The proportion of the intermittent and non "dispatchable" renewable resources such as solar or wind power will increase in the future. A higher flexibility will be required for the "controllable" generation facilities to guarantee the power production and consumption balance and particularly the combined cycle. Numerous studies have been done to reduce the start-up time for combined cycle, but some grid variations necessiate faster power response known as primary and secondary frequency control which consist of power variations added to the power reference. To deliver a faster and bigger powier response it is worthwhile to consider the use of the steam turbine. The throttling influences the efficiency and the live steam pressure such that the problem is intrinsically multivariable and requires coordination of the gas and steam turbine loads. The paper presents a solution based on advanced control strategies to improve the response of the combined cycle plants. The technique used is a Model Predictive Control (MPC) based on a linear model of the process which reproduces the effect of the gas turbine load and the throttle of the steam turbine valve on the electrical power and live steam pressure. Results obtained in simulation are also given and show the participation of the steam turbine for primary responses.