ABB ret650, Applications Manual

Page: 209 / 392

hence the designation “reverse reactance” or “negative reactance”. Effectively this
means that, whereas the line voltage drop compensation in figure
73 gave a voltage
drop along a line from the busbar voltage U
B
to a load point voltage U
L
, the line
voltage drop compensation in figure 76 gives a voltage increase (actually, by
adjusting the ratio X
L
/R
L
with respect to the power factor, the length of the vector
U
L
will be approximately equal to the length of U
B
) from U
B
up towards the
transformer itself. Thus in principal the difference between the vector diagrams in
figure
73 and figure 76 is the sign of the setting parameter X
L
.
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
77 below shows this situation with
T1 being on a higher tap than T2.
IEC06000491_3_en.vsd
Load
T1 T2
U
B
UL
I
T1
I
T2
U
B
RI
T1
jX
L
I
T1
I
cc
-I
cc
(I
T1
+I
T2
)/2
I
T1
I
T2
R
L
I
T2
jX
L
I
T2
I
CC
...T2
I
CC
...T1
IL
IEC06000491 V3 EN-US
Figure 77: Circulating current caused by T1 on a higher tap than T2.
The circulating current I
cc
is predominantly reactive due to the reactive nature of
the transformers. The impact of I
cc
on the individual transformer currents is that it
increases the current in T1 (the transformer that is driving I
cc
) and decreases it in
T2 at the same time as it introduces contradictive phase shifts, as can be seen in
figure
77 . The result is thus, that the line voltage drop compensation calculated
voltage U
L
for T1 will be higher than the line voltage drop compensation
calculated voltage U
L
for T2, or in other words, the transformer with the higher tap
position will have the higher U
L
value and the transformer with the lower tap
position will have the lower U
L
value. 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.
1MRK 504 169-UEN A
Section 11
Control
Transformer protection RET650 2.2 IEC 203
Application manual