Hello dear Aamir, to study the frequency variation in power system as a result of load or generation change the fourth order model is the most widely used. You can represent the equivalent inertia for your machines. Please have a look at the following papers:

https://www.researchgate.net/publication/303405010_Robust_Stabilization_of_Load_Frequency_Control_System_Under_Networked_Environment

I hope this will help you.

Best Regards

Ashraf

Article Robust Stabilization of Load Frequency Control System Under ...

The answer for your question is Automatic Generation Control (AGC). You may refer to my papers available in RG. Also so much literature is available on this in RG itself.     

as Krishnarayalu answered, AGC is your problem. Some academical references are:

1 - chapter 11 of Kundur's book - have some numerical examples and block diagrams, so you can implement those in matlab/simulink and evaluate your solution

2 - chapter 9 of Machowski, Bialek and Bumby's book - also have numerical examples of load-frequency control problem.

regards.