| With the continuous increased capacity of installed wind power, the effects of wind power generation on the grid are more and more considerable. As a consequence, the grid codes issued by more and more power system operators specify that the wind turbines should withstand certain voltage disturbances without tripping, to do this the wind turbine systems must continuously develop and improve their performance. A large number of wind turbine systems are increasingly being installed in remote areas in China. Many wind farms are located in the terminal of power transmission systems, whose connections might be weak. The presence of voltage unbalance and voltage harmonic distortion is more severe in these weak transmission lines. Such issues may affect safe and reliable operation of DFIG wind turbines (Doubly Fed Induction Generator, DFIG). Hence, the grid compatibility of DFIG wind turbine systems should be improved during grid voltage unbalance and distortion.In order to improve the grid compatibility of DFIG wind turbine systems, the studies of this dissertation are focused on:1. Control strategy of DFIG for improving DFIG grid compatibility under grid voltage distortions conditions;2. Control strategy to reduce the impacts of unbalanced grid voltage on DFIG wind turbines,which is in order to improve the reliability of wind turbine and also to meet the modular design requirements of DFIG converters;3. Phase-locked loop (PLL) control strategy for providing precise phase and frequency of the grid even under non-ideal grid voltage conditions.In Chapter1, the state-of-the-art of wind power technologies is presented. The grid connected requirements of wind power systems issued by power system operators are also given. Further, the dissertation summarizes the existing control methods for the DFIG converters under distorted and unbalanced grid voltage conditions.In order to improve the compatibility of the DFIG wind turbine systems to the distorted grid voltage, the operation characters, current control performance and the improved control strategy are presented and developed in Chapter2. Firstly, the impacts of the grid harmonic voltage on the DFIG and the current contol system are evaluated. Then, a stator current harmonic suppression method, which is using a6th order resonant controller to eliminate5th and7th order current harmonics, is proposed. The analysis shows that the impacts of the5th and7th order voltage harmonics on the stator current as well as the electromagnetic torque are effectively removed. Finally, simulation and experimental results are presented to valiate the analysis.Based on the trade-off between the steady-state performance, the dynamic performance, and the stability of the proposed stator current harmonic control system, a systematic optimized design procedure of the resonant controller parameters is presented in Chapter3. In order to further improve the dynamic performance, two types of phase compensations are used to increase phase margin of the control system, i.e.1) resonant controller with lead angle compensation;2) resonant controller combined with a lead-lag compensator.Chapter4is focused on the compatibility improvement of the DFIG wind turbine systems to the unbalanced grid voltage. The negative-sequence impedance of the DFIG system is calculated when using the conventional current control method with PI-controller. Then the rejection capability to the unbalaned grid voltage is assessed. Based on the analysis, a stator current balance control method using a2nd harmonic resonant controller is proposed in order to increase the negative-sequence impedance. The experimenatl results show that the fundamental negative sequence stator current caused by unbalanced grid voltage is suppressed and the electromagnetic torque fluctuation is significantly reduced.Meanwhile, the unbalanced grid voltage also causes a large second-order harmonic current in the dc-link capacitors as well as dc-voltage fluctuation, which potentially will degrade the lifespan and reliability of the capacitors in voltage source converters. This dissertation proposes a novel dc-capacitor current control method for Grid Side Converter (GSC) to eliminate the negative impact of unbalanced grid voltage on the dc-capacitors. In this method, a dc-capacitor current control loop, where a negative-sequence resonant controller is used to increase the loop gain, is added to the conventional GSC current control loop. The rejection capability to the unbalanced grid voltage and the stability of the proposed control system are discussed in detail. A modular implementation method of the proposed control strategy is developed for the DFIG controller. Finally, experiments are presented to validate the theoretical analysis.As it is difficult for the conventional software PLL to achieve good steady-state and fast dynamic performance simultaneously under non-ideal grid conditions such as voltage harmonic distortion and imbalance, a synchronous reference frame based PLL method, which uses delayed signal cancellation to extract the positive sequence components from unbalanced grid voltage in natural abc frame, is analyzed in Chapter5. The loop filter consists of a PI controller and a first order low-pass filter for high attenuation of harmonic components. The structure of the PLL method is simple. The experimental results show that this PLL method has a fast dynamic response, and an accurate detection of the grid voltage frequency and phase.From the aforementioned analysis, it is seen that the grid compatibility of DFIG wind generation systems is able to be significantly improved by using the advanced control technology under harmonic distorted and unbalanced grid voltage conditions, which can lead to more reliable and safer operation of DFIG wind turbines. |
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