Under the situation of global energy transformation and vigorous development of new energy, the proportion of offshore wind power in the amount of power generation from new energy resources has been increasing, and the research on offshore wind power related technologies has become increasingly important. For large-scale development of offshore wind power, on one hand, it is necessary to improve the grid connection reliability of the wind power itself. On the other hand, it is also necessary to develop new technical roads to improve the transmission efficiency, make full use of the transmission capacity of existing transmission lines or relieve the pressure on the construction of offshore transmission channels. Therefore, this paper will focus on offshore wind power, carrying out relevant research on offshore wind power and multi-terminal, low-frequency transmission technology based on modular multilevel matrix converter (M3C). This paper proposes feasible topologies and a switching control strategy for the multi-terminal, low-frequency transmission system, which achieve coordinated control among multiple M3Cs. A simulation model of the wind power multi-terminal, low-frequency transmission system is built on PSCAD/EMTDC simulation platform to verify correctness and effectiveness of the topology and the control strategy.

The modular multilevel matrix converter (M3C) based low frequency alternating current (LFAC) system is an attractive solution for the flexible interconnection in urban power grids. However, M3C stations show negative damping in certain frequency bands due to their fast control system dynamics. It is necessary to analyze the resonance stability when M3C stations are connected to the power grid. This paper aims at the resonance stability analysis of Hangzhou M3C based LFAC system. Firstly, port impedance models are built for both power frequency sides and low frequency sides by frequency domain scanning. Secondly, the resonance modes in the normal condition and some extreme conditions are calculated through the s-domain admittance matrix (s-DAM) method. The analysis indicate that all resonance modes have positive damping and no resonance instability risk exists in Hangzhou LFAC system.

Compared with HVAC or HVDC, low frequency AC (LFAC) technology demonstrates specific advantages on the offshore wind farm integration. Firstly, this paper discusses the selection principle of the upper limit of operating frequency of low-frequency system. Then the current along the submarine cable is calculated by MATLAB. According to the simulation results, the maximum transmission frequency of the cable line is determined as 20Hz. On this basis, taking into account the transmission capacity and difficulty of equipment manufacturing, 20Hz is finally selected as the optimal transmission frequency of offshore wind farm based on LFAC, which provides guidance for future designs of equipment and demonstration engineering.