In order to meet the higher demand of dynamic reactive power support is put forward in the Ultra High Voltage Direct Current(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. In this paper, the limiting model of excitation system for the new-generation synchronous condensers under low excitation and strong excitation conditions are established. Then, the expression of sub-transient reactive power for the new-generation synchronous condenser is derived and the reactive power characteristics considering the influence of excitation regulation are also obtained. In addition, the transient reactive power demand of the high-voltage direct current(HVDC) transmission system is also analyzed. Finally, the simulation model of the new-generation synchronous condensers access to UHV DC power grids are built based on the PSCAD software. The simulation results verify the support effect of the new-generation synchronous condensers on DC transient reactive power.