Mitigation of Subsynchronous Control Interactions Using Multi-Terminal DC Systems

This article presents a control strategy to mitigate subsynchronous control interactions in doubly-fed induction generator (DFIG)-based wind farms. The strategy makes use of a multi-terminal dc (MTDC) system to damp subsynchronous oscillations (SSOs) by adding a supplementary damping control (SDC) to MTDC grid converters. The SDC is focused on stations with modular multilevel converters. This converter topology currently allows the integration of several high-voltage and high-power MTDC applications into modern power systems. Additionally, the case in which SDCs are added to both MTDC grid converters and DFIG converters is analyzed. The SDC is designed using an identified model of the system. The identification method injects probing signals from the converter terminals and obtains an identified model by measuring the response of the system. This method facilitates the control tuning and implementation of the SDC in practical systems in which either the equations and parameters of some equipment can be difficult to know precisely or the model of commercial equipment is not available.