ABB REC670 Applications Manual page 427

1MRK 511 401-UUS A
Bay control REC670 2.2 ANSI
Application manual
I
cc....T2
T1
I
cc....T1
I
I
T2
T1
V
B
I L
Load
V
L
ANSI06000491 V1 EN-US
Figure 176: Circulating current caused by T1 on a higher tap than T2.
The circulating current I
transformers. The impact of I
increases the current in T1 (the transformer that is driving I
the same time as it introduces contradictive phase shifts, as can be seen in figure 176.
The result is thus, that the line voltage drop compensation calculated voltage V
will be higher than the line voltage drop compensation calculated voltage V
in other words, the transformer with the higher tap position will have the higher V
value and the transformer with the lower tap position will have the lower V
Consequently, when the busbar voltage increases, T1 will be the one to tap down, 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:
T2
is predominantly reactive due to the reactive nature of the
cc
on the individual transformer currents is that it
cc
Section 14
R I
L T2
jX I
L T2
V
-I
I
B
cc
T2
(I +I )/2
T1 T2
I
I
T1
cc
en06000491_ansi.vsd
) and decreases it in T2 at
cc
Control
RI
T1
jX I
L T
1
for T1
L
for T2, or
L
L
value.
L
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