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5 SETPOINTS
Power transformers that are built in accordance with ANSI and IEC standards are required to identify winding terminals and
phase relationships among the windings of the transformer.
ANSI standard C.37.12.70 requires that the labels of the terminals include the characters 1, 2, and 3 to represent the
names of the individual phases. The phase relationship among the windings must be shown as a phasor diagram on the
nameplate, with the winding terminals clearly labeled. This standard specifically states that the phase relationships are
established for a condition where the source phase sequence of 1-2-3 is connected to transformer windings labeled 1, 2
and 3 respectively.
IEC standard 60076-1 (1993) states that the terminal markings of the three phases follow national practice. The phase rela-
tionship among the windings is shown as a specified notation on the nameplate, and there may be a phasor diagram. In this
standard the arbitrary labeling of the windings is shown as I, II, and III. This standard specifically states that the phase rela-
tionships are established for a condition where a source phase sequence of I - II - III is connected to transformer windings
labeled I, II and III respectively.
The source phase sequence must be stated when describing the winding phase relationships since these relationships
change when the phase sequence changes. The example below shows why this happens, using a transformer described in
IEC nomenclature as "Yd1" or in GE Multilin nomenclature as "Y/d30."
The above figure shows the physical connections within the transformer that produce a phase angle in the delta winding
that lags the respective wye winding by 30°. The winding currents are also identified. Note that the total current out of the
delta winding is described by an equation. Now assume that a source, with a sequence of ABC, is connected to transformer
terminals ABC, respectively. The currents that would be present for a balanced load are shown below.
Note that the Delta winding currents lag the Wye winding currents by 30°, which is in agreement with the transformer name-
plate.
GE Multilin
Courtesy of NationalSwitchgear.com
A
B
I
I
A
I
I
a
l
I
=
I
-
I
I
l
l
a
a
c
a
b
Figure 5–2: EXAMPLE TRANSFORMER
I
A
I
C
Figure 5–3: PHASORS FOR ABC SEQUENCE
745 Transformer Management Relay
5.2.3 PHASE SHIFTS ON THREE-PHASE TRANSFORMERS
C
I
B
C
I
b
l
c
l
=
I
-
I
I
=
I
-
l
l
l
b
b
a
c
c
c
I
a
l
-
I
b
l
I
c
I
c
l
I
B
-
I
a
l
5.2 AUTO-CONFIGURATION
N
I
b
l
I
a
-
I
c
l
I
b
l
I
b
5
5-7
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