Research On Stability Of Hydropower-dominant Power System And Control Strategy Of Hydropower Units

Due to the power reverse regulation caused by the water hammer phenomenon,the hydropower units provide negative damping in the load-frequency regulation process.When hydropower occupies a large proportion in the power system,the dynamic response of the power system is determined by hydropower units’characteristic.The damping characteristic is fully expressed.Thus,serious faults such as low-frequency-oscillations appears frequently in hydropower-dominant power system and are great threatens to the stable operation of the system.This paper analyzed the stability,dynamic characteristic,and control strategy of hydropower-dominant system in several ways.The linear models and nonlinear models of several hydropower units have been established for further study.To build the nonlinear model of units without complete characteristic curves and reduce the complexity of nonlinear modeling,a simplified nonlinear modeling method of hydroturbine is proposed.The simplified model is established with only field test data.With enough precision,this model is available in engineering modeling and simulation.A new regulation mode is proposed for the problems of opening regulation mode and power regulation mode.A modified Phillips-Heffron model is used to analyze the damping characteristic of four regulation models.The frequency range where these modes provide positive damping is studied.The stability of power angle is researched via eigenvalue analysis.The theorical analysis found that power regulation mode has better stability than opening regulation mode when parameters are set reasonably in some circumstances.The simulation of typical faults based on nonlinear models has validated the results.For the frequency stability in hydropower-dominant system,this paper has built a hydro-thermal double-unit system.The impact of various factors including hydropower occupation,governor parameters,hydroturbine coefficients,T_W of hydropower unit and occupation of direct current transmission on frequency stability is deeply studied.The results illustrated that a bigger hydropower occupation,governor parameters,T_W of hydropower unit and occupation of direct current transmission would decrease the system stability.The system stability in different operating conditions depends on the changing characteristic of coeffients e_y and e.To explore the mechanism of frequency instability of hydropower-dominant system,this paper has analyzed the frequency response characteristic by the motive equations of generator rotor.The frequency stability of a power system completely composed of hydropower units is almost the same with that of an isolated system of only several hydropower units.It is pointed that the parameters of units in hydropower-dominant system should be similar with the parameters of islanding operation mode.The frequency stability when part of hydropower units participate in primary frequency regulation is better than that when all hydropower units participate in primary frequency regulation.The Gravitational Search Algorithm(GSA)is used to optimize the regulating parameters of hydropower units in double-unit system.The results of optimal parameters demonstrated that small regulating parameters of hydropower units with a bigger hydropower occupation,a bigger T_W and smaller hydroturbine coefficient e_y would lead to a better stability.The parameter optimization of multi-unit system has validated the analysis.Base on the parameter optimization results,some suggestions are given to improve the evaluation rules of primary frequency regulation.According to the dynamic characteristic of frequency,a frequency graded regulation strategy is proposed in this paper.Simulation results of four-unit system illustrated the frequency graded regulation strategy would accelerate the time of frequency reaching stable value.Furthermore,the setting strategy of frequency regulation grade of units with different dynamic characteristic is discussed.The optimal parameters of units with different characteristics or frequency regulation grades are obviously different.It demonstrates the differentiated primary frequency regulation evaluation rules is necessary for hydropower units with different characteristic or frequency regulation grade in hydropower-dominant power system.