Different operation modes of the ultra-high voltage direct current(UHVDC)system yield different power constraints and direct current(DC)variables at both converters in power flow modeling

which need to be differentiated. Under the hierarchical integration mode

the inverter connects two alternating current grids with different system parameters and operation conditions

thus the parameters of the converters at the high and low terminals should be set independently. The power flow model of the UHVDC system is proposed. The balanced and unbalanced operation modes are differentiated. Based on the system's symmetry of each operation mode

the independent DC variables are decided and the equivalent power of the converters are found

thus to derive the power flow constraint equation. For the hierarchical integration mode

the operation modes are reclassified by the capacities of the total system and the transmission power to the two receiving-end grids

and the power flow model is proposed. According to the power transfer schemes

the changes of the DC parameters are derived

thus to compare the impacts of them on the grid power flow. The two-region IEEE RTS-96 test system is applied to validate the feasibility and accuracy of the proposed model.