GE 845 Instruction Manual page 247

CHAPTER 4: SETPOINTS
NOTE:
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
Figure 4-57: Filtering and calculating transformer 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.
The restraint current is calculated as a maximum of the same internally compensated
currents:
Refer to the Transformer Setup chapter for the winding currents magnitude and phase
shift compensations performed by the 845 relay.
Id I I I
1 2 3
comp comp comp
Ir max( I , I , I
1 2 3
comp comp
PROTECTION
)
comp
Eq. 1
Eq. 2
4–143