  In this paper, we consider a hierarchical control based DC microgrid (DCmG)
equipped with unknown input observer (UIO) based detectors, where the potential
false data injection (FDI) attacks and the distributed countermeasure are
investigated. First, we find that the vulnerability of the UIO-based detector
originates from the lacked knowledge of true unknown inputs. Zero trace
stealthy (ZTS) attacks can be launched by secretly faking the unknown inputs,
under which the detection residual will not be altered, and the impact on the
DCmG in terms of voltage balancing and current sharing is theoretically
analyzed. Then, to mitigate the ZTS attack, we propose an automatic and timely
countermeasure based on the average point of common coupling (PCC) voltage
obtained from the dynamic average consensus (DAC) estimator. The integrity of
the communicated data utilized in DAC estimators is guaranteed via UIO-based
detectors, where the DAC parameters are perturbed in a fixed period to be
concealed from attackers. Finally, the detection and mitigation performance of
the proposed countermeasure is rigorously investigated, and extensive
simulations are conducted in Simulink/PLECS to validate the theoretical
results.
