New topology for more efficient AC/DC converters for future offshore wind farms

1. New topology for more efficient AC/DC converters for future offshore wind farms New topology for more efficient AC/DC converters for future offshore wind farms Stephan Meier, Staffan Norrga, Hans-Peter Nee Department of Electrical Engineering Division of Electrical Machines and Power Electronics Royal Institute of Technology Stockholm, Sweden Stephan Meier Electrical Machines and Power Electronics Norpie 04

2. New topology for more efficient AC/DC converters for future offshore wind farms Contents • VSC transmission • Adjustable speed wind turbine generators • New AC/DC converter topology Principle of operation Commutation principles Modulation algorithm Basic waveforms Advantages Challenges Stephan Meier Electrical Machines and Power Electronics Norpie 04 • Conclusions

3. New topology for more efficient AC/DC converters for future offshore wind farms VSC transmission - Principle • High Voltage Direct Current (HVDC) system based on Voltage Source Converters (VSC) Stephan Meier Electrical Machines and Power Electronics Norpie 04 • No capacitive cable charging currents • Provides isolation between the offshore installation and the mainland AC grid

4. DC cable can have any length required ffarm is independent of fgrid: Frequency control of the wind turbine generators Controllable active and reactive power flow New topology for more efficient AC/DC converters for future offshore wind farms VSC transmission - Advantages Stephan Meier Electrical Machines and Power Electronics Norpie 04

5. High costs for the complex converters New topology for more efficient AC/DC converters for future offshore wind farms VSC transmission - Disadvantages Stephan Meier Electrical Machines and Power Electronics Norpie 04 • High switching losses due to high-frequency PWM switching

6. New topology for more efficient AC/DC converters for future offshore wind farms Adjustable speed wind turbine generators • Low acoustic noise at low speed operation • Cut-in wind speed • Maximum output power • Constant output power • Cut-off wind speed Stephan Meier Electrical Machines and Power Electronics Norpie 04 • Improved system efficiency • Pitch control limits the power at rated conditions • Reduced mechanical stresses and improved power quality

7. New topology for more efficient AC/DC converters for future offshore wind farms HVDC based VSC systems • New AC/DC converter topology • Direct-In-Line ASG (full size back-to-back VSC) • Doubly-fed induction generator ASG (back-to-back VSC is rated at approx. 20 % of rated power allowing a wide speed range) • Individual direct HVDC connection Stephan Meier Electrical Machines and Power Electronics Norpie 04

8. Distribution grid: - MF AC bus Offshore platform: - Single-phase VSC • Main MF transformer • Main circuit breaker Installation in the wind turbine: • Line filter - MF transformer • Cycloconverter - Circuit breaker New topology for more efficient AC/DC converters for future offshore wind farms New AC/DC converter topology Stephan Meier Electrical Machines and Power Electronics Norpie 04

9. New topology for more efficient AC/DC converters for future offshore wind farms Features • Soft commutation is achieved for all semiconductor valves. • The number of VSC phase legs is reduced to one. • The cycloconverters enable variable speed operation of the wind turbines. Cheap and well-established fast thyristors are used. • Single-phase MF transformers Stephan Meier Electrical Machines and Power Electronics Norpie 04

10. New topology for more efficient AC/DC converters for future offshore wind farms Cycloconverter commutation Stephan Meier Electrical Machines and Power Electronics Norpie 04

11. New topology for more efficient AC/DC converters for future offshore wind farms VSC commutation Stephan Meier Electrical Machines and Power Electronics Norpie 04

12. New topology for more efficient AC/DC converters for future offshore wind farms Modulation Carrier-based modulation: • Maintain soft commutation • Ensure proper transformer operation: Constant VSC commutation intervals • Provide the desired PWM patterns for the cycloconverter Stephan Meier Electrical Machines and Power Electronics Norpie 04

13. New topology for more efficient AC/DC converters for future offshore wind farms Simulated waveforms at rated operation (i) Stephan Meier Electrical Machines and Power Electronics Norpie 04 Main transformer voltage and current

14. New topology for more efficient AC/DC converters for future offshore wind farms Simulated waveforms at rated operation (ii) Stephan Meier Electrical Machines and Power Electronics Norpie 04 Cycloconverter phase voltage and current

15. New topology for more efficient AC/DC converters for future offshore wind farms Advantages • Application of squirrel-cage induction generator. • Cheaper single-phase MF transformers with reduced weight and volume. • Significant reduction in series-connected IGBTs in the VSC. Stephan Meier Electrical Machines and Power Electronics Norpie 04 • Significant reduction of switching losses. • Application of comparably cheap and well-established fast thyristors in the cycloconverter.

16. New topology for more efficient AC/DC converters for future offshore wind farms Challenges • Design of the MF transformer: Transformer insulation has to withstand high voltage derivatives. • Effect of the square-wave MF voltage on the AC cables. • Design of an appropriate design system: - on the wind farm level - on the converter level Stephan Meier Electrical Machines and Power Electronics Norpie 04

17. New topology for more efficient AC/DC converters for future offshore wind farms Conclusions • The proposed AC/DC converter offers full adjustable speed operation of the wind turbines. • Low initial costs. • Reduced switching losses due to soft commutation. Stephan Meier Electrical Machines and Power Electronics Norpie 04 • The application of VSC transmission in the grid connection of wind farms becomes far more attractive.

18. New topology for more efficient AC/DC converters for future offshore wind farms Questions? Stephan Meier Electrical Machines and Power Electronics Norpie 04 Acknowledgment: The authors would like to express their gratitude to Vind-Forsk and the Swedish Energy Agency for financial support.