Research On The Control Strategy Of Doubly-Fed Wind Power Generation System

In recent years, as one of the renewable energy, wind power generation technology has been developed rapidly all over the world. There are different types of generators used in wind power generation system, and doubly-fed induction generator (DFIG) is one of the major generators which is used most widely.With the development of DFIG wind power generation, research of optimal control strategy, fault ride-through control strategy and wind farm level unit optimization that participate in power grid schedule are new focuses in wind power research.In view of the PWM converter coordination control of DFIG wind power generation system, this paper gives a multi-objective adaptive coordination control strategy for PWM converter starting from the aspects of the grid voltage, reactive power demand and system loss, which can provide adaptive control for one-unit system and satisfy multiple targets. In the meantime, this strategy can also be used in wind farm control system to realize wind farm participating in active and reactive power adjustment of the grid.In view of grid fault, this paper studys on the fault ride-through of DFIG, presenting a strategy, which can effectively reduce overflow problem of rotor side converter which is caused by power grid fault, achieve its fault crossing and improve power quality which enables DFIGs participate in grid power quality adjustment.In view of how the DFIG wind farm takes part in grid active power and voltage adjustment, this paper presents a voltage coordinated control strategy, giving reactive power optimal distribution method in wind farm control level. The strategy uses grid dispatching command, reactive power demand and the running state of each turbine as the objective function model, and distrbute reactive power command of each turbine in a better way, realizing the reactive power optimization in system level.This paper studys on problems which including double PWM converters optimized coordination control, fault ride-through control and reactive power optimization control. The main contents are as follows:1. Detailed analysis of DFIG operation principle are conduct simplified mathematical model, Thevenin's equivalent circuit. And power characteristics, power limits and classic double closed-loop control strategy are studied which are the theoretical basis and simulation foundation of optimizing control strategy of DFIG research.2. The theoretical and mathematical model of double PWM converters which is the key in the process of operating performance of wind power system are analyzed. Mathematical model of PWM converter based on voltage source in different coordinate system is derived and the steady state characteristics and double closed-loop control strategy are studied. A coordination control strategy of dual PWM converters based on internal model control is presented which has good anti-interference performance and robustness. The strategy is on the basis of the double closed loop control, using feed-forward internal model correction link and synchronous rotating coordinate system under the current PI regulator, making both rotor side and grid side converters to take part in DFIG excitation control. It can enable the PWM converter perform better, using the converter capacity limit to the maximum, besides, it can improve the anti-interference ability, dynamic response and efficiency of wind power system.3. According theoretical analysis of fault ride-through strategy when grid failure happens is done, involving DFIG operation features of grid symmetrical and asymmetrical fault cases. Based on positive sequence and negative sequence current inner loop structure, a fault ride-through control strategy applied to power grid asymmetric fault is raised. It can inhibition active and reactive power fluctuations caused by asymmetry power grid failure, increase system security and realize the system fault crossing. Computer simulation is done to validate the effectiveness of the proposed control strategy.4. At present, wind farm generally adopt constant power factor control mode, that does not make full use of reactive power regulation ability of DFIG to participate in the wind farm voltage control. To solve this problem, a piecewise layered wind farm voltage coordination control strategy is proposed, which send reactive command to each DFIG to adjust the reactive power in wind farm via reactive power optimization algorithm in wind farm control level. According to the simulation results of wind farm model, the effectiveness of the new wind piecewise layered voltage coordination control strategy is verified.5. Detailed simulation model of wind power system consisting of DFIGs is built in the MATLAB/Simulink platform, through which the simulation effectiveness of the control strategy proposed is verified. Meanwhile, some physics experiment of the power characticts of DFIG wind power generation system is done. All the simulation results show that the proposed control strategy is effective.