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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
Gordon McFadzean TNEI Services Ltd, United Kingdom
Co-authors:
Gordon McFadzean (1) P Catherine Cleary (1) F Steve Dixon (1) Stephanie Hay (1)
(1) TNEI Services Ltd, Manchester, United Kingdom
Poster Award Winner

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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

Gordon is a lead member of the grid connection team at TNEI, advising clients on optimal strategies and liaising with network operators throughout the grid connection process. He has a strong academic background and significant project experience, particularly in power systems analysis and compliance studies for renewable developments. Gordon has been involved in many strategic projects, including a technical note on grid connections for Ofgem and a review of the charging arrangements in the UK offshore wind sector. Recently, he led the cost benefit modelling for TNEI’s work on medium voltage DC technology.

Abstract

The potential benefits of direct-to-shore mvdc connections for offshore wind

Introduction

Could Medium Voltage DC technology remove the need for a substation offshore? TNEI Services, in partnership with Scottish Power and Scottish Enterprise, are performing exploratory research into the prospective market for MVDC technology, via stakeholder engagement and technical case studies. This paper highlights the potential for cost reductions if a large offshore wind farm was to use direct to shore MVDC transmission at ±60kV. A cost benefit model has been prepared which considers capital and operational costs using the latest available information. The results of this modelling are supported by wider market analysis and engagement with industrial and academic stakeholders.

Approach

Five offshore array/transmission topologies were considered in this work. To facilitate a comparison between these topologies, a detailed cost-benefit analysis was undertaken. Capital costs and operational costs for the entire electrical system were estimated over a typical project’s lifetime. Operational costs, including the cost of lost revenue due to electrical losses and unavailability, were considered by calculating the net present value of each MWh of energy. This analysis included all the key components of the system including, cables, switchgear, transformers and onshore and offshore substations. Industry need was demonstrated through engagement with a wide range of stakeholders.

Main body of abstract

A 1000MW offshore wind farm was considered, between 50 and 200km offshore. The offshore topologies considered were:
1. Conventional 220kV HVAC transmission, 33kV AC arrays
2. Conventional ±320kV HVDC transmission, 33kV AC arrays
3. ±60kV MVDC transmission, ±60kV MVDC arrays
4. ±320kV HVDC transmission, ±60kV MVDC arrays
The results of the lifecycle cost comparison are:
• Direct MVDC topology is the most cost effective for any distances above 105km
• Capex accounts for less than half of the cost for the Direct MVDC topology
• Costs due to electrical and unavailability losses are very significant, dependent on the transmission distance
• At a distance of 150km the total electrical system cost (including opex and capex) is 3% lower than the next cheapest topology (conventional 220kV HVAC)
• Direct MVDC is also more cost effective than conventional HVDC for all distances below 200km.
The sensitivity of the results to a number of factors was also explored. Reducing the cost of energy from offshore wind made the direct MVDC case attractive over a greater range of distances, starting from 61km, whereas increasing the cost of energy had the opposite effect. Improvements in converter cost and availability had a similar impact on the results.
The implications of the results are discussed with input from academic, industry and supply chain stakeholders. For example, one prominent offshore wind developer has already considered MVDC technology but advised that testing & demonstration and a comprehensive cost benefit analysis are needed before MVDC can be viably considered.

Conclusion

Medium Voltage DC technology offers the potential to significantly reduce capital cost and project risk by removing the need for an offshore substation. A full lifecycle cost analysis has shown direct MVDC connection for offshore wind to be a cost competitive alternative to both 220kV AC and HVDC, at distances from ~100km offshore. The reduction in the capital cost of an MVDC solution provides savings of over 50% on the upfront cost for a 1000MW wind farm located 100km offshore. Project risk and unavailability is also favourably impacted as critical equipment is brought back onshore with the direct MVDC solution.


Learning objectives
After the presentation of this paper, attendees will be familiar with the proposed topology of a direct MV DC connection to shore and understand the lifecycle cost comparison process, including:
• Main differences in capital cost for an MVDC solution
• Impact on Losses
• Impact on Unavailability
• Industry appetite for MVDC technology