The power collection and integration system of medium voltage DC in the large PV is more efficient than the traditional AC collection system, which is the potential area to develop the photovoltaic resources in the future. As one of the key components of power collection and integration system with medium voltage DC, DC transformer is analyzed in detail in this paper, including the technical requirements, structure and control method from the collection system. Then, the corresponding fault clearance methods are proposed when the short circuit fault happens in the low voltage or high voltage side. Finally, a simulation for fault operation analysis of DC transformer is included to verify the feasibility and effectiveness of the proposed method.