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Generator Requirements and Functionality for Ocean Energy Converters

Explore the key requirements and functionality of generators used in ocean energy converters, including device-specific needs, mechanical robustness, corrosion protection, and grid connection codes.

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Generator Requirements and Functionality for Ocean Energy Converters

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  1. Generator Requirements and Functionality for Ocean Energy Converters Dr. Dara O’Sullivan Hydraulics & Maritime Research Centre, UCC 21st Oct 2010

  2. Overview • Evolution of generator technology • Common requirements of OE generators • Device specific requirements of OE generators • Generator functionality within OE devices

  3. Traditional Generators • Synchronous • Fixed speed • Fault current – 3-4 pu • Field regulated reactive power Turbine

  4. Technology Migration Asynchronous Driven by Wind Turbine Development Two-speed Variable Speed Variable Speed Gearless

  5. Technology Drivers • Energy Capture – MPPT • Mechanical Robustness – drive-train torque loads • Grid Connect Codes • Reliability

  6. What about OE Generators? • Are the issues the same as wind? • What are the differences? = ?

  7. Generator Requirements Device Specific Common Speed Range Environmental Torque Rating Grid Connect Custom or OTS

  8. Operation and Maintenance • High cost – minimise requirement! • Tidal – Expensive to perform at best • Wave – weather windows

  9. Brushed Generators • Typical brush life – 3-8k hrs • Twice yearly maintenance/replacement – corresponds with wind industry best practice • Brushes are considerably more inaccessible in many OE devices • Brush film formation disrupted in oscillatory current flows – significantly enhanced wear Brushed generators may be unsuitable for OE devices

  10. Corrosion • Saline air exposure • Not air conditioned generator enclosures • PMGs sensitive • Special coatings available e.g. VACCOAT

  11. Mechanical Design • Significant heave and pitch accelerations in many devices e.g. OWC: • Pitch: 7deg/s2 • Pitch: 4 m/s2 • 2x bearing rating due to gyroscopic loads • Robustness important

  12. Grid Connection • Voltage Limits • Voltage Distortion • Flicker • Reactive Power Control Requirements • Active Power Control Requirements • Fault Ride-Through

  13. Voltage Distortion LV HV MV Local Voltage Pelec t t 10s

  14. Generator Grid Connect Options Fully variable speed Reactive power control Controlled fault ride-through Limited variable speed Limited reactive power control Controlled fault ride-through ‘Fixed’ speed No reactive power control No fault ride-through Fixed speed Reactive power control Controlled fault ride-through

  15. Device Specific Requirements • Oscillating Water Column • Hydraulic Point Absorber • Overtopping Device with Hydro PTO • Direct Drive Point Absorber • Tidal Turbine • Tidal Oscillating Hydrofoil

  16. Oscillating Water Column • Air turbine with limited range of efficient operation • Variable speed • OTS design • High peak-to-average torque ratio

  17. Hydraulic Point Absorber • Generator design strongly linked to hydraulic circuit design • Fixed speed possible • OTS possible • Hydraulics can absorb torque pulsations Wavebob Prototype

  18. Overtopper Wave Dragon prototype • Conventional hydro turbines • High pole or gearbox coupled • Rating equals maximum mean power • Fixed speed possible

  19. Direct Drive Point Absorber • Direct drive linear or rotary generator • Custom design • Must have power electronics conversion • Low speed and high torque • High part load efficiency very important

  20. Tidal Devices • Tidal turbines • Marine equivalent of wind turbines • Higher gear ratios • Oscillating hydrofoils • Similar to hydraulic point absorbers

  21. Functionality • Determines how the generator sits within the power conversion chain • Power conversion • Prime mover efficiency optimisation • Power smoothing • Damping control

  22. Summary

  23. Thank YouQuestions?

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