The work presented in this paper deals with obser-vability analysis of a synchronous machine connected to an infinite bus. The dynamical behaviour of the latter is represented by a one axis fifth order model including regulators with unknown reference signals. Different possible outputs scenarios are checked to achieve observability conditions. Then, an observer based on Extended Kalman Filter (EKF) algorithm is proposed by considering the reference signals as a part of its state. The observer performance is tested with and without regulators saturations.