The information provided is rather limited, but I'm assuming that you removed the generator from your loop and replaced it with a constant value. This will result in an error for your PI controller, which is normal. However, the PI controller integrates the error to remove the steady state error introduced by the high gain feedback loop. Assuming you are not running your simulation in real-time, the integrated value will become very high very quickly, which can result in simulation errors. This does not happen for the generator model, because the Pi controller can actually influence the speed and reduce the error to zero.
Hope this helps, Dennie
