The negative branch impedance in the transformer sequence circuit model

The negative branch impedance of the transformer´s T branch model was investigated as to its effects on current reversals in the delta tertiary winding and transformer neutral. Along with the negative branch values small positive values allowed current reversals in the transformer´s neutral. Given the importance of accuracy the model should be developed with zero sequence impedances derived from zero sequence testing. It was also pointed out how transformer design affects the T branch model. Generally the negative or low impedance is on the low voltage side of the transformer model. However, different winding configurations can eliminate or even move the negative impedance to the high voltage side. The latter case being possible or even likely for transformers with high H/X turns ratios (e.g. 345/34.5/13.8 kV).