Add to ORKGThe main theme of my Bachelor thesis was protection of the transformer, measured case was orient to differential protection of the transformer, functions, stabilization and release characteristic. The proposed method of testing module SPCD3D53 differential protection SPAD 346C which was made by ABB. Setting of release characteristics on the protection were made for real power transformers. Testing device OMICRON CMC 256 plus was used to measurement, which was same as protection connected to personal computer. All the PC’s settings operations were realized by recommended software. The measurements were made for three cases, when differential current was in one, two or three phases. Results of measurement are mentioned in tables and graph