ABB RELION 670 Series Applications Manual page 585

1MRK 504 152-UEN B
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
T2.
T1
U
B
UL
IEC06000491 V3 EN-US
Figure 348: Circulating current caused by T1 on a higher tap than T2.
The circulating current I
transformers. The impact of I
current in T1 (the transformer that is driving I
introduces contradictive phase shifts, as can be seen in figure 348. The result is thus, that the
line voltage drop compensation calculated voltage U
drop compensation calculated voltage U
higher tap position will have the higher U
position will have the lower U
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:
1. that the busbar or load voltage is regulated to a preset target value
2. that the load is shared between parallel transformers in proportion to their ohmic short
circuit reactance
If the transformers have equal percentage impedance given in the respective transformer MVA
base, the load will be divided in direct proportion to the rated power of the transformers when
the circulating current is minimized.
This method requires extensive exchange of data between the TR8ATCC function blocks (one
TR8ATCC function for each transformer in the parallel group). TR8ATCC function block can
either be located in the same IED, where they are configured in PCM600 to co-operate, or in
Application manual
348 below shows this situation with T1 being on a higher tap than
I ...T2
CC
T2
I ...T1
CC
I
I
T2
T1
IL
Load
is predominantly reactive due to the reactive nature of the
cc
on the individual transformer currents is that it increases the
cc
value. Consequently, when the busbar voltage increases, T1 will
L
R I
L T2
-I
cc
I
T2
(I +I )/2
T1 T2
I
T1
I
cc
) and decreases it in T2 at the same time as it
cc
for T1 will be higher than the line voltage
L
for T2, or in other words, the transformer with the
L
value and the transformer with the lower tap
L
Section 14
RI
T1
jX I
jX I
L T2
L T1
U
B
IEC06000491_3_en.vsd
SEMOD159053-159 v5
Control
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