GE 845 Instruction Manual page 269

CHAPTER 4: SETPOINTS
NOTE:
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
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.
Figure 4-47: Differential/Restraint Characteristic for main transformer
d
Pickup
The above figure shows the differential/restraint characteristic of the main transformer
percent differential protection.
The decision for operation or no operation is complemented by detection of CT saturation,
followed by currents directional check. The saturation flag (SAT) can only be initiated
during an external fault, providing the magnitude of any of the restraints is bigger than a
threshold average using the Break1 and Break 2 setting programmed in the menu. While
at the same time, the differential/restraint ratio is below a slope average calculated based
on Slope 1 and Slope 2 settings. The directional flag is not checked unless the SAT flag is
initiated. Normally during internal faults, the trajectory of the differential/restraint ratio
does not cross the average calculated breakpoint threshold, in which case the directional
flag is not checked. The directional flag is checked during external faults (SAT flag
triggered), where due to saturation, the differential/restraint trajectory is able to enter the
operating region. Usually the angle between the currents during external fault and CT
saturation remains bigger than 90 degrees, in which case the percent differential
protection does not operate. However, while in the operating region, the protection would
operate, on evolving faults (external to internal fault), as the directional flag would be high
(angle less than 90 degrees).
Ir max( I , I , I
1 2 3
comp comp
Operate
Slope 2
PROTECTION
)
comp
Block
Slope 1
restraining
r
Eq. 2
4–141