If something, a system, a generator, (anything!), is trying to control an output variable but, in-fact,
has no control of the output variable it will normally go all the way in the direction the error points.
Does this make sense to you Mark?
Example: A valve is looking at the water level in a tank. The tank has a hole in the bottom too big for the supply the valve is controlling to fill the tank back up to the desired level. The valve is going to be wide open ALWAYS but it won't ever affect the level it's trying to control to.
A generator working against a LARGE power network is in the same situation. Whatever the network voltage is one generator is not going to change it. It simply doesn't have the
authority to force a change. So, the regulator controlling the field which is attempting to control the output voltage is going to go to a maximum or minimum depending on the voltage setting it's asked to control to. If the grid is 20kV and the generator voltage control is set to 20.001kV the regulator will go full ON trying to drag the entire network up one more volt.
In some cases this could fry the regulator or burnout the field as it will just sit pegged the entire time. It could and likely would trip the unit off line.
In this situation the generator control is changed to an output variable it does have control over. VARs, or power, or current, all variables that can be controlled against a rigid fixed voltage.
Keith Cress
kcress -