GE 745 Instruction Manual page 105

CHAPTER 5: SETPOINTS
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
The source phase sequence must be stated when describing the winding phase
relationships since these relationships change when the phase sequence changes. The
example below shows why this happens, using a transformer described in IEC
nomenclature as "Yd1" or in GE Multilin nomenclature as "Y/d30."
The above figure shows the physical connections within the transformer that produce a
phase angle in the delta winding that lags the respective wye winding by 30°. The winding
currents are also identified. Note that the total current out of the delta winding is described
by an equation. Now assume that a source, with a sequence of ABC, is connected to
transformer terminals ABC, respectively. The currents that would be present for a balanced
load are shown below.
Note that the delta winding currents lag the wye winding currents by 30°, which is in
agreement with the transformer nameplate.
Now assume that a source, with a sequence of ACB is connected to transformer terminals
A, C, B respectively. The currents that would be present for a balanced load are shown
below:
FIGURE 5–2: Example transformer
FIGURE 5–3: Phasors for ABC sequence
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