ABB RELION 620 Series Technical Manual page 530

Section 4
Protection functions
524
this requires the interposing CTs to handle the vector group and/or ratio mismatch
between the two windings/feeders.
The accuracy limit factor for the interposing CT must fulfill the same requirements as
the main CTs. Please note that the interposing CT imposes an additional burden to the
main CTs.
The most important rule in these applications is that at least 75 percent of the short-
circuit power has to be fed on the side of the power transformer with only one
connection to the protection relay.
GUID-799588E3-C63F-4687-98C5-FF48284676DF V1 EN
Figure 267: Differential protection of a three-winding transformer and a
transformer with two output feeders
TR2PTDF can also be used for the protection of the power transformer feeding the
frequency converter. An interposing CT is required for matching the three-winding
transformer currents to a two-winding protection relay.
The fundamental frequency component is numerically filtered with a Fourier filter,
DFT. The filter suppresses frequencies other than the set fundamental frequency, and
therefore the protection relay is not adapted for measuring the output of the frequency
converter, that is, TR2PTDF is not suited for protecting of a power transformer or
motor fed by a frequency converter
1MRS757644 F
620 series
Technical Manual