ABB 615 series Technical Manual page 522

Section 4
Protection functions
516
TR2PTDF
G
3~
a)
GUID-B326703C-3645-4256-96AD-DA87FC9E9C67-ANSI V1 EN
Figure 257: Differential protection of a generator-transformer block and short cable/
line
87T can also be used in three-winding transformer applications or two-winding
transformer applications with two output feeders.
On the double-feeder side of the power transformer, the current of the two CTs per phase
must be summed by connecting the two CTs of each phase in parallel. Generally 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.
87T
3dI>T
1MAC059074-MB A
87T
TR2PTDF
3dI>T
Distance
e.g. 500 m
b)
615 series ANSI
Technical Manual