Sparse Generalized-Admittance AC Power Flow for Fast Contingency Analysis and Remedial-Action Assessment

Abstract

Repeated AC power-flow calculations are common tasks in transmission system operations, in particular for contingency analysis and remedial-action assessment. For large networks, their cumulative runtime can become a computational bottleneck in operational processes. This paper develops a sparse reformulation of the generalized bus-admittance power-flow method known as PFPD. Loads and generators are represented by fixed shunt admittance terms on the diagonal of the admittance matrix, while corrective nodal currents are iteratively computed to match the constant-power and regulated-voltage constraints. In contrast to the original formulation, the slack bus is treated as a fixed-voltage boundary and removed from the admittance block decomposition. The resulting iteration reuses sparse LU factorizations of the initial generalized non-slack matrix and its PQ block, without explicitly forming dense inverse matrices. The same formulation admits two initialization modes: an unsolved base case using the same flat-start shunt heuristic as in the primary PFPD method, and repeated post-action calculations initialized from a solved operating point. Localized topology and parameter changes are expressed as low-rank admittance updates and the resulting admittance matrices are inverted by means of the Woodbury identity. An optional local-network formulation can provide an warm start before full-network refinement. N-1 experiments on the 1354-bus and 9241-bus PEGASE systems and the 6717-bus synthetic Texas system yield average per-contingency speedups between 14.4 and 18.0 relative to pandapower. On transformer tap-position actions, the method is 2.8--2.9 times faster than pandapower and 7.2--29.9 times faster than PowSyBl OpenLoadFlow.