CHAPTER 4: SETPOINTS SYSTEM
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–77
Phase Shift Compensation
Phase Compensation Reference: The percent differential protection from the 889 relay
uses the Delta winding as a reference to perform phase shift compensation. If the
transformer has only “Wye” connected windings, the phase currents from the first Wye
winding in the setup are used as a reference.
The power system phase sequence must be set 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”.
Figure 4-16: Example Transformer
The Example Transformer figure shows the physical connections within the transformer
that produce a phase angle in the delta winding lagging 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.
Figure 4-17: Phasors for ABC Sequence
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 in
the next figure: Phasors for ACB Sequence.
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