| Under the guidance of the "low-carbon transformation" goal,new power systems have flourished.Pumped storage energy plays an important role in promoting the consumption of renewable energy such as wind power and photovoltaic due to its dual operation characteristics of power generation and energy storage.On account of the advantages including rapid power regulation,flexible power storage and fast conversion of operating conditions,the novel variable-speed pump-turbine units(VSPTUs)is expected to gradually replace the traditional fixed-speed pump-turbine units(FSPTUs)and become the core energy conversion equipment of pumped-storage plants in the future.The improvement of regulation performance determines that the VSPTUs may inevitably face more frequent transition processes.The drastic variations of the pressures and speed may seriously threaten the safe operation and regulation effect of the power plant.Consequently,based on the contradiction between the flexible operation and the safety of system,this paper focuses on the transient stability performances of VSPTUs.The main research contents and conclusions are as follows:1.Focusing on the characteristic of flexibly adjustable speed for VSPTU,taking the correlation mechanism between the steady-state operation parameters as the starting point,this paper studies and analyzes the impact of initial operating speed on the dynamic stability of the units during conversion of operating conditions.(1)Revealing the correlation between the steady-state speed and other operating parameters.To achieve this aim,the radial basis function interpolation method is applied to extract the parameters at different operating condition and the results are compared and analyzed.The results show that under the same output power,the rotational speed of pumpedstorage units is inversely proportional to the flow and torque,respectively.The ratio of rotor speed,flow discharge and torque at the best efficiency points to the rated condition is 95.63%,98.11% and 104.57%,respectively.All of the optimum rotor speed for the VSPTUs under the recommended steady-state operating conditions is lower than the rated rotor speed.(2)Describing the impact of the variations of initial rotor speed on the stability of the unit during the transition process.To achieve this aim,the load rejection,as the most danger conversion of operating conditions,is taken as the research case.The impact of the initial speed on the stability of system is evaluated by quantifying the dynamic evolution characteristics of the key index parameters such as the speed,volute pressure.The results show that the optimum speed operation can reduce the maximum speed increasing by 3.24%-8.10%during load rejection,the maximum volute pressure rising by 1.74%-10.63%.The reduction of initial speed can effectively improve the safety and stability performances of the unit during load rejection.(3)The research is focusing on the pumped-storage system with different type of units sharing common pipeline.The load rejection process is still taken as a research case to analyze the hydraulic interference characteristics between different types of units during the fast conversion of operating conditions.Based on an objective evaluation method,the scheme selection and operation suggestions are made for the process of successive load rejection.The results show that obvious hydraulic interference may occur when the pumped storage system with different units sharing common pipeline performing load rejection.The minimum pressure for draft tube is less than 0 for VSPTU as it suffering the disturbance of FSPTU.The phenomena can be avoided by reasonably selecting the interval time for successive load rejection.2.Focusing on the dual regulation of speed and excitation for VSPTU,taking the power regulation mode independently from turbine output as the starting point,the power generation characteristics of the VSPTU in the process of hydraulic interference are studied and analyzed.(1)Describing the pressure disturbance law of the multi-units pumped-storage system.To achieve this aim,taking the factors of various disturbance laws such as the type of interference unit operating conditions,the hydraulic system pipeline layout,and the branch pipe asymmetric layout scheme to form the simulation cases for comprehensively evaluating the pressure disturbance characteristics of the pumped storage system and summarize the disturbance laws.The results show that canceling the common pipeline of the water diversion system can greatly reduce the interference degree of the disturbed units.The head variation can be reduced by 81.41%-88.56%.Meanwhile,the pressure disturbances in the process of hydraulic interference can be effectively alleviated by selecting the long-branch corresponding unit as the disturbance unit.(2)The power regulation effect of VSPTU in the process of hydraulic interference is evaluated under different disturbance conditions.Meanwhile,the influence of power regulation on the stability performance of unit is studied and analyzed by taking the FSPTU as a comparison case.The results show that the independent power regulation characteristics ensure that the VSPTU can significantly improve the power output stability of the unit and effectively isolate the impact of unstable hydraulic factors on the stability of power system.In addition,the anti-interference power regulation of may affect the stability of the mechanical system,the maximum speed deviation under s exceed 7.5% of the rated speed.3.Focusing on the fast power regulation response of VSPTUs,taking the contradiction between regulation speed and operation stability as the starting point,,the dynamic response of frequency regulation of fixed and variable speed units is compared based on different frequency disturbance cases.(1)Accurately evaluating the comprehensive frequency regulation performance of VSPTUs located in isolated grid.For illustrating the advantages of frequency regulation performance for VSPTU,the simulations of FSPTUs and VSPTUs in frequency regulation processes are made and evolution by applying the key index parameters such as system frequency,unit speed and working head are done.The results show that the independent and fast power regulation response of the VSPTUs makes it far better than the FSPTUSs in frequency regulation performance.The maximum deviation of the system frequency when the FSPTU is applied for frequency regulation is more than 5-7 times of the corresponding result of the VSPTU.Meanwhile,the power regulation by electrical response may cause the large torque in the transient processes and lead to an obvious speed oscillation range,the ratio between the speed range of the two units can exceed 10% of the rated speed at most.(2)For effectively solving the problem of large speed oscillation in the frequency regulation process of VSPTUs.To achieve this aim,the speed pulling-back control method is introduced.Through the comparison of simulation results,the control principle and effect of the control method on speed variations are comprehensively analyzed,and the optimal parameters of the controller are selected based on an evaluation method.The results show that the speed variation of 3.01% of the rated speed can be reduced at most by using the speed pulling-back control method.The optimal controller parameters can be set as Ka=11.5 and Kb=4.5. |
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