Sub- and Super-Synchronous Interactions Between STATCOMs and Weak AC/DC Transmissions With Series Compensations

With the increasing integration of power electronic converters into the power system, the interactions between converters and their adjacent transmissions bring emerging oscillation issues. A new type of sub- and super-synchronous interactions (S-2 SI) between STATCOMs and the weak AC/DC grid were detected in China Southern Grid. As an oscillation incident never reported before, as far as the authors know, its characteristics were quite different from the previous ones. Field measurements as well as simulation studies have revealed that the various factors contributing to the S-2 SI include the number of online STATCOMs, the strength of the AC system (in terms of effective short-circuit ratio), the status of series compensation, and the loading level of the HVDC. The mechanism of such oscillation has been clarified and it is caused by the S-2 SI between STATCOMs and weak AC grids, where sub- and super-synchronous components are involved in a coupled way. To fully represent such a coupling effect, the concept of sub- and super-synchronous coupled impedance model (IM), or S-2 SC-IM, is proposed and developed in order to establish a stability criterion for the S-2 SI. Based on the IM-based method as well as the time-domain simulations, the impacts on S-2 SI from the various factors are quantitatively analyzed. The results show good consistence among theoretical analysis, electromagnetic simulations, and field measurements. Therefore, the effectiveness of the developed models and the stability criterion has been well verified.