Offshore wind power generation has received much attention as a key technology for further implementing renewable energy sources to realise carbon neutrality by 2050. Offshore wind farms offer the advantage of large-scale deployment potential. However, since the offshore wind farms are located far from demand centers in most cases, a proper transmission method should be selected for long-distance high-power transmission.

Unlike AC transmission, HVDC (high-voltage DC) transmission consumes no reactive power in transmission lines and allows power delivery using cables with lower current ratings. HVDC transmission has an economic advantage, particularly for long-distance transmission. Submarine HVDC systems are a promising option for connecting offshore wind farms with onshore power grids.

While various activities have been conducted on power transmission from offshore wind farms to onshore power grids, a multi-terminal HVDC system effectively transfers power to multiple onshore demand areas.

This paper proposes a control method for sharing offshore wind power fluctuations between onshore terminals in a three-terminal bipole submarine HVDC system. The paper also reports the results of simulations to confirm the proposed control scheme. The amount of power sent from offshore wind farms and the power system stability of the onshore grids were considered in the simulations.

The simulations were conducted with PSCAD V5.0 to verify whether the prescribed reference values properly shared the wind farm power fluctuations. The offshore wind farm was modelled as an equivalent three-phase current source with constant power output. A detailed synchronous generator model, including an excitror controller and governor controller, was connected to each onshore AC grid to examine grid frequency variations.

The initial condition for the simulation assumed the offshore wind farm output power to be 500 MW. The received power references were 50 MW for onshore terminal 1 and 450 MW for onshore terminal 2. Then, the offshore wind farm output power was changed to 300 MW. Subsequently, the received power references for onshore terminals were updated to 30 MW and 270 MW, keeping the ratio of power reference for terminal 1 and that for terminal 2.

The simulation results confirmed that the proposed control method properly shared power between the onshore two terminals even when the wind farm output power rapidly changed. The results also showed that the grid frequency was kept within a range of 50 ± 0.2 Hz in the onshore grids 1 and 2.

Future work will involve more detailed modelling of offshore wind farm output characteristics and further validation of power sharing between the onshore two terminals.