5-8
745 Transformer Management Relay
GE Multilin
5.2 AUTO-CONFIGURATION
5 SETPOINTS
5
Now assume that a source, with a sequence of ACB is connected to transformer terminals A, C, B respectively. The cur-
rents that would be present for a balanced load are shown below:
Figure 5–4: PHASORS FOR ACB SEQUENCE
Note that the Delta winding currents leads the Wye winding currents by 30°, (which is a type Yd11 in IEC nomenclature and
a type Y/d330 in GE Multilin nomenclature) which is in disagreement with the transformer nameplate. This is because the
physical connections and hence the equations used to calculate current for the delta winding have not changed. The trans-
former nameplate phase relationship information is only correct for a stated phase sequence.
It may be suggested that for the ACB sequence the phase relationship can be returned to that shown on the transformer
nameplate by connecting source phases A, B and C to transformer terminals A, C, and B respectively. This will restore the
nameplate phase shifts but will cause incorrect identification of phases B and C within the relay, and is therefore not recom-
mended.
All information presented in this manual is based on connecting the relay phase A, B and C terminals to the power system
phases A, B and C respectively. The transformer types and phase relationships presented are for a system phase
sequence of ABC, in accordance with the standards for power transformers. Users with a system phase sequence of ACB
must determine the transformer type for this sequence.
5.2.4 PHASE ANGLE CORRECTION
The following diagram shows the internal connections of the Y/d30
°
transformer of our example:
Figure 5–5: WYE / DELTA (30
°
LAG) TRANSFORMER
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 corre-
sponding 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).
I
I
I
I
I
I
I
I
I
I
-
-
-
I
I
A
a
b
c
c
l
b
l
a
l
c
l
a
l
b
l
B
C
WINDING 1 (WYE)
WINDING 2 (DELTA)
a
b
c
A
B
C
N
