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Conceptualization and Multi-Objective Optimization of the Electric System of an
Airborne Wind Turbine
J. W. Kolar et al.
Swiss Federal Institute of Technology (ETH) Zurich Power Electronic Systems Laboratory
www.pes.ee.ethz.ch
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Pareto-Optimal Design of Airborne Wind Turbine Power Electronics
J. W. Kolar, T. Friedli, F. Krismer, A. Looser, M. Schweizer, P. Steimer, J. Bevirt
Swiss Federal Institute of Technology (ETH) Zurich Power Electronic Systems Laboratory
www.pes.ee.ethz.ch
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Outline
► ETH Zurich ► Power Electronic Systems Laboratory (PES) ► Out-of-the-Box Wind Turbine Concepts
Pumping Power Kites Airborne Wind Turbines
► Feasibility of AWT Electrical System
Electrical System Structure Multi-Objective Optimization (Weight vs. Efficiency) Controls Aspects
► Summary
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Profile of ETH Zurich Dept. of ITET Power Electronic Systems Lab
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Departments of ETH Zurich
ARCH Architecture BAUG Civil, Environmental and Geomatics Eng. BIOL Biology BSSE Biosystems CHAB Chemistry and Applied Biosciences ERDW Earth Sciences GESS Humanities, Social and Political Sciences HEST Health Sciences, Technology INFK Computer Science ITET Information Technology and Electrical Eng. MATH Mathematics MATL Materials Science MAVT Mechanical and Process Engineering MTEC Management, Technology and Economy PHYS Physics USYS Environmental Systems Sciences
Students ETH in total
18’000 B.Sc.+M.Sc.-Students 3’900 Doctoral Students
21 Nobel Prizes 413 Professors 6240 T &R Staff
2 Campuses 136 Labs 35% Int. Students 90 Nationalities 36 Languages
150th Anniv. in 2005
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Research in EE @ D-ITET
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► Balance of Fundamental and Application Oriented Research
Power Semiconductors
Power Systems
Advanced Mechatronic
Systems
High Voltage Technology
Power Electronic Systems
High Power Electronics
Energy Research Cluster @ D-ITET
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► Balance of Fundamental and Application Oriented Research
Power Semiconductors
Power Systems
Mechatronic Actuator Systems
High Voltage Technology
Power Electronic Systems
High Power Electronics
Energy Research Cluster @ D-ITET
►
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DC-AC Converter
D. Bortis
Y. Lobsiger B. Wrzecionko
R. Burkart H. Uemura
Power Electronic Systems Laboratory Johann W. Kolar
AC-DC Converter
F. Vancu
R. Bosshard S. Schroth
Ch. Marxgut
Pulsed Power
T. Soeiro
DC-DC Converter
J. Mühlethaler
T. Andersen D. Boillat
U. Badstübner M. Kasper St. Waffler
G. Ortiz
Multi-Domain Modeling
Industry Relations R. Coccia / B. Seiler
AC-AC Converter
M. Schweizer N. Widmer
A. Müsing
F. Giezendanner I. Kovacevic
A. Stupar
Mega-Speed Drives
T. Baumgartner A. Looser A. Tüysüz
Magnetic Levitation
M. Steinert T. Reichert
B. Warberger C. Zingerli F. Zürcher
Secretariat M. Kohn
Administration P. Albrecht / P. Maurantonio
Computer Systems C. Stucki
Electronics Laboratory P. Seitz
28 Ph.D. Students 4 Post Docs
Power Electronic Systems Laboratory
Leading Univ. in Europe
F. Krismer
►
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Power Electronics
Cross-Departmental
Mechanical Eng., e.g. Turbomachinery, Robotics
Microsystems Medical Systems
Economics / Society
Actuators / EL. Machines
PES Research Scope
• Micro-Scale Energy Systems • Wearable Power • Exoskeletons / Artificial Muscles • Environmental Systems • Pulsed Power
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Industry Collaboration
• Automotive Systems • More-Electric Aircraft • Renewable Energy • Semiconductor Process Technology • Medical Systems • Industry Automation • Etc.
