Hello wuqn
Droop is usually a governor setting, not an Automatic Voltage Regulator setting.
Droop controls the amount that the prime mover slows down as the load is increased on an islanded set. Droop in combination with the speed setting controls the amount of load a paralleled set will take.
Droop is usually measured in RPM.
If you were to mis-apply the term to an AVR, it would describe the drop in output voltage as the output current increases.
By level, I assume that you mean the voltage setpoint. On a single set, this controls the voltage of the set.
On a paralleled set, this controls the power factor of the set. A higher voltage setting produces more VARS. It also controls the system voltage in the ratio of the size of the set to the rest of the system.
A co-generation set connected to a large utility, the effect of the AVR setting on the system voltage will be negligible.
A small islanded plant with two equal generators in parallel, the effect of a small increase in the AVR setting of one set on the system voltage will be roughly 1/2 the effect that would result if the set was running alone.
Gain; This is another description of droop, but with different units.
Gain is a ratio usually described by a whole number.
The same characteristic may be described as proportional band.
Proportional band is usually described by a percentage.
Again, more commonly applied to a governor than to a voltage regulator.
Negative sequence currents are caused by load conditons. Neither AVR settings nor governor settings will cause negative sequence currents.
yours
You may have a generator winding failing to ground.
There are a couple of things that you can do.
A zero sequence current implies the presence of a zero sequence ct. This will probably be a window ct with all the load conductors passing through it.
Make sure all the conductors pass through the window, including the neutral conductors.
Check to see if anyone has added a small load directly to the generator and bypassed the window ct with either the phase conductors or the neutral conductor.
Take the generator out of service and megger the windings.
There is a strong possibility that a winding in the generator is failing. At this point you may be faced with the cost of a rewind. If you wait, an internal fault may become an arcing fault and destroy the stator of the generator. The remedy at that point is often to replace the entire generator end.
yours
waross - wuqn said negative sequence current, not zero sequence current.
Negative sequence current could be caused by unbalanced (e.g. single phase) loads on your system. I would think that a negative sequence current of 2% is actually quite low. A common limit on negative sequence voltage is around 2%, and this can imply negative sequence currents of 10% or more.
It's unlikely that the AVR is causing your NPS current. Not impossible, but unlikely. The most probable cause has already been mentioned: load imbalance. Load current imbalance could result from voltage imbalance due to poor connections, or from single phase loading.
How big are the machines? 2% NPS current is a lot on a large HV machine, but probably insignificant on a small LV set.
