Large electrical systems have always been strongly associated with the use of synchronous machines for power generation and load service, with other benefits, possibility of providing dynamic control of the system voltage profile and frequency regulation. Regarding frequency control, synchronous machines aggregate inertial response, i.e., convert part of the kinetic energy stored in their rotors into electrical energy (and vice-versa), whenever there is an imbalance between load and generation. This behavior determines the initial Rate of Change of Frequency (RoCoF), determined mainly by two factors: a magnitude of the active power unbalance and the global inertia of the system, that is dependent on the number of synchronized generators and the inertia of each generator. The increase of wind and solar photovoltaic plants in electrical energy matrix, which are usually connected to the system through inverters and do not aggregate inertial response to the system, will impact on frequency regulation and RoCoF values. In this scenario, the main objective of this thesis is to present a quantitative analysis of the evolution of RoCoF in the Brazilian Electrical System during outages of large generation blocks. RoCoF values are analyzed from a global point of view, through the frequency at the center of inertia of the system, and from a local point of view, through buses that represent the Brazilian Electrical Subsystems.