This paper describes quantitatively the magnetic field inside the windings of a HTS transformer. The results are obtained by means of finite element method simulations, using the electromagnetic software package Flux2D. The transformer design has been made by ŠKODA in the European project BIG-POWA. The ratings of the transformer are U1 = 400 V, I1 = 47 A, U2 = 133 V, I2 = 140 A (rms values). The magnetic field in the superconducting windings has been calculated in the steady state regime at 50 Hz. It is non-uniformly distributed along the width of the windings and reaches a magnitude of 80-100 mT. It is shown that the field near the end of the windings can be reduced by using ferromagnetic rings, placed in the lower and upper part of the transformer. The effect of the magnetic field on the critical current density in the Bi-2223/Ag tapes, with which the windings are made of, has been evaluated.