Checks whether a transformer differential (87T) relay would operate for a given pair of CT-secondary currents, using the standard percentage-bias (percentage differential) principle: differential current Id = |I1−I2| is compared against a dual-slope restraint characteristic based on the bias/restraint current Ir = (I1+I2)/2.
Standards followed
Percentage-bias / percentage-differential protection principle, consistent with IEEE C37.91 transformer protection guidance
What it can do
- Differential (operate) current and restraint (bias) current from the two CT-secondary currents.
- Dual-slope restraint characteristic: minimum pickup, a low first slope (covers tap-changer range and CT mismatch), and a steeper second slope above a configurable knee restraint current (stays secure during CT saturation on close-in through-faults).
- Trip / no-trip verdict.
Example problem
I1=1.05pu, I2=0.98pu, minimum pickup 0.3pu, slope1 25%, knee at 2.0pu restraint, slope2 60%.
- Id = |1.05−0.98| = 0.07pu.
- Ir = (1.05+0.98)/2 = 1.015pu — below the 2.0pu knee, so slope1 applies.
- Threshold = 0.3 + 0.25×1.015 = 0.554pu.
- 0.07pu < 0.554pu → no trip (normal load/inrush condition, correctly restrained).
No trip.
Want the full step-by-step version?
Currents must already be referred to a common base (CT ratio and vector-group/phase-shift compensated) before entering them here — this tool checks the restraint characteristic itself, not the CT/vector-group compensation.