High-voltage, high-frequency and high-power transformer having a core (1) on which a primary winding (2) is disposed on which a secondary winding (4) is disposed in an insulated manner, whereupon the entire assembly is housed and mounted in an insulator (3), wherein the insulator (3) is made up of two parts or halves (6) and (7) symmetrical with respect to a transverse vertical plane, each part having a hollow tubular element (3.1) housed inside an outer housing (3.2) of each half of the insulator, defining in each part an annular space (3.3) comprised between the outer wall of the tubular element (3.1) and the inner wall of the outer housing (3.2), where the secondary or high-voltage winding is disposed, the insulator (3) presenting in its outer housing (3.2) a slot (5), which is situated at zero volts level, and though which the oil penetrates towards the secondary winding.