ABB RELION REX640 Technical Manual page 695

1MRS759142 F
unwanted and false differential currents. The main reasons for unwanted differential
currents are:
• Mismatch due to varying tap changer positions
• Different characteristics, loads and operating conditions of the current
transformers
• Zero sequence currents that only flow on one side of the power transformer
• Normal magnetizing currents
• Magnetizing inrush currents
• Overexcitation magnetizing currents.
TR2PTDF is designed mainly for the protection of two-winding transformers.
TR2PTDF can also be utilized for the protection of generator-transformer blocks
as well as short cables and overhead lines. If the distance between the measuring
points is relatively long in line protection, interposing CTs can be required to reduce
the burden of the CTs.
Figure 396: Differential protection of a generator-transformer block and short
cable/line
TR2PTDF 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.
REX640
Technical Manual
Protection functions
695