| High-carbon energy is facing exhaustion and its huge consumption causes environmental pollution.These problems restrict the sustainable development of China's modern industry and economy.It is urgent to develop and utilize various low-carbon clean renewable energy sources.With the maturity of wind power generation technology,large-scale development and utilization of wind power have become an important way to solve the energy shortage and environmental pollution problems in China.With the release of two "13th Five-Year Plan" of wind power at the end of 2016,China's wind power industry will usher in another development climax.Doubly-fed induction generator(DFIG)is mostly used in the large wind farms in China.Converter control technology and fault ride-through capability of DFIG are the key factors to decide whether the grid can accept large-scale wind power.Therefore,it is an important subject to study the converter control technology and the transient behavior of DFIG during grid fault.For this reason,this dissertation will focus on the following studies,such as the dynamic and transient control performance improvement of converter,the transient behavior analysis of DFIG during grid failure and the impact of auxiliary damping control on the transient behavior of DFIG.The main contents of this dissertation are as follows:(1)In order to improve the dynamic and transient control performance of rotor side converter(RSC),a control strategy of RSC based on nonlinear adaptive disturbance rejection controller(NADRC)is proposed.Firstly,the dynamic mathematical model of DFIG for control strategy design is established,control performance of traditional control strategies for RSC is briefly evaluated,and the reasons affecting the dynamic and transient control performance of RSC are analyzed.Then,NADRC is designed in detail,and its control parameters are designed by using pole placement technology.Finally,steady state,dynamic state and transient state performance of the proposed control strategy for RSC is evaluated by time domain simulation.The proposed control strategy for RSC has the following advantages: current tracking no overshoot and quickly stabilizing;improving the dynamic and transient control performance of RSC;achieving single-parameter adjusting characteristics,and shortening the adjusting time of controller.(2)In order to quickly suppress the capacitor voltage fluctuation of the dc-link caused by external disturbances,a control strategy of grid side converter(GSC)based on capacitor current direct control is proposed.Firstly,a vector mathematical model for GSC control research is established.Then,the control performance of the traditional GSC control strategy is briefly evaluated,and the reasons for capacitor voltage fluctuation of the dc-link bridge are discussed.Finally,the ability of the proposed control strategy for GSC to suppress capacitor voltage fluctuation of the dc-link bridge is assessed by time domain simulation.The proposed control strategy for GSC has the following advantages: using capacitive current as control variable in the inner current loop to improve the dynamic and transient control performance of GSC;using voltage feed-forward compensation to eliminate the delay of current feed-forward compensation;introducing "tracking-differentiator" technology to deal with the load disturbance dynamics of GSC.(3)In order to further understand the transient behavior of DFIG during power grid fault,a general model and method for transient behavior analysis of DFIG during power grid fault are proposed.Firstly,the dynamic model of rotor windings of DFIG under normal grid voltage is constructed,and the characteristics of rotor open-circuit voltage and rotor current are analyzed.Then,according to the principle of flux conservation and superposition,the transient analytical expressions of stator flux,rotor open-circuit voltage and rotor current are derived.Finally,the validity of the proposed general analytical model and method for rotor transient behavior and the correctness of the analytical formula for the rotor transient current are verified by time domain simulation.The proposed general transient behavior analysis method has the following advantages: using vector analysis,the results obtained are concise;the general rotor transient behavior analysis model and method are suitable for different types of power grid faults;the derived analytical formula for the rotor transient current has high accuracy and is suitable for engineering applications.(4)In order to suppress subsynchronous resonance in wind-fire bundled transmission system,an auxiliary damping control strategy based on stator voltage vector orientation control of RSC is proposed.Firstly,based on the first standard model of IEEE,a wind-fire bundled transmission system model is established,and the influence of wind power plant with DFIGs on the subsynchronous resonance characteristics of the original system is discussed.Then,starting from the mechanism of restraining system subsynchronous resonance,an auxiliary damping controller is designed and its parameters are optimized by using particle swarm algorithm.Finally,the effectiveness of the proposed auxiliary damping control strategy to suppress the system subsynchronous resonance and its effect on the transient behavior of DFIG are evaluated by time domain simulation.The proposed auxiliary damping control strategy has the following advantages: using reactive power regulation capability of DFIG to improve the system subsynchronous resonance damping;optimizing the additional damping controller parameters by using particle swarm algorithm and the system subsynchronous resonance damping being optimal;little affecting on the transient behavior of DFIG for the additional damping control. |
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