GE T60 Instruction Manual page 258

5.4 SYSTEM SETUP
Note that the delta winding currents lag the wye winding currents by 30° (in agreement with the transformer nameplate).
Now assume that a source, with a sequence of ACB is connected to transformer terminals A, C, and B, respectively. The
currents present for a balanced load are shown in the Phasors for ACB Phase Sequence diagram.
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Note that the delta winding currents leads the wye winding currents by 30°, (which is a type Yd11 in IEC nomenclature and
a type Y/d330 in GE Multilin nomenclature) which is in disagreement with the transformer nameplate. This is because the
physical connections and hence the equations used to calculate current for the delta winding have not changed. The trans-
former nameplate phase relationship information is only correct for a stated phase sequence.
It may be suggested that phase relationship for the ACB sequence can be returned the transformer nameplate values by
connecting source phases A, B and C to transformer terminals A, C, and B respectively. Although this restores the name-
plate phase shifts, it causes incorrect identification of phases B and C within the relay, and is therefore not recommended.
All information presented in this manual is based on connecting the relay phase A, B and C terminals to the power system
phases A, B, and C respectively. The transformer types and phase relationships presented are for a system phase
sequence of ABC, in accordance with the standards for power transformers. Users with a system phase sequence of ACB
must determine the transformer type for this sequence.
If a power system with ACB rotation is connected to the Wye winding terminals 1, 2, and 3, respectively, from a Y/d30 trans-
former, select a Power Rotation setting of ACB into the relay and enter data for the Y/d330 transformer type.
e) MAGNITUDE COMPENSATION
Transformer protection presents problems in the application of current transformers. CTs should be matched to the current
rating of each transformer winding, so that normal current through the power transformer is equal on the secondary side of
the CT on different windings. However, because only standard CT ratios are available, this matching may not be exact.
In our example, the transformer has a voltage ratio of 220 kV / 69 kV (i.e. about 3.188 to 1) and a compensating CT ratio is
500 A to 1500 A (i.e. 1 to 3). Historically, this would have resulted in a steady state current at the differential relay. Interpos-
ing CTs or tapped relay windings were used to minimize this error.
5-112
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Figure 5–28: PHASORS FOR ABC SEQUENCE
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Figure 5–29: PHASORS FOR ACB SEQUENCE
T60 Transformer Protection System
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828718A1.CDR
5 SETTINGS
GE Multilin