Investigation On Enhanced Operation Of DFIG Wind Power System Using Direct-resonant Approach Without Phase-locked Loop

As the new development of the global energy,the wind resources have experienced a rapid growth recently.The doubly-fed induction generator(DFIG)-based wind power generations have been widely employed due to their outstanding merits,including the smaller power converter,the lower cost and so on.However,since the power converters in the DFIG-based wind power generation can just purely generate active fundamental currents,the operation adaptability and the power quality may not meet the grid requirements.Among the voltage disturbances,the voltage unbalance has more significant impacts on the DFIG wind power generation.Therefore,it is essential to investigate the DFIG performance during network unbalance and develop the corresponding techniques.Against the increasing requirements of the adaptive operation and the power quality management,several detailed investigations on DFIG's rotor-side converter(RSC)and grid-side converter(GSC),including the direct resonant approach,the no-PLL-based DPC scheme and the no-PLL-based grid-tied-current control scheme,have been highlighted in this paper.Then,some novel and important conclusions can be summarized as follows.1.For the vector control(VC)of DFIG's RSC and GSC,an improved VC scheme with the direct-resonant approach is proposed.The resonant controller with the infinite amplitude gain at the assigned frequency is used to directly regulate the current,the power and the torque.Due to the frequency selectivity of the resonant controller,these ac and dc-coupled signals can be directly controlled without the sequential separations.Besides,the complex calculations of the negative sequence conmmanded currents are eliminated.This approach can also reduce the parameter dependency of the control system.Then,the enhanced control and operation of DFIG's RSC and GSC are developed using the direct-resonant approach.It can achieve the modulized operation and the coordinated operation of DFIG's RSC and GSC during network unbalance.2.Focused on the direct power control of DFIG's RSC and GSC,a novel direct power control(DPC)without PLL is proposed.The DPC scheme is implemented in the virtual synchronous reference with its phase angle ?1=?1\t(?1=100? rad/s).Since a virtual phase angle is used for coordinate transformations instead of the PLL estimated phase angle,the phase-locked loop(PLL)can be eliminated in the control system.The potential instability issues caused by the PLL are also avoided.Based on the direct resonant approach,the improved DPC scheme without the sequential separations is developed for DFIG's RSC and GSC during frequency variation and network unbalance.Then,the natural flux linkages caused by the frequency variation would damp rapidly and the negative sequence currents are under control.The power quality of the DFIG system are improved and enhanced.3.The grid-terminal-current control scheme without PLL for DFIG's RSC and GSC is proposed.In this control scheme,the commanded currents are calculated on the basis of the instantaneous power theory without the generator parameters.It is also implemented in the virtual synchronous reference frame with its phase angle ?1=(?1=100? rad/s).Thus,this control scheme can reduce the parameter dependency and avoid the potential instability caused by the PLL.Then,based on the grid-terminal-current control scheme of the DFIG,an improved grid synchronization with no parameter dependency is developed.This approach can simply accomplish the satisfactory grid synchronization during voltage unbalance and frequency variation.Finally,based on the direct resonant approach,the adaptive operation of DFIG's RSC and GSC is achieved and then the power quality of the DFIG system is enhanced.