ABB 615 series Technical Manual page 523

1MAC059074-MB A
615 series ANSI
Technical Manual
GUID-799588E3-C63F-4687-98C5-FF48284676DF-ANSI V1 EN
Figure 258: Differential protection of a three-winding transformer and a transformer
with two output feeders
87T 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, 87T is not suited for protecting of a power transformer or motor fed by a frequency
converter
Section 4
Protection functions
517