| Summary: | Large wind farms are usually located in remote and offshore areas. High voltage transmission systems that have long transmission distances are used to deliver the wind power to main grids. Weak AC grids have high impedance, low short circuit ratio (SCR) and/or low inertia compared to strong AC grids. The voltage stability of weak AC grids is a challenging issue that needs to be considered. This thesis compares weak and strong AC grids based on the voltage stability analysis. The steady-state characteristics of the weak AC grids are investigated. The power transfer characteristics of the wind farms that are connected to weak AC grids are studied under different voltage control technologies. The mitigation of the voltage recovery problems for weak AC grids is proposed by supplementary voltage control. The main characteristics of a weak AC grid are determined using P-V and V-Q curves. Different short circuit levels of the AC grid are presented with an increase in grid load and active power generation. Weak AC grid has a poor voltage stability limit and a low reactive power margin, which make the grid close to voltage instability. A static model is developed to study a test system including wind farm, AC grid, and reactive power compensators. Variable-speed wind turbines are examined under different control modes (power factor control, AC voltage control and reactive power control) using full power converters to increase the limit of transferred wind power to weak AC grids. Reactive power compensators of STATCOM, SVC, and fixed capacitor are compared to the full power converters. The capability of transferring power using STATCOM and SVC is greater than the full power converters. A dynamic model for the wind farm connected to the AC grid is developed and a STATCOM. The AC grid is modelled using two methods: as an ideal voltage source behind a Thevenin impedance and as a synchronous generator. A reactive power versus AC voltage droop is designed in STATCOM. The effectiveness of the STATCOM control is tested to increase the power transferred to the weak AC grids. The new supplementary voltage control is proposed using the full power converters with DC chopper considering three-phase to ground fault. Although the DC chopper is inadequate to keep the transient stability, a fast voltage control of the STATCOM is utilized to support the DC chopper in weak AC grids. The voltage recovery is improved using this controller after fault clearing.
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