  Power system simulations that extend over a time period of minutes, hours, or
even longer are called extended-term simulations. As power systems evolve into
complex systems with increasing interdependencies and richer dynamic behaviors
across a wide range of timescales, extended-term simulation is needed for many
power system analysis tasks (e.g., resilience analysis, renewable energy
integration, cascading failures), and there is an urgent need for efficient and
robust extended-term simulation approaches. The conventional approaches are
insufficient for dealing with the extended-term simulation of multi-timescale
processes. This paper proposes an extended-term simulation approach based on
the holomorphic embedding (HE) methodology. Its accuracy and computational
efficiency are backed by HE's high accuracy in event-driven simulation, larger
and adaptive time steps, and flexible switching between full-dynamic and
quasi-steady-state (QSS) models. We used this proposed extended-term simulation
approach to evaluate bulk power system restoration plans, and it demonstrates
satisfactory accuracy and efficiency in this complex simulation task.
