ABB RELION 620 Series Technical Manual page 446

Section 4
Protection functions
440
the interposing CT, the accuracy limit factor must fulfill the same requirements as the
main CTs. The interposing CT imposes an additional burden to the main CTs.
87T3
3dI>T
G
3~
a)
GUID-6C603D81-D4DB-416A-A0D8-43BB4267B98F V1 EN
Figure 204: Differential protection of generator-transformer block and short cable or
line
87T can be used in three-winding transformer applications or two-winding transformer
applications with two output or two input feeders.
If the CTs connected to the two output or input feeders have the same ratio, follow the
description below.
On the double-feeder side of the two-winding power transformer, the currents from the
other feeder are connected to the third three-phase current set of the protection relay
(Current group 3 type to be set as "Wnd 2 restraint").
It is also possible that the double-feeder side is on the input side of the transformer
(Current group 3 type to be set as "Wnd 1 restraint").
If the CTs connected to the two output or input feeders have a different ratio, follow the
description below.
In three-winding transformer applications, currents from the winding 3 are always
connected to the third three-phase current input set of the protection relay (Current group
3 type to be set as "Tertiary").
1MAC504801-IB E
87T3
3dI>T
Distance
e.g. 500 m
b)
620 series ANSI
Technical Manual