The existing calculation methods of transient overvoltage on the commutation bus due to the ultra-high voltage direct current(UHVDC)blocking have no real solution in the case of extremely low short-circuit ratio. Based on the resonant circuit after DC blocking

this paper deduces and establishes the expression of the relationship between transient overvoltage and resonant frequency. Referring to the definition of short circuit ratio

the concept of reactive power short-circuit ratio is proposed. Then

a method for calculating transient overvoltage after DC blocking is proposed with wider application range. It is found that the trend of transient overvoltage on the commutation bus is nonlinear and accelerates with the decrease of reactive power short circuit ratio. Finally

based on the DIgSILENT simulation platform

the DC transmission standard test system of International Council on Large Electric Systems and Xinjiang Tianshan-Zhongzhou DC transmission system of China are built to verify the proposed calculation method of transient overvoltage. The simulation results show that the proposed calculation method is effective and applicable in the system with extremely low short-circuit ratio.