Frequency Control of Offshore Wind Farms with Series-connected Diode Rectifier Units-based HVDC Connection

This paper presents a direct frequency control for offshore wind farms (OWFs) connected to a high voltage direct current link (HVDC) through series-connected diode rectifier units (DRUs). The proposed frequency control principles allow the achievement of a controlled frequency within the OWF in which it is not needed to place the capacitor banks at the DRU stations. The frequency control principles are derived using a voltage source converter (VSC)-STATCOM placed at one of the DRU stations. The AC-voltage magnitudes are clamped by the DRUs, provided that the HVDC link voltage is imposed by the onshore inverter. In addition, the VSC-STATCOM is used for all the DRUs harmonic current compensation, resulting in passive AC-filters being removed from the DRU stations. The performance of the proposed control system is validated through a detailed switching model simulation using the operation of a delta-connected cascaded H-bridge STATCOM.