SYSTEM-LEVEL CONTROL OF INVERTERS IN DC MICROGRID

Abstract

Multi-terminal dc (MTdc) medium voltage distribution systems will leverage the integration of distributed energy resources interfaced by dc-dc converters. Faster and robust controllers are hugely needed for accurate voltage control. This thesis proposes and analyses a control scheme for the converter control layer of these dc-dc interfacing converters in the MTdc system. This controller should reject any disturbance at the converter terminals and stabilize the output voltage, without any knowledge of the network model. An Active Disturbance Rejection Control scheme (ADRC), by capitalizing on the control structure of a Linear Quadratic Gaussian (LQG) control with a set-point trajectory generator has been proposed. The ADRC method is based on the model of the virtual disturbance, which represents the external and internal disturbances at the converter terminals. In the proposed control structure, the ADRC parameters are systematically designed. Being an ADRC scheme, the proposed converter controller is inherently robust to external disturbances and plant model uncertainties. In addition, the proposed control structure ensures the robustness of the ADRC scheme to variations of the control parameters. Furthermore, the systematic design of the ADRC parameters allows the verification of the theoretical assumptions of the ADRC convergence. In this way the convergence of the proposed control structure is analytically demonstrated, i.e., the converter control layer yields the known nominal value of the voltage output of the dc-dc converter. Moreover, convergence of a multi-converter system is proven, by showing analytically that the system interactions perturbing the dynamics of each converter can be accurately represented by the local virtual disturbance model at the converter terminals. The theoretical proof is also validated by a small signal stability analysis for the single terminal as well as the Multi Terminal dc grid. Using modal analysis, the stability of the MTdc grid is verified and interactions between the ADRC controlled systems are investigated. ADRC equations are derived for buck, boost and DAB converters and MATLAB/Simulink models are developed to test those models. The buck, boost and DAB converter are used at source terminals and buck converter is used at load terminal. The PID controller is used to control the load converter.