Improvement of the ground-fault detection in field windings of synchronous machines with static excitation based on third-harmonic voltage phase-angle comparison

This paper presents the theoretical approach and experimental tests of a new algorithm for detecting ground faults in rotor windings of synchronous generators with static excitation. This location method requires the installation of a grounding resistance in the secondary winding of the excitation transformer. First, the third-harmonic reference voltage is obtained by the voltage measurements in the excitation system. Then, the third-harmonic voltage at the grounding resistor is compared to the reference voltage. The ground fault is detected when the phase angle between those voltages is under the trip setting value. This new algorithm has been tested in a 5 kVA Synchronous generator with satisfactory results.