ABB RELION REX640 Technical Manual page 613

1MRS759142 C
REX640
Technical Manual
the phase shift and turns ratio. The numerical microprocessor based differential
algorithm implemented in TR2PTDF compensates for both the turns ratio and the
phase shift internally in the software.
The differential current should theoretically be zero during normal load or external
faults if the turns ratio and the phase shift are correctly compensated. However, there
are several different phenomena other than internal faults that cause 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.
GUID-272C2173-1BA1-49BF-B89D-CE18FFF5C1F6 V1 EN
Figure 337: 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
Section 4
Protection functions
607