A Novel Distributed Direct-Voltage Control Strategy for Grid Integration of Offshore Wind Energy Systems Through MTDC Network

Although HVDC transmission systems have been available since mid-1950s, almost all installations worldwide are point-to-point systems. In the past, the lower reliability and higher costs of power electronic converters, together with complex controls and need for fast telecommunication links, may have prevented the construction of multiterminal DC (MTDC) networks. The introduction of voltage-source converters for transmission purposes has renewed the interest in the development of supergrids for integration of remote renewable sources, such as offshore wind. The main focus of the present work is on the control and operation of MTDC networks for integration of offshore wind energy systems. After a brief introduction, this paper proposes a classification of MTDC networks. The most utilized control structures for VSC-HVDC are presented, since it is currently recognized as the best candidate for the development of supergrids, followed by a discussion of the merits and shortcomings of available DC voltage control methods. Subsequently, a novel control strategy-with distributed slack nodes-is proposed by means of a DC optimal power flow. The distributed voltage control (DVC) strategy is numerically illustrated by loss minimization in an MTDC network. Finally, dynamic simulations are performed to demonstrate the benefits of the DVC strategy.