GE 745 Instruction Manual page 107

CHAPTER 5: SETPOINTS
5.2.5 Zero-sequence Component Removal
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
Under balanced conditions, the winding 2 phase current phasors lag the corresponding
phase current phasors of winding 1 by 30°. With CTs connected in a wye arrangement
(polarity markings pointing away from the transformer), the corresponding phase currents
cannot be summed directly to obtain a zero differential current, since corresponding
phasors will NOT be 180° out-of-phase.
Traditionally, this problem is solved by connecting the CTs on the wye side of the
transformer (winding 1) in a delta arrangement. This compensates for the phase angle lag
introduced in the delta side (winding 2).
The 745 performs this phase angle correction internally based on the following setpoint.
Set
S2 SYSTEM SETUP TRANSFORMER

The 745 supports over 100 two and three-winding transformer types. Table 5–1:
Transformer types on page 5–13 provides the following information about each
transformer type:
As shown in the "Y/d30°" entry of the transformer types table, the phase angle correction
(or phase shift) introduces 30° lag in winding 1. This lag is described in Phase shifts on page
5–24. This table provides the following information about each phase shift type:
1. If zero-sequence current can flow into and out of one transformer winding (e.g. a
grounded wye or zig-zag winding) but not the other winding (e.g. a delta winding),
external ground faults will cause the differential element to operate incorrectly.
Traditionally, this problem is solved by delta connecting the CTs on the wye side of a
wye/delta transformer so that the currents coming to the relay are both phase
AUTO-CONFIGURATION
to "Y/d30°".
TRANSFORMER TYPE

5–11