ASDEX Upgrade, an experimental tokamak for nuclear fusion research, requires an electrical power up to a few hundred MVA for a time period of 10 – 15 s. The power and energy is provided by isolated networks based on flywheel generators. About twenty power transformers must be energized on a daily basis to get the pulsed power supply operational for plasma experiments. Nuisance interactions of transformer inrush currents with the protection system of the synchronous machines and capacitor banks used for reactive power compensation must be avoided. To investigate the effects of inrush transients in present and future configurations, measurements were performed and a simulation model based on an equivalent magnetic circuit representation of the transformer was derived. The paper describes the model, results of simulation and measurement and shows which measures were taken to keep transients in the isolated network at an acceptable level.