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An autotransformer is used as the main type of UHV transformer, but the internal fault model of autotransformer provided by most simulation software is not available. To find out the existing problems of the application of the differential protection of UHV transformer, a three winding autotransformer model is firstly built according to the existing Unified Magnetic Equivalent Circuit transformer model provided by EMTDC software. Furthermore, the internal fault model of UHV transformer is set up. By virtue of above models, the energization and internal faults are simulated to validate the well-applied differential protection using second-harmonic blocking. It can be proven that the second-harmonic ratios of three phase inrushes of the UHV power transformer are all below 10% in some energization scenarios. In this case, the maloperation of the differential protection by means of 15%~20% second-harmonic restraint ratio cannot be avoided. On the other hand, the second-harmonic ratio of the fault current is relatively high in the beginning of fault inception for some light faults, leading to the short time delay of operation of protection. This discovery will encourage the research and development of the novel scheme of the main protection of UHV transformer.