  Power system protection schemes today rely on currents rising by several
orders of magnitude when faults occur. In inverter-dominated power systems, a
fault current might be just a few percent larger than normal, making fault
detection more difficult. One solution is for the inverter to slightly perturb
its output current and/or voltage, i.e., to inject an auxiliary signal, so as
to make the system's behavior under faults easier to distinguish from normal.
In this paper, we optimize auxiliary signals for fault detection with distance
relays. We begin with a standard auxiliary signal design problem for generic
static systems. We use duality to reformulate the problem as a bilinear
program, which we solve using the convex-concave procedure. We implement the
framework in an example based on distance protection, in which the auxiliary
signal is negative sequence current.
