METHODOLOGY AUDIT — TRANSFORMER DIFFERENTIAL VIRTUAL LAB
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OVERALL VERDICT
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The revised V2 model is methodologically suitable for diploma-level concept demonstration and comparative virtual-lab exercises. It is not suitable for actual transformer-protection setting, relay commissioning, or publication-grade transient validation.

CRITICAL ISSUES FOUND IN THE FIRST VERSION
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1. The transformer kVA and voltage ratings were only displayed; they did not affect the calculations.
2. Relay currents were generated directly from arbitrary multiples of 5 A, without transformer rated-current or CT-ratio conversion.
3. Internal phase and earth faults used unexplained fixed coefficients.
4. A trip time was calculated using an invented expression that was not tied to a manufacturer relay characteristic.
5. CT saturation was represented without clearly stating that it was only a simplified magnitude-loss approximation.
6. The transformer rating text overlapped the relay area and was not clearly visible.

CORRECTIONS IMPLEMENTED
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1. Three-phase rated winding currents are calculated from I = S/(sqrt(3)V).
2. HV and LV CT primary ratios are entered explicitly with a fixed 5 A secondary rating.
3. CT secondary currents are calculated from actual winding currents and CT ratios.
4. Numerical compensation refers both sides to a common 5 A base and corrects the selected tap mismatch.
5. Internal faults use a user-defined HV contribution percentage; the two in-zone contributions add in the differential element.
6. The dual-slope biased operating threshold is continuous at the breakpoint.
7. Manufacturer-specific trip-time prediction was removed. The lab reports operating margin instead.
8. Inrush blocking uses the entered second-harmonic percentage and threshold only for the inrush scenario.
9. Assumptions and methodological boundaries are displayed inside the laboratory.

ACCEPTABLE SIMPLIFICATIONS
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- Single RMS-current magnitude per side rather than phase-by-phase phasors.
- Vector-group phase compensation is assumed already completed by the numerical relay.
- External fault is represented by selected through-current in per unit.
- CT saturation is represented by reduced fundamental-current magnitude, not a transient waveform.
- Internal earth fault does not include an explicit zero-sequence network or restricted-earth-fault element.

WHAT THE LAB MAY VALIDLY DEMONSTRATE
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- Current balance in healthy operation.
- Stability of percentage-biased differential protection during external through current.
- Possible false differential current caused by CT error, mismatch, or severe saturation.
- Differential operation for an in-zone internal fault.
- Security improvement from numerical ratio/tap compensation.
- Inrush security from second-harmonic restraint.

WHAT MUST NOT BE CLAIMED
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- Exact fault-current magnitude for a real transformer installation.
- Exact relay operating time.
- Exact CT transient saturation behaviour.
- Complete phase-shift, zero-sequence, or vector-group performance.
- Suitability of the displayed settings for a real transformer.