Coherency-based low order models for shaft systems of turbine-generator sets

Accurate low order lumped models that represent the low frequency torsional motion of turbine-generator sets are needed for determining shaft torsional responses resulting from subsynchronous resonance conditions, electric power system faults and planned/unplanned switching actions in the electric network. This paper presents a coherency-based method that resolves a high order inertia-spring lumped model into a low order inertia-spring lumped model, while preserving the selected group of natural torsional frequencies and their associated mode shapes. Forced eigen-frequency matching and conservation of angular momentum form the basis of the iterative procedure developed in the paper. Numerical examples are included to illustrate the capabilities of the proposed method in determining accurate low order dynamic equivalents.