A method for producing a cooling channel in a winding for a welding transformer , the method comprising the steps of holding, by a holder , a winding module (35A; 35B) which comprises a first winding element , a second winding element and a base element (353; 353B), wherein each one of the first and second winding elements (351; 352) protrudes from the base element (353; 353B) and wherein the first and second winding elements (351; 352) are positioned spaced to each other at the base element (353; 353B) so that the winding module (35A; 35B) has a t-cross-section, wherein the first winding element comprises at least one window (3511; 3512) for accommodating a part of a core of the welding transformer , wherein the second winding element comprises at least one window (3521; 3522) for accommodating a part of the core of the welding transformer , and wherein the base element (353; 353B) comprises grooves separating the base element (353; 353B) into sections such that at least one of the sections is connected to the first and second winding elements (351; 352) and such that at least one of the sections is connected to only one of the first and second winding elements (351; 352), and sealing a plug (60; 60A, 60B, 60C) into a first hole of the cooling channel , the cooling channel being provided in the first and second winding elements as well as the base element (353; 353B) and being suitable for flowing a coolant through the first and second winding elements (351; 352) as well as the base element (353; 353B), which first hole is open to the outside of one of the elements , by controlling a movement of the plug (60; 60A, 60B, 60C) relative to the first hole so that a solid-state welding joint (600; 601; 602) is produced by material of the plug (60; 60A, 60B, 60C) and material of the winding module (35A; 35B), in which the first hole is positioned, wherein the holder holds the winding module (35A; 35B) such that a movement between the winding module (35A; 35B) and the holder is prevented while the step of sealing is performed. holding, by a holder , a winding module (35A; 35B) which comprises a first winding element , a second winding element and a base element (353; 353B), wherein each one of the first and second winding elements (351; 352) protrudes from the base element (353; 353B) and wherein the first and second winding elements (351; 352) are positioned spaced to each other at the base element (353; 353B) so that the winding module (35A; 35B) has a t-cross-section, wherein the first winding element comprises at least one window (3511; 3512) for accommodating a part of a core of the welding transformer , wherein the second winding element comprises at least one window (3521; 3522) for accommodating a part of the core of the welding transformer , and wherein the base element (353; 353B) comprises grooves separating the base element (353; 353B) into sections such that at least one of the sections is connected to the first and second winding elements (351; 352) and such that at least one of the sections is connected to only one of the first and second winding elements (351; 352), and sealing a plug (60; 60A, 60B, 60C) into a first hole of the cooling channel , the cooling channel being provided in the first and second winding elements as well as the base element (353; 353B) and being suitable for flowing a coolant through the first and second winding elements (351; 352) as well as the base element (353; 353B), which first hole is open to the outside of one of the elements , by controlling a movement of the plug (60; 60A, 60B, 60C) relative to the first hole so that a solid-state welding joint (600; 601; 602) is produced by material of the plug (60; 60A, 60B, 60C) and material of the winding module (35A; 35B), in which the first hole is positioned, wherein the holder holds the winding module (35A; 35B) such that a movement between the winding module (35A; 35B) and the holder is prevented while the step of sealing is performed. <EOS>