Abstract: According to the relationship between currents in the primary and secondary windings of four-limb three phase to four phase transformers, three differential protection connection schemes are proposed. To meet the requirement to correct operation of differential protection in all situations, the three schemes are compared in the aspects such as the selection and connection of current transformers and the number of auxiliary current transformers, and so on. The analysis results show that there is the best one among the three schemes, in which only one auxiliary is needed.