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
Electrical Machines
and Power Electronics
Norpie 04
Stephan Meier, Staffan Norrga, Hans-Peter Nee
Department of Electrical Engineering
Division of Electrical Machines and Power Electronics
Royal Institute of Technology
Stockholm, Sweden
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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
2
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
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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
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
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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 costs for the complex converters
•
High switching losses due to highfrequency PWM switching
5
New topology for more efficient AC/DC converters for future offshore wind farms
Adjustable speed wind turbine generators
a) Cut-in wind speed
b) Maximum output power
c) Constant output power
d) 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
6
New topology for more efficient AC/DC converters for future offshore wind farms
HVDC based VSC systems
1) New AC/DC converter
topology
2) Direct-In-Line ASG (full
size back-to-back VSC)
Stephan Meier
Electrical Machines
and Power Electronics
Norpie 04
3) Doubly-fed induction
generator ASG (backto-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 topology for more efficient AC/DC converters for future offshore wind farms
New AC/DC converter topology
Distribution grid: - MF AC bus
Offshore
platform:
- Singlephase VSC
- Main MF
transformer
- Main circuit
breaker
Stephan Meier
Electrical Machines
and Power Electronics
Norpie 04
Installation in the wind turbine:
- Line filter
- MF transformer
- Cycloconverter - Circuit breaker
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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
9
New topology for more efficient AC/DC converters for future offshore wind farms
Cycloconverter commutation
Stephan Meier
Electrical Machines
and Power Electronics
Norpie 04
10
New topology for more efficient AC/DC converters for future offshore wind farms
VSC commutation
Stephan Meier
Electrical Machines
and Power Electronics
Norpie 04
11
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
12
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
13
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
14
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.
•
Significant reduction of switching losses.
•
Application of comparably cheap and well-established fast
thyristors in the cycloconverter.
Stephan Meier
Electrical Machines
and Power Electronics
Norpie 04
15
New topology for more efficient AC/DC converters for future offshore wind farms
Challenges
Stephan Meier
Electrical Machines
and Power Electronics
•
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
Norpie 04
16
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.
•
The application of VSC transmission in the grid connection
of wind farms becomes far more attractive.
Stephan Meier
Electrical Machines
and Power Electronics
Norpie 04
17
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.
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