Impact Analysis Of DFIG Wind Turbines Considering Virtual Inertia Control On System Small-signal Stability

With the putting forward of the grid-friendly wind power plant, the guidelines of grid interconnection of wind farms require that the DFIG should have ability to respond to the change of system frequency. Though introducing the virtual inertia control will improve the interconnection performance of DFIG,it will certainly influence the stability of power grid. Therefore, there is an urgent need for research on the impact of DFIG wind turbines considering virtual inertia control on system small-signal stability. The main contribution of this paper is as follows:First, an interconnected power system model of DFIG considering PLL and virtual inertia control is created. Then, based on the above model, the impacts of PLL and virtual inertia control on power system small-signal stability are explored theoretically by analytic method. The results show that the damping ratio of power system affected by participation factor of virtual inertia which is influenced by PLL. Besides, the smaller the PI parameter of PLL is, the smaller the participation factor of virtual inertia is, and the bigger the damping ratio of electromechanical oscillation is. This is opposite to the impact of DFIG without considering virtual inertia control under the action of PLL on system stability. The rationality of interconnected power system model and the correct of analysis results have been verified by the simulation of IEEE four-machine two-regional test system.The order of DFIG model is so large that is not conducive to analysis, in order to solve this problem, a reduced model for small-signal stability is proposed in this paper. First, the decay constant of various components are calculated, and according to the relationship between the speed of the attenuation of each state variable, the dynamic characteristics of various components of DFIG are explored from the time scale. On this basis, taking into account the electromechanical coupling characteristics, the 3-order simplified model retaining virtual inertia control and phase-locked loop dynamics is derived. Finally, the dynamic model of reduced model accessed to multi-machine interconnected systems is built. The simulation results of IEEE four-machine two-regional test system show that the reduced model can describe the dynamic characteristics of the system accurately under the small disturbance. And compared with the detailed model, the simplified model greatly reduces the computational amount.Aiming at the stochastic excitation caused by wind power fluctuation and the parameter variations caused by uncertainty of wind speed, a new stability analysis method considering wiener noise for stochastic systems with unknown parameters and excitations is proposed. Based on the above reduced model, the analytic function of stochastic parameters and state matrix is derived using sensitivity analysis method, and the interconnected power system model considering stochastic parameters and wiener noise is created. Then, a class of Lyapunov functional considering stochastic factors is established, and the robust stability criterion of stochastic system is deduced. The simulation results of IEEE four-machine two-regional test system and New England 10-machine 39-bus test system illustrate that the proposed method can distinguish the stability under stochastic parameter and excitation and can be applied to complex multi-machine system.