Investigation On Grid-connected Stability And Synchronization Control Strategy Of DFIG Wind Power System

With the continuous advancement of the low-carbon energy development strategy,the installed capacity of wind power system has experienced fast growth.The doubly-fed induction generator(DFIG)based wind power system has the advantages of the small converter capacity and no risk of permanent magnet demagnetization,which is popular for the commercial wind power solution around the world.Compared with the synchronous generator,the wind power system will not achieve synchronization through the actual physical rotor structure,but rather rely on synchronization control strategy such as phase-locked loop(PLL)to obtain the grid frequency and phase.However,the synchronization control strategy is altering the steady-state and transient characteristics of high-penetration renewables in a profound way,resulting in the different risk of instability for wind power generation compared with the traditional power system.On the one hand,due to the reverse distribution of wind power resources and power loads in China,wind power systems are usually connected to the grids with low short circuit ratio(SCR),the power quality compensation and the long transmission equipment,then the wide-band oscillations occur frequently.The sub/super-synchronous oscillation is within the controller bandwidth,which is significantly affected by the synchronization control strategy.On the other hand,the wind turbines require a longer grid fault tolerance time to maintain the continuity of output power.Nevertheless,under the influence of large disturbances such as voltage sags,the synchronization control strategy of DFIG system may not accurately track the phase of the grid and lose the synchronism,leading to the chain accidents and unintentional collapse.Against the aforementioned background,several detailed investigations on grid-connected stability and synchronization control strategy of DFIG wind power system are presented in this thesis.The main work and innovation achievements are as follows:1.Aiming at the influence of synchronization control strategy on the DFIG system’s small disturbance stability,the different modulation modes for direct current(DC)dynamics,and the contribution of generator-side impedance and grid-side impedance are obtained.Then the generator-side impedance model of DFIG system is established by the sequence-domain transformation.The complex vector-PLL(CV-PLL)is proposed to remove the frequency coupling,which simplifies the difficulty of impedance characteristic analysis and reshaping.The broadband negative resistance characteristics of DFIG system considering the current controller and PLL are studied based on frequency band division,and the dominant factors of low-frequency,middlefrequency and high-frequency negative resistance are clarified by solving real-part region and splitting impedance subsystem.The grid-connected synchronization method of the DFIG system without voltage orientation is illustrated,and the influences of PLL-less reference frames on the broadband negative resistance and small disturbance stability are revealed.2.Aiming at the influence of synchronization control strategy on the DFIG system’s large disturbance stability,the transient flux and the loss of synchronization(LOS)phenomenon for DFIG system under voltage sags are analyzed.Furthermore,a simplified analysis model is proposed under transient synchronous time scale,and the mechanism of LOS fault is clarified based on the voltage-angle curves.By comparing the swing equation and phase portrait of synchronous generator,some parameters influence laws of transient synchronization stability under different impedance ratio are elaborated,and the instability margin considering the transient damping and initial speed under high impedance ratio grid is studied.The significant coupling between phase angle dynamics and reactive currents in ultra-weak power grid(SCR<1.5)are presented.With the considerations of anti-movement phenomenon,the transient waveform and abnormal PLL output frequency are analyzed.3.This thesis points out some novel schemes for synchronization control of DFIG system considering the small and large disturbance stability.Compared with the influence laws of PLL parameters,it is noticed that the design of PLL parameters cannot take into account both the small and large disturbance stability for DFIG system.According to the key influencing factors of PLL on DFIG system stability,such as coordinate transformation of the rotor current,reference value calculation matrix,transient damping and initial speed,some stability improvement schemes are proposed to reduce the wide-band oscillations and risk of LOS.And these proposed schemes are tested against the broadband impedance passivity-based stability assessment.On this basis,the proposed stability improvement schemes are compared in terms of parameter design difficulty,robustness with parameter deviations,reference command tracking ability,adaptability under ultra-weak power grid,rotor’s overcurrent protection and the risk of LOS.Finally,the one of the suitable scheme for the comprehensive improvement of the DFIG system’s small and large disturbance stability under weak power grid is selected.