Generator-fed transformer energization is governed by generator subtransient reactance, excitation response, speed/governor dynamics and available prime-mover torque as well as generator kVA. Inrush can cause severe voltage dip or torque disturbance on an islanded source. A dynamic study should use the actual generator and excitation models, transformer saturation and the intended energization sequence.
Equal generator kVA can conceal different voltage responses
Subtransient and transient reactances, time constants and excitation limits influence the voltage during an abrupt current demand. Two generators with the same apparent-power rating can therefore behave differently when energizing the same transformer. Request the actual machine and exciter data, including model bases and limiters. A short-circuit current value alone does not show how the voltage recovers or whether the excitation system encounters a ceiling during the event.
Separate fast electrical response from slower speed recovery
The earliest current and voltage behavior requires the generator electrical model and transformer saturation. Subsequent real-power demand, losses and connected-load response interact with the prime mover and governor. The relevant timescales can therefore extend beyond the first few cycles. Use a model combination that captures the phenomena governing the decision, and retain the generator's pre-event load, reactive output and available operating margin rather than assuming an unloaded machine.
Check the complete island and restoration plan
Existing auxiliaries, contactors and converter controls may fail to ride through a voltage dip even when the generator itself remains stable. Assess those loads alongside transformer inrush, frequency recovery and protection behavior. Compare the intended transformer sequence and load pickup against the same criteria, including a failed or delayed recovery. There is no generally valid generator-to-transformer kVA ratio that replaces this assessment for every machine, transformer and island configuration.
Technical reference framework
[1] CIGRE Technical Brochure 568 — Transformer energization in power systems: A study guide.
[2] IEC 60076-8:1997 — Application guide.