GE 845 Instruction Manual page 211

CHAPTER 4: SETPOINTS
NOTE:
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
Figure 4-23: 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.
Figure 4-24: Menu for transformer windings setup
As shown in the menu for transformer windings setup, the 30° lag of the Delta winding
currents, will result in a 0° phase shift applied to Delta currents (Delta –phase reference),
and a 30° phase angle correction (phase shift) applied to the winding 1 currents (Wye
winding). These angle corrections are described in the table as Phase shift.
SYSTEM
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