Power stability analysis of UHVDC systems hierarchically connected to AC systems

With the development of ultra-high-voltage direct current (UHVDC) transmission systems, the receiving AC systems face the problems of power flow transferring, integrating capability and voltage stability. A novel UHVDC hierarchical connection (HC) mode has been proposed and adopted in several real projects in China. The more complex electric coupling relationship of the entire systems made the power interaction characteristics different from those of previous multi-infeed HVDC systems. Thus, it is important to analyze the power stability of such systems in a systematic manner to avoid adverse interactions. In this study, the equivalent model of HC-UHVDC systems is established and several evaluation indices concerning power stability are discussed and improved. Furthermore, the power integration capability and reactive power interaction are evaluated based on real data. The results of this research can be used to provide technical support to real HC-UHVDC projects.