Bidirectional Synthetic Inertia of a High-Voltage Direct-Current Transmission System With Battery Energy Storage Support

The proliferation of renewable energy sources and high-voltage direct-current (HVDC) transmission is creating low-inertia scenarios in power systems, compromising stability and causing severe frequency excursions and high rates of change of frequency (ROCOF) during disturbances. Existing HVDC control strategies offer limited inertia emulation through DC voltage variation and lack efficient coordination for simultaneous frequency support in the interconnected AC grids. We propose a coordinated control strategy deploying bidirectional synthetic inertia using the HVDC link's DC capacitors, complemented by battery energy storage systems (BESS). The inertia contribution in each AC grid is dynamically decomposed into a weighted sum of active power from the local BESS, local DC capacitors, and the remote AC grid. This approach respects BESS state of charge (SOC) constraints to extend lifespan and mitigates DC voltage oscillations by incorporating remote AC grid support when voltage or SOC limits are exceeded. The strategy was experimentally evaluated using the control hardware-in-the-loop methodology. The results demonstrate, in general, satisfactory performance in maintaining a stable voltage in the HVDC link while reducing frequency transients in both AC grids. The evaluation of the overall bidirectional performance indicates that the strategy achieves an average reduction in maximum frequency excursion of 29% and maximum ROCOF of 32.5%, compared to the base case without inertial support. The overall performance is confirmed to improve significantly as the BESS resource becomes more available, validating the advantages of the proposed coordination.