Impact of Strong Dynamic Couplings between VSC-based Generation Units and Power Systems on Power System Electro-mechanical Oscillation Modes

The impact of a VSC-based generation unit on power system electro-mechanical oscillation modes (EOMs) is two-fold, changing power flow conditions and dynamically coupling with power systems. This paper focuses on the latter one. Usually the dynamic couplings between VSC-based generation units and power systems are thought to be weak due to the excellent performance of vector control. However, it is discovered in this paper that the dynamic couplings would be tremendous if the DC-link voltage control mode of the VSC coincided with any of the EOMs. Under the special condition of modal coincidence, damping of the affected EOM would be degraded substantially. Furthermore, it is found that the strength of dynamic couplings at the point of modal coincidence is positively correlated to the penetration level of the VSC-based generation unit. The effect of damping deterioration is worsened with the increasing of penetration level of the VSC-based generation unit when modal coincidence occurs. Both theoretical analysis and case study are presented in the paper to validate these findings.