| Abstract |
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 systemand hence the microgrid voltage and frequency become more susceptible under power variations. DCmicrogrids are specially affected by this phenomena as systems with rotating inertia can not be directlyconnected. In the case of ac microgrids, an alternative is to employ strategies that emulate the behavior ofclassical generators, such as virtual synchronous machines (VSM), because they are capable of emulatingthe inertial behavior with power converters. Inspired by the operation concept of classical generatorsand VSM techniques, in this paper an autonomous virtual-capacitor control is designed for dc microgrids,which provides synthetic capacitance to the system. One of the most interesting advantages of theproposed strategy over conventional approaches is that the rate of change of voltage can be decreased bysimply varying the virtual-capacitor. In addition, the transient and steady-state behavior of the convertercan 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 orderto determine the values of control parameters and delimit the stability boundaries for different points ofoperation, we analyze the stability of the proposed strategy by looking at the dominant eigenvalues ofthe small-signal state-space model. Simulation results are included to highlight the advantages of thevirtual-capacitor technique over classical strategies and to carry out the parametric stability analyses. |
