Influence of tertiary stabilizing windings on zero-sequence performance of three-phase three-legged YNynd transformers. Part II: Tank overheating hazard and short-circuit duty

- The influence of the stabilizing winding on tank overheating hazard and short-circuit duty of three-phase three-legged YNynd transformers is presented.
- Relative position of the stabilizing winding, the presence of magnetic shunts and the effect of primary and secondary neutral grounding are key factors that must be carefully evaluated.
- Zero-sequence ampere-turn balance analysis offers a qualitative indication of overheating hazard.
- Tank overheating hazard can be adequately evaluated by means of well-designed zero-sequence temperature rise tests.
- In most of the cases, stabilizing windings may be omitted in wye-wye core-form transformers reducing very significantly mechanical requirements and without tank overheating hazard.

This paper analyzes the influence of stabilizing windings in three-phase three-legged YNynd transformers on tank overheating hazard and on the short-circuit performance of such transformers in the event of unbalanced conditions involving zero-sequence currents as asymmetrical faults. It complements a previous companion paper that presented experimental zero-sequence measurements and equivalent circuit calculations for three power transformers. Those previously calculated zero-sequence equivalent circuit parameters are used here to evaluate the comparative performance of YNynd power transformers when the delta stabilizing winding is closed and when it is open. Additionally, it analyzes the effects of other variables, such as relative position of the stabilizing winding, presence of magnetic shunts in the tank walls, and system grounding.