Til Kristian Vrana

Main grid frequency support strategy for VSC-HVDC connected wind farms with variable speed wind turbines

A wind farm (WF) connected by voltage source converter-based HVDC (VSC-HVDC) results in a completely ‘inertia-less’ in offshore AC grid . AC frequency is determined by control of the VSC-HVDC on the WF side, independently of the main grid frequency. Moreover, the rotational speed of a variable speed wind turbine is independent of the AC frequency of the offshore AC grid. These two aforementioned features result in ‘inertia-less’ system, that is absence of mechanical energy storage to accomodate for abrupt changes in generation and load. An ‘inertia-less’ system does not have a load-frequency response similar to traditional AC grid with inertia, hence making it difficult for the wind turbines to contribute to frequency support of the main grid. This paper develops an artificial coupling of the wind farm grid and the main grid frequencies, for main grid frequency support by the wind farm. Artificial frequency coupling is achieved by use of droop controllers for the DC voltage control. A case of study, using PSCAD, demonstrates the effectiveness of artificial frequency coupling for the main grid frequency support.

Review of HVDC component ratings: XLPE cables and VSC converters

Data on HVDC Technology has been collected, to give an overview on the available technology for building HVDC grids. The data include existing transmission schemes as well as ongoing projects until 2020, but ideas and concepts have not been included. Unlike some existing reviews of HVDC technology, the focus was on assessing HVDC grid components, rather than HVDC transmission projects. This is why cables and converters are treated separately, even though they might belong to the same HVDC link. The data indicate that VSC bipole systems of at least 2 GW rating should be available, even though no existing or ongoing project has such a high power rating.

Benefits of asymmetric HVDC links for large scale offshore wind integration

The large remote offshore wind clusters that are planned in the North Sea will most likely be connected with a meshed HVDC grid. Power will mostly flow from the offshore wind clusters to shore, creating asymmetrical requirements for the HVDC links that will consist of several parallel HVDC systems. To realise an asymmetrical link, some of those systems could be designed unidirectional, resulting in possible changes and simplifications (especially to the protection system). Assessment of a future scenario has shown that 42% of the HVDC systems can only be operated unidirectional. The remaining systems could in theory be used both directions, but power flow optimisation has shown, that this will in many cases not happen. A first cost calculation has shown that almost 6% of the investment cost can be saved when asymmetric design is implemented. This indicates the need to consider asymmetric design and to develop unidirectional HVDC systems.