High amplitude overvoltage will occur in the AC system after the DC block in the islanded operation mode of Yunnan-Guangdong ultra-high voltage direct current (UHVDC) sending system. This is a key problem of islanded operation. The dynamic process of voltage can be divided into several phases. This paper analyzes the major factors influencing the overvoltage level in different phases. The overvoltage in hundreds of milliseconds after disturbance in phase two is lower than that of phase one

but the duration is longer and the criterion of voltage is higher. The voltage level of this phase is notably affected by excitation control of generators. The short-circuit ratio is closely related to the overvoltage level and the short-circuit ratio differs from phase to phase. The voltage control of the generator high-voltage-side increases the short-circuit ratio of phase two after disturbance and mitigates the overvoltage by reducing the electrical distance between the constant voltage source and the system. The rational voltage level and reactive power distribution in normal operation is guaranteed.