Study on Operation Characteristics of UHVDC Power System with Hierarchical Connection to AC Grid

The UHVDC quasi-steady-state equations of the hierarchical connection mode are proposed in this paper. Then the operation characteristics of UHVDC power system with hierarchical connection to AC grid are analyzed. The AC/DC system fault interaction mechanism of UHVDC hierarchical control has been analyzed and simulated with the practical power grid cases. The results show that the hierarchical connection mode can effectively improve the stability of AC/DC systems. Furthermore, the impacts of DC restart and continuous commutation failure on AC/DC transmission capability have been simulated with practical power grid cases. The results show that for areas without strong reactive power supply, hierarchical connection mode can improve voltage stability, and enhance the transmission capacity of the DC system. It has proven that using hierarchical connection mode in the UHV DC receiving-end grid can improve the voltage support ability of UHV DC system, and reduce the commutation failure risk.