ABB RET670 Applications Manual page 676

Section 3
IED application
670
Plant with capacitive shunt compensation (for operation with the circulating
current method)
If significant capacitive shunt generation is connected in a substation and it is not
symmetrically connected to all transformers in a parallel group, the situation may
require compensation of the capacitive current to the ATCC.
An asymmetric connection will exist if for example, the capacitor is situated on the LV-
side of a transformer, between the CT measuring point and the power transformer or at
a tertiary winding of the power transformer, see figure 284. In a situation like this, the
capacitive current will interact in opposite way in the different ATCCs with regard to
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.
1MRK504116-UUS C
Application manual