1. 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. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

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. 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

13. 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

14. 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

15. 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.

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. 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.