► 16 International Industry Partners
► Core Application Areas
PES Research Budget
2/3 Industry Share
Strategic Research
Industry Related Research
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General Research Approach
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► Mapping of Design Space into System Performance Space
Abstraction of Power Converter Design
Performance Space
Design Space
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Mathematical Modeling and Optimization of Converter Design
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► Sensitivity to Technology Advancements ► Trade-off Analysis
Technology Sensitivity Analysis Based on η-ρ-Pareto Front
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“Out-of-the-Box” Wind Turbine Concepts Power Kite & Ground-Based EE-Generation
Power Kite & On-Board EE-Generation
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Conventional 100kW Wind Turbine
► Characteristics
- Tower 35m/18 tons - Rotor 21m / 2.3tons - Nacelle 4.4 tons
■ Large Fraction of Mechanically Supporting Parts / High Costs
►
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► Helium or Hydrogen Inflated ► Magnus Effect - Additional Lift
Air Rotor Wind Generator
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Revolutionize Wind Power Generation Using Kites / Tethered Airfoils
■ Wing Tips / Highest Speed Regions are the Main Power Generating Parts of a Wind Turbine
[2] M. Loyd, 1980
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■ Sails up to 5000m2
■ Filled with Compressed Air
■ Adjustable Height of 500m ■ Autopilot Force Control ■ Sail Stored in Compact Form ■ 320m2 2MW Prop. Power
► 98% of All International Goods Carried via Sea
► 98% of All Cargo Vessels Powered by Diesel Engines
Support Ship Propulsion by Large Towing Kite
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Support Ship Propulsion by Large Towing Kite
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Support Ship Propulsion by Large Towing Kite
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Controlled Power Kites for Capturing Wind Power
■ Wing Tips / Highest Speed Regions are the Main Power Generating Parts of a Wind Turbine
► Replace Blades by Power Kites ► Minimum Base Foundation etc. Required ► Operative Height Adjustable to Wind Conditions M. Loyd, 1980
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► Wind at High Altitudes is Faster and More Consistent ► Operate Kites at High Altitudes or Even in the Jet Stream
0
.
0
0
.
2
0
.
4
0
.
6
0
.
8
1
.
0
1
.
2
1
.
4
1
.
6
1
.
8
2
.
0
120m
2 kW/m2 0
700m
Source:
Controlled Power Kites for Capturing Wind Power
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Controlled Power Kites for Capturing Wind Power
► Wind at High Altitudes is Faster and More Consistent ► Operate Kites at High Altitudes or Even in the Jet Stream
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Pumping Power Kites
Ground-Based EE-Generation
Source: M. Diehl / K.U. Leuven
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Basics of Power Kites
■ Generated Force Could be Converted into Useful Power by Pulling a Load / Driving Turbines via a Tether
► Kite´s Aerodynamic Surface Converts Wind Energy into Kite Motion
Source: M. Diehl / K.U. Leuven
►
►
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Pumping Power Kites
► Maximum Power
Source: M. Diehl / K.U. Leuven
M. Loyd, 1980
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Pumping Power Kites for Capturing High Altitude Wind Power
► Lower Electricity Production Costs than Current Wind Farms ► Generate up to 250 MW/km2, vs. the Current 3 MW/km2
► Research at the
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Pumping Power Kites for Capturing High Altitude Wind Power
Carousel Configuration
► Lower Electricity Production Costs than Current Wind Farms ► Generate up to 250 MW/km2, vs. the Current 3 MW/km2
► Research at the
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Offshore Pumping Power Kites
