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

Contents VSC transmission Adjustable speed wind turbine generators 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

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

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

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

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

HVDC based VSC systems New AC/DC converter topology 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

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

Features Soft commutation is achieved for all semiconductor valves. 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

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

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

Modulation Carrier-based modulation: Maintain soft commutation 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

Simulated waveforms at rated operation (i) 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

Simulated waveforms at rated operation (ii) 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

Advantages Application of squirrel-cage induction generator. 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.

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

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.

Questions? Acknowledgment: 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.