  This paper proposes a methodology for enhancing power systems resiliency by
proactively splitting an interconnected grid into small self-sustaining islands
in preparation for extreme events. The idea is to posture the system so that
cascading outages can be bound within affected areas, preventing the
propagation of disturbances to the rest of the system. This mitigation strategy
will prove especially useful when advance notification of a threat is available
but its nature not well understood. In our method, islands are determined using
a constrained hierarchical spectral clustering technique. We further check the
viability of the resultant islands using steady-state AC power flow. The
performance of the approach is illustrated using a detailed PSS/E model of the
heavily meshed transmission network operated by PJM Interconnection in the
eastern USA. Representative cases from different seasons show that variations
in power flow patterns influence island configuration.
