  Ground faults in converter-based grids can be difficult to detect because,
unlike in grids with synchronous machines, they often do not result in large
currents. One recent strategy is for each converter to inject a perturbation
that makes faults easier to distinguish from normal operation. In this paper,
we construct optimal perturbation sequences for use with the Multiple Model
Kalman Filter. The perturbations maximize the difference between faulty and
fault-free operation while respecting limits on performance degradation.
Simulations show that the optimal input sequence increases the confidence of
fault detection while decreasing detection time. It is shown that there is a
tradeoff between detection and degradation of the control performance, and that
the method is robust to parameter variations.
