5–12 745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
AUTO-CONFIGURATION CHAPTER 5: SETPOINTS
corrected and void of zero-sequence current. Because the 745 software mimics the
CT delta connection, the zero-sequence current is automatically removed from all
Wye or zig-zag winding currents of transformers having at least one delta winding.
2. External ground faults also cause maloperation of the differential element for
transformers having an in-zone grounding bank on the delta side (and the wye
connected CTs on the same side). Traditionally, this problem is solved by inserting a
zero-sequence current trap in the CT circuitry. The 745 automatically removes zero-
sequence current from all delta winding currents when calculating differential current.
Where there is no source of zero-sequence current (e.g. delta windings not having a
grounding bank), the 745 effectively removes nothing.
3. Autotransformers have an internal tertiary winding to provide a path for third-
harmonic currents and control transient overvoltages. Also, many two-winding wye/
wye transformers have a three-legged core construction that forces zero-sequence
flux into the transformer tank, creating an inherent delta circuit. In both these cases,
there is zero-sequence impedance between the primary and secondary windings. The
745 removes zero-sequence current from all wye/wye and wye/wye/wye transformer
windings to prevent possible relay maloperations resulting from these two conditions.
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