A PRACTICAL APPROACH TO INCLUSION OF ELECTROMAGNETIC FIELD NONLINEARITIES IN THE DYNAMIC MODELING OF LARGE TURBOGENERATORS WITH EMPHASIS ON THE INTERACTION BETWEEN MACHINE DESIGN AND SYSTEM STABILITY ASPECTS

A rigorous mathematical model for large turbogenerators is developed. It includes the representation of the stator, field, damper bars as well as the polehead damping. An approach has been developed to account for the influence of magnetic saturation on an instantaneous basis on the values of machine inductances and the damping resistances.; The effect of magnetic saturation on machine inductances is considered by means of numerical analysis of magnetic fields in two dimensions coupled with the concept of ampereturn drop functions. This technique and its adaptation to turbogenerators are outlined. General expressions for the inductances associated with the model are derived in terms of machine dimensions and material properties. Validation of the approach is carried out, using an existing 659 MVA, 4 pole turbogenerator as an example, by comparing its unsaturated as well as saturated values of a few typical reactances (inductances) with corresponding data obtained by employing the established calculation techniques. Excellent agreement is found; A technique is adapted to calculate the effect of instantaneous changes of the saturation status and rotor speed variations on the values of equivalent damping resistances of the solid iron rotor and damper bars. Curve fitting techniques are employed in order to use the generated data for dynamic simulation of turbogenerators. The technique is applied to the 659 MVA turbogenerator and results are given.; Subsequently, all the concepts and techniques developed are applied to a nonlinear dqo discrete-time dynamic model of turbogenerators. This model is employed in simulating a number of transients to which the 659 turbogenerator is assumed to have been subjected. Two of the important findings of this modeling approach are: (1) The reduction of about 18% in peak transient electromagnetic torques obtained from the present model in comparison with that obtained by using the classical saturation factor concept, and (2) the large magnitude variation of the equivalent damping resistances of the solid iron rotor during a dynamic period which was obtained based on the data generated from a two dimensional nonlinear numerical eddy current solution model of turbogenerators. Coupled with the varying saturation effect on inductances, the wide variations in the damping resistances pose considerable doubt as to the wisdom of treating machine dynamics with 'fixed' transient and subtransient time constants and reactances.