Modeling of Saturated Cylindrical-Rotor Synchronous Machines
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It has been recognized recently that the magnetic coupling between the direct and quadrature axes of saturated synchronous machines (cross-magnetizing phenomenon) plays an important role in their analysis using the two-axis frame models. To determine the parameters which represent this cross-magnetizing effect, the machine saturation curves in the various intermediate axes are needed. If a synchronous machine has two windings: one in the direct axis and the other in the quadrature axis, saturation curves in the intermediate axes can be obtained by exciting the machine from both these windings simultaneously under open-circuited condition. However, there is usually no field winding in the quadrature axis of industrial synchronous machines and, thus, the saturation curves in the intermediate axes cannot be obtained by the open-circuit test. This thesis presents an analytical method for determining the saturation curves in the intermediate axes of cylindricalrotor synchronous machines from their measured d- and q-axis saturation curves. The accuracy of this method is verified by comparing the measured saturation curves in the intermediate axes of a specially-designed cylindrical-rotor synchronous machine, which has d- and q-axis field windings, with those calculated using the proposed method. The thesis also presents a method for determining the parameters representing the cross-magnetizing effect from the calculated intermediate-axis saturation curves of synchronous machines. By applying these methods also to the cases of salient-pole synchronous machines, their generality is demonstrated. Moreover, a modified two-axis frame model of saturated synchronouns machines which includes the cross-magnetizing effect is presented in this thesis. The accuracy of this modified model is verified by comparing the measured active and reactive power/load angle curves of a cylindrical-rotor and a salient-pole synchronous generators with the curves obtained analytically by applying the proposed modified two-axis frame model.