Research On Sub-Synchronous Oscillation In Power System With DFIG-Based Wind Farms

With the increase of the wind power installed capacity and the application of series compensation technology in large scale wind power transmission, sub-synchronous oscillation (SSO) accidents caused by series capacitor compensation have happened in many doubly fed induction generator (DFIG)-based wind farms in China and other countries. Therefore, research on influencing factors and damping method of SSO in power system with DFIG-based wind farms is of remarkable value and practical significance. The main research contents are as follows:(1) The working principle and structure of wind power generation system with DFIG-based wind farms are studied. The dynamic model which are suitable for the research on SSO in DFIG farms are derived and rotor side and grid side controller model based on stator voltage vector oriented are established.(2) The linearized model and state matrix are derived. Given the algorithmic flow of eigen-structure analysis method, the eigen-structure analysis program is compiled. Based on participation factors and the mechanism on SSO in wind farms, the SSO modal and shaft modal are identified. The time-domain simulation program is compiled to verify the validity and accuracy of eigen-structure analysis results.(3) Parameters that may affect SSO in wind farms are systematically divided into three types, which are uncertain parameters, certain uncontrollable parameters and certain controllable parameters. The research indicates that wind speed, rotor speed, active power produced by wind turbine, series compensation level, line resistance and transformer resistance, reactive power produced by DFIG stator, proportional coefficient in inner and outer loop of RSC have great impact on SSO in wind farms, which can respectively provide a reference to damping controller design, power system planning and design, and controller parameter optimization. The influencing mechanism of rotor side controller on SSO in wind farms is studied. The rotor side controller can be equivalent to a rotor side resistance and the influence of proportional coefficient in inner and outer loop of RSC on SSO in wind farms is explained by IGE. The contradiction reason between the findings of outer loop proportional coefficient in previous literatures is explained by the study of mutual influence between inner and outer loop proportional coefficient. The correctness of the influencing mechanism study is verified by eigen-structure analysis.(4) Four damping methods on SSO in wind farms are studied. A method based on reactive power step control in an emergency state to damp SSO in wind farms is proposed and verified through time-domain simulation, which is a practical method to damp SSO in an emergency state and only need to control the reactive power reference value. The phase extraction method based on eigen-structure analysis is studied and the auxiliary damping controller is designed considering the random changes of wind speed. Band-stop filter fixed in the output of RSC controller which can damp the designed stop-band frequency range of SSO is studied and verified through time-domain simulation. Besides, the feasibility of the bypass damping filter to damp SSO in wind farms is first studied and an appropriate bypass damping filter which can damp the whole sub-synchronous frequency is designed.