Ground fault detecting apparatus and method for detecting ground fault of field circuit and exciting circuit by detecting ground fault current flowing from ground to neutral point of exciting circuit

An apparatus for detecting a ground fault on an exciting circuit and a field circuit of an AC excited variable speed rotating machine, the exciting circuit outputting an exciting voltage of controlled frequency and/or an amplitude, and the field circuit outputting a field current produced by the exciting voltage, the apparatus comprising:

• a ground resistance connected between a neutral point of said field circuit and ground;
• a current detecting means for detecting a ground current flowing between the ground and said neutral point through said ground resistance and outputting a detected signal;
• and
• a ground fault detecting means for extracting a component of a fundamental wave signal from the detected signal and for detecting said ground fault according to the extracted component and a frequency of the fundamental wave signal with constant detecting sensitivity regardless of a change of the exciting voltage, the fundamental wave signal controlling the frequency and/or an amplitude of the exciting voltage.