The Japanese government has announced policies to achieve carbon neutrality by 2050 as a countermeasure to recent abnormal weather and global warming. To realize carbon neutrality, the power sector must maximize the installation of renewable energy sources.

Offshore wind farms are expected to be widely implemented since offshore locations offer better wind conditions than onshore sites. However, it is a significant challenge that regions with high potential for offshore wind farms are often far from power demand centers. A high-voltage DC transmission (HVDC) system is a promising transmission method for long-distance transmission.

To utilize the HVDC system to improve power system stability, this paper proposes providing a two-terminal HVDC system between an offshore wind farm and an onshore power grid, with a damping control function to mitigate power oscillation caused by onshore grid disturbances. Simulation results to confirm the proposed method are also reported.

The simulation model consists of an offshore wind farm, a two-terminal DC transmission system, a synchronous generator adjacent to the HVDC system, double-circuit AC transmission lines, and an infinite bus power source.

A power oscillation damping (POD) controller was added to the onshore terminal of the HVDC system to suppress a power swing caused by AC grid disturbances, such as a ground fault. In the POD controller, frequency fluctuations in the onshore grid are detected, and the detected fluctuations are modified by a phase compensator and added to the d-axis current command value for the AC current controller. The proposed control method supplies additional power from a DC capacitor of the onshore terminal to the onshore grid. The damping control method stabilizes the nearby synchronous generator by suppressing its frequency and power fluctuations.

Simulation studies were conducted using PSCAD V4.6 to verify the proposed control method. The simulation modelled the offshore wind farm as an equivalent three-phase current source that outputs constant power. Equivalent three-phase AC voltage sources modelled the AC/DC converters of the HVDC system. The rated power output of the wind farm, HVDC system, and synchronous generator are 500 MW, 500 MW, and 200 MW, respectively. The HVDC transmission distance is 100 km, and the rated DC voltage is 500 kV. The calculation time step was set to 20 microseconds.

The simulation conditions were set for a three-phase-to-ground fault on a circuit of the AC transmission line. The active power flow into the AC transmission line, the DC voltage and the active power output of the HVDC system were observed.

The simulation results showed that the HVDC system stabilizes the power flow into the AC transmission line by adjusting the active power output to suppress power oscillation caused by the grid disturbances.

The full paper will also propose a damping control method for power oscillation using the reactive power output of the onshore terminal of the HVDC system, and report the verification simulation results.