Criteria for neutral stability of wye-grounded-primary broken-delta-secondary transformer circuits

This paper presents the results of an investigation made to determine and evaluate the effect of the factors affecting neutral instability of Y-grounded potential transformer circuits. The method used employs the medium of miniature system representation. The results therefore include the effect of saturation. The effects of the shape of the saturation curve, normal operating voltage, resistance of the winding, system capacitance, and impedance in the broken A secondary or across each phase are illustrated. Regions of instability are shown which should be avoided in applying Y-connected potential transformers as ground fault detectors. The analysis shows by means of oscillograms the conditions under which incorrect indications of faults may be obtained.