Controle desacoplado entre um sistema de transmissão VSC-HVDC e as turbinas de um parque eólico offshore em conexão CC série

Abstract

This dissertation presents a decoupled control strategy between a VSC-HVDC transmission system and the turbines of an offshore wind farm with generating units connected in series by the DC terminals of its converters. In this strategy, the control of the generating units is done independently of the inverter control of the onshore substation, used in the VSC-HVDC transmission. This strategy also has other advantages compared to other strategies proposed in the literature. In the configuration with generating units in DC series connection, unlike other configurations proposed in the literature, an offshore substation to increase the voltage to the transmission levels is not required. Despite the previous advantage, this type of system is not explored in the literature and it has not yet been used in any commercial project. Each generating unit comprises a wind turbine, a permanent magnet synchronous generator, and a neutral point diode clamped (NPC) voltage source converter, which control the currents delivered by the generators, thus regulating the power supplied to the transmission system. In addition, a third harmonic equalization and suppression strategy is used for capacitor voltages of NPC converters. In order to ensure safe operation, a voltage controller is used so that the DC voltages of the NPC converters do not exceed the maximum value. On the other side of the transmission system, the mains connection is made by a modular multilevel converter (MMC). This converter processes all the energy transmitted by the DC cables by controlling the current in the DC terminals through the control of the currents injected in the AC network. In addition, a circulating currents suppression strategy is used in the MMC. A voltage controller is also used in the MMC, but to ensure that the DC voltage does not exceed the minimum value. Simulations results obtained with the PSCAD/EMTDC program are used to verify the operation of the proposed control strategy for different operation conditions.