Description
- DATX100 Under balanced conditions, the winding 2 phase current phasors lag the corresponding
phase current phasors of winding 1 by 30°. With CTs connected in a wye arrangement
(polarity markings pointing away from the transformer), the corresponding phase currents
cannot be summed directly to obtain a zero differential current, DATX100 since corresponding
phasors will NOT be 180° out-of-phase.
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Traditionally, this problem is solved by connecting the CTs on the wye side of the
transformer (winding 1) in a delta arrangement. DATX100 This compensates for the phase angle lag
introduced in the delta side (winding 2).
The 745 performs this phase angle correction internally based on the following setpoint.
Set S2 SYSTEM SETUP TRANSFORMER TRANSFORMER TYPE to “Y/d30°”.
The 745 supports over 100 two and DATX100 three-winding transformer types. Table 5–1:
Transformer types on page 5–13 provides the following information about each
transformer type:
As shown in the “Y/d30°” entry of the DATX100 transformer types table, the phase angle correction
(or phase shift) introduces 30° lag in winding 1. This lag is described in Phase shifts on page
5–24. This table provides the following information about each phase shift type:
5.2.5 Zero-sequence Component Removal
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 DATX100 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.
External ground faults also cause maloperation of the differential element for
transformers having an in-zone grounding DATX100 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.
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, DATX100 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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