Environmental and energy issues have become a concern worldwide. Power supply in remote island grids mainly depends on diesel generators, which require expensive fuel transportation from the mainland to the island and cause high CO2 emissions. Implementing renewable energy sources on a large scale on the island may solve its environmental and energy problems. However, renewable energy sources may make it challenging to maintain the power supply-demand balance and suppress grid frequency fluctuations.

A HVDC (high-voltage DC transmission) system connecting the mainland and the remote island is considered to solve these power supply-demand imbalance and frequency variation problems. The HVDC system is more economical than AC transmission for long-distance submarine cable transmission. The HVDC system has two converter terminals and a DC cable, allowing bidirectional power flow. When the renewable energy sources have a low power supply, power is sent from the mainland to the island. On the other hand, when the renewable energy sources supply excessive power, power is exported from the island to the mainland.

This paper proposes a control method for the HVDC system connecting the mainland to a remote island with many renewable energy sources. Load variations and fluctuations in the output of renewable energy sources cause a power supply-demand imbalance and frequency fluctuations in the island grid. The proposed control method, in coordination with the synchronous generator, is designed to maintain the power supply-demand balance and suppress frequency fluctuations in the island grid.

The proposed control method was verified by 24-hour-long simulations using Simulink CAD data running on a real-time simulator OPAL-RT (OP4510). The island grid had three types of power sources: a synchronous generator, a solar power plant and a wind power plant. An HVDC system interconnected the mainland and island grids. The synchronous generator was assumed to have a relatively small rated capacity. A sunny and occasionally cloudy weather condition was applied for simulations.

The proposed control method for the HVDC island-side terminal uses a constant-voltage and constant-frequency controller. A droop function between the power and frequency is provided for the controller. The frequency reference value is adjusted based on the power supply-demand imbalance in the island grid, which is measured by an energy management system on the island. This control method coordinates the operation of the HVDC system and synchronous generator to share the required power supply and stabilize the island grid frequency.

The simulation results confirmed that the proposed control method can maintain supply-demand balance and suppress frequency fluctuations for 24 hours, even when the output power of renewable energy sources fluctuates significantly. Thus, the proposed method is an effective solution for installing large-scale renewable energy sources.