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5.2 AUTO-CONFIGURATION
The following diagram shows the internal connections of the Y/d30° transformer of our example:
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,
5
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. Under S2 SYSTEM
SETUP/TRANSFORMER, set:
TRANSFORMER TYPE:
Y/d30°
The 745 supports over 100 two and three-winding transformer types. Table 5–1: TRANSFORMER TYPES on
page 5–10 provides the following information about each transformer type:
transformer type notation as it appears on the display
TRANSFORM
WDG
ER TYPE
#
Y/d30°
1
2
5-8
WINDING 1 (WYE)
A
B
a
b
WINDING 2 (DELTA)
Figure 5–5: WYE / DELTA (30° LAG) TRANSFORMER
winding connection (wye, delta, or zig-zag) and ground CT assignment
angle by which a winding lags Winding 1
CONNECTIO
VOLTAGE
N
PHASORS
WYE
(gnd 1/2)
DELTA
30° lag
745 Transformer Management Relay
5.2.5 PHASE ANGLE CORRECTION
C
N
c
diagrams showing the phase rela-
tionship of voltage phasors, where
↑ (the arrow head) indicates the
PHASE
reference phase
SHIFT
30° lag
phase angle correction (or phase shift)
0°
that is performed internally to calculate
differential currents
5 SETPOINTS
GE Power Management
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