Approximate Slow Subsystems for Preserving the Conservative Property of Two-Time-Scale Power Systems

The paper concerns the reduced order modeling of nonlinear two-time-scale power systems by approximate slow subsystems. In particular, we investigate the issue if the full order power system model is conservative, whether the slow subsystems will also be conservative. For linearised models, the slow subsystems obtained from truncated asymptotic expansions are symmetrisable, implying that energy functions can be construted from a Lagrangian. For the nonlinear systems with linearized fast dynamics slow subsystems obtained from the first three terms of an asymptotic expansion shown to possess energy functions. For higher order approximate slow subsystems, this property does not seem to hold.