Research On Control Strategies Of DFIG-based Wind Power System For Adapting To The Distorted Grid

The growing installation capacity of the wind energy, brings increasing concerns about the wind power generation systems’output power quality and operating reliability to satisfy the demand of accessing grid network. Most of large wind farms are installed in the terminal of power transmission systems in China, where might be considered as weak grid. The grid voltage supply may suffer from harmonics and unbalances frequently. To maintain the stability of the power grid and the safe operation of wind power generation system, the wind power system access guide in China-《Technical regulation for wind power system access》 and 《Technical rule for connecting wind farm to power network》, require the wind power generation system connected to the grid can continue to operate, and the output power quality should satisfy the relational standards, even under unbanlancd and harmonically distorted grid voltage conditions. As the doubly fed induction generator (DFIG) wind power system is most popular in commerce, considering the actual conditions in China, this dissertation research the enhanced control technique of DFIG wind power generation system under unbanlancd and harmonically distorted grid voltage conditions. The studies of this dissertation are focused on:1) control capability of GSC’s and DFIG’s traditional vector control, improved vector control of GSC, DFIG and DFIG wind power system under harmonically distorted grid voltage conditions;2) control capability of GSC and RSC traditional integral sliding mode control, and resonate-based sliding mode control of GSC, RSC and DFIG wind power system under unbanlancd and harmonically distorted grid voltage conditions. The main research and contribution can be outlined as follows:1) Under harmonically distorted grid voltage conditions, the mathematical model of the DFIG wind power system was given, which describes the current, instantaneous power, electromagnetic torque. This dissertation analyzed the impacts of grid side inductance and parameters of the controller on the capability of harmonic current suppression under harmonically distorted grid voltage conditions, and proved the limitation of GSC and RSC traditional vector control under this grid conditions. Finally, both simulation and experimental results verify the correctness.2)This dissertation proposed the improved vector control of GSC、RSC and DFIG wind power system under harmonically distorted grid voltage conditions, which includes:i) proportional-integral-resonant (PIR)-based improved vector control of the GSC, achieved the control targets of sinusoidal output currents or stable output active/reactive power; ii) stator current resonant controller-based improved vector control of DFIG, achieved the control targets of sinusoidal stator currents, sinusoidal rotor currents, stable stator active/reactive power or stable stator reactive power and electromagnetic torque; iii) coordinated control strategy of the DFIG wind power system, achieved the control targets of sinusoidal total output currents and stable electromagnetic torque, stable system output active/reactive power and electromagnetic torque. Finally, both simulation and experimental results validate the availability of the proposed control strategy.3) In order to enhance the DFIG system’robustness with respect to parameter variations, this dissertation designed GSC and RSC integral sliding mode control, and gived the design procedure of the control parameters. This dissertation analyzed the control capability of GSC and RSC traditional integral sliding mode control under unbanlancd and harmonically distorted grid voltage conditions, and proved that tracking errors will exist with integral sliding mode control strategy. Finally, both simulation and experimental results verify the correctness.4) In order to eliminate the tracking errors of sliding mode control under unbanlancd and harmonically distorted grid voltage conditions, the resonant controllers were added in the switching surface. Furthermore, the resonant-based sliding mode control strategies of GSC and RSC were proposed. With the resonant-based sliding mode control strategy of GSC, the control targets of sinusoidal output currents or stable output active/reactive power were achieved with the calculations of corresponding references. Then, with the resonant-based sliding mode control strategy of RSC, the control targets of sinusoidal stator currents, stable stator active/reactive power or stable stator reactive power and electromagnetic torque were also achieved with the calculations of corresponding references. Finally, the experimental results validate the availability of the proposed control strategy.