The UHVDC sending-end power grid with large amount of renewable energy has serious transient overvoltage problems. Firstly, the structure of renewable power systems and the overvoltage protection logic of the convertor-based renewable generators are introduced. Then, the transient overvoltage characteristic of the high renewable power-penetrated UHVDC sending-end power grid under the DC blocking fault and the commutation failure is analyzed. And the mechanism of cascading overvoltage tripping of the renewable generators is analyzed in the meantime. On this basis, the local and global transient voltage rise severity indexes based on the multiple-two-element table are established under single fault and the multiple-fault set, respectively. Finally, the effectiveness of the proposed indexes is shown by some case studies of the BPA simulation system of Qinghai Power Grid in China.