This work is aimed at extending a discussion offered concerning a paper presented at the 1993 IEEE/PES Winter Meeting (Boldea and Nasar, 1987) which attempted to show that the "differential leakage" was not required and, in general, seriously questioned the need for a third-order model. Unfortunately, the simplifications were possible only using a per-unit system not compatible with industry standards and current engineering practices for saturation analysis. In this paper, mathematical reasoning is combined with maximum-likelihood estimation from actual SSFR data to show that, for network structures preserving the main-field magnetizing impedance, the differential leakage contributes significantly to the dynamic response and proves mandatory for adequately modeling rotor impedance. Besides, the claim made that a third-order model may not be sufficiently parsimonious is not home out by further verification of the author's own data end extensive experience with real SSFR data from a 1100 MVA turbine-generator.
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