H∞-Based Robust Controller for Step-Down Non-Isolated DC/DC Converter Loaded by Constant Power Load in DC Microgrids

Multi-converter-based DC distributed power system (DPS) is most commonly used in DC microgrids. The structure of such systems often consists of many DC/DC switching converters, which loaded by other converters. In such interconnected systems, the cascaded connection of the source converter with its load-side counterpart is a prevailing connection form. The load converter acts as a constant power load (CPL) for its source circuit when it controls its output tightly. Due to the negative incremental resistance characteristic induced by CPL, instability is imposed on the system. Moreover, factors such as variations of input voltage and load as well as the system’s parameter uncertainties can adversely affect the system stability. Hence, to achieve robust and safe performance in these systems, the stability issue and the guarantee of a constant output voltage are of great importance. To realize this goal, this paper presents a well-designed H∞ technique-based robust controller for a cascaded DC DPS consisting of a DC-DC converter operating in continuous conduction mode and loaded by a CPL, taking into account system parameters uncertainty and significant variations in loading and the input voltage. To that end, a small-signal transfer function of step-down non-isolated DC/DC Converter loaded by a CPL is firstly derived via the averaged state-space modeling. Then, by using the unstructured uncertainty modeling approach, all the uncertainties and perturbations considered are added for developing the controller model. The system modeling and the proposed controller design are explained in detail and its effectiveness and robustness against uncertainties are validated by simulation results.