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11kV Transformer IDMT Protection Curve & Discrimination Sizer

Prevent catastrophic upstream DNO 11kV feeder tripping during downstream LV faults. This tool evaluates IEC 60255 inverse definite minimum time (IDMT) curves, computes operating times across fault ranges, and evaluates the engineering guideline Δt ≥ 0.35s grading margin (NPAG / BS EN 60255-151) between 11kV relays and 400V air circuit breakers.

BS EN 60255-151 Discrimination Gate: To achieve total selectivity, upstream relays must maintain a grading margin of Δt ≥ 0.35 seconds over downstream devices, accounting for circuit breaker opening time (80ms), relay overshoot (50ms), and CT saturation tolerances per NPAG Chapter 9.

1. Upstream 11kV Overcurrent RelayIEC 60255 Characteristic CurveCurrent Transformer (CT) Primary RatingRelay Current Setting Tap (Is in Primary Amps)Current pickup setting on the 11kV protection relay (typically 100% of CT).Shifts the IDMT curve vertically (typically 0.10 to 0.25).

2. Downstream 400V Incomer & Fault LevelSecondary Fault Current at 400V Busbar (kA)Prospective 3-phase symmetrical fault current at the main LV switchboard.Downstream Incomer Total Clearing Time (seconds)400V ACB short-time delay (Isd) trip time + contact separation time.Transformer Rating

Coordination & Selectivity Audit

Discrimination Grading Margin (Δt)CALCULATING...

Upstream 11kV Relay Operating Time--

Downstream 400V Clearing Time--

Reflected 11kV Fault Current--

Standard Grading Margin Threshold≥ 0.35 seconds

Evaluating protection grading margin...

TIME-CURRENT DISCRIMINATION (LOG-LOG)11kV Relay vs 400V ACB

11kV Upstream Relay 400V Downstream ACB Operating Δt

IEC 60255 IDMT Relay Operating Formulation

BS EN 60255-151:2009 & NPAG Chapter 9

AM2 PORTFOLIO & DESIGN AUDIT FORMATTRANSPARENT FIRST-PRINCIPLES MATHS

Scenario: 1000 kVA 11kV/400V packaged substation, 18 kA symmetrical fault on 400V busbar, 400V ACB clears in 0.15 s:

Fault Reflection to 11kV: 18,000A × (400V / 11,000V) = 654.5A on the 11kV primary feeder.
Relay Pickup (PSM): With a 100/5A CT and a 100A plug setting (Is = 100A), the Plug Setting Multiplier is: PSM = 654.5A / 100A = 6.55.
Standard Inverse Tripping Time: Under BS EN 60255 (SI: k=0.14, α=0.02):t = [0.14 / (6.55^0.02 - 1)] × 0.15 = 0.548 seconds
Grading Margin (Δt): Δt = 0.548s - 0.150s = 0.398 seconds (398 ms).
Verdict: Because Δt (398 ms) ≥ 350 ms, full discrimination is achieved. The 400V ACB will isolate the fault cleanly without causing the upstream 11kV DNO feeder circuit breaker to nuisance trip.

Transformer Magnetising Inrush Advisory: When energising power transformers, transient magnetising inrush currents typically reach 8 to 12 times full load current for 100ms. For transformer primary feeders, Extremely Inverse (EI) or modern digital relays with 2nd harmonic restraint (typically set to 15% to 20% 100Hz ratio) should be selected to avoid nuisance tripping upon energisation.

Engineering Standards: BS EN 60255-151:2009 • NPAG Discrimination Practice • BS 7671 Section 536