| Wind power generation,which mainly based on doubly-fed induction generator(DFIG)based wind power generation system,is one of the important parts of the current use of the new energy.The main research contents of this paper are the control strategy research of the DFIG-based wind power generation system under the non-ideal grid condition.The non-ideal grid conditions consided in this paper include grid voltage drop,harmonic voltage and unbalanced voltage.The main research contents and conclusions are as followings:(1)The mathematical model of the single-mass wind turbine,doubly-fed machine and back-to-back PWM converters are created in the three-phase stator stationary reference frame and two-phase synchronous rotating frame(dq-axis),respectively.Based on that,the matrix model of the full-order DFIG wind power generation system are established in which the current of stator,rotor and grid-side converter is chosen as the state variable,the machine terminal voltage and rotor voltage is chosen as the input variables.Finally,the power relationship of DFIG wind power generation system is analyzed.(2)A 2MW DFIG wind power system is built in the PLECS platform and then the simulation is carried out under the non-ideal grid condition,the simulation results show that when the grid voltage drop to the 20% of the normal value at the most,the rotor current pulse will reach about 3 times the normal value,the power and the electromagnetic torque pulsation amplitude will reach about 2 times.(3)For the grid voltage sag,a method based on improved rotor-side converter control algorithm and the DC Crowbar is presented.With this control strategy,the rotor transient overvoltage is suppressed by 50%,the transient power and electromagnetic torque pulse is suppressed by about 40%.For the grid voltage harmonics,an advanced phase-locked loop algorithm is designed to filter out the harmonic component,negative sequence component and dc component.Therefore,the PLL can accurately extract the voltage phase of the grid in the case of harmonic and unbalanced voltage.Based on the advanced PLL,the harmonic voltage amplitude and phase extraction algorithm are proposed,then the harmonic voltage compensation algorithm(HVCA)is proposed and applied in a LCL-type three-phase inverter to compensate the grid harmonic.The simulation results show that the proposed method can ensure that the DFIG stator terminal voltage is the ideal sine,which can eliminate the impact of the grid voltage harmonic on the DFIG wind system fundamentally.For the unbalanced voltage,the algorithm of calculating the voltage drop amplitude is designed based on the advanced PLL.Then,the unbalanced voltage compensation algorithm(UVCA)is designed balance the DFIG stator terminal voltage.From the simulation results it can be find that the control strategy can help the terminal voltage returning to normal within 25 ms after the grid voltage imbalance occurres and achieving the normal operation of the DFIG wind power system under unbalanced grid voltage.(6)A three-phase LCL-type inverter experiment platform is designed.The effectiveness of the experimental platform is verified by the resistive load.Then,the harmonic control algorithm is studied under the nonlinear load condition.Finally,the output characteristics of rotor side converter of DFIG wind power system are approximated by this experiment platform.The simulation and experimental results show that the three-phase inverter experiment platform can effectively simulate the steady-state process of sub-synchronous and super-synchronous operation and the dynamic-state process of DFIG operation state transition. |
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