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Conference programme 

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Poster session

Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Lars Martin Hytten DNV GL, The Netherlands
Co-authors:
Cornelis Plet (1) F P Lars Hytten (1) Ben Hendriks (1) Wolfgang Ebigt (1) Mischa Vermeer (1)
(1) DNV GL Energy, Arnhem, The Netherlands

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Poster
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Abstract

Power frequency optimisation for offshore wind farms

Introduction

Future offshore wind farm cost savings can be realized through an integrated design approach in which a wind farm is designed as one system rather than the sum of standardised components. This requires a thorough understanding of the way in which technological changes affect costs. This paper presents a robust cost model which relates the levelized cost of energy to the wind farm’s export and/or array power frequency. A case study is presented which illustrates how a deviation from standardised power frequencies can lower costs, increase export distance and enable novel drive train concepts with improved reliability and efficiency.

Approach

A comprehensive and verified levelized cost of energy model is adapted to include the effects of a change in power frequency on electrical components, support structures, installation techniques and energy production of offshore wind farms. This model is used to compute the levelized cost of energy for a number of different benchmark wind farm typologies over a range of power frequencies to find the frequency at which return on investment is maximised. In addition, novel technological concepts enabled by a change of power frequency are studied. The analysis is completed by charting the risk landscape associated with introducing unqualified technologies.

Main body of abstract

This presentation focuses on using an offshore wind farm’s power frequency as a design parameter which can be changed to realize cost benefits rather than it being a given constant. The use of frequency converter stations to connect distant offshore wind farms allows the power frequency of the wind farm’s export and/or array circuits to deviate from the standardised 50 Hz. A change in power frequency has a direct effect on the required weight of magnetic components as well as power losses in all electrical components. In a chain-reaction, structural requirements for support structures and lifting capability requirements of installation vessels are affected. By including these effects into a robust cost model, it is possible to find the power frequency which maximises return on investment.

In case of an HVDC export circuit, the offshore converter station determines the frequency of the array circuits. Here the optimal frequency is higher than 50 Hz since cost savings are mostly realized by reducing the weight of the offshore AC substation. Using a back-to-back onshore converter station, the power frequency of the export circuits and the array circuits can be determined. In this case the optimal frequency is below 50 Hz in order to minimise reactive power flow in the export circuit. The lack of an offshore converter station improves reliability. Using a lower frequency has the added benefit that losses are lower and it enables a novel DFIG based drivetrain concept by reducing the required gear ratio, improving both efficiency and reliability.


Conclusion

The presented analysis shows that significant gains in terms of cost, efficiency and reliability can be realized by utilizing a non-standardised power frequency based on the offshore wind farm’s characteristic. The implementation of the suggested approach is possible with existing technology. This example of an integrated design approach illustrates that cost reductions in offshore wind energy exist in viewing the wind farm as one system rather than the sum of individual components. In addition, the analysis shows that offshore applications require specialised offshore technologies, rather than adapted onshore technology.


Learning objectives
The analysis will provide wind farm developers and designers with a tool to optimise offshore wind farm performance. It provides insight into the role of power frequency in the cost model for offshore wind energy, enabling the user to make well-informed choices about the possibilities of using a non-standardised or a standardised but non-50 Hz frequency such as 16,7 Hz and 25 Hz.