| With the substantial increase of wind power into the power grid, grid-connected wind turbines (WTs) have an increasing influence on the stability of power system. Therefore, countries in the world with wind energy been well developed have successively introduced grid codes by which the stability of WTs has been restricted definitely. As been widely recognized, the constraint of WTs’behavior under grid fault is a serious challenge to wind turbines in the grid codes. The key requirements of the constraint can be summarized as follows. WTs are not only required to "withstand" all kinds of grid faults, but also should have the ability to "support" the recovery of the faulted power. From the perspective of the common types of grid failure, the most directly and significantly harmful ones to the grid-connected WTs are the voltage unbalance, harmonic distortion and symmetry/asymmetry swell or dip. From both theoretical research and engineering application points of view, thus, it is valuable to investigate the operational characteristics of grid-connected WTs under such typical grid faults, as well as putting forward the corresponding control strategies.Against this background, this thesis, detailedly and systematically, study the operational characteristics of the widely installed double-fed induction generator (DFIG) based WTs under these common grid faults, i.e., grid voltage unbalance, harmonically distortion, dip and swell. Especially, great attention has been put on the expanded application of resonant (R) controller. And theoretical analysis, simulation and experimental validations have been carried out. Moreover, an improved control strategy for the grid-friendly WTs has been proposed, which can satisfy the latest grid codes. This thesis focuses on two sides. The first one is the modeling analysis and resonant (R) control strategies of DFIG-based WTs under generalized unbalanced and harmonically distorted grid voltage. The other one is the high voltage ride through (HVRT) technology of DFIG-based WTs under grid voltage swell condition. Conclusively, the main research and contribution of this thesis can be outlined as follows.1. The key points of the resonant (R) controller applied to DFIG’converters have been investigated systematically, which settles the theoretical foundation for the resonant controller’s further expansion in wind farms. Three types of resonant controllers are compared, i.e., the proportional resonant (PR), vector proportional integral (VPI) and proportional integral resonant (PIR). Their basic characteristics and applicable situations are then summarized, from the aspects of their ability to regulate the fundamental and harmonic current, computational complexity and the frequency adaptive capacity. Taking the PR controller as an example, three applicable problems of the resonant controller are systematically settled, i.e., parameters tuning, phase compensation, discretization. Especially, based on the root locus and frequency-domain analysis method, this thesis proposes an integral tuning way for both the fundamental and harmonic resonant controllers. Meanwhile, a grid voltage synchronous signal detection method is put forward, based on the resonant controller as well.2. From the mathematical modeling of a DFIG under unbalanced and distorted grid voltage condition, this thesis, for the first time, evaluates the influences of such grid fault on DFIG’s current unbalance and distortion, the instantaneous active and reactive power and electromagnetic torque, with analytical expressions provided in detail. Moreover, based on the established DFIG model, an improved resonant control strategy is thus put forward. The following issues have been discussed on this subject. Firstly, an integral mathematical model of the DFIG during grid voltage distortion with5th and7th harmonic components considered, is put forward. And the influences of the low-order harmonics on DFIG stator’s instantaneous active, reactive power and electromagnetic are evaluated. Then, based on the new DFIG model, the corresponding optional control objectives and current command algorithms are proposed. Furthermore, a resonant current control scheme is presented for the DFIG-based WT during such harmonic grid voltage condition.3. For the first time, this thesis expands the modeling method aforementioned to a more complex grid voltage condition, i.e., voltage unbalance and distortion with5th and7th harmonics included. And the DFIG’s instantaneous active, reactive power and electromagnetic power are thus remodeled, which clarifies the reasons that cause so bad ripples in the power and torque. Importantly, a coordinated control scheme is proposed for DFIG’s both grid-side and rotor-side convertors. At the same time, based on the established mathematical model, the current references of DFIG"s grid-side and rotor-side convertors have been simplified and the control structure has been optimized as well, which makes the control scheme much easier to be set in real WTs. Moreover, a generalized mathematical model of DFIG under generalized grid harmonic conditions is reestablished. The generalized model can not only be used to analyze the traditional harmonics, i.e.,5th,7th harmonics, but also can be applied to the assessment of fractional harmonics or even low frequency oscillation. The generalized model is with so strong universality that the proposed modeling idea is sublimated.4. The transient response process of a DFIG-based WT during grid voltage swell and dip failure conditions has been studied systematically. And according to the grid code, a control and protection scheme of a DFIG-based WT’s HVRT and LVRT has been proposed. The following issues are concluded. Firstly, the major hazard of the grid voltage swell fault on a DFIG-based wind turbine is well analyzed, with the main factors affecting the WT’s HVRT capability been given. And accordingly, a feasible way to realize DFIG’s HVRT operation is thus put forward. To satisfy the reactive current output requirements of the grid code, a proposed control and protection scheme is designed to meet the DFIG HVRT operation. Meanwhile, the technical difficulties and key points of a DFIG’s LVRT operation have been well summarized. And on the basis of the existing wind turbine protection modules, a complete and coordinated control and protect strategy for DFIG-based WT’s HVRT and LVRT is finally presented.5. A new programmable grid fault emulating power has been developed in the lab, which can simulate voltage symmetric/asymmetric well/dip, unbalance, harmonic distortion, frequency drift, phase angle jump, amplitude fluctuations and so on. It can also simulate a complex adverse grid, which may combine two to three kinds of the faults aforementioned. To be the key point, the fault’s severity, duration, and other parameters can be adjusted flexibly. Meanwhile, a5.5kW DFIG prototype has also been developed to validate the correctness of proposed mathematical model and the effectiveness of the proposed resonant (R) control strategy under such unbalanced and harmonically distorted grid voltage conditions. The reliability and stability of the coordinated control for DFIG-based WTs’HVRT and LVRT are verified as well. |
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