Research on mechanism of large-scale wind turbines off-grid and preventive relay protection strategy

Abstract

Currently, researches on wind turbines off-grid mainly focus on change of voltage and reactive power after fault in wind system. However, due to uncertainty of wind farm, there is another possibility: large-scale wind turbines off-grid caused by active power fluctuation. The paper will study this problem. As is known to us, large-scale blackout accidents in power system are mostly caused by power oscillation. In addition, power imbalance is the root cause. In conventional system, power imbalance is usually caused by a sudden fault, while wind turbine is the unbalanced power resource for wind system. Although output power of single wind turbine is a trivial, the large installed capacity could have a great impact on the wind farm. Considering regulation inertia of pitch angle and power distribution in case of uncertain disturbance, the paper focuses on the influence of frequency fluctuation on the output torque of the wind turbines. It is confirmed that output power of wind turbine brings positive feedback to each other in some cases, namely cluster effect and butterfly effect of wind turbines will make power fluctuation with greater convergence and larger amplitude. In this case, if unbalanced power exceeds the critical point, wind turbines will be out of step from part to the whole wind farm. As a result, large-scale wind turbines off-grid will happen. Simulation model by MATLAB testifies the interaction mechanism between wind turbines. Finally, relay protection strategy is proposed to identify large-scale wind turbines off-grid.