In this paper, development of the special converter transformer model (with 5 windings) and testing results of its differential protection is presented. The special converter transformer is mainly used for supply of rectifiers or converters, FACTS devices etc. This type of transformer can introduce non-standard fixed phase angle shifts such as 20o, 15o, 12o, 10o, etc. this is obtained by unique connections of the secondary windings (e.g. polygon, zig-zag, extended delta). The converter transformer must be protected by a relay being able to compensate for non-standard phase shifts. Otherwise the error magnitude would be very high (0.34 pu for 20o phase shift) and problem with sensitivity and stabilization of protection can take place. An appropriate simulation model of the converter transformer with five windings has been developed in MATLAB/Simulink environment. Next, the testing results of differential protection (7UT87 Siemens device) operation for various disturbances are presented. The testing was performed with use of Omicron tester and prepared simulation files in COMTRADE format.