| The power grid with high proportion of hydropower units is prone to ultra-low frequency oscillation(ULFO).The reason is that the turbine has obvious water hammer effect.The negative damping effect caused by it can lead to the damping characteristics of the power system becoming worse and serious phase lag,and the unreasonable parameter setting of the speedregulating system further destroys the stability of the system.In the context of asynchronous interconnection of regional power grid in China,the problem of ULFO in the transmission end power grid with intensive hydropower will become more and more serious.Therefore,the study of ULFO is of great significance to ensure the safe and stable operation and subsequent development of China’s power grid.First of all,based on the small disturbance stability analysis of power system,this paper integrates the speed-regulating system,electro-hydraulic servo system,hydraulic turbine and generator linearization model in power flow calculation software PSD-BPA to get the model of primary frequency control(PFC)closed-loop control system of hydropower unit,and then derives the analytical expression of damping torque coefficient of the prime mover system,and analyzes the influence degree of PID parameters of governor on the system damping and the negative damping effect caused by water hammer effect,which provide a reliable theoretical basis for the study of ULFO in this paper.Next,this paper introduces the concept of root locus family and the general drawing method in detail,establishes a typical single machine load model used to study the ULFO,and based on this model,draws the root locus family curve of the system,and analyzes its applicability in the study of ULFO.According to the requirements of PFC of hydropower units,with the help of the dominant pole method,a PI parameter optimization scheme of governor is proposed.A 10-generator 39-node sample system is built by BPA program to verify the scheme proposed in this paper.The results show that the system after PI parameter optimization can effectively suppress the ULFO and meet the requirements of PFC.Then,when using the root locus family to optimize the governor parameters to suppress the ULFO,it is found that in some hydropower systems,the stability region is relatively small,so a correction controller is proposed to optimize the lag phase and improve the performance of the system.In this paper,the control effects of the lead correction,lag correction and lag-lead correction controllers are studied.After comparison,the lead correction controller is selected to suppress the ULFO,which is put into the single-machine system.The parameter setting scheme of the controller is proposed by using the root locus synthesis method.In the single-machine system and the modified 4-machine 2-area system with high proportion of the hydropower unit,the effectiveness of the controller to suppress the ULFO is verified by simulation.Finally,this paper takes Yunnan power grid as the actual system to study the ULFO,and uses the specific data of BPA program of 2017 summer operation mode to simulate and reproduce the ULFO phenomenon of Yunnan power grid,and carries out simulation tests on the influence of the ULFO from the aspects of water hammer effect of turbine,PFC of governor and parameter setting,the results show that the stronger the water hammer effect is,the greater the frequency oscillation will be,but the research on the suppression of the ULFO should be more focused on the speed-regulating system,and then put forward the optimization sequence strategy of the governor parameters based on the sequence of the damping torque coefficient,and the feasibility of the optimization strategy is verified by the simulation of a specific example. |
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