Mechanical-electrical-grid model for the doubly fed induction generator wind turbine system considering oscillation frequency coupling characteristics

With the evolution of renewable energies, many doubly fed induction generators (DFIGs) are being connected to the power grid, whose operation and grid-connection stability have a major impact on the power grid. Currently, most studies focus on either modeling the mechanical-electrical section or the electrical-grid section, and discussions have been limited to shaft oscillation or frequency coupling problems. In this study, a mechanical-electrical-grid model of a DFIG was established to examine the impacts of wind speed and system control parameters on electrical damping and grid-connection stability. The accuracy of the proposed model and validity of the analyses were verified using simulations. The following were observed: (1) In the case of changing wind speeds, the wind speed and the applied control model determine the shaft oscillation of DFIG, whereas the grid-connected impedance on the rotor side is dependent on the wind speed. (2) At a constant wind speed, changes in control parameters under different control modes affect the dynamic characteristics of the drive train differently, whereas the grid-connected impedance on the rotor side is primarily determined by the proportional gain of the inner/outer loop of the control system. The conclusions drawn from this study can further improve the safe and stable operation of DFIG wind power generation systems as well as their connection to the power grid.