Research On Operation And Control Of Hybrid Wind Farms Under Unbalanced Grid Voltage Conditions

With rapid development of wind power industry in China, it has been a trend to construct large-capacity of hybrid wind farms by combining different wind power generation systems. Due to the flexible power control capability and variable speed wind turbines, doubly fed induction generators(DFIG)- and permanent magnet synchronous generators(PMSG)-based wind power generation system have become dominant among the present wind power generation system. At present, most wind farms are installed in rural or offshore areas where the grid is relatively weak, and voltage unbalance condition occurs more frequently. During network unbalance, the operation performance of DFIG-based wind power generation system and PMSG-based wind power generation system is distinctly different. In order to improve the adaptability of a hybrid wind farms, the hybrid wind farms combining both DFIG-based wind power generation system and PMSG-based wind power generation system are researched in this paper. In addition, the operation performance of the hybrid wind farms during network unbalance is analyzed, and the control strategy under unbalanced grid voltage conditions is investigated.The simplified model of the hybrid wind farm with DFIG- and PMSG-based wind power generation system is firstly developed. Based on this, the fundamental principle and the implementation method of a generator-based wind power generation system in the hybrid wind farms outputting negative-sequence current to suppress voltage unbalance factor at the point of common coupling(PCC) is analyzed in detail. The control effect is verified by simulation and experimental results.Then, the ability of grid-side converter(GSC) and rotor-side converter(RSC) of DFIG-based wind power generation system outputting negative-sequence current under unbalanced grid voltage conditions is studied. During network unbalance, DFIG-based wind power generation system is controlled to suppress double supply frequency oscillations in electromagnetic torque or active power output. With the needed negative-sequence current taken into account, the controllable operation area with different voltage unbalance factors and active power outputs for various control targets can be achieved. The analyses are verified by simulation results.In addition, the ability of GSC of PMSG-based wind power generation system outputting negative-sequence current is also deduced. The reference value of negative-sequence current of GSC for restraining double supply frequency oscillations in active power output and grid-connected current during network unbalance is derived, which is considered to obtain the controllable operation area with different voltage unbalance factors and active power outputs for various control targets. The analyses are also verified by simulation results.According to the controllable operation areas and the maximum values of negative-sequence of DFIG- and PMSG-based wind power generation system for various control targets under unbalanced grid voltage conditions, the coordinate control strategy of the hybrid wind farms during network unbalance with different voltage unbalance factors and active power outputs is primarily proposed. Under different operation conditions, DFIG- and PMSG-based hybrid wind power generation systems are controlled to achieve various control targets, and the control effects are demonstrated by simulation results. The simulation results show that the power quality and the unbalanced voltage tolerance ability of the hybrid wind farms can be both improved by implementing the proposed coordinate control strategy, which maintains the stable operation of the hybrid wind farms under unbalanced grid voltage conditions.