Solving the Divergence Problem in AC-QSS Cascading Failure Model by Introducing the Effect of a Realistic UVLS Scheme

AC-Quasi-Steady-State (AC-QSS) models for cascading failure often suffer from divergence issues. Presently, this is handled by uniform load shedding (ULS) or its variants till convergence is achieved. In reality, a pre-defined undervoltage load shedding (UVLS) scheme sheds loads in blocks below a voltage threshold and the system reaches a post-UVLS equilibrium, if it exists. This may not be captured by the post-ULS equilibrium, which in turn can lead to a different path of cascade propagation. We use the ULS equilibrium as the initial state and pose the problem of reaching post-UVLS equilibrium as an optimization problem. Due to the nature of the problem, an exhaustive search can be used that has exponential complexity in inputs. To solve this, we propose a heuristic that reaches a post-UVLS candidate equilibrium. The proposed algorithm is tested on a 2383-bus Polish system.