This problem is really too big, and some principles can only be understood by drawing. I suggest you find a book, Principles of Automatic Devices in Power Systems, edited by Xiong. There is a chapter mainly about automatic frequency adjustment of power system, which is very detailed. It is difficult to cover how to estimate the automatic frequency modulation of large power grid. There are many formulas to be derived, but they must be useful, because my graduation thesis is: preventing the system from low-frequency oscillation.