This paper proposes a post-fault control technique for H-bridge multilevel modular converters (MMC), which in combination with a restoration framework, can achieve fast operation recovery in multi-terminal direct current (MTDC) networks under different dc fault types. The studied network consists of four voltage-source converters (VSC) for high voltage dc (HVDC) transmission. A meshed MTDC grid topology is used for the connection of two asynchronous grids with two offshore wind farms (OWFs). The effect of different dc fault types on the grid restoration time is evaluated. Following the proposed restoration steps, the grid is able to restore its operation within 34 ms, if a pole-to-pole fault occurs. To accelerate the discharging process of the grid during a pole-to-ground fault, a control technique is proposed that allows momentarily the connection of the two dc pole cables through the MMC-VSC switch valves. In this way, the dc grid is discharged faster and thus, the operation can be faster restored. The stresses to which the switches of the submodules are subjected are also analysed in this paper. The study showed that, with the proposed strategy, an H-bridge MMC-based network can restore its operation after a pole-to-ground dc fault within 158 ms without the need for expensive dc breakers.@en

The aim of this paper is to present the current status of the offshore wind industry and to identify trends in Offshore Wind Projects (OWPs). This was accomplished via a thorough analysis of the key characteristics – commissioning country, installed capacity, number of turbines, water depth, project area, distance to shore, transmission technology and investment cost – of the commissioned and under construction European OWPs. Furthermore, the current status of the several countries outside of Europe was also investigated. The analysis revealed that the European offshore wind power grew on average 36.1% yearly since 2001. Currently, there are 7748 MW installed and 3198 MW under construction distributed among 76 OWPs situated in European waters. These projects are spread among ten countries, with the highest share of offshore projects belonging to the northern European countries. The UK has 46% of the total installed European offshore wind capacity with 26 projects, Germany ranks second with 16, while Denmark is third with 13 projects. These countries constitute 88% of the European offshore capacity. The analysis also showed that, although the installed capacity of the OWPs is growing, the projects׳ area is not increasing at the same pace due to the release of turbines with higher rated capacities which allow projects to increase their power nameplate without proportionally increasing the number of turbines. The average distance to shore and the water depth are both increasing throughout the years. Although the average investment cost per project is rising with the higher distances to shore and water depths, the multi-GW plans of the northern European and Asian countries indicate that the industry will continue to grow. The European Union targets of having 40 GW of offshore wind capacity deployed by 2020 in Europe and 150 GW by 2030 may represent plausible scenarios since the required growth is below the European.@en

There is a growing interest in the development of high-voltage multi-terminal direct current (MTDC) networks for integration of renewable energy sources, reinforcement of existing ac networks and feeding remote locations. Although many studies have investigated power flow control strategies for MTDC networks, few studies have looked into the possible sources of load flow errors. The aim of this paper is to analyze the effect of errors in the OPF operation of these future HVDC grids for three different sources: voltage measurement errors;variation of cable resistance with temperature and error in the prediction of the generated power from renewable sources. To perform the analysis a direct current OPF tool is used to compare two different control strategies, namely droop control and the distributed voltage control (DVC) strategy. The OPF object function is the minimization of losses in the MTDC network, which is based on the recently built 5-terminal HVDC grid in Zhoushan, China.

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In the future, floating wind turbines could be used to harvest energy in deep offshore areas where higher wind mean speeds are observed. Currently, several floating turbine concepts are being designed and tested in small scale projects; in particular, one concept allows the turbine to move after installation. This article presents a novel layout optimization framework for wind farms composed of moveable floating turbines. The proposed framework uses an evolutionary optimization strategy in a nested configuration which simultaneously optimizes the anchoring locations and the wind turbine position within the mooring lines for each individual wind direction. The results show that maximum energy production is obtained when moveable wind turbines are deployed in an optimized layout. In conclusion, the framework represents a new design optimization tool for future offshore wind farms composed of moveable floating turbines.

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