Study On Mechanism Analysis And Suppression Methods Of Ultra-low Frequency Oscillation In Power Grid With High Proportion Of Hydropower

With the rapid development and wide application of DC transmission technology,China’s regional power grid interconnection has entered a new era of asynchronous interconnection.However,the huge changes in the scope and structure of the power grid make the problem of ultra-low frequency oscillation more prominent in Yunnan and southwest power grids with high proportion of hydropower,which seriously threatens the safe and stable operation of the power grid.Not only is the frequency range of ultra-low frequency oscillation lower than that of low frequency oscillation,but their mechanism properties are also different: low frequency oscillation is the problem of angle stability caused by the continuous relative swing between the rotors of the unit;ultra-low frequency oscillation is the problem of frequency stability caused by the instability of the primary frequency regulation process of the unit.Therefore,under the background of asynchronous interconnection,it is urgent to analyze the mechanism of the increasingly serious ultra-low frequency oscillation in the regional power grid with high proportion of hydropower,and propose effective suppression methods.In this paper,by the linearization of appropriate models,the small-signal analysis model of ultra-low frequency oscillation of two-machine single-load system is derived,and the accuracy is verified by simulation identification.Based on the eigenvalue calculation,the influence of the parameters of the hydraulic turbine-governor system on the mode stability of ultra-low frequency oscillation is analyzed.Based on the damping torque analysis method,the influence of the parameters of the hydraulic turbine-governor system on the damping characteristics of ultra-low frequency oscillation is analyzed.Compared with the analysis results of eigenvalue calculation,the applicability of damping torque analysis method for studying the influence of the hydraulic turbine-governor system on ultra-low frequency oscillation is verified.A multi-objective optimization method of hydraulic turbine governor parameters to suppress ultra-low frequency oscillation is proposed in this paper.The conflict between the performance of primary frequency regulation and the suppression effect of ultra-low frequency oscillation of hydraulic turbine-governor system is analyzed by derivation.Considering the change of water-hammer effect,the suppression effect of ultra-low frequency oscillation and the performance of primary frequency regulation are taken as optimization objectives at the same time,and the influence of governor parameters on low frequency oscillation damping characteristics is considered as a constraint.A multiobjective optimization method of hydraulic turbine governor parameters is proposed to balance the suppression effect of ultra-low frequency oscillation and the performance of primary frequency regulation,which is solved by NSGA-Ⅱ and evaluated by TOPSIS.The effectiveness of the proposed method is verified by simulation in a four-machine twoarea system.In order to reduce the negative effect of optimal governor parameters on the performance of primary frequency regulation,an optimal design method of SVC additional frequency controller is proposed in this paper.The small-signal analysis model with SVC additional frequency control of ultra-low frequency oscillation is derived,and the influence of SVC additional frequency control on ultra-low frequency oscillation is analyzed by eigenvalue calculation.The FOPID controller is used to design the SVC additional frequency controller,and the parameter optimization problem of the SVC additional frequency controller is modeled as the optimization problem of damping ratio of ultra-low frequency oscillation mode under multiple operation modes,which is solved by CPSO algorithm.In a improved four-machine two-area system,the suppression effect of ultra-low frequency oscillation of the SVC additional frequency controller,the interaction between the SVC additional frequency controller and the node voltage,and the factors influencing the suppression effect of ultra-low frequency oscillation are analyzed.The suppression effect of ultra-low frequency oscillation of the SVC additional frequency controller is further verified in a improved 10-machine 39-bus system.