Grid-connected operation of direct-drive wave energy converter by using HVDC line and undersea storage system

Oscillatory nature of the ocean waves lead to intermittent power generated by the wave energy converters (WECs). As a result, WECs face a major barrier for grid integration. The recently introduced undersea storage system (USS) offers a viable solution to overcome this barrier by smoothing the WEC output power fluctuations. Another, but related, issue is that since the output voltage of WECs are relatively at the low-voltage range and they are deployed far away from the coast, the internal grid in wave farm is required to collect the produced energy by all WECs and deliver it to a high-voltage grid via a voltage step-up equipment. In this paper, a modular multi-level converter (MMC)-based isolated dc/dc converter is proposed for voltage step-up at the collecting point and transferring the harvested power to a high-voltage dc (HVDC) line. Simulation studies of a grid-connected direct-drive WEC along with the USS are carried out. The considered WEC is interfaced with the MMC-based converter by an ac/dc converter. The USS is employed to regulate the dc-link voltage of the MMC converter and WEC output power fluctuations. The regulated dc power is transferred through an HVDC line and converted to ac on the onshore cite.