  A major risk of geomagnetic disturbances (GMDs) is cascading failure of
electrical grids. The modeling of GMD events and cascading outages in power
systems is difficult, both independently and jointly, because of the many
different mechanisms and physics involved. This paper introduces a relaxation
based modeling of GMD-induced cascading failures:~the dc approximation-based
DCSIMSEP solver was adapted to simulate cascading as a result of GMDs, the full
set of ac power flow equations were relaxed to guarantee optimality, and the
reactive power losses were modeled while keeping the problem convex. The
developed algorithm was implemented in PowerModelsGMD.jl - an open-source
software specifically designed to model and analyze geomagnetic hazards - and
demonstrated to work on the RTS-GMLC-GIC-EAST synthetic test network.
