Back when I was manufacturing generators, I developed a couple of inter-related Excel-based tools for determining capability curves: block load rejection, decrement curves, reactive capability, waveform/harmonic content and analysis, and bus transfer (e.g. reclosure) calculations. Unfortunately, since they were created while employed by a certain entity, they have some proprietary geometry and insulation details embedded in them - and the programming remained with that employer.
The actual calculations to create any of them aren't all that difficult, just iterative (from at least one known operating point). However SOME detail of the machine characteristics is required (time constants, reactances, etc.) - which can often be determined from operating conditions and a bit of (internal) geometry knowledge.
