Mission Brief
Protect the Transformer
A 20 MVA, 69/13.8 kV Dyn11 transformer at a FortisBC substation is being re-commissioned after an OLTC overhaul. The previous settings nuisance-tripped on energizing and once let a through-fault rattle the differential. Your job: commission the full transformer protection — the 87T differential done right (vector-group compensation, dual-slope restraint, harmonic blocking) plus the backup, thermal and overexcitation functions — and prove it on the test bench.
- Understand what the differential measures and why it would false-trip
- Compensate the vector group and remove zero-sequence
- Set the dual-slope restraint to ride CT mismatch + tap travel
- Arm 2nd-harmonic (inrush) and 5th-harmonic (overexcitation) blocking
- Set the 50/51 backup and prove the scheme on secondary injection
What it measures — current in equals current out
A differential element trusts Kirchhoff's law: whatever current flows into the transformer must flow out. It compares the HV and LV currents and watches the difference. Zero difference = healthy or through-fault. A real difference = current is escaping inside the protected zone — an internal fault.
Why it would false-trip — three lies
Three things make the two currents look different when nothing is wrong. Each has a fix:
- CT ratio mismatch + tap changer — the HV and LV CTs never match perfectly, and the OLTC moves the ratio. Fix: the percentage (biased) slope.
- Vector group — a Dyn transformer phase-shifts the current 30°, so even balanced load looks unbalanced. Fix: phase + zero-sequence compensation.
- Magnetizing inrush — energize the transformer and a huge one-sided current flows into the HV only. Fix: 2nd-harmonic restraint.
The fix — a dual-slope restraint characteristic
Plot differential current against restraint (bias) current. The relay only operates above the line. The first slope rides over CT mismatch and tap travel; the steeper second slope keeps it stable when a heavy through-fault saturates the CTs. Drag the point: internal faults climb into operate, through-faults stay restrained.
drag the point — operate above the dual-slope line (slope1 20% · slope2 60%)
Compensate the vector group first
Before the relay can subtract the two currents it must line them up: correct the clock-number phase shift and strip the zero-sequence off any grounded-wye winding. Numerical relays do this in software once you tell them the vector group.
Vector Group — Dyn11
Dyn11: compensate the -30° phase shift and remove zero-sequence on the wye side before the differential subtraction.
Don't trip on energizing — harmonic restraint
Inrush is rich in 2nd harmonic; overexcitation is rich in 5th. The relay measures the harmonic content of the differential current and blocks when it is high — that is how it tells a fault from a transformer just being switched in. Slide the harmonics and watch the block engage.
Inrush & Overexcitation — Harmonic Restraint