ABB REG670 - Page 566

A comparison with figure 284 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 284 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
287 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
284 and
figure 287 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
288 below shows this situation with T1 being
on a higher tap than T2.
IEC06000491_3_en.vsd
Load
T1
T2
U
B
UL
IT1
IT2
U
B
RIT1
jXLIT1
Icc
-Icc
(IT1+IT2)/2
IT1
IT2
RLIT2
jXLIT2
ICC...T2
ICC...T1
IL
IEC06000491 V3 EN
Figure 288: 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 288. 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.
Section 15 1MRK 502 071-UEN -
Control
560 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual