Optimized Grid Integration of Point Absorber Converters with Grid-forming Energy Storage System

Abstract

Wave energy is a promising form of renewable marine energy, offering higher energy density and greater predictability compared to other renewable sources like wind and solar. With an estimated potential of 70,356 TWh per year near global coastlines by 2024, it presents a significant opportunity for sustainable power generation. However, despite its significant potential, wave energy is still in a relatively early stage of development compared to more established renewable technologies like wind and solar power. Only a limited number of wave energy converters (WECs) have achieved successful grid integration. Challenges persist, particularly in providing adequate voltage and frequency support for offshore AC grids in applications such as isolated weak grids or long-distance WEC connections using diode-rectifier-based HVdc transmission lines. Additionally, the inherent variability of ocean waves leads to fluctuating power outputs, complicating grid integration. Integrating energy storage systems (ESS) has emerged as a potential solution to absorb and stabilize these oscillations.

This paper introduces a grid-forming control framework for a WEC-ESS system designed to stabilize offshore grid voltage and frequency while mitigating power fluctuations. A wave-to-wire model is developed using the open-source WEC-Sim tool, comprising three key components: a wave power capture system (point absorber WECs), an electromechanical energy conversion system (including the power take-off system and generator-side converter), and an electrical power regulation system (comprising the DC bus capacitor, grid-side converter, and ESS). This integrated model enables comprehensive simulation of the wave energy conversion process, offering valuable insights for performance analysis and optimization. The proposed framework employs a virtual synchronous generator (VSG) method for ESS control to enhance grid voltage and frequency support, while advanced WEC control strategies optimize energy capture efficiency. Furthermore, this study investigates two ESS configurations: decentralized ESS (DESS), where storage is distributed across individual WECs, and centralized ESS (CESS), where storage is located at the wave farm's terminal. Simulation results indicate that the WEC-ESS system with grid-forming control remains stable under voltage and frequency fluctuations in weak grids. Moreover, by leveraging the self-smoothing nature of power aggregation from multiple WECs, the CESS configuration requires a smaller ESS capacity while still maintaining grid stability and power quality. In contrast, the DESS system requires a larger ESS capacity, resulting in higher costs. This comparison highlights the potential for cost savings with centralized energy storage solutions in large-scale wave energy farms.