Abstract: At At present, the current transformers in the non-earthing system of 35 kv and below in substations usually adopt two-phase incomplete star connection, the b-phase current is calculated by the vector sum of the AC-phase current, which can reflect various short-circuit faults between phases, but the sensitivity of the protection is worse than that of the three-phase full-star connection, because it reduces the equipment by one third, therefore, it is widely used in ungrounded system. In this paper, a case of main transformer skip trip caused by two-phase incomplete star-shaped current transformer circuit abnormality is analyzed to further understand the diversity of its wiring mode during working, in order to facilitate similar failure analysis and related preventive measures.