AC Grid Forming by Coordinated Control of Offshore Wind Farm connected to Diode Rectifier based HVDC Link - Review and Assessment of Solutions

Diode Rectifier (DR) based HVDC transmission can significantly reduce system costs and foot print of an HVDC power station for Offshore Wind Farms (OWFs), but as a consequence, the control of the offshore AC grid becomes challenging. The replacement of the offshore Voltage Source Converter (VSC) by a passive DR leads to major research questions pertaining to the control of the offshore AC grid. To achieve the wind farm operation and seamless power extraction, many grid forming solutions have been devised. This paper reviews a few of the major control solutions for AC grid forming and operation of DR-HVDC based OWFs, following which a comparison is made between them. Then two of these solutions are selected and implemented in a study case OWF model. By using the simulation results and further analysis, the approaches adopted by these two solutions are elaborated further and the various challenges regarding the operation and control of the DR-HVDC based OWF are highlighted.