PROgress on Meshed HVDC Offshore Transmission Networks (PROMOTioN)

This PhD is part of one of Horizon 2020 Research Program’s biggest energy project: PROgress on Meshed HVDC Offshore Transmission Networks (PROMOTioN).

New goals concerning electric generation from renewable energy sources have motivated the proliferation of offshore wind power plants (offshore WPPs, OWPPs). Such projects require new investments to be made in electric power network infrastructure connecting OWPPs with onshore grids through submarine power ca- bles. These costly, long-term investments are needed to fully exploit Europe’s offshore wind resources, and have to be carefully planned, considering the reliability and robustness of the marine facilities.

Considered the most cost-effective cable-based solution for long-distance bulk electric power transmission to shore, high-voltage direct current (HVDC) is seen as the main offshore transmission technology. With the proliferation of offshore HVDC connections, meshed offshore grids (MOGs) are emerging as a re- sponse to the transmission systems’ needs for increased flexibility and reliability, but the high cost of power converter technology is, among others, hindering the deployment of HVDC-MOGs.

While line-commutated converters (LCCs) were the first solution to convert electric power between high- voltage, alternating current (HVAC) and HVDC, voltage-source converters (VSCs) have made the most pro- gress. Not requiring a voltage source on the offshore side and offering more controllability, VSCs have ena- bled the point-to-point connection of offshore WPPs to onshore AC networks. Moreover, they have become the preferred HVDC converter technology: the grid forming units, upon which most connection requirements are based. However, new control strategies taking advantage of the type 4 (full-converter) wind turbines’ own power converters have been recently shown to enable the use of diode rectifiers (DRs) for connecting WPPs to HVDC networks. Such solutions offer reduced offshore converter station size; reduced conduction losses; reduced investment, installation and maintenance costs, and increased reliability. However, they re- quire fundamentally different wind turbine (WT) and WPP controls.

As part of the Horizon 2020 Research Program’s current biggest energy project: PROgress on Meshed HVDC Offshore Transmission Networks (PROMOTioN), this PhD project will develop, analyse and test the interoperability of the WT and WPP controls with DRs to connect the OWPPs to HVDC networks. The project will focus on OWPPs comprised of type 4 WTs, connected via DRs to point-to-point HVDC transmis- sion networks with VSCs onshore (DR-VSC-HVDC). The project aims at contributing in the following research areas.

Operation and control of offshore AC networks

The operation and control of WPP-based offshore AC networks behind DRs will be studied for different op- erational scenarios, including start up and island operation. Different strategies for controlling the WT front- end VSCs of DR-VSC-HVDC-connected OWPPs will we considered.

Ancillary services

The contribution of DR-VSC-HVDC-connected OWPPs to the provision of ancillary services to onshore AC networks will be analysed, considering communication and control limitations. Focus will be given to fre- quency support and power oscillation damping (POD) services. The compatibility of corresponding higher- level controls devised for VSC-HVDC-connected OWPPs will be examined and alternatives to them will be explored.

Contact

Oscar Saborío-Romano
PhD student
DTU Wind Energy
+45 93 51 17 60