4–140 845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
PROTECTION CHAPTER 4: SETPOINTS
Percent Differential
The 845 relay provides one Overall Percent Differential element per setpoint group. The
setpoints from the protection menu define a dual slope, dual breakpoint differential/
restraint characteristic. The filtering and calculation of transformer differential and
restraint currents, and differential current 2nd and 5th harmonics are as shown in the
figure: Filtering and calculation of differential and restraint currents.
NOTE:
The K2 CT bank is connected to Winding 1, which is the High-Voltage winding of the GSU
connected to the system. In contrast, the K1 CT bank is connected at the neutral side of
the generator, and must be configured as Winding 2 (K1).
Figure 4-46: Filtering and Calculation of differential and restraint currents
The 845 continuously calculates per-phase differential and restraint currents, and
compares the ratio between the two values with a user pre-defined differential/restraint
characteristic. The purpose of the characteristic is to define the zone of differential
protection operation and the zone of no operation. The differential characteristic provides
setpoints for sensitivity, dependability, and security for all types of faults, and is
programmed by the user to reflect a variety of transformer differential protection
applications.
The differential current is calculated per-phase as a vector sum of the currents from all
windings after magnitude and angle compensation.
Eq. 1
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