Investigation of Subsynchronous Oscillation in a DFIG-Based Wind Power Plant Connected to MTDC Grid

This paper investigates a new type of subsynchronous oscillation (SSO) event observed in a real-world doubly fed induction generator (DFIG)-based wind power plant (WPP) connected to a multiterminal dc (MTDC) grid in Northern China. The system under study is unique and, to date, is the world's largest four-terminal DC transmission network, which connects far-away wind and photovoltaic power plants to regional load centers. For the first time, this paper utilized field-measured data to analyze the characteristics and reveal the underlying mechanism of the SSO in doubly fed induction generators (DFIGs) connected to HVDC grids. The analysis is carried out using the impedance network model established from the impedances obtained from field recorded data. The investigations on the field data concluded that the SSO is essentially caused by the subsynchronous interaction between the DFIGs and the MTDC grid without the presence of a physical series/shunt compensation or weak AC grid. Instead, it is found that the SSO is triggered due to the converter-grid resonance where the MTDC grid presented itself as a virtual capacitor and the DFIGs showed negative resistance at the subsynchronous frequency range.