Advanced Static Var Compensators (ASVCs) are high power electronics based devices used to provide fast variable reactive power compensation to power networks. They should be properly controlled to ensure fast and continuous reactive power to meet a certain fluctuating load demand and enhance the transient stability of the power system. The effectiveness of these compensators depends on the choice of the control strategy. In this paper, we deal with the application of the Internal Model Control technique (IMC) and the State Feedback Control (SFC) concept to adjust the ASVC Var flow with the ac transmission network. These controllers are evaluated under a variety of operating conditions where performances and robustness have been analyzed and compared to a conventional PI controller. Simulation results in the case of a non linear model show that SFC and IMC controllers, suitable for real time implementation, lead to improved transient response and hence provide fast reactive power compensation to ac transmission networks.