| In recent years,there have been many ultra-low frequency oscillation(ULFO)events in the power grid with a high proportion of hydropower units,which has seriously affected the safe and stable operation of the actual power grid.Therefore,it is of great significance for the frequency stability of power system to study the mechanism and influence factors of ULFO in high-proportion hydropower output system.Firstly,based on the Phillips-Heffron model,the mechanism of ULFO is analyzed by means of eigenvalue analysis and damping torque method.The research on the speed regulation system of turbo generator and hydro-generator in Yunnan power grid is carried out.The relationship between the demarcation frequency and the damping characteristics of the governing system and the influence of different parameters on the damping characteristics and phase-frequency characteristics of the governing system are analyzed,which provides a theoretical basis for subsequent simulation studies.Secondly,the oscillation characteristics of ULFO are studied based on the singlemachine system.The relationship between the time constant of water hammer effect and the stability of the system is discussed from the aspects of Rawls criterion,eigenvalue method and time domain simulation.And the damping torque method is used to analyze the influence of PID parameters on the damping torque of the governing system.A PID parameter optimization strategy based on particle swarm optimization is proposed.The PID parameters are analyzed in the single-machine system and the 4-machine 2-area system.The damping characteristics of the hydropower unit speed control system before and after optimization are optimized,and the effectiveness of the proposed optimization strategy for suppressing ULFO problem is verified by simulation.Next,the geographical connection mode and typical operation mode of the Nuozhadu and Chusui DC island system are introduced.The frequency adjustment process of the DC frequency controller in the island system is analyzed.The Bode diagram method is used to study the DC frequency controller.The effect of parameters on the stability of the island system.The coordinated control of DC frequency controller and governor in the Nuozhadu and Chusui DC island systems is studied.The simulation results show that the proposed control method can effectively suppress ULFO in the island system.Finally,under the PSD-BPA simulation platform,ULFO appearing in Yunnan Power Grid are studied based on the 2017 BPA offline simulation data of Yunnan Power Grid.According to the characteristics of Yunnan Power Grid after asynchronous operation,ULFO phenomenon of Yunnan Power Grid is reproduced by BPA offline data simulation.The comparison between frequency and active power of hydropower plants and thermal power plants shows that ULFO is strongly correlated with the hydropower unit speed control system.Then damping torque method is used to analyze the damping characteristics of the speed regulation system of hydropower units in Yunnan power network.The simulation results show that the optimization of PID parameters and the adjustment of FLC and primary frequency modulation dead zone can effectively suppress ULFO problem in Yunnan power grid after asynchronous interconnection. |
PDF Full Text Request