Power flow modelling to UHVDC line and its hierarchical connection mode

At one converter station, compared with the high‐voltage direct current (HVDC) line, the conversion units of the ultra HVDC (UHVDC) line are doubled and controlled independently, possibly yielding the unbalanced operation modes, separate settings to the DC variables of the positive and negative poles, and different equivalent powers and mismatch equations (MEs). Under the hierarchical connection mode (HCM), the inverters connect two AC voltages. Hence, the operation modes are not the same as the UHVDC line, and the parameters of the inverters at the high and low terminals are to be set independently. In this study, the power flow model of the UHVDC line is newly proposed. Considering the unbalanced modes and the HCM, the DC variables, the equivalent powers of the converters, and the MEs are decided. The elements of the extended Jacobian matrix are defined. The initial values and specified values are derived. The modified IEEE RTS system and a provincial system in the eastern China are applied to validate the feasibility and accuracy of the proposed models. The research provides the basis for the operation and economic dispatch of the AC/UHVDC systems.