Small Signal Stability of UHVDC Hierarchical Connection to AC Grid

UHVDC hierarchical connection to AC grid is a specially-designed mode in China to enhance the support from the receiving AC grid to HVDC. Besides the merits to optimize power flow, such a connection mode features stronger stability than the mode in which all power is fed into the grid with a single voltage grade. In this paper, the stability of UHVDC hierarchical connection is quantitatively discussed based on the proposed mathematical model for small signal analysis, where the interactions between the converters through both AC and DC system, as well as the effects of HVDC control are taken into account. Meanwhile the sensitivity of stability to the supporting strength of AC system and the parameters of control system is demonstrated based on simulation cases built in PSCAD. Hopefully the results can facilitate the proper decision on the control parameters and the investigation on the advantage of UHVDC hierarchical connection.