Abstract The frequency of an electrical power system is controlled by generators doing primary frequency regulation, which stands for maintaining the stability and reliability in short times and secondary frequency regulation, also known as Automatic Generation Control, which its main goal is to maintain the frequency in a set-point. This last is done remotely from the control centre of an electric power system operator in most cases, and to carry it out supervisory control and data acquisition -automatic generation control- energy management system. These systems are used that make use of current international standards of the International Electrotechnical Commission that adapt in any case to many supervision and control mechanisms. Using those standards supposes the use of a range of available options; particularly, the commands used to control electric power generators fall into 2 possible options, and to evaluate responses under disturbances, a study of the modelling of generators under automatic generation control becomes necessary. A study of the response in closed loop control of automatic generation control is discussed to determine its models under different scenarios. Then, this is the input to determine responses to simulated deviations on the commands due to known failures or disturbances.