  In this work, we present a novel consensus-based secondary control scheme for
current sharing and voltage balancing in DC microgrids, composed of distributed
generation units, dynamic RLC lines, and nonlinear ZIE (constant impedance,
constant current, and exponential) loads. Situated atop a primary voltage
control layer, our secondary controllers have a distributed structure, and
utilize information exchanged over a communication network to compute necessary
control actions. Besides showing that the desired objectives are always
attained in steady state, we deduce sufficient conditions for the existence and
uniqueness of an equilibrium point for constant power loads -- E loads with
zero exponent. Our control design hinges only on the local parameters of the
generation units, facilitating plug-and-play operations. We provide a voltage
stability analysis, and illustrate the performance and robustness of our
designs via simulations. All results hold for arbitrary, albeit connected,
microgrid and communication network topologies.
