Study On Mechanism And Suppressing Countermeasures Of Ultra-low Frequency Oscillation Caused By Hydropower Units

China has implemented the strategy of “West-to-East Power Transmission”,“Nortel Power Transmission to South China” and “National Networking”.In recent years,with the widespread application of HVDC as a cost-effective,high-capacity and long-distance transmission technology,some ultra-low frequency oscillations with frequencies less than 0.1Hz have appeared in the actual power grid.The ultra-low frequency oscillation is not the relative oscillation between the generator rotors,but the frequency oscillation caused by the small disturbance of the system during the primary frequency modulation process,which belongs to the frequency stability category.The output of hydropower units in Yunnan Power Grid is about 71%~75%.Due to the water hammer effect in hydropower units,the damping and stability of the system will be deteriorated if the turbine-governor control system parameters are set unreasonably.If the proportion of hydropower units is too high,the ultra-low frequency oscillation will occur in the system.Therefore,the study on mechanism and suppressing countermeasures of the ultra-low frequency oscillation caused by hydropower units can guarantee the safe and stable operation effectively in Yunnan Power Grid under the condition of asynchronous networking.Firstly,based on the small interference analysis of power system and combined with the generator equations,excitation system equations,turbine-governor control system equations of hydro turbine units and steam turbine units,load equations and network equations,the linearized state space model of multi-machine system is obtained.The ultra-low frequency oscillation mode is solved,and then its oscillation mode and damping ratio are analyzed.Based on the linearized state space model of multi-machine system,it provides a reliable theoretical basis for the study of ultra-low frequency oscillation by using torque analysis and eigenvalue analysis.Secondly,the mechanism of ultra-low frequency oscillation is analyzed based on the mechanical damping torque and Nyquist vector margin of the turbine-governor control system.The influence of the turbine-governor control system parameters on the system stability is also discussed.The correctness of the linearized state space model is verified in a 3-machine 9-node system.The ultra-low frequency oscillation performance of electromagnetic power and mechanical power in the system is analyzed.And the ultra-low frequency oscillation mode is solved by the linearized state space model of the 3-machine 9-node system.The characteristics of ultra-low frequency oscillation are studied based on the participation factor and root locus by using the characteristic analysis method.Finally,based on the principle of phase compensation,a design method of the governor power system stabilizer is proposed to suppress the ultra-low frequency oscillation.By analyzing the mechanical damping torque and Nyquist vector margin of the turbine-governor control system with GPSS,the principle of GPSS to suppress ultra-low frequency oscillation is explained.The parameters of GPSS in single-machine and multi-machine systems are set based on the phase compensation method.And the effectiveness of GPSS in suppressing ultra-low frequency oscillation is verified by simulation in a single-machine single-load system,a 4-machine 2-area system and a 13 hydropower plants with large rated capacity in Yunnan Power Grid.