► Operated at Altitudes of 200…800m ► Conventional Offshore Location or Floating Platforms
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Offshore Pumping Power Kites
► Operated at Altitudes of 200…800m ► Conventional Offshore Location or Floating Platforms
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Airborne Wind Turbine
On-Board EE-Generation
Source: M. Diehl / K.U. Leuven
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Alternative Concept – Airborne Wind Turbine
► Power Kite Equipped with Turbine / Generator / Power Electronics ► Power Transmitted to Ground Electrically M. Loyd, 1980
►
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Alternative Concept – Airborne Wind Turbine
► Power Kite Equipped with Turbine / Generator / Power Electronics ► Power Transmitted to Ground Electrically
Source:
M. Loyd, 1980
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Basic Physics of Wind Turbines
► Maximum Achievable acc. to Lanchester / Betz ► High Crosswind Kite Speed Very Small Turbine Area
►
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Comparison of Conventional / Airborne Wind Turbine
■ Numerical Values Given for 100kW Rated Power
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SkyWindPower AWT Concept
► Tethered Rotorcraft – Quadrupole Rotor Arrangement ► Inclined Rotors Generate Lift & Force Rotation / Electricity Generation
Artist´s Drawing of 240kW / 10m Rotor System
■ Named as One of the 50 Top Inventions in 2008 by TIME Magazine
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► Reinforced Tether Transfers MV-Electricity to Ground ► Composite Tether also Provides Mechanical Connection to Ground
AWT Concept
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AWT Concept
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Demonstration Plan
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Flight Mode - Parked
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Future Prospects
■ Example for Thinking “Out-of-the-Box” !
Source: M. Diehl / K.U. Leuven
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Future Prospects Future Prospects
Source: M. Diehl / K.U. Leuven
■ Example for Thinking “Out-of-the-Box” !
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Technical Feasibility of AWT Electrical System ► AWT Electrical System Structure ► Multi-Objective Optimization (Weight vs. Efficiency) ► Controls Aspects
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AWT Basic Electrical System Structure
► Rated Power 100kW ► Operating Height 800…1000m ► Ambient Temp. 40°C ► Power Flow Motor & Generator
■ El. System Target Weight 100kg ■ Efficiency (incl. Tether) 90% ■ Turbine /Motor 2000/3000rpm
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Design of Electrical Power System
► Clarify Practical Feasibility of AWT Concept ► Clarify Weight/Efficiency Trade-off / Multi-Objective Optimization / PARETO-Front
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Tether Design DC Voltage Level η-γ-PARETO Front
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Tether DC Transmission Voltage Level
► Pth,1 = 100kW / lth = 1000m ► Strain Relief Core – Kevlar (Fth = 70kN, d=5mm) ► Cu or Al Helical Conductors - ½ Uth Isolated ► Outer Protection Jacket (3mm)
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► Tether Voltage Vth,1 = 8kV
Tether η-γ-PARETO Front
■ Total Weight of Tether: 320kg
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System Overview
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Possible AWT Electrical System Structures
►
►
► Low-Voltage or Medium-Voltage Generators / Power Electronics ► Decision Based on Weight/Efficiency/Complexity
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Generator / Motor Design Dimensions
Number of Pole Pairs η-γ-PARETO Front
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Generator / Motor η-γ-PARETO Front
► Medium Voltage vs. Low Voltage Machine Vth,1 = 8kV
■ LVG: Diameter 17cm (excl. Cooling Fins) / Width 6.0cm / p = 20 / η = 95.4% / Weight 5.1kg
