Transformer Protection Settings Calculator (Inrush Based)

Transformer Protection Settings Calculator

Industrial-grade Protection Coordination tool. This system analyzes the physics of your transformer's Inrush Current (using Holcomb/Specht models) and automatically generates recommended Relay Settings (ANSI 50/51/87). Adheres to IEEE C37.91 and NEC 450.3 guidelines.

Quick Presets:

1. Source / Grid Data

System Voltage ($V_{LL}$) (V)

Short Circuit MVA ($S_{sc}$)

Source X/R Ratio

2. Transformer Specifications

Rated Power ($S_{tx}$) (kVA)

Impedance ($Z\%$)

Core Material

Residual Flux ($B_r$) (%)

Pre-Insertion Resistor (Ω)

Switching Angle (deg)

Protection Recommendations

Suggested Relay Settings

ANSI 50 (Instantaneous)

0 A

ANSI 51 (Time O/C)

0 A

ANSI 87 (Harmonic)

Physics Data

Peak Inrush	0 A
Decay (Time Constant)	0 ms
Voltage Dip	0%

Inrush vs. Protection Limits

The Green Dotted Line represents the recommended Instantaneous Trip level. The Red Zone is the calculated inrush envelope.

Standards & Logic Trace

ANSI 50: Avoiding the "Hair Trigger"

The Instantaneous Overcurrent element (50) is designed to trip immediately for severe faults. However, Inrush Current can look exactly like a fault in magnitude.

IEEE C37.91 Standard: Recommends setting the instantaneous unit above the peak inrush current to avoid nuisance tripping. A margin of 1.5x to 1.6x is often used to account for DC offset asymmetry and relay measurement tolerances.

ANSI 51: Riding the Decay

The Time-Overcurrent element (51) protects against prolonged overloads. It typically picks up at 125% to 150% of full load amps (FLA) per NEC 450.3.

The Logic: Inrush current decays exponentially. The relay must have enough time delay to "ride through" this decay period. Our tool calculates the time ($t_{ride}$) it takes for the inrush RMS to fall below the pickup setting.

ANSI 87: The Harmonic Fingerprint

Differential relays compare current entering and leaving. Inrush only enters, so it looks like a fault. However, inrush is asymmetrical and rich in 2nd Harmonics (100Hz/120Hz).

Logic: We calculate the 2nd harmonic content based on the peak saturation flux. Standard practice is to restrain (block) tripping if the 2nd harmonic exceeds 15% to 20% of the fundamental.

Underlying Physics

                             Core Saturation: Energization forces magnetic flux to $2 \cdot \phi_{max}$. The iron core saturates, permeability drops, and the winding acts like an air-core inductor ($X_{air}$), drawing massive current limited only by source impedance ($Z_{src}$).
                        