Phase Shifts On Three-Phase Transformers - GE 745 TRANSFORMER MANAGEMENT RELAY Instruction Manual

5.2 AUTO-CONFIGURATION
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 repre-
sent the names of the individual phases. The phase relationship among the windings must be shown as a pha-
sor 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 con-
nected 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 relationship 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 stan-
dard specifically states that the phase relationships 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 rela-
tionships 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 Power Management nomenclature as "Y/d30."
5
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.
5-6

5.2.4 PHASE SHIFTS ON THREE-PHASE TRANSFORMERS

A B
I
A
I
a l
I = I - I
a a l c l
a b
Figure 5–2: EXAMPLE TRANSFORMER
I
A
I
C
Figure 5–3: PHASORS FOR ABC SEQUENCE
745 Transformer Management Relay
C
I I
B C
I I
b l c l
I = I - I I = I
b b l a l c c
c
I
a
- I
b l
I
c
I
c l
I
B
- I
a
5 SETPOINTS
N
- I
l b l
I
a
l
- I
c l
I
b l
l
I
b
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