Operação de um parque eólico offshore com as unidades de geração conectadas em série com um sistema de transmissão VSC-HVDC

Abstract

In the most used transmission configurations for offshore wind farms, the generating units are connected in parallel, transmitting the energy to an offshore substation, where the voltage is increased and then transmitted to the coast by subsea cables, what can be done using alternate or direct current. A promising transmission configuration, but not used commercially or experimentally, have the generating units connected in series through the output terminals of their rectifiers. In this sense, it is possible to increase the voltage without the need of an offshore substation, which probably reduce the costs and increase the overall efficiency. In this work, the generating units are composed of a wind turbine, a permanent magnet synchronous generator, a three-phase rectifier and a dc solid state transformer, which has a full-bridge inverter, an air core transformer and a diode rectifier. The transformer is essential to ensure galvanic insulation between the generating unit components and the dc link. On the receiving end, a modular multilevel converter is used as an inverter, which process all the energy supplied by the wind farm and inject it into the mains. The first contribution of this work is the design of a dc solid state transformer, with proper dimensions to the installation in a wind turbine, using the FEMM software. Another contribution is a solution related to the modulation strategy of the modular multilevel converter. It is used to mitigate problems related to the unbalanced voltages on the positive and negative poles of the dc link, caused by the ground position. Finally, it is investigated the operation of a system with two or more sets of series connected units in parallel. In this configuration, there are overcurrent problems in the sets and overvoltage in the generating units. In order to mitigate these problems, it is proposed a control strategy based on the idea of virtual admittance. Simulations in the PSCAD/EMTDC software are used to verify the operation of the series-connected wind farm, as well as the proposed control strategies.