The research work developed in this thesis has been mainly devoted to the observation and sensorless control problems of electrical systems. Three major contributions have been carried out using the high - gain concept and output feedback adaptive nonlinear control for online emergency power supply. In this thesis, we dealt with the synthesis of sampled high - gain observers for nonlinear systems application to permanent synchronous generators and doubly - fed induction generator. We particularly focus on two constraints: sampling effect and tracking unmeasured mechanical and magnetic state variables. The first contribution consists in a high gain observer design that performs a relatively accurate estimation of both mechanical and magnetic state variable using the available measurements on stator currents and voltages of PMSM. We propose a global exponential observer having state predictor for a class of nonlinear globally Lipschitz system. In a second contribution, we proposed a novel non – standard high gain observer design for non-injective feedback relation application to variable speed DFIGs based wind power generation systems. Meanwhile, a reduced system model is analyzed, provided by observability test to check is it possible synthesis state observer for senseless control. In last contribution, an adaptive observer for states and parameters estimation are designed for a class of state - affine systems application to output feedback adaptive nonlinear control of three-phase AC/DC boost power converter for online emergency power supply systems. Basically, the problem focused on the cascade nonlinear adaptive controller that is developed making use Lyapunov theory. The parameter uncertainties are processed by the control laws under backstepping design techniques with a capacity of adaptation.