Control Strategy Of Doubly-fed Induction Generator Based On Voltage Source Output

Fossil energy created current industrial civilization,while inevitably it brings about a series of environmental problems such as haze.The rise of wind power technology can provide an effective scheme to solve environmental problems.With the rapid development of doubly fed wind power generation technology,the single machine capacity and total wind power installed capacity of the motor are increasing.For the traditional large-scale wind farms generally often use PQ control to regulate the wind power output,the doubly fed generator is present as the current source characteristic.Under the current source mode,the speed of the wind turbine and the grid frequency have been decoupled.Therefore,compared with the traditional synchronous generator,the contribution of the wind turbine to the inertia of the power system is almost zero.The Point of Common Coupling of the wind farm will show the characteristics of low inertia,weak damping,and the supporting role of AC power system to the wind farm will be weaker and weaker,which will have a bad effect on the steady state and transient characteristics of the system.In this paper,a mathematical model of a doubly fed generator and a stator side converter is set up in a three-phase stator stationary coordinate system and a two phase synchronous rotating coordinate system.According to the mathematical model of the motor and the stator side converter and stator side converter design based on DC voltage loop and current loop double closed-loop vector control strategy and a rotor side converter control strategy under the islanding operation mode of a double fed generator with structure symmetric external voltage loop and rotor current inner loop The transfer function between the given voltage instruction and the output voltage is derived from the control strategy of the DFIG rotor side converter,and the Bode diagram of the transfer function is plotted.The effectiveness of the designed control structure is verified theoretically.Then the simulation experiment platform of the doubly fed generator is designed,and the control strategy of the island is further verified through the experimental results.Based on the control strategy of the rotor side converter under the isolated island mode,the active power-frequency,reactive power-amplitude droop outer loop is added,and the voltage source droop grid-connected output scheme of the doubly fed generator set is designed.The small signal expression of the power transfer function including the motor model is derived.and the system bode diagram is drawn according to the small signal expression,which theoretically verifies the effectiveness of the droop grid connected control strategy.Then through the experimental results,the voltage source droop grid-connected output control strategy is further verified.The problem of harmonic voltage distortion caused by the nonlinear elements in the system is very common,whether it is working in the island mode or voltage source droop grid-connected output mode.Aiming at this problem,a harmonic voltage suppression strategy based on harmonic voltage proportion feedforward is designed,and the transfer function between harmonic current source and output voltage is deduced.The harmonic gain of harmonic current source to output voltage is compared before and after harmonic voltage suppression is adopt.Combined with the phase margin of the open loop transfer function of the system,a reasonable method for solving the proportional feedforward coefficient is given.Finally,the suppression effect of the proposed control strategy on the harmonic voltage is verified by the experiment.