Exploring the technical limits of HVAC and HVDC for offshore wind transmission
Image: Fredrik Öhlander/Unsplash
High-voltage alternating current (HVAC) and high-voltage direct current (HVDC) are the two most widely used transmission technologies for offshore wind farms. HVAC is generally preferred for shorter distances due to its lower installation costs. In contrast, HVDC requires costly offshore converter platforms, making it less economical for shorter links.
However, as transmission distances increase, HVAC systems face several technical challenges. Long HVAC cables generate excessive reactive power, particularly at low operating points. This necessitates the installation of shunt reactors for compensation, as well as static synchronous compensators (STATCOMs) to enhance voltage stability and meet grid connection requirements. These additional components increase complexity and cost, making HVDC the preferred choice for longer transmission distances.
Evolved converter technology enhances appeal of HVAC systems
Recent advances in converter technology have made the use of onshore converters a more economically viable alternative, further enhancing the appeal of HVAC systems.
“As converter technology has evolved, we wanted to explore the technical limits of HVAC-connected offshore wind farms and compare them to HVDC solutions,” explains Anant Narula, research specialist at Chalmers University of Technology. “For the past two years, we’ve been developing analytical models to study both small-signal and large-signal stability in offshore wind farms.”
Efficient analytical models applicable to any power-electronics-dominated power system
The research is grounded in mathematical modeling, which offers an efficient method for evaluating system stability. While simulation tools such as electromagnetic transient (EMT) software are also effective, they are computationally intensive and time-consuming.
The study's findings demonstrate that the analytical models closely replicate the behavior observed in EMT simulations, making them reliable tools for identifying the break-even point between HVAC and HVDC technologies in offshore applications.
Initial small-signal analyses were carried out using classical dq-impedance-based models, but the research team later developed a novel method based on the Power Response Matrix.
This new approach offers several advantages:
- It enables detailed studies of control interactions and small-signal stability in power-electronics-dominated systems.
- It allows analysis of key properties for converter systems, such as inertia contribution, resonance damping, and other dynamic behaviors.
- It provides physical insight into system behavior rather than relying solely on abstract mathematical formulations.
- It supports the use of black-box models, which is particularly beneficial for modeling complex, multi-vendor systems where full model details may not be available.
“These analytical models are applicable to any power-electronics-dominated power system—not just the systems studied in this project,” says Anant Narula. “We’ve also aimed to make the analysis more physically intuitive, rather than purely mathematical, so it’s accessible and useful to practicing power systems researchers and engineers. This approach has been well received by our project partners.”
The research is part of the SESBC project, "High Voltage AC Transmission Systems for Grid Connection of Offshore Wind Farms", which concludes in September 2025. The project has already produced several publications and tools. Interested parties are encouraged to contact the research team for more information.
Published papers
Evaluation and Comparison of Small-Signal Characteristics of Grid-Forming Converter Systems in Two Different Reference Frames:
https://ieeexplore.ieee.org/abstract/document/10976625
Empowering offshore wind with ES-STATCOM for stability margin improvement and provision of grid-forming capabilities:
https://www.sciencedirect.com/science/article/pii/S0378779624006874
Power-Response Matrix-Based Modeling of Converter Systems for Small-Signal Analysis:
https://ieeexplore.ieee.org/abstract/document/10860906
Investigation of Control Parameters’ Impact on Damping Property of Grid-Forming Converters:
https://ieeexplore.ieee.org/abstract/document/10860863
External Inertia Emulation to Facilitate Active-Power Limitation in Grid-Forming Converters:
https://ieeexplore.ieee.org/abstract/document/10637739
Contact
Partners
Chalmers University of Technology, Hitachi Energy, Svenska kraftnät, DNV, Swedish Energy Agency