Research On Frequency Optimization Control Technology Of DFIG In Wind-hydro Power System

The double fed wind turbine adopts variable speed constant frequency control,which is the mainstream model in the wind farm and it doesn’t have the performance requirements of primary frequency regulation for grid connection,because of the maximum power point tracking control.Yunnan power grid as an example,with the increase of wind power penetration,primary frequency regulation by conventional power supply can not work well for system frequency stability.At the same time,the problem of ultra-low frequency oscillation caused by water hammer effect in large hydropower stations is serious.Above all,the problem of frequency stabilization of wind-hydro power system should be solved quickly.This thesis focusing on the primary frequency regulation of wind-hydro power system where the frequency regulation of wind is deficiency and the phenomena of hydroelectric power is inverse,analyzed the characteristics of frequency control and built the simulation model.This thesis proposed the frequency optimization control of DFIG,which is suitable for wind-hydro power system.In view of the above problems,the following research is carried out:At first,the common frequency control methods for wind turbines are researched.It is found that the primary frequency regulation of combined rotor kinetic energy control and reserve power control is more suitable for large-scale wind power integration.Then,based on the difference analysis of primary frequency regulation process of wind power and hydropower,the result is that the hydroturbine can provide continuous power,but power inverse had an effect on frequency stability,and the wind turbine frequency control responsed quickly but the duration of the speed recovery is long,causing the frequency of the second drop easily.Therefore,researching parameter tuning of DFIG frequency control is necessary,especially in the wind-hydro system,and consideration must be given to the reverse hydropower with higher proportion.The method of proportional and differential control with fixed parameters of DFIG is usually adopted.However,during the frequency adjustment,the equivalent inertia of wind turbines will be drastically reduced,and when the rotor speed reaches the target value,the speed will return to normal immediately.If the wind power frequency control characteristics and the power inverse of hydroturbine are mismatch.the frequency unstabitily situation will be exacerbated.For the advantages of the quck response of wind turbine frequency control to remedy power inverse of hydropower,and give full play to the stable power support capability provided by the hydropower units,this thesis proposed a method of parameters optimization integrated the rotor kinetic energy control and load reducing control,which applying the quick response principle to set the droop control coefficient,and utilizing the water hammer effect time and energy balance condition as the basis for setting the virtual inertia coefficient,while increasing the difference of real time power as the feedback quantity,for dditional power frequency control correction.For the red mud wells test example,it’s verified that the doubly fed wind turbine has a frequency modulation ability,and it shows that the control of the fixed parameter pitch angle with large fluctuation,is hardly affected by wind speed,and has the high requirement for mechanical device,compared with the frequency optimization control proposed in this thesis.In order to reduce the calculation order and avoid the unconvergence problem caused by the nonlinearity,the wind farm is usually modeled by a single or multi machine equivalent.The study shows that the cable lines in the wind farm will affect the dynamic characteristics of the output of the wind farm.Therefore,in the view of the collecter parameter equivalent problem of complex topological wind farm,an equivalent method is proposed in this thesis based on the principle of power loss consistency,through the simulation and analysis,it is proved that the equivalent modeling method in this thesis can not only ensure the consistency of the wind power loss,but also the integration voltag,and improves the accuracy of equivalent modeling.Finally,through the typical equivalent system simulation,comparing the frequency optimization control of DFIG for wind-hydro power system and fixed parameter control,it is verified that the frequency optimization control is beneficial to the system frequency stability as soon as possible,and the frequency control ability of the hydropower station is fully utilized,and the second droop of the frequency is reduced.In the analysis of Yunnan power grid,based on the comparison of the primary frequency coordination characteristics between Leiying wind farm and Liyuan hydropower station,the results show that when the wind farm with frequency optimization control,wind power rapid frequency can make up the affect of power inverse of hydroturbine,and then,it proves that the coordinated frequency control of the wind and hydropower can satisfy the performance frequency requirements of the wind farm and improves the frequency stability of the wind-hydro power system.