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

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

Presentation Contents

• Technical Paper Background• Existing Systems

– HVAC transmission systems– HVDC systems

• Proposed HVDC System• Selected Challenges• Conclusions• Next Steps

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

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…

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

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

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

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

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

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…

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

Thank you for listening!

Narec Contact DetailsWebsite: www.narec.co.uk

Technical Paper Authors:chong.ng@narec.co.uk

paul.mckeever@narec.co.uk