The Study Of Torsional Vibration Between Wind Turbines And Grid

This paper studies on the principles and influencing factors of torsional vibration between wind turbines and the grid. The existed research results presented by other researchers have been summarized and evaluated and based on the summarization, small signal stability analysis is proposed to study the torsional vibration stability of a simple wind power system-single machine to infinite bus system (SMIB system). Aiming at two types of wind turbine generators, which are fixed-speed wind turbines and variable-speed wind turbines equipped with doubly-fed induction generators (DFIG), the math models of the main elements of SMIB system have been developed, including shaft three-mass model, stator model, parallel-compensation capacitor model, RLC model of transmission line with series-compensation capacitor, infinite bus model and controller model of doubly-fed wind generator. The small signal models of all the elements listed above have been established in the form of state-space blocks in the software MATLAB/SIMULINK, and the linearized models of the whole SIMB system of the two types of wind turbines have been set up by linking the elements'blocks with block I/O interfaces. The SIMB models are initialized at a steady-state operation point, and the system eigenvalues are calculated among which the eigenvalue standing for the shaft torsional vibration mode has been determined. In the analysis, some wind turbine parameters or transmissions-line ones are varied in order to observe their effects on the frequency and damping characteristics of the shaft torsional vibration mode. Using eigenvalue analysis method, we conclude that: (1)As to the SIMB system of fixed-speed wind turbines, the frequency and damping characteristics of the shaft torsional vibration mode vary with the series-compensation level of the transmission lines, and the damping effect become the weakest when the electric resonance frequency of RLC transmission lines plus one of the shaft natural torsional vibration frequency equals to operation frequency(50Hz), which might causes instability on the torsional vibration mode. On the contrary, the parallel-compensation level has nearly no impact on the frequency and damping characteristics of the shaft torsional vibration mode. (2)As to the SIMB system of DFIG wind turbines without rotor speed control loop, the shaft torsional vibration is dominant by its own motion state, with nothing to do with the state of electric parts. That means the shaft torsional vibration mode is highly independent and the parameters variation of electric parts and controllers barely affects its frequency and damping characteristics. (3)As to the SIMB system of DFIG wind turbines which includes a rotor speed control loop, the connection between mechanic parts and electric parts is significantly strengthened by the new control loop. The rotor speed control loop enhances the damping effect of the system on the shaft torsional vibration mode, thus improving system torsional vibration stability.