A DC/DC converter is a vital component for offshore DC grids, in which a medium frequency transformer (MFT) is the main element. Therefore, it is necessary to optimise the core material and the design method so that the loss of the MFT would be reduced. In this study, the multi‐objective genetic algorithm is proposed, by which two MFTs with different core materials are both optimised. The magnetic flux density of the transformer core and the current density of the windings are introduced as the optimised variables. Also, the key equations for transformer design are put forward as the objective functions. Moreover, with constant iteration of the process of segmentation, parallel selection, mergence, recombination and variation, the Pareto optimal solution is determined. Furthermore, to verify the optimisation results, the temperature field and electric strength of the amorphous alloy MFT are calculated and simulated by finite‐element analysis. Finally, a MFT prototype with an amorphous alloy core is built, and the experiment is carried out.

The modular multilevel converter (MMC) is now the most attractive topology for medium and high voltage power conversion applications with several advantages over the traditional voltage source converter (VSC). However, due to a large number of sub-modules (SMs) in the MMC, system reliability is a big challenge in its practical application, where each SM may be considered as a potential point of failure. In this paper, a reliability evaluation based on the Markov model is proposed for the MMC. The failure rates of the power electronic devices and SMs are firstly analyzed. Then, the Markov model and the state transition equation of the system are built in detail. A general reliability evaluation function is established, in which the mean time to failure and reliability evaluation of the MMC with redundant SMs are also discussed. Finally, a practical direct current (DC) distribution example for reliability evaluation is analyzed, and the results verify that the reliability evaluation based on the Markov model could provide a useful reference for project design.

With the increasingly severe form of global fossil energy, to further address the excessive use of fossil energy and the environment pollution and other issues, wind power has become a hot research scholars in recent years. Offshore wind power has become an important source of energy for sustainable development in the future because of its advantages in land area, wind energy resources and stability, and low noise pollution. In recent years, the pure-DC wind farm based on DC collection-DC transmission has been subjected to a comparison. Considering the difficulty in control of a large number of converters, a distributed control strategy is proposed to ensure the voltage stability of DC collection bus for DC offshore wind power. Firstly, the key converter modular multilevel converter(MMC) is modeled and analyzed, and the calculation formulas of voltage and current inside MMC are obtained. Secondly, the voltage and current double loop control is designed for the collection converter, and the power coordination control is adopted in the outer loop to ensure the consistent output power between individual wind turbines. To verify it, a multi-typhoon multi-condition collection simulation model is built in MATLAB/ SIMULINK. The results show that the proposed control strategy of offshore wind power DC collection multi-link converter could effectively maintain the DC bus voltage stability.