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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
Simon Catmull RES Offshore, United Kingdom
Co-authors:
Simon Catmull (1) F P Richard Chippendale (2) James Pilgrim (2) Gail Hutton (1) Priank Cangy (2)
(1) RES Offshore, Kings Langley, United Kingdom (2) University Of Southampton, Southampton, United Kingdom

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Poster
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Presenter's biography

Biographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited

Simon Catmull received the Bachelor’s degree in Mechatronics from the University of Sussex in 2003 and then went on to obtain a Master’s degree in Renewable Energy Systems and Technologies from Loughborough University in 2010. He has been employed by RES Offshore since 2010 and currently works as a Project Engineer supporting the development, construction and operation of wind farms on- and offshore.

Abstract

Cyclic load derivation for the rating of offshore wind farm cables

Introduction

High voltage AC subsea cables represent a significant proportion of the capital expenditure associated with offshore wind farms. As wind farms move further offshore the proportion of total capital expenditure becomes greater. Both raising the transmission voltage or moving to HVDC transmission of power offer the possibility of reducing the number of cables or minimising the conductor size of the cables needed to carry the rated output of a wind farm. An alternative approach is to exploit the variability of wind farm output and size the subsea cables according to the load that they are likely to experience.


Approach

Long term wind speed time series data were obtained for a hypothetical offshore wind farm from a meso-scale model of the UK wind climate. This was converted to a load time series for a generic offshore wind farm and subjected to simple statistical analyses and finite element modelling in order to derive an equivalent cyclic load profile for use within IEC60853: “Calculation of the cyclic and emergency current rating of cables”. The ratings obtained using the steady state and cyclic rating methodologies were compared and the implications upon the export system considered.




Main body of abstract

By its nature, the power generated by offshore wind farms varies substantially with time. Despite this fact, the cable systems which connect such wind farms back to the onshore grid are typically sized based on a maximum, continuous current rating. In order to develop techniques by which the sizing of such cable connections might be optimized, this paper presents a method for deriving equivalent cyclic load curves from realistic, time variant generation data. These cyclic load profiles may be used with the conventional IEC 60853-2 cable rating calculation, allowing the cable to be sized for a more realistic duty cycle. The example calculations presented show that this reduction in conservatism permits the use of smaller cable sizes, helping to drive down project costs. The implications of using such methods are discussed, highlighting the trade off between cost and conservatism.

Conclusion

Comparisons between the steady state and cyclic rating were shown for a thermally challenging location section of the subsea cable route for an offshore wind farm. The cyclic rating calculated using IEC60853 was shown to be in good agreement with results obtained through 2D FE analysis. The ampacity increase of 22% as a result of considering cyclic ratings over the traditional continuous rating method is sufficiently great that the target rating could be met with two export cables of conductor cross-sectional area of 1000mm2. This would remove the need to change the conductor size for all, or part, of the cable.


Learning objectives
The objective of this work was to determine a means by which the cyclic rating calculation method could be
applied to offshore wind farms for the purpose of reducing the capital expenditure costs associated with the export system.