| A large-scale development and utilization of wind power has been an important component of China's energy development strategy.As the wind resource and distribution of load demand is inconsistent,the large-capacity and long-distance outward transmission of wind power is necessary.Series compensation capacitor technology with the advantage of reducing transmission line loss and increasing line transmission capacity,can be used as a technical support for large-scale wind power transmission.However,the wide application of this technology has the possible problem of leading to wind power subsynchronous oscillation.The occurrence of sub-synchronous oscillation seriously has affected the safe and stable operation of large-scale wind power base and its outgoing system.In order to solve the problem of subsynchronous oscillations(SSO)caused by DFIG(Doubly-Fed Induction Generator)wind energy system interconnected with a series compensated electric network,the following aspects are studied in this paper:1.DFIG wind energy system interconnected with a series compensated electric network.Regarding the wind farms with DFIGs interconnected with electric network by the series capacitor compensation,this paper establishes the system's mathematical models,including the aerodynamic model of wind energy capturing,wind turbine model of driving train,doubly-fed induction generator model,dc capacitor's dynamic process,rotor side converter control model,grid side converter control model,transmission line model etc.Dq decoupling control is realized by dq coordinate transformation,and the PI control mode of rotor side converter and grade side converter is described in detail.2.Impedance-based Nyquist stability criterion was applied to analyze and verify the influence of wind farm's sub-synchronous oscillation caused by wind velocity,series compensation level.Then the complex torque coefficient method was applied for detailed analysis,small value signal increment was added on the generator rotor to achieve the electrical damping curve,finally,the time domain simulation method is used to verify the analysis results intuitively.The smaller the wind speed is and the higher the series compensation,the more possible that the sub-synchronous oscillation induced by induction generator effect.What's more,The greater the proportional and integral parameters of the current inner loop of the rotor side converter,the more possible leading to sub-synchronous control interactions induced by the wind turbine controller,which is wind farm specific sub-synchronous oscillation forms.3.Based on the analysis of the synchronous oscillation mechanism of the wind farm,the method of additional damping controller is used to suppress the subsynchronous oscillation.The frequency of the lowest point of the electrical damping,and the angle to be compensated are obtained by the complex torque coefficient method to calculate the phase-shifting parameters of the damping controller.The parameters of the additional damping controller are optimized by the particle swarm intelligence algorithm.And the damping controller can compensate the electrical damping and improve the stability of the system,so that the system can run in the stable first quadrant.The tim domain simulation method is used to verify the effect of additional damping controller on the subsynchronous oscillation of wind farm.4.In this paper,the method of replacing the traditional PI control with RBF neural network control is proposed for electromagnetic torque Te outer loop control of the rotor side converter.The self-learning ability of the RBF neural network of torque loop makes the PID controller adaptive,which can adapt to the different series compensation and different wind speeds of the complicated operation conditions in DFIG wind farm.The subsynchronous oscillation would be induced by the traditional PI control parameters,the RBF neural network controller designed in this paper has been proved a good restraining effect on the subsynchronous oscillation. |
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