Strong dynamic interactions between multi-terminal DC network and AC power systems caused by open-loop modal coupling

This study examines the AC/DC dynamic interactions in an AC power system with an integrated multi-terminal DC network (MTDC) based on a closed-loop interconnected model of the MTDC/AC power system, wherein, the MTDC and the AC system are modelled as two interconnected open-loop subsystems. Analysis in this study indicates that when a complex pole of the open-loop MTDC subsystem is close to an electromechanical oscillation mode (EOM) of concern in the open-loop AC subsystem on the complex plane, this open-loop modal coupling may cause strong dynamic interactions between the MTDC and the AC system. It is very likely that when the open-loop modal coupling occurs, the small-signal stability of the MTDC/AC power system may considerably degrade. In this study, New England test system integrated with an MTDC for wind power transmission is used to demonstrate and validate analysis and conclusions made. Both the master-slave control and the DC voltage droop control for the MTDC are examined. Study cases show that the open-loop modal coupling caused considerable damping degradation of EOM of concern, imposing serious threat to power system small-signal stability.