| Due to the impact of the environment,the output power of wind turbines is fluctuant and irregular.The power system needs to have more reserve capacity for frequency adjustment,which not only increases the cost of power system frequency regulation,but also a large range of wind turbines are connected to the power grid,which weakens the inertia of the power grid and reduces the stability of the power grid system.Therefore,how to coordinate the active power output of variable-speed wind turbine and super capacitor energy storage to make them have the ability of inertia and frequency response has become a hot research topic.First of all,the topology diagram of doubly fed wind turbine with super capacitor energy storage centrally included is introduced,the mathematical model of doubly fed variable speed wind turbine is constructed,the maximum power tracking point and transmission system model are given,and the motor side and grid side models are described in detail.Due to the existence of converters at the motor side and grid side of doubly fed variable speed wind turbine,when the system power fluctuates,it shows weak inertia or no inertia of frequency adjustment.For the doubly fed variable-speed wind turbine connected to the power grid,the higher the permeability of the wind turbine,the slower the frequency response rate of the system and the worse the recovery stability in case of power disturbance;Based on the working principle of super capacitor energy storage system,the mathematical model of super capacitor energy storage is established.Secondly,based on the two typical operating states of the doubly fed variable speed wind turbine and the partitioned rotor power control,this paper expounds several common load shedding control modes of the doubly fed variable speed wind turbine and the principle of super capacitor energy storage participating in frequency modulation,and further analyzes the frequency response characteristics of the variable speed doubly fed wind turbine and the necessity of the wind storage participating in the system frequency modulation.Under different working conditions,Different frequency modulation modes of variable speed wind turbine and super capacitor energy storage frequency modulation are simulated and verified.Finally,based on the characteristics of district control of doubly fed variable speed wind turbine,the advantages and disadvantages of various frequency modulation methods and the frequency modulation characteristics of super capacitor energy storage,considering the economy and safety of load shedding operation and energy storage of doubly fed variable speed wind turbine,a joint frequency modulation control strategy of variable speed wind turbine and super capacitor energy storage based on wind speed segmentation is proposed;Discrete Fourier transform(DFT)is used to decompose the differential power,so that the wind turbine and super capacitor energy storage at each wind speed can respond to the differential power at different time scales,so as to achieve the purpose of joint frequency modulation of wind turbine and energy storage;To avoid the inconsistency of state of charge(SOC)changes of different groups of supercapacitors,which leads to overcharge and discharge of supercapacitors.The consistency algorithm is introduced to ensure the balance of SOC changes of supercapacitors.The simulation results show that the multi-scale coordinated optimization strategy based on DFT change and differential active power is conducive to accurate,fast and stable frequency modulation of multi wind speed wind turbine and super capacitor energy storage,and is conducive to solving the problem of low inertia and no inertia of power system after variable speed wind turbine is connected to the grid,which improves the economy of wind turbine grid connection;Using the consistency algorithm,the change of SOC of energy storage is equal,which effectively solves the problem of overcharge and discharge of energy storage,and improves the safety and economy of energy storage. |
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