Research On Mechanism And Suppression Measures Of Low Frequency Oscillation Induced By Governing System

Low frequency oscillation (LFO) seriously affects the safety and stability of power grid operation, power system stabilizer (PSS) based on the negative damping mechanism can inhibit LFO effectively. However, LFO still occurs in power grid with PSS, theory and practice show that the wave motion of prime mover and governing system will induce LFO. In recent years, the application of digital electro-hydraulic control system (DEH) makes the impact of turbine and its governing system on LFO more significantly.In2011, LFO occurred when primary frequency regulation test was applied in a steam turbine power plant of the South Power Grid, with LFO alarm of many tie-lines in500kV grid, which affected the stability of power grid seriously. The mathematical models of boiler, turbine, generator, excitation and governing system were established according to the actual operation of the plant in accident condition. The coordination control system (CCS), the higher-level control system of governing system was accounted. The simplified model of CCS was established based on the thermal control model to make it suitable for power system dynamic analysis. The simulation platform was established in PSCAD/EMTDC based on the existing models, and the LFO event was reproduced.The principle, control mode and composition of the governing system were analyzed. The system model was simplified according to the research needs. The block diagram of power system was established. The damping characteristics of governing system with different control methods were analyzed by damping torque method. Results showed that improper parameter settings of governing system would induce LFO. The range of parameter value was calculated based on the operation requirements of power grid to ensure the stability of power grid.As primary frequency regulation institution, governing system has an effect on dynamic stability as well as frequency characteristics. Studies show that the excessive pursuit of primary frequency regulation performance will make dynamic stability undermined. The coordination optimization between primary frequency regulation and dynamic stability was discussed. A new type of intelligent optimization algorithms was applied to parameter optimization of DEH, to guarantee primary frequency regulation performance and dynamic stability simultaneously. The result was proved by time-domain simulation.