| In response to the ever-increasing problem of energy security and environmental degradation, to develop wind power has become an important choice for China's sustainable energy development strategy. Due to the actiual power grid condition of our country, many wind generators are located in the abnormal power transmission system with presence of steady-state voltage unbalance and harmonic distortion and transient voltage sag. Unlike the traditional synchronous generator, wind generator such as DFIG (Double Fed Induction Generator) is connected into the power grid by semiconductor converter, which is sensitive to the gird disturbance under the current control strategies.With the continuous increased capacity of insatalled wind power, the disconnection of wind generators form grid may cause significant power loss for the power system, which may result in voltage and frequency unstability of the wind power system. As a consequence, the grid codes issued by system operator specify that wind generators should have strong fault ride-through capability under the above two mentioned grid disturbance. Hence, this paper studies the control strategies of Double Fed Induction Generator (DFIG) under the two above extreme external power grid condition, which is helpful for the wind power's integration and power system's operation.The main contribution of the dissertation can be summarized as follows:This paper introduces an island power system with large scale wind power and aluminum smelting load. The simplified equivalent model of aluminum smelting load (ASL) considering harmonic characteristics is proposed based on the analysis of the mechanism of power system voltage distortion caused by ASL. On this basis, dynamic model of DFIG and multi-terminal VSC-HVDC transmission system is established. Also, harmonic transfer characteristics in multi-terminal VSC-HVDC are analyzed under unbalanced and distorted grid voltage condition. Furthermore, electromagnetic transient characteristics of DFIG are analyzed when transient voltage drop happens.This paper proposed an advanced auxiliary control system of an energy storage device (ESD) to improve the DIFG's transient voltage support capability. Firstly, on the analysis of the working principle of ESD auxiliary control systems, the paper proposes a two-layer design strategy that can assess the current capacity of storage-side converter and determine the appropriate distribution of the rotor demagnetizing current reference. Then, the coordinated control strategies are designed for ESD control system and DFIG control system. Simulation results show that the proposed ESD auxiliary control systems and control strategies can significantly improve the DFIG fault ride-through capability and transient voltage support capability.Two novel transient reconfiguration solutions and coordinating control strategy for the power converters to enhance the fault ride through (FRT) and transient voltage support capabilities of a doubly-fed induction generator (DFIG) with an energy storage device (ESD). First, based on demagnetizing control strategy, a novel transient configuration where the connection of the grid side converter (GSC) is reconfigured such that it is connected to the rotor circuit in parallel with the rotor side converter (RSC), while another transient configuration where the GSC is connected to the grid in series with the stator of DFIG by a series transformer based on stator voltage compensating control strategy is proposed. The operational principles of the two proposed transient configuration for enhancing the fault ride through capabilities are analyzed, and the coordinated control designs of the proposed transient configuration are designed. The effectivenesses of the proposed two transient reconfiguration of DFIG are verified on the actual island power sytem with high wind power integration.A coordinated optimization control method for DFIG under voltage unbanlance and distortion is proposed. Taking into accout the voltage control capability of RSC, the control is able to overcome the disadvantages of single-objective optimization method and improve the operation performance of DFIG. The assessment indexs including the output power, electromagnetic torque, stator current, and rotor current of DFIG is dedeloped. And then, an optimization model is established with the objective of minimal indexs and the constraints of voltage control capability of RSC and each assessment index. In order to sovle the optimization model to obtain the optimal control strategy of rotor current, an intelligent optimization method is used. Then, the control system of the DFIG based on PI-R current controller under voltage unbalance and distoration is designed. The actual isolated sytem is used to validate that the proposed method is effective to consider muti-objectives to optimize the performance of DFIG. Additionally, when the system voltage unbanlance and harmonic distortion is serious, the proposed method can also improve the the performance of DFIG.A coordinated control method of multi-terminal VSC-HVDC and wind farm systems for eliminating dc-link voltage ripple and AC side voltage distorotion of WFVSC of the multi-terminal VSC-HVDC is proposed.. Two coordinated wind power control strategies for the multi-terminal VSC-HVDC are developed to miantian its power balance and voltage stability, which is sutiable for the connection of isolated power grid and wind farm. And then, the GSVSC is proposed to dampen the fluctuations of DC voltage and AC voltage distortion of WFVSC. Simulation results of the actual isolated system indicated that the proposed GSVSC is effective to eliminate the pulsation components of the DC voltage of the VSC-HVDC and to remove the harimonic voltage component from the AC side voltage of WFVSC, and consequently, the the influence of the voltage unbalance and distortion of the isolated power system on the operation of he multi-terminal VSC-HVDC and wind farm systems can be restrained. |
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