ESL Gunnar. Should read tuning.
Hi cloonypan;
What exactly is the problem that you think that you have. I haven't worked on as many generators as some of the people here, maybe only about 50 or more. I have never had to adjust the "droop" of an AVR.
When a block load hits a set, the frequency drops until the governor can respond and get enough fuel into the cylinders to hold the extra load and come back up to speed. When the frequency drops, transformers and motors may go into magnetic saturation and the current will increase greatly. Even though the increased current is reactive and should not present a real load, there are I2R losses with any current. With the combined resistance of feeder conductors and motor and/or transformer windings, the load of the I2R losses associated with saturation may hinder or prevent the generator prime mover from returning to the set speed. The saturation currents may also fairly quickly burn out motors and transformers.
To address this issue almost all modern AVRs for smaller sets (less than about 2 MW as far as I know) have a feature called Under Frequency Roll Off or UFRO. The UFRO feature typically allows about a 3 Hz drop in frequency and for any further frequency drop will lower the voltage in proportion to the frequency. Thus if a load drops the frequency to 28.5 Hz ([60Hz-3Hz]/2=28.5Hz) you can expect the AVR to drop the voltage to 50% of the normal voltage.
It's doing its job.
You are probably seeing a UFRO effect. Trying to eliminate it by changing the droop setting will probably add instability to the AVR performance.
Bill
--------------------
"Why not the best?"
Jimmy Carter