Abstract: In order to meet the higher demand of dynamic reactive power support is put forward in the UHV DC power transmission project, the new-generation synchronous condensers are applied in the UHV DC/AC. However, in order to provide bidirectional transient reactive power characteristics during AC and DC grid fault condition, especially to reduce the commutation failure phenomenon at the inverter station, there are great changes in the main body and excitation control system of the new-generation synchronous condensers compared with the same-capacity generators. Therefore, according to the requirement of reactive power in the grid and the requirements of safe for the new-generation synchronous condensers, the limiting model of excitation system for the new-generation synchronous condensers under low excitation and strong excitation conditions are established.Based on this, the closed loop transfer function of the excitation current is derived. According to the requirement of the excitation limit model, the analytical expressions of the transient reactive power of the new-generation synchronous condensers are obtained during the high and low voltage of the grid.Finally, the simulation model of the new-generation synchronous condensers access to UHV DC power grids are built based on the PSCAD software. The transient reactive support capability of the new-generation synchronous condensers under grid fault condition are verified.