Grid-Forming Offshore Wind Farm Integration Through LCC-Assisted Diode Rectifier

Abstract

A line commutated converter (LCC)-assisted DR (LCCA-DR) is proposed for the integration of offshore wind farms (OWFs). The LCCA-DR involves connecting a small-capacity LCC in series with the DR on the dc side. This configuration adds a new control freedom for the active power transmission of the high voltage direct current (HVdc) link, allowing for local power balancing control of OWFs. As a result, the wind turbine converters (WTCs) can be designed with classical P/f and Q/V-based grid forming (GFM) control schemes, functioning as virtual synchronous generators (VSGs). In conjunction with the VSG-controlled GFM-WTCs, the LCCA-DR acts as a virtual governor (VG) that effectively regulates the frequency of the offshore ac grid, enabling synchronized operation. The VSG-VG control framework facilitates power decoupling and provides advantages in terms of control performance and stability. Since the adjustment of the HVdc link's active power requires only minimal variation in the dc voltage, the LCC's proportion in the hybrid converter is very small. This ensures that costs, reactive power, harmonic filtering, and losses do not significantly increase compared to traditional pure-DR stations. The effectiveness of the proposed solution is verified through time-domain simulations.