Shunt VSC Based Subsynchronous Damping Control for DFIG-based Wind Farms Connected to an MMC-HVDC System

An emerging subsynchronous oscillation (SSO) associated with wind generators and voltage-sourced converter (VSC) based HVDC has been observed around the world. It is revealed that this new type of SSO is caused by the subsynchronous control interaction among power electronic converters and the grid. In this paper, the stability criterion of SSO is examined using frequency-dependent impedance models. Then a shunt VSC based subsynchronous damping control (SVSDC) has been proposed to reshape the impedance characteristic of the whole system and thus mitigate the SSO. The stability criterion and the control has been applied to a typical system with DFIG-based wind farms connected to a modular multilevel converter (MMC) based HVDC transmission. Impedance model based analyses as well as electromagnetic simulations have verified the effectiveness of the criterion and the control scheme. For its flexibility of capacity and deployment, the proposed SVSDC offers a promising option to address the SSO issues associated with wind generators and/or VSC-HVDCs.