Fault Handling Capabilities of Grid-Forming Wind Turbines in Offshore Wind Farms Connected With MMC HVDC System

Offshore wind power generation equipped with conventional grid following controls challenges the power system stability by reducing the inertia of the grid and weakening the AC offshore grid. To overcome this vulnerability a promising solution could be equipping the wind turbines with grid-forming (GFM) control schemes. Many methodologies have been presented in the literature for GFM inverters. In spite of this, their applications for WTGs have not been thoroughly explored. This paper presents the study of fault handling capability of wind turbines with several grid-forming control strategies. In this context, four different control schemes i.e. Visynch, P/f droop, Q/f droop and conventional grid following control are considered. The performance of these control strategies is assessed and compared during a three-phase symmetrical fault on the point of common coupling of an offshore wind farm. A case study which consists of two wind turbines connected to the main converter of an offshore substation equipped with grid-forming control is also considered. The results indicate that, during faults, each of the four control strategies (Visynch, P/f droop, Q/f droop, and Gfol) effectively limits the current and restores normal operation. A significant advantage of Q/F droop control is its reduced sensitivity to variations in system and fault parameters, adjustable frequency response, and independence from existing Phase-Locked Loops (PLLs). On the other hand, the P/f droop control provides greater voltage support during faults due to its voltage controllers. However, post-fault oscillations result from the stiff voltage controller. Visynch control, on the other hand, shows a lower margin of stability. The advantages and weaknesses of each control scheme is revealed using the simulation results.