  There are tradeoffs between current sharing among distributed resources and
DC bus voltage stability when conventional droop control is used in DC
microgrids. As current sharing approaches the setpoint, bus voltage deviation
increases. Previous studies have suggested using secondary control utilizing
linear controllers to overcome drawbacks of droop control. However, linear
control design depends on an accurate model of the system. The derivation of
such a model is challenging because the noise and disturbances caused by the
coupling between sources, loads, and switches in microgrids are
under-represented. This under-representation makes linear modeling and control
insufficient. Hence, in this paper, we propose a robust adaptive control to
adjust droop characteristics to satisfy both current sharing and bus voltage
stability. First, the time-varying models of DC microgrids are derived. Second,
the improvements for the adaptive control method are presented. Third, the
application of the enhanced adaptive method to DC microgrids is presented to
satisfy the system objective. Fourth, simulation and experimental results on a
microgrid show that the adaptive method precisely shares current between two
distributed resources and maintains the nominal bus voltage. Last, the
comparative study validates the effectiveness of the proposed method over the
conventional method.
