A power three-phase transformer with smooth voltage regulation under load has a common magnetic core M with division into nine limbs arranged in a plane. The three main limbs 2, 5 and 8 are used for winding the primary phase windings W₁. The six additional limbs 1, 3, 4, 6, 7 and 9 house the DC biasing windings Wd. Secondary phase windings W₂ are located on nine limbs: three on the main limbs 2, 5, 8 and two on additional limbs 1, 3, 4, 6, 7 and 9. The compensating winding W_k is wound on all additional limbs. Deep smooth regulation of the secondary voltage of the transformer is carried out by changing the value of the DC current in the six DC biasing windings Wd.