| In southwest China,there are abundant hydropower resources,so the content of hydropower units is relatively high in the power system.However,the frequent occurrence of ultra-low frequency oscillations during the asynchronous network operation of large-scale hydropower units seriously affects the stability and security of the power grid.In view of the phenomenon of ultra-low frequency oscillation with a long oscillation period and an oscillation frequency within 0.1Hz,it is of great significance for the safe and stable operation of the power grid to study its mechanism and put forward corresponding measures to effectively suppress the ultra-low frequency oscillations.First,this paper introduces the power system model,and expounds the principle of the eigenvalue analysis method.On the basis of the influence of the governor on the damping characteristics of the system,the primary frequency modulation process is further analyzed.The relationship between the amplitude and phase margin of the bode diagram of the open-loop system and the close oscillation frequency and damping of the system is verified by simulation.It provides a theoretical basis for the following research on ultra-low frequency oscillation.Second,analyzing the generation mechanism of ultra-low frequency oscillation,the secondary frequency modulation(AGC)is built.Based on the eigenvalue method,it is linearized and different parameters are set to simulate two modes: ultra-low frequency oscillation caused by governor action and ultra-low frequency oscillation caused by AGC process.The change of damping characteristics of the system in governor mode and AGC mode is analyzed by modal diagram.For the AGC process,the influence of frequency deviation coefficient and integration coefficient on system damping is summarized through the root locus diagram of characteristic roots.Finally,the simulation is carried out in a four-machine system to verify the relationship between the generation mechanism of ultra-low frequency oscillation and the damping of the system,and it is concluded that the deviation of mechanical power and electromagnetic power of each unit is not the same,but the frequency deviation of the whole system is unified.Then,the influence of PID governor parameters on system damping torque is analyzed,and a new PID parameter optimization method is proposed.Firstly,the continuous state constraint condition which is originally in the form of inequality is changed into the integral form of time,and then it is smoothed so that the gradient calculation of PID parameter can be carried out.Then,based on the optimal control software MISER3.2,the parameter optimization of the nonlinear system is accomplished by obtaining the derivative of the objective function and the constraint condition with respect to the PID parameter.Compared with the traditional algorithm,this method does not need to linearize the nonlinear system,so it can get more practical parameter setting.The parameters obtained by simulation in a stand-alone system are better than the traditional PSO algorithm in terms of the number of oscillations,overshooting,adjustment time and the improvement of system damping.Finally,the simulation is carried out in the multi-machine system to further verify that the mechanism of ultra-low frequency oscillation is due to the primary frequency modulation process,and the speed regulation system provides negative damping,and the relationship between the installed capacity of the system and the suppression of ultra-low frequency oscillation is obtained.The priority is given to setting the PID parameters of the high-capacity units to better suppress ultra-low frequency oscillation. |
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