A Novel Differential Protection Method for Current Limiting Control of Non‐Isolated DCTs in Multi‐Voltage Level DC Grids

The DC transformer (DCT) is critical equipment used to interconnect DC grids with different voltage levels. Unlike AC transformers, DCT fault behavior is significantly affected by its internal active components and topology, making conventional differential protection methods inadequate. This paper proposes a novel differential protection method for the non‐isolated DC modular multilevel transformer (DCMT), which employs the AC and DC components of the DCMT current to establish composite differential criteria. First, a fault analysis model of a multi‐voltage level DC grid with DCMT is established, and fault currents at different fault locations in the DCMT can be quantitatively analyzed. Subsequently, the differential current characteristics of the DCMT under various fault conditions are analyzed, revealing the limitations of traditional differential protection methods. On this basis, a composite criterion is proposed to identify internal and external faults. Furthermore, considering the impact of fault severity on fault identification, an adaptive strategy for DCMT is designed using the current phase‐plane, which enhances the sensitivity of turn‐to‐turn fault (TTF) identification. Finally, a simulation model of a multi‐voltage level DC grid containing DCMT is constructed using the PSCAD/EMTDC simulation platform to verify the effectiveness and correctness of the proposed method.