ABB REC670 Applications Manual page 398

Section 14
Control
392
hence the designation "reverse reactance" or "negative reactance". Effectively this
means that, whereas the line voltage drop compensation in figure
voltage drop along a line from the busbar voltage U
the line voltage drop compensation in figure
by adjusting the ratio X
vector U will be approximately equal to the length of U
L
transformer itself. Thus in principal the difference between the vector diagrams in
figure 172 and figure 175
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 being on a higher tap than T2.
I ...T2
CC
T1
I ...T1
CC
I I
T2
T1
U
B
IL
Load
UL
IEC06000491 V3 EN-US
Figure 176: Circulating current caused by T1 on a higher tap than T2.
The circulating current I
the transformers. The impact of I
increases the current in T1 (the transformer that is driving I
T2 at 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 U for T1 will be higher than the line voltage drop compensation
L
calculated voltage U for T2, or in other words, the transformer with the higher tap
L
position will have the higher U
position will have the lower U
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.
175
/R with respect to the power factor, the length of the
L L
is the sign of the setting parameter X
T2
is predominantly reactive due to the reactive nature of
cc
on the individual transformer currents is that it
cc
value and the transformer with the lower tap
L
value. Consequently, when the busbar voltage
L
1MRK 511 401-UEN A
172 gave a
to a load point voltage U
B
gives a voltage increase (actually,
) from U up towards the
B B
.
L
176 below shows this situation with
R I
L T2
jX I
L T2
U
-I
B
cc
I
T2
(I +I )/2
T1 T2
I
T1
I
cc
IEC06000491_3_en.vsd
) and decreases it in
cc
Bay control REC670 2.2 IEC
Application manual
,
L
RI
T1
jX I
L T1