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Collection System Design for DC Series Wind Farms
The recent inauguration of the London Array, a 640-MW wind farm located 35 km offshore, amply serves to illustrate the emphasis on use of renewable and offshore energy solutions. Dc series and series-parallel wind farms have been proposed by researchers to facilitate HVdc connection with the onshore grid, eliminate a mandatory offshore platform for step-up conversion, and save costs compared to ac collection systems.
A techno-economic design method for the dc series and series-parallel wind farms is proposed in this dissertation. The method uses the calculation of losses, reliability and equipment costs for a base-case 300-MW wind farm located 150 km offshore to arrive at an optimal design. A dual active bridge (DAB) converter with soft switching capabilities and a nonlinear control system that minimizes reactive power in the high-frequency transformer (HFT) is proposed. The DAB converter and the control system models are derived, designed, and implemented. In order to study system operation, start-up and control, a system model of a 120-MW dc series-parallel wind farm located 150 km offshore is developed. The present work proposes two designs of the dc wind turbine, a). Without storage (Type-D1), and b). With integrated storage (Type-D2). The above-mentioned studies are conducted with the collection system voltage held constant at 70 kV. The system model includes the complete model of the HVdc converters and the grid interconnection. The system model is also used to investigate transient behaviors of various wind turbines, collection system topologies and transmission system during faults.
The optimal design of dc series-parallel wind farm uses six turbines in series. The optimal design has approximately 22% losses, average system availability of 80% and allows a simultaneous disconnection of three wind turbines in a stack. Due to the wind conditions the turbine output voltage is expected to vary significantly. With the use of the proposed DAB converter, the minimum conversion system efficiency is seen to be 92%. The system studies indicate simultaneous start-up to be used as the preferred strategy. It is also shown that system operation requires over-voltage and under-voltage limitation strategies. The system fault studies indicate that the pole to pole faults anywhere in the system are severe and may lead to loss of control of the turbine main converter.