Analysis Of Small-Signal Stability Of Power Systems Considering Wind Power Fluctuations

In recent years, with the development of wind power technology, policy support from government, the increasingly grim energy crisis and an objective demand of environmental protection, the installed capacity of wind power is rising sharply year by year. At the same time, the weak or negative damping of inter-area oscillation which is usually caused by the interconnection of power grids has attracted widespread attention. Although preliminary studies have been done on the impact of wind power on small signal stability, there’s much room left for analysis on the impact of wind power fluctuations on small signal stability. This paper provides different types of wind turbine mathematical model, analyzes the impact of wind power on low frequency oscillation and then calculates the sensitivity of eigenvalue damping ratio to wind farm active power, by which the damping ratio distribution of low frequency oscillation within typical wind farm active power distribution is worked out.At first, Mechanical system models of wind turbines are established in the paper, including aerodynamic system model, mechanical drive system model and pitch angle control system model. Then an induction generator mathematical model of ordinary asynchronous wind turbine is established. As for doubly fed wind turbine the paper provides an equivalent model of generator and converter, while for permanent-magnet direct-driven wind turbine, the paper establishes a permanent-magnet synchronous generator model and a relevant converter control system.Secondly the paper gives an assessment to the impact of connecting wind farms to different types of grid system on damping characteristics, in which three kinds of single wind turbine infinite-bus system are built. Afterwards, establishing the system state equations, which are used to analyze small signal stability, an analysis expression on system state matrix is worked out. Besides, the paper studies the oscillation modes with eigenvalue analysis and gives an analysis on its mechanism. Take the classic system of Generator-4Area-2as an example, the paper analyzes the impact of different types of wind farm or wind farm with different active power connecting grid system on its original oscillation modes. In another example, the New-England Generator-10Bus-39system, the impact of wind power access with different ports on damping characteristics is studied in the paper, compared with characteristics shows when no wind power connects the grid system.Last but not least, in order to study system damping characteristics within the fluctuation range of active power on wind farm, a calculation formula about the sensitivity of eigenvalue damping ratio to wind farm active power is deducted, and the calculation results are verified by two area system compared with disturbance simulation. The sensitivity of damping ratio shows accurately the trend and extent of the impact from varied wind farm active power on damping characteristics. Guided by the above calculations, the extreme damping ratios of low frequency oscillations within active power range on wind farm are solved by iterative method and the damping ratio distribution can be further deducted. Moreover, by analyzing the active power status on wind farm when damping ratio in oscillation modes maximizes, the damping ratio’s variation law under fluctuations of wind active power can be deducted in details.