Rotor Oscillations of a synchronous generator due to under excitation 1

[constantins]

What exactly causes rotor oscillations of a synchronous generator (while delivering complex power) during stages of under excitation.

Many thanks
Constantins

 

[wbd]

Underexciting a generator where it exceeds the generator capability curve results in overheating the structural parts of the stator end winding and ends of the stator core and results in pole slipping (generator instability).

 

[electricpete]

I'm assuming you're talking about stability of the generator in avoiding unstable rotor angle swings while connected to a grid.

As you may be aware a system will be less stable as the real power transported approaches the capacity of the line. Power transmitted by a long line (inductance) is approx sin(delta)*|E1||E2|/XL where E1 and E2 are voltages on each end of the line and delta is the angle between them. Syncronism is lost if delta exceeds 90 degrees since power handling capability starts decreasing at that point. In the underexcited case E1 will be lower and E2 will also likely be lower (depends on system reactive power balance), resulting in a lower power handling capability for the line.

So for a given amount of real power required to be transmitted, it is more stable to operate with higher voltage.

Note that real situations are of course much more complex... several transmission lines and internal reactance of generator. But in general the conclusion still holds... higher voltage causes increased ability to transmit real power.