Discussion of Third Harmonic Voltage Characteristics of Large Hydro Generator Stator Winding and Improvement of Ground Protection

Abstract

Large hydro-generators require a 100% protected area for stator ground protection. Because the fundamental voltage criterion has low sensitivity near the neutral point, a high sensitivity third harmonic voltage criterion is needed to ensure that the generator can accurately respond to stator ground fault. This paper analyzes the third harmonic voltage characteristics of two large hydro-generators, calculates the sensitivity of the existing third harmonic criterion, and points out that the impedance parameters of the grounding transformer should be considered. In view of the lack of sensitivity of the third harmonic criterion for large hydro-generators, this paper proposes a stator ground protection criterion based on the third harmonic voltage variation, which has zero action quantity under the generator non-fault conditions and zero braking quantity after a ground fault occurs, and thus has high sensitivity and can effectively respond to high-resistance ground faults. PSCAD/EMTDC simulation results show that the proposed method has high sensitivity at different fault locations and transition resistances, which can provide theoretical support for the improvement of stator grounding protection in practical engineering.