Investigation of Rotor Stray Currents in Large Salient-Pole Generators: Experimental Validation

This article investigates the presence and impact of stray currents in rotor windings of large salient-pole generators, where field measurements from several large hydrogenerators have shown that unexpected stray currents can significantly affect machine sensor operations and readings, leading therefore to incorrect diagnostics and even failures. This study aims to contribute to a better understanding of the circulation of stray currents in rotor pole windings during machine operation. It highlights some interesting information contained in the stray current that could offer critical insights into machine operations. An experimental setup in a real environment, with in-service conditions, was reproduced to test a 310 MVA, 13.8 kV, 128.6 rpm-60 Hz (56 poles) hydrogenerator poles with a nominal field current of 2475 A. The tested poles in a real environment with a 2000T press and static excitation of 3000 A DC have been used to reproduce and follow the path of these stray currents under insulation failures. Correlating the obtained results with more field measurements collected in hydrogenerators of 166 and 285 MVA shows that the impact of overvoltages generated by the excitation system, due to thyristor turn-off recovery currents, may have caused a malfunction of some air-gap sensors. It will be shown that pole winding inter-turn insulation failure could be more sensitively detected using AC and even HF parasitic voltage. Moreover, the comprehension of the stray current circulation in a pole winding under real inter-turn short circuit (ITSC) condition could be improved by understanding the remaining current that still contributes to the magnetomotive force as demonstrated by experimental results.