A Hybrid Flexible Neutral Grounding Mode for Large Generators

Single-phase ground faults frequently occur at the generator stator windings. To realize reliable arc suppression, a dual-frequency active arc-suppression strategy has been widely considered. However, the existing method needs an inverter with large capacity, which is large in volume and high in cost, causing limited engineering application prospects. To reduce the capacity, a novel hybrid flexible neutral grounding method for large generators is proposed in this paper. It combines the dual-frequency active arc-suppression strategy with the arc-suppression coil-grounding mode. Thus, most of the fundamental components in the fault current are compensated by the arc-suppression coil, and the rest are compensated by the active arc-suppression device. The capacity of the inverter can be greatly reduced under the premise of reliably realizing arc suppression. To identify transient and permanent ground faults after arc suppression, the phase-angle relation between the third-harmonic voltage and current variation at the generator neutral point is used to form the fault-type identification criterion. The third-harmonic quantities can avoid the blind area at the neutral point caused by fundamental quantities. Simulation and experimental test results verify the effectiveness of the proposed method.