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VSC-HVDC power transmission systems based on extruded cables are being commissioned with higher rated voltages and longer distances in recent years. In China, the rated voltage of commissioned VSC-HVDC cable projects has been increased from 160 to 200 kV, and then to 320 kV in the past 5 years. To investigate the feasibility of discovering insulation defects of an XLPE HVDC cable system using partial discharge detection under DC voltage, a test cable loop was built using the same batch of 320 kV HVDC XLPE cable used to construct the Xiamen ± 320 kV VSC-HVDC project. Artificial defects of insulation wound and semi-conductive layer tips were designed and constructed at the joint. The two types of defects were effectively detected by the HVDC partial discharge test, and the measured results show distinguished discharge characteristics. Suggestions for determining trigger level and partial discharge inception voltage under DC voltage for partial discharge detection of extruded HVDC cables are given. [ABSTRACT FROM AUTHOR]