Controle coordenado de um parque eólico offshore considerando um sistema de transmissão MMC-HVDC operando nos modos on-grid e off-grid

Abstract

This work presents a control strategy for the integration of an offshore wind farm to the onshore electrical grid through an HVDC transmission system based on modular multilevel converters. The proposed algorithm allows the operation of the HVDC system connected (on-grid) or isolated (off-grid) from the mains. Regardless of the operating mode, the MMC rectifier works as a grid-forming converter for wind generating units, imposing voltage to the collector network. During the grid-connected operation, the MMC inverter regulates the dc-link voltage while the generating units are controlled to track the maximum power of the wind turbines. On the other hand, in case of disconnection from the grid, the MMC inverter starts to regulate the ac voltage of the connection bus, maintaining the energy supply for the local loads. Meanwhile, the generator control stops to track the maximum power of the turbines and starts to regulate the dc-link voltage, avoiding an excessive injection of power in the HVDC link. In addition, the pitch angle control of the wind turbine is used to keep the generator speed at a fixed value. Mathematical models are developed for each mode of operation allowing the design of the converters’ controllers. All control loops were designed in the synchronous reference frame. Digital simulation results obtained with a model implemented in PSCAD/EMTDC are used to demonstrate the performance of the designed controllers and to validate the effectiveness of the proposed algorithm.