GE Multilin 745

Setpoints
http://www.GEindustrial.com/multilin
5–10
GE Multilin
Auto-Configuration745
Transformer Management Relay
Zero-Sequence
Component Removal
1. If zero-sequence current can flow into and out of one transformer winding (e.g.
a grounded Wye or zig-zag winding) but not the other winding (e.g. a Delta
winding), external ground faults will cause the differential element to operate
incorrectly. Traditionally, this problem is solved by Delta connecting the CTs on
the Wye side of a Wye/Delta transformer so that the currents coming to the
relay are both phase 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 insert-
ing a zero-sequence current trap in the CT circuitry. The 745 automatically
removes zero-sequence current from all Delta winding currents when calculat-
ing differential current. Where there is no source of zero-sequence current (e.g.
Delta windings not having a grounding bank), the 745 effectively removes noth-
ing.
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 malopera-
tions resulting from these two conditions.