Analysis and Diagnosis of Inner-Turn Short Circuit of UHV Converter Transformer Windings

As the core component of the UHV DC transmission system, the internal failure of the converter may cause serious power failure. This paper focuses on the sudden abnormal increase of total hydrocarbons in a UHV converter, and comprehensively utilizes oil chromatography analysis, on-site diagnostic test, disassembly inspection at the factory, and multi-physics field simulation verification to investigate the fault mechanism of the short-circuiting between strands of the valve-side coil of the converter. In the study, with the help of low-frequency current test and load loss test, the possibility of magnetic circuit failure is excluded, and the fault is locked in the electrical circuit of the winding. After returning to the factory for disassembly and inspection, it was found that the valve-side coil was short-circuited between the strands due to the poor winding process. Simulation based on a two-dimensional axisymmetric field coupling model shows that the current density at the short-circuit point reaches 3.5 times of the normal value. The study shows that the short-circuit between strands caused by wire displacement is the root cause of the gas production abnormality, and the coupling of the leakage field and the circulating current exacerbates the local overheating. The research results provide an important theoretical basis for the design optimization and condition maintenance of UHV converters.