Part 3 of the protection series. Setting the 11 kV relay of a 1.6 MVA transformer: above inrush, below the IEEE C57.109 damage curve, and with an instantaneous element that ignores LV faults.
| Transformer | 1.6 MVA, 11/0.4 kV, Dyn, uk 6 % — HV full-load current 84.0 A |
| Relay | IEC Standard Inverse, pickup 100 A (1.19 × FLC), TMS 0.10 |
| Maximum LV three-phase fault | 37.4 kA at 0.4 kV → 1,360 A seen at 11 kV (16.2 × FLC) |
| Inrush (assumed) | 8 × FLC for 0.1 s = 672 A (12 × FLC = 1,008 A for comparison) |
Set above the maximum LV fault seen from HV, so the HV relay does not trip for faults the LV breakers should clear, and above inrush.
With a Dyn transformer an LV phase-to-earth fault appears in only two HV lines, at 1/√3 of the ratio-transformed value.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| No trip on inrush | t > 0.1 s at 672 A | 0.36 s | ✓ PASS |
| Below damage curve at max through-fault | t < 4.76 s | 0.26 s | ✓ PASS |
| Instantaneous above LV fault | > 1,360 A | 1,800 A | ✓ PASS |
Key insight: A transformer HV relay sits between three limits: inrush (lower bound in time), the damage curve (upper bound), and selectivity with the LV incomer. The instantaneous setting must stay above the largest LV fault seen from HV.
Every input in this example is editable in the live calculator — no signup needed.
Open Protection Coordination Suite (full workbench) calculator →Typically 8–12 × FLC for 0.1 s for distribution transformers; use the manufacturer's value when available.
For category II–IV transformers IEEE C57.109 adds a mechanical-duty curve for frequent faults; the calculator shows the thermal part and notes the limit.