Analysis on the Operating Characteristic of UHVDC New Hierarchical Connection Mode to AC System

The UHVDC system plays an important role in smart grids. In this paper, a new topology structure of UHVDC hierarchical connection mode to different AC systems is analyzed with a view to improving system transmission abilities. Then a new mathematical model is proposed to calculate the MIIF under different UHVDC hierarchical connection modes. The effect of coupling impedance, equivalent impedance and different DC control mode adopted by two group inverters are illustrated to analyze the MIIF. The method to calculate the MISCR of the multi-infeed HVDC system is also applied to HISCR of UHVDC hierarchical connection mode with the MIIF value. In UHVDC hierarchical connection mode, total power to different hierarchical active current layer can be allocated reasonably to change parameter of the received power grid. Thus, the commutation failure can be analyzed according the MIIF change values of UHVDC hierarchical connection mode to the AC system. Therefore, it may increase the risk of commutation failure in the other converter when one converter is in a state of commutation failure. The correctness of the proposed method is verified by PSCAD simulation. The simulation results are illustrated to verify the operating characteristics of the system.