Stable equivalent models with minimum phase transfer functions in the dynamic aggregation of voltage regulators

The dynamic aggregation is a step of the calculation of coherency-based dynamic equivalents. If the dynamic aggregation of voltage regulators is solved as an unconstrained optimization problem, negative time constants may appear in the solution, which characterize unstable equivalent models and/or nonminimum phase equivalent transfer functions. The squared-variable transformation is considered to solve this problem. The solutions of the dynamic aggregation problem of voltage regulator models obtained with both the original formulation and the squared-variable transformation are compared. A criterion is also proposed to select the best equivalent voltage regulator model for each coherent group. The New England system, including salient-pole synchronous machines with subtransient effects and excitation system models, is considered in the studies.