1
Stephan Meier, Staffan Norrga, Hans-Peter Nee
Department of Electrical EngineeringDivision of Electrical Machines and Power Electronics
Royal Institute of TechnologyStockholm, Sweden
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
New topology for more efficient AC/DC converters for future offshore
wind farms
2
• VSC transmission
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
Contents
• Adjustable speed wind turbine generators
• New AC/DC converter topologyPrinciple of operationCommutation principlesModulation algorithmBasic waveformsAdvantagesChallenges
• Conclusions
3
• High Voltage Direct Current (HVDC) system based on Voltage Source Converters (VSC)
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
VSC transmission - Principle
• No capacitive cable charging currents
• Provides isolation between the offshore installation and the mainland AC grid
4
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
VSC transmission - Advantages
ffarm is independent of fgrid: Frequency control of the wind turbine generators
DC cable can have any length required
Controllable active and reactive power flow
5
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
VSC transmission - Disadvantages
• High switching losses due to high-frequency PWM switching
• High costs for the complex converters
6
• Low acoustic noise at low speed operation
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
Adjustable speed wind turbine generators
a) Cut-in wind speed
b) Maximum output power
c) Constant output power
d) Cut-off wind speed
• 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
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
HVDC based VSC systems
1) New AC/DC converter topology
2) Direct-In-Line ASG (full size back-to-back VSC)
3) Doubly-fed induction generator ASG (back-to-back VSC is rated at approx. 20 % of rated power allowing a wide speed range)
4) Individual direct HVDC connection
8
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
New AC/DC converter topology
Installation in the wind turbine:- Line filter - MF transformer- Cycloconverter - Circuit breaker
Offshoreplatform:- Single-
phase VSC- Main MF
transformer- Main circuit
breaker
Distribution grid: - MF AC bus
9
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
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
10
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
Cycloconverter commutation
11
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
VSC commutation
12
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
Modulation
Carrier-based modulation:
- Maintain soft commutation
- Ensure proper transformer operation: Constant VSC commutation intervals
- Provide the desired PWM patterns for the cycloconverter
13
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
Simulated waveforms at rated operation (i)
Main transformer voltage and current
14
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
Simulated waveforms at rated operation (ii)
Cycloconverter phase voltage and current
15
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
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.
• 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
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
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
17
New topology for more efficient AC/DC converters for future offshore wind farms
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
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.
• 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
Stephan Meier
Electrical Machines and Power Electronics
Norpie 04
Questions?
Acknowledgment:The authors would like to express their gratitude to Vind-Forsk and the Swedish Energy Agency for financial support.