When disturbance or fault occurs to AC/DC hybrid power system, the transformer and current transformer (CT) will produce complex transient interactive process, which makes it difficult for relay protection to identify faults and operate correctly. In recent years, there are many unusual mal-operations of the differential protection for transformer, generator and transmission line already in operation caused by the adjacent switching-on transformer, which have seriously threaten the safe and stable operation of the power grid. The transformer inrush current, sympathetic current and CT transient saturation are researched based on mechanism analysis, digital simulation and dynamic test. The mechanism of transformers and CT transient interactive process is discovered and relevant countermeasure is proposed. A simulation platform is also built to help to select the types of CT and analyze accident. This work is supported by Key Project of Smart Grid Technology and Equipment of National Key Research and Development Plan of China (No. 2016YFB0900600) and National Natural Science Foundation of China (No. 51277084).