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This paper addresses the transient response of instant ground fault in Yun-Guang hybrid UHVDC/AC power transmission system. A detailed bipolar UHVDC electromagnetic transient model is established by PSCAD/EMTDC software. The control strategies for UHVDC system are investigated and the main factors of commutation failure are analyzed. A lot of simulation results demonstrate that Yunnan rectifier AC system fault can not cause UHVDC commutation failure. Metallic ground fault of Guangdong inverter AC system will result in UHVDC commutation failure but enough transition resistance can reduce the chance of it. When one pole ground fault of UHVDC line occurs, DC current of the fault pole has an obvious overshoot and non-fault pole is influenced a little. UHVDC can come back to normal operation once the instant ground fault is cleared.