This book provides a detailed study of the active control methods for large-scale offshore wind farms connected via flexible high-voltage direct current (VSC-HVDC) transmission systems. Firstly, it introduces the basic structure and fundamental control of offshore wind farms connected via VSC-HVDC systems, and proposes a vector modeling method for them. Furthermore, it analyzes the fault characteristics of offshore wind farms connected via VSC-HVDC systems under different fault conditions, and proposes an active fault suppression method based on energy control. Finally, it introduces the method of offshore wind farms connected via VSC-HVDC systems to support the grid frequency. From basic concepts to self-active safety control, and then to active support control of the grid, this book systematically introduces the active control methods of large-scale offshore wind farms connected via VSC-HVDC systems. In particular, it introduces some advanced control methods from the perspective of energy. This book is a useful reference for undergraduate and graduate students interested in offshore wind farms and VSC-HVDC, researchers studying fault ride-through and active frequency support of offshore wind farms connected via VSC-HVDC systems, as well as engineers.