Integration of wind power plants connected to HVDC via diode rectifiers

The use of offshore high-voltage direct-current (HVDC) terminals based on voltage source converters (VSCs) has made it possible to develop HVDC-connected offshore wind power plants (OWPPs) with the prevailing grid-following approach to the control of wind turbines (WTs). In such approach, WTs rely on other units (e.g. VSC-based offshore HVDC terminals) forming their alternating-current (AC) network, i.e. controlling the AC network voltage magnitude and frequency. That is the case of the German North Sea clusters, which represent the only HVDC-connected OWPPs to date. However, the
amount of HVDC-connected OWPPs is widely expected to increase, as the size and distance from shore of new OWPPs increase, and the associated costs decrease. In quest of lowering costs further, diode rectifiers (DRs) have been recently suggested as a viable alternative for connecting OWPPs to HVDC, prompting growing interest from both industry and academia. Since diodes are passive devices, however, such offshore HVDC terminals are inherently devoid of the grid-forming capability of VSCs. WTs have therefore been suggested as feasible candidates to take over such duty. This, however, entails fundamentally different WT controls, changing their control approach from that of grid-following units to that of grid-forming units. In this work, different grid-forming control schemes for the front-end (line-/grid-side) converters of type-4 (full-converter) WTs in DR-connected OWPPs are presented. The energisation of such OWPPs is then discussed, and a novel energisation method is proposed and demonstrated. Finally, the capability of such WTs and OWPPs to modulate their production so as to provide ancillary services to onshore AC networks is assessed. In doing so, the compatibility of corresponding plant-level controls and concepts devised for VSC-connected OWPPs is examined. The assessment includes the provision of frequency support (FS) and power oscillation damping (POD) using signals directly communicated to such OWPPs (communication-based). It also includes the proposal and demonstration of two methods for providing frequency support using signals estimated from local measurements (communication-less). The simulation results corroborate the feasibility of the proposed energisation method, which constitutes a reliable and robust alternative with minimal additional hardware. They also indicate that the DR connection technology does not impact the capability of OWPPs—in most of their operating range—to provide ancillary services to onshore AC networks by means of plant-level controls and concepts similar to those devised for VSCconnected OWPPs.