Phenomenological models of large synchronous machines from short-circuit tests during commissioning-a classical/modern approach

To date, synchronous-machine short-circuit oscillograms have been analyzed, either numerically or graphically by applying the basic simplified concepts set forth in ANSI Standard 115. This paper attempts to evaluate the pitfalls of this approach by comparing its results with those obtained using modern multiresponse estimation techniques. Within the two-axis theory framework, the phenomenological models considered are the ANSI dynamic constants, operational impedances and equivalent circuits. Each model containing a damper winding in each axis is identified independently from the three phase currents using a constrained nonlinear estimation scheme; the cost function is chosen as a suitable norm of the weighted-covariance matrix of residual errors. Short-circuit data obtained during the commissioning of 360 MVA synchronous condensers at Manitoba Hydro´s Dorsey substation are extensively analyzed for retrieving the pertinent stability constants. It is found that the graphically-oriented approach, even carefully executed on a microcomputer in accordance with Std-115 recommendations, still proves somewhat unsatisfactory, whereas modern estimation techniques applied to the same data provide consistent parameters for all models, even for the weakly excited q-axis