Generalized Turbine Model Based Analysis And Control Strategy Research Of Ultra Low Frequency Oscillation Of Hydropower Units

Low frequency oscillation of power system is always a serious hidden danger for the safe and stable operation of power system.It is a complex theoretical and engineering scientific problem that has not been effectively solved.Its frequency oscillation range is generally 0.1Hz～2.5Hz.However,in recent years,some oscillation events with frequency lower than the above-mentioned range have appeared in the operation process of the power grid with a large proportion of hydropower units in China,which is called ultra-low frequency oscillation.Ultra low frequency oscillation（UFO）plays an important role in many cascading power system failures,which poses a great threat to the stable operation of power system.The analysis of the mechanism and control strategy of the ultra-low frequency oscillation is of great theoretical value and practical significance for the maintenance of the safe and stable operation of the power grid and its long-term development in the future.First of all,in recent years,hydropower units account for a large proportion of China’s power grid,and there are many long-time and ultra-low frequency oscillation phenomena in the operation process,the relevant studies have shown that the turbine model has an important impact on the ultra-low frequency oscillation behavior of hydropower units.In this paper,the transient component of turbine output torque is modeled and simplified on the basis of the generalized basic equation of turbine,and the dynamic process of turbine is fully considered on the premise of ensuring the simplicity of turbine model.A flow factor T_pq is defined,which is closely related to the inherent parameters of the turbine and the variation of the generating flow,and a generalized turbine model with full consideration of the flow factor is established.The comparison between the generalized turbine model and the ideal turbine model shows that the generalized turbine model can effectively improve the accuracy of transient process simulation such as“water hammer”effect.Secondly,based on the generalized turbine model,the influence of flow factor on the ultra-low frequency oscillation behavior of hydropower units is analyzed from the damping characteristics of the system and the stability of Routh criterion.The analysis results show that compared with the ideal turbine model,the flow factor in the generalized turbine model can provide greater damping torque for the system,and when it is greater than the critical stability value,the system can maintain stable state.A single machine system is built in Simulink and four-machine and two-region system is built in PSCAD simulation software for simulation verification.The simulation results show that the flow factor under the generalized turbine model is conducive to suppress the ultra-low frequency behavior of hydropower units,and with the increase of the flow factor,the suppression effect of ultra-low frequency oscillation is more obvious.Finally,according to the demand of restraining ultra-low frequency oscillation in the power grid with large hydropower units,a PID parameter optimization method of turbine governor is proposed,which can not only meet the requirement that the generator speed can be stabilized in the shortest time after disturbance,but also improve the damping level of the system in ultra-low frequency band,so that the system has stronger anti-interference ability under different disturbance conditions.Based on the fast and efficient optimization strategy of beetle antennae search algorithm,an improved beetle antennae particle swarm optimization algorithm is constructed and applied to PID parameter optimization of hydraulic turbine governing system.Through Schaffer test function and simulation in single machine and multi machine system,the optimization effect of the improved algorithm proposed in this paper is verified.The results show that the improved beetle antennae particle swarm optimization algorithm can effectively improve the damping level of hydraulic turbine governing system in ultra-low frequency band,and it is more effective and superior than the beetle antennae search algorithm and particle swarm optimization algorithm,which provides new ideas and technical means for suppressing the problem of ultra-low frequency oscillation through the optimization of the speed control system parameters.