For the specification of winding insulation of transformers, it is important to know the electrical stresses to which the winding can be exposed during fast transient oscillations. These oscillations occur during switching operations performed by circuit breakers, or when gas-insulated substations (GIS) are used. Therefore one of the priorities is to use a high-frequency transformer model capable to simulating fast transient oscillations in the windings. The model presented requires only information about the geometry of the winding and the core, as well the electrical and magnetic parameters for the used materials. In the transformer model, the frequency-dependent core and copper losses are included. Numerical computations are performed with and without the core losses being taken into account. Two types of measurement are taken to verify the validity of the model. First, the voltage transients are measured and computed by the application of a step impulse with a rise time of 50ns. Then, the transformer is switched by a vacuum circuit breaker, and the multiple reignitions, which contain oscillations with a wide frequency range, are analysed. The results verify that the model is suitable to simulate the voltage distribution in transformer windings over a wide frequency range.