other people that could use it when you aren’t. [You could earn money, and the flour mill operator might find it economical to use your water power than to produce his own.]
There needs to be a idler wheel on each belt that connects to the big shaft by tension, so that if there are problems, a piece of rotating equipment can be taken off line to prevent its damage, or possibly damage to the other connected power sources and loads.
Because the big shaft is not perfectly stiff, it has a tendency to twist from torsional forces as various waterwheels and grinding stones operate. Small changes in speed over the shaft length stress the shaft—ideally not to the point of damage, but if allowed to persist could be destructive.
Adding water wheels to the shaft {or removing grindstones} has a tendency to speed up the shaft, where removing water wheels or adding grindstones has a tendency to slow the rotating “system” down.
Does that make any sense?
[This message has been edited by Bjarney (edited 08-15-2003).]
