ABB RELION RET670, Applications Manual

Page: 780 / 1016

the calculation of circulating currents. The capacitive current is part of the imaginary
load current and therefore essential in the calculation. The calculated circulating
current and the real circulating currents will in this case not be the same, and they will
not reach a minimum at the same time. This might result in a situation when
minimizing of the calculated circulating current will not regulate the tap changers to
the same tap positions even if the power transformers are equal.
However if the capacitive current is also considered in the calculation of the circulating
current, then the influence can be compensated for.
Load
T1
I
L
T2
I
cc....T2
I
cc....T1
V
B
V
L
I
T1
I
T2
I
C
I T
2
-I
C
Load
T1
I
L
T2
I
cc....T2
I
cc....T1
V
B
V
L
I
T1
I
T2
I
T2
I
T1
I
T1
I
C
en06000512_ansi.vsd
ANSI06000512 V1 EN-US
Figure 370: Capacitor bank on the LV-side
From figure 370 it is obvious that the two different connections of the capacitor banks
are completely the same regarding the currents in the primary network. However the
CT measured currents for the transformers would be different. The capacitor bank
current may flow entirely to the load on the LV side, or it may be divided between the
LV and the HV side. In the latter case, the part of I
C
that goes to the HV side will
divide between the two transformers and it will be measured with opposite direction
Section 15
1MRK 504 163-UUS A
Control
774 Transformer protection RET670 2.2 ANSI
Application manual