###### Abstract  
Modular multilevel converter-high-voltage direct current (MMC-HVDC) systems are a key technology for integrating large-scale offshore wind farms due to their flexibility, controllability, and decoupled active and reactive power characteristics. However, offshore wind farms rely on power electronic converters, resulting in low inertia, which can worsen frequency fluctuations and affect system stability during major disturbances. Additionally, the decoupled power control of MMC-HVDC systems limits wind farms' inertia contribution to the AC grid, exacerbating inertia deficiency. To address this, a coordinated inertia support strategy is proposed, utilizing a DC voltage-frequency mapping method that enables wind farms to perceive frequency variations without communication and rapidly provide inertia response. This strategy coordinates wind farms and MMC-HVDC systems to enhance frequency support. Simulations demonstrate that the proposed strategy overcomes MMC-HVDC's decoupling effect, accelerates frequency recovery, and improves the inertia response speed, achieving faster power support and higher peak power output, thereby enhancing frequency stability. Furthermore, PSCAD/EMTDC simulations were conducted to analyze the transient characteristics of MMC-HVDC under AC-side faults, verifying that braking resistors (BRs) effectively suppress DC overvoltage, reducing wind farm power curtailment and improving system security. This study provides a new approach for frequency stability control in MMC-HVDC-based offshore wind integration and serves as a reference for further optimization of inertia support and fault protection strategies. MMC-HVDC; offshore wind farm; fault characteristics; grid forming; inertia support +
Footnote †: This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)]([https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/))). Footnote †: This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/by/4.0/](https://creativecommons.org/licenses/by/by/4.0/)]([https://creativecommons.org/licenses/by/by/4.0/](https://creativecommons.org/licenses/by/by/4.0/))). Footnote †: This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)]([https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/))). Footnote †: This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/by/4.0/](https://creativecommons.org/licenses/by/by/4.0/)]([https://creativecommons.org/licenses/by/by/4.0/](https://creativecommons.org/licenses/by/by/4.0/))). Footnote †: This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons. As wind farms expand and send electricity over longer distances, the transmission technology of wind power has also advanced. MMC-HVDC systems are a new generation of \"voltage source DC transmission technology\" with the highest flexibility and best controllability, which can improve the fault ride-through capability of new energy stations [2; 3; 4]. Because of their advantages, such as no commutation failure, active and reactive power decoupling, and stable operation, they play a significant role in improving the interaction between AC and DC and improving the stability of the power grid. They have become the main technical solution for large-scale offshore wind farms to be connected to the grid [5; 6; 7].