Analysis of Magnetic Saturation in a Salient-Pole Synchronous Machine after Sudden Three-Phase Short Circuit

Abstract

In order to evaluate magnetic saturation at sub-transient and transient states of a large-sized salient-pole synchronous machine, the currents and leakage inductances after sudden three-phase short circuits are ana-lyzed by a two-dimensional non-linear transient finite element method (FEM). The currents are calculated both by direct and indirect methods. The direct method uses the FEM and considers fluctuation of the magnetic saturation after the short circuit. The indirect method solves the Park's differential equations without the fluctuation. Results of the direct method agree well with measurements. When the pre-short voltage is large, fluctuation of the leakage inductances after the short is large and it greatly affects values of the short-circuit currents. As the pre-short voltage becomes large, the damper leakage inductances become saturated at first, then the field leakage inductances and finally the armature leakage inductances. Although the indirect method considers saturation at the pre-short condition, it gives inaccurate results and they are nearly the same as the results by the linear FEM. In general, it is thought that saturation of the leakage flux should be considered where sub-transient conditions with large disturbance are analyzed.