Research On Control Scheme Of Doubly-fed Induction Generator System Under Unbalanced Grid Voltage

Because of its structural features, the doubly-fed induction generator (DFIG)based wind power system is very sensitive to the impact of the grid, even a smallunbalanced grid voltage will affect the stablity of DFIG, and delivery a large numberof harmonics to the grid. As the installed capacity of wind turbines is increasinglarger amd lager, their impact on the grid can not be ignored. When the grid voltagefault occurs, the traditional control scheme based on normal grid conditions couldnot meet the increasingly stringent requirements of wind power. Therefore,reserches on the operation and control on DFIG under grid fault conditions had beena new hotspot. This paper studied the operation of DFIG wind power system underunbalanced grid voltage, designed a new passivity-based controller (PBC) for bothrotor-side and grid-side converters of DFIG, then the simulation results andexperiments verified the effectiveness of the proposed control schemes.Firstly, the character of unbalanced grid voltage was analyzed and the gridsynchronization information was accessed by the improved Phase Locked Loop(PLL). Then, the operation of the DFIG wind power system was analyzed. Base onthe models of DFIG system under balanced grid voltage, the positive and negativemodels for both rotor-side and grid-side converters of DFIG under unbalanced gridvoltage were established, and the complete instantaneous power models of DFIGhad been simplified.Secondly, the control sheme of DFIG under unbalanced grid voltage wasstudied. In the positive and negative synchronous rotating frame (SRF), the currentreferences were calculated by different control targets, and the inner loop wascontrolled by Passivity-based dual-current controller. Thereby, the DFIG could keepoperating steadely; track the maximum wind power and delivery stable power togrid, when DFIG operated under unbalanced grid voltage.Thirdly, the passivity-based dual-current control schemes for grid sideconverter of DFIG under unbalanced grid voltage were proposed. When grid voltageunbalance, both positive and negative components would emerge, the PBC was usedto control the dual-current loop. Combine with the voltage control, the grid sideconver could be connected to grid in unity power factor; the DC bus voltage couldkeep steady and the energy of grid side converter could flow in both directions.When the whole DFIG system was integrated, the operation state of rotor-sideconver would affect the grid-side converter. Therefore, combined the control ofgrid-side and rotor-side converters, a coordinate control scheme was proposed toeliminate the fluctuations in DC bus voltage. Simulation results of the proposedcontrol schemes verified its correctness and effectiveness. Finally, the grid-side converter experiments under unbalanced grid voltagewere studied on the experimental platform for a small wind power system. Thesoftware program based on DSP2812was designed. The main routine flowchart wasgiven and the processes of the AD sampling and fault protection subroutine wasdescribed. The PWM master interrupt subroutine, which contain the controlalgorithm of the grid-side converter under unbalanced grid voltage, was illustratedmore detail. Experimental results showed that the proposed method could controlthe grid-side converter connected to grid by propose control targets, even it operatedunder unbalanced grid voltage. The experimental research has proven thecorrectness of the theory once again.