A utility once specified a 50 MVA transformer using the old "constant reactance" model. Their protection study predicted a peak fault current of 15 kA. They bought a 20 kA breaker. After manufacturing, the transformer was tested per IEC 60076-6, revealing that during asymmetric saturation, the effective reactance dropped 22%, yielding a true peak current of 19.5 kA. The breaker was operating at 97.5% of its rating—no margin.
The problem? It varies with:
While not the primary focus, IEC 60076-6's treatment of asymmetric saturation directly applies to GIC events (solar storms). A half-cycle saturation from GIC behaves identically to the first half-cycle of an asymmetrical fault. Engineers using this standard can model transformer behavior during geomagnetic disturbances without needing separate solar storm guides. iec 60076-6