How to create Frequency disturbance in a grid

Hi everyone!

I have to design for my thesis an algorithm for frequency support. I have some ideas but I do not know how to create the frequency disturbance in the grid. I usually use the sources from SimPowerSystems but if I change the frequency from the dialog, then I cannot correct (probably because it is a stiff source).

So I guess I have to model the entire grid. I tried to set up a generator but the output was zero. I have also tried to use some generators from the distributed resources but I cannot set them right for my system... Is there any other way to model a simple power system or to create a disturbance in the frequency?

Thank you in advance,

George