High voltage ac-transmission systems for grid-connection of offshore wind farms

This project aims to explore and compare two key technologies used to transmit electricity from offshore wind farms to the shore: HVAC (High Voltage Alternating Current) and HVDC (High Voltage Direct Current). The goal is to understand the practical limitations of HVAC, particularly over long distances, and identify how it compares to HVDC. Offshore wind farms are far from the coast, and transmitting the electricity they generate poses technical challenges that need careful analysis to ensure the system remains stable and reliable.

General Problem and Approach

Offshore wind power plants (OWPPs) generate electricity from wind turbines located far away from the shore, and this electricity needs to be transported over long distances to the onshore grid. HVAC is commonly used for shorter distances, but it may become less efficient or unstable over long distances, especially when compared to HVDC, which is better suited for long transmission distances. However, HVDC systems are more expensive and complex to implement. The project seeks to determine where HVAC reaches its limits and when it makes sense to switch to HVDC.

To investigate this, the project focuses on developing analytical methods to evaluate the stability of HVAC-connected offshore wind farms. The analysis covers different scenarios and operating conditions, including normal system operations and fault conditions. A key part of the study is to understand the root causes of harmonic instability. In simple terms, harmonic instability refers to unwanted fluctuations in the electrical system that can lead to power quality issues and even failures. By identifying the factors causing these instabilities, the project aims to improve the reliability of HVAC systems for offshore wind farms.

Ancillary Services and Energy Storage

Offshore wind farms are not only required to generate power but also provide ancillary services to the grid, such as voltage control or energy balancing. These services help keep the grid stable, especially when the wind farm is connected to a weaker grid or when other wind farms are nearby. The project will evaluate how much energy storage is required to provide these ancillary services effectively. This is critical because energy storage systems can help smooth out fluctuations in power generation, particularly in renewable energy systems like wind farms, where the output can vary depending on wind conditions.

Expected Outcomes

The project’s primary outcome will be a comprehensive guideline on the use of HVAC systems for offshore wind farms. It will outline the necessary hardware, control strategies, and stability requirements to extend the use of HVAC in these applications. Additionally, the project will compare HVAC solutions with HVDC to determine the technical break-even point between the two technologies. This comparison will help decision-makers understand when it is better to switch from HVAC to HVDC for long-distance power transmission.

In summary, the project will provide valuable insights into optimizing the transmission of electricity from offshore wind farms, ensuring that future renewable energy systems are both stable and efficient. By identifying key system parameters and control strategies, the project aims to enhance the reliability of offshore wind power plants, making them more viable for widespread use in energy grids.

Involved in the project: 

Anant Narula, Massimo Bongiorno, Mebtu Beza, Jan R. Svensson, Daniel Karlsson

Partners involved in the project: 

Hitachi Energy, Svenska kraftnät, DNV, Vattenfall

Funders involved in the project: 

Energimyndigheten, Hitachi Energy, Svenska kraftnät