Analysis on voltage stability of hybrid system with UHVDC hierarchical connection to AC grid

Ultra high voltage direct current (UHVDC) hierarchical connection to AC grid mode has the advantages of leading flow reasonable distribution and improving voltage support capability of receiving-end system. But the DC transmission system based on line-commutated inverter has the inherent voltage stability problem. This paper focuses on the voltage stability of AC-DC system under UHVDC hierarchical connection mode, deduced the computing method of voltage stability factor under hierarchical connection mode, analyzed the voltage stability characteristic of AC-DC system while adopting different control mode, and verified the algorithm rationality by simulating in actual small system. Analysis shows that adopting constant current or power on the rectifier side and constant voltage on the inverter side control mode is most beneficial for voltage stability of AC-DC system.