In grid-connected mode, DC microgrids can exchange power with the grid to avoid power
imbalance and energy storage units excessive charge or discharge. In islanded mode,
DC microgrids power imbalance can no longer be solved by the grid. This can result in
energy storage units charging or discharging beyond acceptable limits, and a complete
network malfunction.
The interconnection of islanded DC microgrids to compensate the absence of the grid
for power exchange is a possible solution. However, as islanded DC microgrids are
characterised by renewable generation and variable loads that make power flow in a
network of interconnected DC microgrids an issue of great concern, a way that permits
power to be exchanged without causing system instability must be ensured. Thus, this
work proposes a two-steps control algorithm that determines droop coefficients and
reference voltages which give the network state-space-based control system the ability to
provide optimal power exchange with guaranteed stability — steady-state bus voltages
are maintained within their predefined tolerance limits; measured powers are as close as
possible to the reference powers; and the system operation is kept stable.
The term stability is used in this report, but the scope of the work presented is focused
on small-signal (small-disturbance) stability.
An algorithm, based on the SOC-proportional approach to determine reference powers for
proportional load sharing when power shortage or excess is found present in the network
of DC microgrids, is also demonstrated herein.