ABB RET670 - Page 670

A comparison with figure 279 gives that the line voltage drop compensation for the
purpose of reverse reactance control is made with a value with opposite sign on X
L
,
hence the designation “reverse reactance” or “negative reactance”. Effectively this
means that, whereas the line voltage drop compensation in figure
279 gave a voltage
drop along a line from the busbar voltage V
B
to a load point voltage V
L
, the line
voltage drop compensation in figure 282 gives a voltage increase (actually, by
adjusting the ratio X
L
/R
L
with respect to the power factor, the length of the vector V
L
will be approximately equal to the length of V
B
) from V
B
up towards the transformer
itself. Thus in principal the difference between the vector diagrams in figure 279 and
figure 282 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 283 below shows this situation with T1 being
on a higher tap than T2.
en06000491_ansi.vsd
Load
T1
I
L
T2
I
cc....T2
I
cc....T1
V
B
V
L
I
T1
I
T2
V
B
V
L
1
RI
T1
jX
L
I
T
1
I
cc
-I
cc
(I
T1
+I
T2
)/2
I
T1
I
T2
V
L
2
R
L
I
T2
jX
L
I
T2
ANSI06000491 V1 EN
Figure 283: 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 283.
The result is thus, that the line voltage drop compensation calculated voltage V
L
for T1
will be higher than the line voltage drop compensation calculated voltage V
L
for T2, or
Section 3 1MRK504116-UUS C
IED application
664
Application manual