Richard: One more question on the brushless alternator. Are there two separate rotors or are all the windings in the same slots.
Yes, they are two completely seperate units on the same shaft. The exciter is significantly smaller and is really another brushless alternator. In the 500 watt range I believe, but it has been many years since I have been involved with them.
Your comment about 20% voltage swing on load change. I hope that didn't occur for more than 1 or 2 cycles. On a slip ring type rotor, they will correct that fast. I used to rebuild regulators for a company that rented 200 - 600 kw plants and they wouldn't accept the regulator if they could see a 6" panel meter move when they dumped on full load in a load bank.
20% is typically worst case and most of the time it is better than that, depending on what the actual load is and how well it will tolerate the dip. Recovery from dip or overshoot was typically measured in milliseconds.
In many cases I might start with an alternator that was double the size required to reduce the dip. In other words I might use a 500KW alternator when the maximum connected load would only be 250KW. Therefore a 100% load change would be 250KW on a 500KW unit. The computers back in the 70's were very sensitive to sudden voltage or frequency changes. I sometimes added a large flywheel to prevent frequency variation when a sudden load was placed on the engine genset.
On AC motor gensets I utilized synchronous motors so that the frequency would not vary if there was a large load change. Synchronous motors run at exactly 1800 rpm regardless of load or voltage change as long as it is within the operating parameters of the motor.
Many generating stations connect into the grid on DC lines so they are isolated from a standard frequency on the grid. I have a friend who is a shift engineer at a power plant with steam turbines. I will ask him how they control the speed to that kind of tolerance with changing load. He has told me how many degrees tolerance they have when switching a generator onto the system. (but I don't remember).
For more detailed information about the North American Power Grid you might want to review this site: http://en.wikipedia.org/wiki/North_American_Electric_Reliability_Council
NERC was formed back in the 60's and I was an active member at one time many years ago.
This pretty well describes the system and how all the AC power grids throughout the country are now tied together into one massive grid. Individual power plants no longer have much effect on the overall grid.