Three-Phase Three-Legged Wye-Wye Transformers With Only One Neutral Grounded and No Stabilizing Winding–Part I: Zero-Sequence Performance

In this article (Part I), a study of the zero-sequence stray losses in wye-wye-connected transformers without a (stabilizing) tertiary winding is presented. The paper shows where and when eddy losses and circulating current losses appear in the transformer structural metallic components. The overheating hazard potential due to the presence of zero-sequence flux is categorized (as severe, serious, and moderate) by the permitted neutral current. The method is successfully applied to (and experimentally verified with) three-phase, three-legged, core type, wye-wye-connected transformers based on factory acceptance tests (FATs) data. It is uncovered that the severity of the overheating is determined by the phase angle difference between the zero-sequence current and the positive-sequence current. This is formulated by the proposed complex current capacity factor (${\bm{CC}}{{{\bm{F}}}_{{\bm{o}}}}$). In the companion paper (Part II), the maximum permitted heating on the transformer structural metallic components due to the zero-sequence current vs. time duration is presented. The idea is to gauge the temperature of the hot spots in structural metallic components by monitoring (magnitude and angle) the neutral current. The ultimate goal of these two papers is to provide engineering practical knowledge to properly design protection procedures for two-winding transformers with wye-connected primary and secondary and one neutral grounded without stabilizing winding.