According to five types of ordinary penetrating faults inside the converter of VSC-HVDC system

including short circuit failure of IGBT valve device

bridge straight-through fault

AC-side single-phase grounding fault

ACside two-phase short circuit fault and DC-side monopole grounding fault

the coordination scheme of fault protection and fault diagnosis for converter is studied. Based on analyzing the system change rules of DC voltage and AC current under the condition of blocking protection

a fault diagnosis algorithm for fault classification and location of converter is proposed

which takes the DC voltage at blocking time and three-phase AC current during two cycles after blocking time as the characteristics signals. Moreover

the voltage and current thresholds for fault classification are determined. A VSC-HVDC system model built in PSCAD/EMTDC is used to validate the proposed fault diagnosis method by simulating serious penetrating faults inside the converter. The simulative results prove that the proposed method can not only identify fault type reliably

but also locate fault position accurately

which is suitable for fault diagnosis for the converter of VSC-HVDC system under the condition of failed valve arm blocking.