Fault Diagnosis and Tolerant Operation Method of Open Circuit Fault in Modular Multilevel DC/DC Converter With Quasi-Two-Level Modulation

The modular multilevel DC/DC converter (MMDC) is a typical high-voltage, high-power DC/DC converter, combining the advantages of quasi-two-level modulation with high DC voltage utilization and strong power transmission capability, having broad application prospects in DC grids. The MMDC is composed of numerous insulated gate bipolar transistor (IGBT)- based switches which may impose an open-circuit fault (OCF). When an OCF occurs in the switch of the MMDC, the internal voltage and current may severely distort, potentially affecting the safe operation of the system. To address this issue, this paper proposes a quasi-two-level modulation based OCF diagnosis and fault-tolerant operation method for an MMDC, which utilizes the signal synthesis of arm current and arm voltage modulation. First, the relationship between fault arm current and modulation wave is revealed. Based on this, a fault diagnosis algorithm is proposed to accurately identify the faulty arm and the fault type, without requiring additional sensors or complex calculations. The MMDC is then controlled in the proposed fault-tolerant mode by actively modifying the modulation wave until the specific faulty submodule is located and bypassed. This reduces voltage and current distortion. Subsequently, the modulation wave automatically returns to normal, allowing the system to quickly recover. Verification results demonstrate that the proposed method accurately and rapidly diagnoses the OCF and achieve faulttolerant operation.