Three-Phase Three-Legged Wye-Wye Transformers with only One Neutral Grounded and no SW - Part II: Zero-Sequence Permissible Temperature

This paper (Part II) presents a study of the zero-sequence permissible temperature rise (T_perm) on metallic structural components in wye-wye-connected transformers without a (stabilizing) tertiary winding. The paper shows the direct heating-time relationship associated to a neutral current in terms of CCF_o. A thermal protection criterion is proposed to control the time scale of the time-neutral current characteristic. The strategy is successfully applied to (and experimentally verified with) ONAN, three-phase, three-legged, core type, wye-wye-connected transformers based on factory acceptance tests (FATs) data. It is uncovered that the maximum permissible heating on the transformer structural metallic components due to the zero-sequence flux is determined by inverse-time CCF_o. This is provided by a family of characteristic curves. The companion paper (Part I) presents where and when eddy losses and circulating current losses are induced in the transformer structural metallic components due to the zero-sequence current. The ultimate goal of these two papers is to provide engineering practical knowledge to design effective protection procedures for two-winding transformers with wye-connected primary and secondary and one neutral grounded.