Design and small-signal stability analysis of a virtual-capacitor control for DC microgrids

At microgrids, classical grid-regulating generators are replaced by converter-interfaced distributed generation (DG) or energy storage systems (ESSs). This causes a reduction of the total inertia of the system and hence the microgrid voltage and frequency become more susceptible under power variations. DC microgrids are specially affected by this phenomena as systems with rotating inertia can not be directly connected. In the case of ac microgrids, an alternative is to employ strategies that emulate the behavior of classical generators, such as virtual synchronous machines (VSM), because they are capable of emulating the inertial behavior with power converters. Inspired by the operation concept of classical generators and VSM techniques, in this paper an autonomous virtual-capacitor control is designed for dc micro-grids, which provides synthetic capacitance to the system. One of the most interesting advantages of the proposed strategy over conventional approaches is that the rate of change of voltage can be decreased by simply varying the virtual-capacitor. In addition, the transient and steady-state behavior of the converter can be defined independently by varying the virtual-capacitance and virtual-impedance, respectively. This feature enables the integration of generation or storage systems with different dynamics. In order to determine the values of control parameters and delimit the stability boundaries for different points of operation, we analyze the stability of the proposed strategy by looking at the dominant eigenvalues of the small-signal state-space model. Simulation results are included to highlight the advantages of the virtual-capacitor technique over classical strategies and to carry out the parametric stability analyses.