GE T60 Instruction Manual page 475

CHAPTER 5: SETTINGS
The differential current is calculated as a vector sum of currents from all windings after magnitude and angle
compensation.
The restraint current is calculated as a maximum of the same internally compensated currents.
The element operates if I > PKP and I
d
restraint factor defined by the relays settings Slope 1, Slope 2, and a transition area between breakpoint 1 and breakpoint
2 settings.
The T60 percent differential element is based on a configurable dual-breakpoint / dual-slope differential restraint
characteristic. The purpose of this characteristic is to define the differential/restraint ratio for the transformer winding
currents at different loading and fault conditions to securely distinguish between external and internal faults. Increased
differential current can occur under load or external fault conditions due to current unbalance between transformer
windings and can be caused by the following:
• Inherent CT inaccuracies
• Onload tap changer operation: it adjusts the transformer ratio and consequently the winding currents
• CT saturation
T60 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
/ I > K, where PKP represents a percent differential pickup setting and K is a
d r
Figure 5-144: Percent differential calculations
GROUPED ELEMENTS
Eq. 5-31
Eq. 5-32
5
5-257