| In recent years,the large-scale wind power generation has increased rapidly due to the outstanding features of high concentrated development,high utilization hours of generation and small occupation of land,beside,the wind power plant use mainly two kinds of generators,including doubly-fed induction generator and permanent magnet synchronous generator.However,compared with the permanent magnet synchronous generator,the doubly-fed generator has more mature technology and lower cost,so that the majority of large-scale wind farms are using doubly-fed generator and this paper uses doubly-fed generator as the research object.because the generating capacity of the large-scale wind farms is great,which make wind power integrate into grid by a way of high-voltage direct current transmission.If the wind farm output active power and HVDC transmission active power can't keep consistent in the process of wind power integration into grid,which can result in active power imbalance on the generation side,and can bring the instability of sending end system frequency,more badly,it may emerge ‘transient instability' due to the voltage and frequency's deviation from normal range.To cure the above problems,this paper makes a research about the frequency stability control method of large-scale wind farms based on VSC-HVDC,and it has important theoretical and application value.The specific research is the following:Firstly,this paper introduced the running mechanism of doubly-fed wind power generation system and its operating characteristics,and analyzed the mathematical model of doubly-fed generator,power relations,the stator flux and voltage orientation theory to seek the mathematic relation of doubly-fed generator stator voltage,output power and its angular frequency.Introducing the operating principle and mathematical model of VSC – HVDC contribute to analyze the internal relation between VSC transmission active power and doubly-fed generator output power,between voltage phase and angular frequency.Secondly,this paper proposed a control method based on VSC-HVDC,which can enhance the stability of frequency and voltage in the large-scale wind power system-connected grid.Combined with the operating characteristic of the large-scale wind power system-connected grid,analyzed the voltage and power balance between doubly-fed generation system and VSC-HVDC system,the transfer function of the angular frequency in doubly-fed generation system and the phase difference of converter reactor both sides' voltage in VSC-HVDC are deduced,meanwhile the function relations between the AC voltage in doubly-fed generation system and the ratio of width modulation about PWM in VSC-HVDC are also deduced.We establish the mathematical model about the frequency and voltage controller based on the above function equation.Under the double reference of the restore stability time of frequency and voltage as well as overshoot,this paper made a parameter tuning work of the frequency and voltage controller,and achieved the optimal control parameters.Finally,simulating the VSC-HVDC control method which can deal with Large-scale wind power frequency and voltage instability by using the professional large-scale power system simulation software DIgSILENT Power Factory.We establish the doubly-fed generation system,VSC-HVDC system and related control system simulation model.Mainly considering the two kinds of typical disturbance in the process of system simulation,such as the wind farm side wind speed change and the typical single-phase earth fault,in order to verify the transient and steady state running performance of large-scale wind power system-connected grid when we use the proposed VSC-HVDC control method.The simulation results show that the proposed control method can greatly enhance the stability of the wind power system frequency and voltage,and improves the safe and stable operation level of system when the large-scale wind power integrates into main grid. |
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