Impedance Modeling and Analysis of Medium-Frequency Oscillation Caused by VSC-HVDC Connected to Local Weak Grid and DFIG-Based Wind Farms

In the northwest of China, a strategy to transmit wind-thermal–bundled power from local grid and doubly fed induction generator (DFIG)–based wind farms through voltage source converter–based HVDC (VSC-HVDC) can be widely applied. However, since is usually weak, new type electrical oscillation in medium-frequency region may occur sending-end converter (SEC) VSC-HVDC with PQ-control. The mechanism this caused by interaction between DFIG, grid, SEC not entirely understood. study, sequence impedance model PQ-control outer loop PLL derived explicit analytic expression, then, explored based on intuitive analysis system frequency characteristics. Compared subsynchronous (SSO) DFIG or inverter, theoretical shows that (MFO) mainly originates due its negative damping effect about 100 200 Hz. addition, impact controller parameters operating conditions characteristics analyzed detail. Finally, correctness validated time-domain simulation.