national renewable energy centre chong ng, principal engineer – reliability & validation
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EWEA 2013 February, 2013, Vienna, Austria. OFFSHORE RENEWABLE PLANT HVDC POWER COLLECTOR AND DISTRIBUTOR. National Renewable Energy Centre Chong Ng, Principal Engineer – Reliability & Validation Paul McKeever, R&D Manager. - PowerPoint PPT PresentationTRANSCRIPT
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National Renewable Energy CentreChong Ng, Principal Engineer – Reliability & Validation
Paul McKeever, R&D Manager
OFFSHORE RENEWABLE PLANT HVDC POWER COLLECTOR AND DISTRIBUTOR
EWEA 2013February, 2013, Vienna, Austria
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Existing•50m blade test •Still water tank•Wave flume•Simulated seabed•Wind turbine training tower•Electrical and materials laboratories
New•3MW tidal turbine drive train - 2012•Offshore anemometry hub - 2012•100m blade test - 2012•15MW wind turbine drive train - 2013•99.9MW offshore wind demonstration site - 2013/14
Narec – Created by Government to stimulate the RE industry, A Controlled and Independent Testing Environment
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Presentation Contents
• Technical Paper Background• Existing Systems
– HVAC transmission systems– HVDC systems
• Proposed HVDC System• Selected Challenges• Conclusions• Next Steps
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Technical Paper Background• UK requires offshore wind to meet its renewable energy generation targets
(2020, 2030, 2050…) – UK Energy Bill … by 2020, 30% from Renewable Energy
• Likely to involve larger turbines (10MW? 20MW?) – FP6 UpWind Project
• Offshore plant would benefit from an appropriate power collection, transmission and distribution technology
– HVDC potentially provides better efficiency, particularly over longer distances– Benefits from power semiconductor and copper cost trends
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HVAC Transmission Systems• Commonly used in many offshore wind farms• Can suffer from excessive reactive current
– Increases cable losses– Reduces power transfer capability– Reactive power compensation required (extra equipment)
• Can suffer from high line losses and excessive voltage drops– Extra cables required– Inter-dependant characteristics need careful consideration
• Transmission voltage level, cable capacitance and charging currents…
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Existing HVDC Systems• Modern HVDC systems generally have advantages such as:
– Lower transmission losses – Fully controllable power flow – No reactive power generation or absorption (‘cable only’ connections) – Reduce/eliminate AC harmonic filter with the latest multilevel converter technologies (e.g. MMC HVDC)
• HVDC transmission systems can be categorised, by the converters used, into three categories:– Line-commutated Converters (LCC), Capacitor Commutated Converters (CCC) and Voltage Source Converters
(VSC) as illustrated below
• Point to point HVDC power transmission – Wind Farm Inter-array?
• What do we want?– A dedicated high efficiency, robust, flexible and low cost power collection, transmission and distribution
technology for use within the wind farm too
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Proposed HVDC System• HVDC power transmission from the point of generation
– Reduce losses and components (i.e. make use of Turbine MV converter and availability of HVDC gird)
• Multi-terminal HVDC system – Increase availability
• Offers flexibility and redundancy
– Reduce cost• Removal of/minimise offshore substation• Reduced cable losses (HV operation)
Grid
HVDC cable MVDC
HVDC
Converter*
GridTransformer
ac
dc
MVDC
HVDC
Hybrid HVDCTransformer
Converter*
Gear G
GeneratorMVDC
HVDC
acdc
Hybrid HVDCTransformer
Converter*
Gear G
GeneratorMVDC
HVDC
acdc
Hybrid HVDCTransformer DC link
capacitor
C1
HT1
HT2
C2
T2C1
HT1
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Proposed HVDC System• Hybrid HVDC Transformer (figure shows
simplified circuit):
– Steps up MVDC to HVDC
– Reduced voltage stress on primary side and current stress on secondary side allows use of “off the shelf” force commutation devices
– Uses magnetic transformer to avoid high conversion ratio
– Potential to require less power capability from switches (30%) when compared with conventional 2-level 3-phase HVDC converter
– Many potential challenges that need full investigation (e.g. switching control, network stability, economic impact, protection and isolation…)
HVDCMVDC
Number ofdevice, n = 4
HFtransformer
MV Module
HV Module
Total number of devices = 4n + 8
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Proposed HVDC System• Switching device comparison:
Proposed Hybrid HVDC Transformer vs. conventional HVDC converter (3-phase 2-level topology)
– Assumptions• n = number of series connected power
switching devices in half of the bridge arm• 6.5kV rated switching devices• VSC-based HVDC converters use 3-phase, 2
(or multi) level converter topology• Assumes 2 devices in series is sufficient to
withstand the MV voltage stress
– 150kVdc example• HVDC side needs n >= 30 devices in series• For conventional VSC-based HVDC systems
– 6n >= 180 devices• For hybrid HVDC transformer
– 4n + 8 >= 128 devices– 29% saving in power semiconductors used
HVDCMVDC
Number ofdevice, n = 4
HFtransformer
MV Module
HV Module
Total number of devices = 4n + 8
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Selected Challenges
• The time to implement– Dependent on development/readiness of the offshore wind industry
• Managing multi-vendor solutions– Will this be a problem?
• Practical implementation (i.e. is it realistic?)– Needs further investigation; this is still a concept
• Will the subsea power cable size increase with no centralised collector?– Shouldn’t increase for similar voltage levels; the overall power stays the same
• Would a platform still be required as a maintenance hub?– A mobile platform could be used for this purpose
• Is there an operational impact?– Turbine operation should be unaffected– System optimum operation and control needs developing
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Conclusions• Potential advantages for offshore wind farm applications
– An alternative to AC and point to point HVDC transmission topologies • Suitable installation in every single power source
– Increases flexibility and redundancy of the entire HVDC system• Positive impact on wind farm availability and O&M costs
– Eliminates/minimises the need for a centralised offshore collection platform
• Potential lower component count at converter level
• Modular component sets across the system– 100MW power block in centralised system vs. 20 x 5MW power blocks in hybrid
HVDC transformer system
• Increased component count at system level (due to de-centralisation)– Balanced by no offshore substation and fewer components, e.g. fewer
power semiconductors and filters…
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Next Steps• Investigate, in detail, the feasibility of this HVDC system
concept– Detailed study of the proposed hybrid HVDC transformer
• Explore the feasibility of the following advantages:– High flexibility leading to ‘independent’ turbines– Additional redundancy and high system availability (no centralised
substation)– High efficiency (power collection and O&M efficiency)– Cost reduction potential– Installation in individual turbines– Optimisation of materials (copper, semiconductor devices…)
• Investigate the use of SiC switching devices– Higher power density and heat tolerance
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Thank you for listening!
Narec Contact DetailsWebsite: www.narec.co.uk
Technical Paper Authors:[email protected]