1 stephan meier, staffan norrga, hans-peter nee department of electrical engineering division of...

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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

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

wind farms

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• VSC transmission


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Contents

• Adjustable speed wind turbine generators

• New AC/DC converter topologyPrinciple of operationCommutation principlesModulation algorithmBasic waveformsAdvantagesChallenges

• Conclusions

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• High Voltage Direct Current (HVDC) system based on Voltage Source Converters (VSC)


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VSC transmission - Principle

• No capacitive cable charging currents

• Provides isolation between the offshore installation and the mainland AC grid

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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

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VSC transmission - Disadvantages

• High switching losses due to high-frequency PWM switching

• High costs for the complex converters

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• Low acoustic noise at low speed operation


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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

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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

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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

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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

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Cycloconverter commutation

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VSC commutation

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Modulation

Carrier-based modulation:

- Maintain soft commutation

- Ensure proper transformer operation: Constant VSC commutation intervals

- Provide the desired PWM patterns for the cycloconverter

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Simulated waveforms at rated operation (i)

Main transformer voltage and current

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Simulated waveforms at rated operation (ii)

Cycloconverter phase voltage and current

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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.

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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

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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.

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Questions?

Acknowledgment:The authors would like to express their gratitude to Vind-Forsk and the Swedish Energy Agency for financial support.

1 stephan meier, staffan norrga, hans-peter nee department of electrical engineering division of...

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