GE 845, Instruction Manual

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4–82 845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
SYSTEM CHAPTER 4: SETPOINTS
Figure 4-21: 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.
Figure 4-22: Phasors for ACB Sequence
Note that the delta winding currents lead the wye winding currents by 30°, (which is a type
Yd11 in IEC 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 transformer nameplate phase relationship
information is only correct for a stated phase sequence.
It is suggested that for the ACB sequence the phase relationship can be returned to that
shown on the transformer nameplate by connecting source phases A, B and C to
transformer terminals A, C, and B respectively. This will restore the nameplate phase shifts
but will cause 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.
The following diagram shows the internal connections of the Y/d30 transformer from our
example.
h f