ABB Relion 670 Series Applications Manual page 376

Section 14
Control
will be approximately equal to the length of V
principal the difference between the vector diagrams in figure
setting parameter X
If now the tap position between the transformers will differ, a circulating current will appear, and
the transformer with the highest tap (highest no load voltage) will be the source of this circulating
current. Figure
T1
V
B
V
L
ANSI06000491 V1 EN-US
Figure 181: Circulating current caused by T1 on a higher tap than T2.
The circulating current I
The impact of I
transformer that is driving I
contradictive phase shifts, as can be seen in figure 181. The result is thus, that the line voltage
drop compensation calculated voltage V
compensation calculated voltage V
position will have the higher V
lower V
L
and when the busbar voltage decreases, T2 will be the one to tap up. The overall performance will
then be that the runaway tap situation will be avoided and that the circulating current will be
minimized.
Parallel control with the circulating current method
Two transformers with different turns ratio, connected to the same busbar on the HV-side, will
apparently show different LV-side voltage. If they are now connected to the same LV busbar but
remain unloaded, this difference in no-load voltage will cause a circulating current to flow through
the transformers. When load is put on the transformers, the circulating current will remain the
same, but now it will be superimposed on the load current in each transformer. Voltage control of
parallel transformers with the circulating current method means minimizing of the circulating
current at a given voltage target value, thereby achieving:
370
.
L
181 below shows this situation with T1 being on a higher tap than T2.
I
cc....T2
T2
I
cc....T1
I
I
T2
T1
I L
Load
is predominantly reactive due to the reactive nature of the transformers.
cc
on the individual transformer currents is that it increases the current in T1 (the
cc
) and decreases it in T2 at the same time as it introduces
cc
value and the transformer with the lower tap position will have the
L
value. Consequently, when the busbar voltage increases, T1 will be the one to tap down,
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) from V up towards the transformer itself. Thus in
B B
R
-I
I
cc
T2
(I +I )/2
T1 T2
I
I
T1
cc
for T1 will be higher than the line voltage drop
L
for T2, or in other words, the transformer with the higher tap
L
177 and figure 180 is the sign of the
I
RI
L T2
T1
jX I
jX I
L T
L T2
1
V
B
en06000491_ansi.vsd
Bay control REC670
Application manual
1MRK 511 401-UUS F
SEMOD159053-159 v5