Studies On Small Signal Stability Of Electric Power System In Respect Of Large Wind Farm Connection

The stability is one of the most important factors effectting the inter-connected bulk power grids. And the weak or minus dampings within a region or between different regions of bulk power grids are the most attracting. As the most mature renewable energy technology, wind power is increasing in power systems. For the situation of power system penetration of large wind power, the impact on the small signal stability and the damping characteristic of power system has not yet been studied adequately. In this paper, linearized status equations of a simple wind power system with wind turbine generators are established, the impact of wind farm on small signal stability and the damping characteristics of power system is studied in a practical inter-connected power system. And the paper presents the stability control models in variable speed wind turbine generator, which improve the damping of the systems. The main contributions of this paper are as follows:(1) The linearized status equations of the aerodynamic model, shaft system model and pitch control model of the wind turbine generator are established for analyzing small signal stability; derives and linearizes the dynamic equations of asynchronous generator, doubly-fed induction generator (DFIG) and permanent magnet synchronous generator (PMSG) with synchronous rotation system as the reference. The paper establishes model of the converter and its control systems in the variable speed wind turbine generators (including wind turbines based on DFIG and PMSG), and derives the corresponding linearized status equations.(2) A simple wind power system composed of a single-unit and an infinite bus is established, including wind turbine with asynchronous generator, DFIG and PMSG. The paper derives and linearizes the status equations of the simple wind power system for small signal stabiliey analysis; reduces the element number of the status matrix of the parameters of the simple systems. The oscillation modes with three kinds of wind turbines generators are studied with eigenvalue analysis and the principle of the oscillation modes is analyzed. The tests of the above small signal analysis demonstrate that wind turbine generators cause the corresponding oscillation modes after being integrated into power grids. These oscillation modes are mainly associated with the mechanical system and the control system of the converter, and have the larger damping coefficients. (3) The paper establishes the mode of practical power grid with wind farms in the software DIgSILENT/Power Factory. According to the different output of the wind farm and the various reserve capacity in the power systems, the impact of the wind farm integrating into power systems on low frequency oscillation modes and damping is investigated, which adopts wind turbine with asynchronous generator, DFIG and PMSG. In the practical power grid study, the conclusion is drawn both in theory and in simulation that the wind farm based on wind turbine with asynchronous generator can improves the damping of the power grid in some extent. But the wind farm based on wind turbine with DFIG or PMSG reduces damping of the systems a little.(4) The paper presents the power system stabilizer model in variable speed wind turbine generators. When low-frequency oscillation in power systems happens, for the wind turbines with DFIG the slip of the generator is taken as the input signal and for the wind turbines with PMSG the frequency of the power systems is taken as the input signal. The variable speed wind turbine generator regulates the electrical power output by controlling the converter, and immits the damping active power associated with low-frequency oscillation to reduce the electrical power oscillation, increase the damping and reinforce the dynamic stability of the systems. In the model of the actual power grid, the power system stabilizer model in the variable speed wind turbine generator is tested in the simulation software. The results demonstrate that the scheme of wind farm increasing damping of the systems is effective and reinforces the dynamic stability of the power system.