National Renewable Energy Centre Chong Ng, Principal Engineer – Reliability & Validation

1. 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

2. Narec – Created by Government to stimulate the RE industry, A Controlled and Independent Testing Environment

3. Presentation Contents • Technical Paper Background • Existing Systems • HVAC transmission systems • HVDC systems • Proposed HVDC System • Selected Challenges • Conclusions • Next Steps

4. 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

5. 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…

6. 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

7. 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)

8. 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…)

9. 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

10. 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

11. 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…

12. 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

13. Thank you for listening! Narec Contact Details Website: www.narec.co.uk Technical Paper Authors: chong.ng@narec.co.uk paul.mckeever@narec.co.uk