Increasing the region of attraction in DC microgrids

Based on the port-Hamiltonian framework, this paper proposes a novel control scheme for stabilising the voltage in DC networks affected by (i) unknown ZIP-loads, i.e., nonlinear loads consisting of the parallel combination of constant impedance (Z), current (I) and power (P) load types, and (ii) unknown (but bounded) time-varying disturbances. Differently from the results existing in the literature, where restrictive (sufficient) conditions on the load parameters, voltage trajectory and voltage reference are assumed to be satisfied, this is the first paper (to the best of our knowledge) proposing a controller that relaxes such conditions and guarantees the exponential stability of the desired equilibrium point, whose region of attraction can be increased by simply tuning the control gains. In the case the network is affected by unknown time-varying disturbances, local input-to-state stability (l-ISS) is ensured. Furthermore, if non-ideal P-loads are considered, excluding the unrealistic possibility that the load absorbs infinite current when the voltage approaches zero, the aforementioned stability results hold globally.