On a weak source, transformer inrush can depress source voltage enough to affect other loads or prevent the transformer voltage from recovering as expected. The study needs source short-circuit strength, transformer saturation/inrush model, residual flux assumptions, breaker closing behavior, system load and voltage criteria. Generator or inverter sources may require dynamic models rather than an infinite-bus assumption.
Source strength is a starting point, not the dynamic model
Short-circuit power provides a useful indication of source impedance at the connection point. An illustrative 100 MVA short-circuit level on a 10 MVA base corresponds to an impedance magnitude near 0.10 per unit before separating R and X. It does not describe the nonlinear transformer current or the response of a generator excitation system. Obtain the minimum credible source strength and the dynamic model required for the source type instead of applying an infinite-bus energization result to a weak island.
Closing instant and residual flux change the current
Flux depends on the integral of winding voltage plus its initial magnetic state. A combination of closing instant and residual flux can drive the core into saturation and produce a strongly asymmetric current. The worst voltage-recovery case need not be identified by a single assumed inrush multiplier. Test a justified range of initial states and breaker pole timing, using a saturation model that the transformer supplier considers suitable for energization analysis.
Judge the effect on the loads already connected
Record voltage depth and duration at the transformer and at sensitive buses, together with frequency behavior where the source is islanded. Existing drives, contactors or generation controls may respond before the transformer current settles. Compare the simulated disturbance with the actual equipment ride-through criteria and protection behavior. Any proposed mitigation—changed sequence, controlled switching or source reinforcement—needs its own verified case rather than being assumed effective from the transformer rating alone.
Technical reference framework
[1] CIGRE Technical Brochure 568 — Transformer energization in power systems: A study guide.
[2] IEC 60076-8:1997 — Application guide.