Determination of Waterwheel Alternator Transient Reactances from Flux Plots

In the transient analysis of salient pole alternators, the reactances X'd and X'q are assumed to be constants. In saturated machines, however, these reactances are functions of the pre-transient load of the alternator. For more exact analyses saturated reactances should be used. The expression of rotor quantities in stator terms, or vice versa, at nonlinear condition is explained. The operational impedances Xd(p), Xq(p) and the various time constants of alternators without amortisseur windings are developed from leakage reactance concepts. After a first approximation of the transient reactances is obtained from the operational impedances, a step-by-step correction procedure is used for finding the operationally dependent direct transient reactance. This is obtained from a simulated dynamical test. Only a few step-by-step corrections are necessary to yield good values for the transient reactances. Since there is a slight difference between Xq and X'q, the analysis will concentrate only to X'd. Notations not explained here are the same as in Concordia [1]. The analysis is applied to a General Electric alternator with nameplate data 82.5 MVA, 16.5 kV, wye-connected, 40 poles and 60 Hz, described in [2].