Multivariable control strategy for autonomous operation of a converter-based distributed generation system

This paper presents a control strategy for the autonomous (islanded) operation of a distributed generation (DG) unit. The DG unit supplies a balanced load through a voltage-sourced converter (VSC). To maintain the autonomous operation in the islanded mode, the DG unit should provide its dedicated load with a sinusoidal voltage with a constant magnitude and a constant frequency. The dynamic model of the islanded DG system is represented by a set of nonlinear equations. Since the objective is to regulate voltage and frequency of the islanded DG about their rated values, the nonlinear model is linearized about the operating point. The obtained linearized model represents a multivariable LTI system with the converter voltages as the inputs, and the load voltage magnitude and frequency as the outputs. Based on the Nyquist array method, a controller is designed to achieve the objectives for the islanded system. To verify the effectiveness of the proposed control strategy, a set of nonlinear simulation case studies are carried out in the MATLAB/SimPowerSystems toolbox. In particular, the performance of the proposed controller with respect to the reference signal tracking and the load resistance perturbations is verified. The simulation results show that the proposed control strategy offers a good robust tracking/performance properties.