GE 845 Instruction Manual page 198

SYSTEM
NOTE:
4–94
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.
Figure 4-40: Wye/Delta (30° lag) Transformer
Under balanced conditions, the winding 2 phase currents lag the corresponding phase
currents of winding 1 by 30°. With CTs connected in a wye arrangement (polarity markings
pointing away from the transformer), even after magnitude compensation, the summation
of the currents from both transformer windings will not result in zero differential current,
because they will NOT be 180° out-of-phase.
Traditionally, this problem was solved by connecting the CTs from the wye side of the
transformer (winding 1) in a delta. This compensates for the phase angle lag introduced in
the delta side (winding 2).
The 845 performs the phase angle correction internally based on the setpoint "Angle With
Respect to W1" from the menu System > Transformer > Transformer Setup > see
"Winding 2", and the same setpoint from the menu System > Transformer > Transformer
Setup > see "Winding 3" for three-winding transformer protection.
The angle of Winding 1 is always 0°, and this setpoint is omitted from the Winding 1 setup
menu.
The 845 supports all standard two-, and three-winding transformer types, and a variety of
non-standard applications, where the selection of winding connection type and selected
lagging angle with respect to Winding 1 are not described in the table of standard
transformer types.
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS