The offshore wind power system is expected to grow rapidly during the upcoming years. Simultaneously, the capacity of wind generators can surpass the rating of 15 MVA, necessitating step-up transformers to match this increased capacity. However, the main challenge is maintaining the compactness and lightness of the transformers while meeting high power demands. With this contribution, we explore the feasibility of a superconducting transformer with very high current density. This study delves into the mitigation of AC losses in a 15 MVA HTS transformer for wind energy applications by means of numerical simulations based on the finite-element method. Employing a 2D axisymmetric T-A formulation coupled with an electrical circuit, we estimate AC losses across hundreds of turns of REBCO tapes within a full transformer at reasonable computational time. The T-A formulation considers the superconducting layer of REBCO tapes as infinitely thin, thus allowing to overcome the problem of simulating a superconductor with an extremely high width-to-thickness ratio. To address the substantial transport current value on the high-voltage side, we utilized several parallel Roebel cables, optimizing parameters such as the number of strands in each cable, inter-strand gaps, and vertical separations between cables. Our findings suggest that altering the winding voltage and hence conductor length is not an effective strategy for reducing AC losses. Furthermore, for mitigating the high AC losses at winding ends, we integrate magnetic flux diverters into the transformer structure. Eventually, we propose two distinct designs operating at temperatures of 20 K and 70 K. This research introduces valuable insights and approaches for designing, optimizing and developing HTS transformers.