In this paper, a novel model for a high-leakage multiwinding transformer is presented. This model is termed the extended T model. The model is developed by relating physical spaces within the multiwinding transformer to the leakage inductances and the mutual couplings of the transformer.The model is currently being investigated for the following areas: leakage and cross-coupling effects in resonant and hard-switched converters, modelling of EMI parasitics and ac resistance components, and modelling of poly-phase transformers, coupled inductors, and machines. In this paper, the basic model is presented and validated.A high-power high-leakage three-winding transformer is first characterized on the bench using open-circuit and series-coupling tests. This test data set yields the transformer inductance matrix composed of self and mutual inductances. The elements of the matrix are then broken down into the leakage and mutual inductances between the three windings. The physical structure of the transformer is then examined, and the new transformer model is created based on this visual examination and the knowledge of leakage and mutual inductances. The inductance matrix is also used to generate two other models: the conventional T model, and the extended cantilever model. Excellent correlation is demonstrated between all three models, PSPICE simulations, and the experimental load tests using an inductive load.