- PMSM – Radial Flux – Internal Rotor - Slotted Stator / Concentrated Windings – Air Cooling - Analytical EM and Thermal Models for Weight / Efficiency Optimization - P = 16kW / 2000rpm
LV Machine HV Machine Thermal Model
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CAD Drawing of LV and MV Machine
► Fixed Parameters and Degrees of Freedom
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Generator / Motor η-γ-PARETO Front
► Selected Design
η = 95.4% γ = 3.1 kW/kg
■ Medium Voltage Machine Not Considered Further
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► Motors Employed for Electric Propulsion of Glider Airplane
Comparison to Commercial Motors
■ Diameter 22cm Width 8.6cm Weight 12kg Pole Pairs 10 Efficiency 91%
Power P = 10kW Speed n = 2200rpm Cooling vL = 25m/s
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System Overview
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Rectifier / Inverter Design Chip Area
Heatsink Volume η-γ-PARETO Front
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Rectifier / Inverter Design
► 2-Level or 3-Level Bidirectional Voltage Source Rectifier
■ Maximization of Heatsink Thermal Conductance / Weight (Volume) - Max. CSPI
- S = 19.3kVA - VDC = 750V - fS,min= 24kHz - TJ = 125°C - Foil Capacitor DC Link
1200V T&FS Si IGBT4s / 1200V SiC Diodes
600V T&FS Si IGBT3s / 600V Si EmCon3 Diodes
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VF [m3/s]
p
F [
N/m
2]
k . pFoperatingpoint
b/n
n = 5
s
d
c
b
PV
pCHANNEL
t
pF,MAX
VF,MAX
Heatsink Optimization
■ Highest Performance Fan ■ Fin Thickness / Channel Width Optimization
VF [m3/s]
p
F [
N/m
2]
k . pFoperatingpoint
b/n
n = 5
s
d
c
b
PV
pCHANNEL
t
pF,MAX
VF,MAX
► Maximize Thermal Conductance / Weight (Volume)
vAir ≈ 5m/s
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Heatsink Optimization
PV /n
Rth,d
Rth,a
Rth,A Rth,A
s
t
c/2
d
Rth,FINRth,FIN
TCHANNEL
VF [m3/s]
p
F [
N/m
2]
k . pFoperatingpoint
b/n
n = 5
s
d
c
b
PV
pCHANNEL
t
pF,MAX
VF,MAX
► Maximize Thermal Conductance / Weight (Volume)
■ Highest Performance Fan ■ Fin Thickness / Channel Width Optimization
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0 0.2 0.4 0.6 0.8 1k = s/(b/n)
0
0.2
0.4
0.6
0.8
1
Rth [
K/W
]
n=6
n=50
XX
n=10
n=34
optimum: n=26 / k=0.65 s=1.0mm / t=0.54mmRth,sub=0.26
sub-optimum: n=16 / k=0.60 s=1.5mm / t=1.0mmRth,sub=0.30
L x b x c= 80x40x40mm3
Al with th= 210W/Km
n = [6, 10, 14, ...., 42, 46, 50]
■ Highest Performance Fan ■ Fin Thickness / Channel Width Optimization
Heatsink Optimization
► Optimum
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Rectifier / Inverter η-γ-PARETO Front
► Selected Design
η = 98.5% γ = 19 kW/kg
■ 3-Level Topology Does Not Show a Benefit
- Switching Frequency Range 24…70 kHz - Heatsink Temperature Range 55…100 °C (Tamb = 40°C)
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System Overview
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8kVDC/750VDC DAB Converter Design Switches / Topology
Transformer η-γ-PARETO Front
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DC/DC Converter Topology
► Bidirectional Energy Transfer - Dual Active Bridge
0.8 kV 8 kV
■ Implementation of Electronic Switches - SiC
- Weight ≤ 25kg - fS = 50…125kHz fS,m = 100kHz - Phase-Shift Control (φ = π/4)
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DC/DC Converter Topology
0.8 kV 8 kV
■ Implementation of Electronic Switches - SiC
► 10kV Si/SiC SuperCascode Switch
- Weight ≤ 25kg - fS = 50…125kHz fS,m = 100kHz - Phase-Shift Control (φ = π/4)
► Bidirectional Energy Transfer - Dual Active Bridge
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Si/SiC Super Cascode Switch
HV-Switch Controllable via Si-MOSFET
* 1 LV Si MOSFET * 6 HV 1.7kV SiC JFETs * Avalanche Rated Diodes
Ultra Fast Switching Low Losses Parasitics
* Passive Elements for Simultaneous Turn-on and Turn-off * Stabilization of Turn-off State Voltage Distribution
Synchronous Switching
MOSFET
JFETs
C / R
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Si/SiC Super Cascode Switch
HV-Switch Controllable via Si-MOSFET
* 1 LV Si MOSFET * 6 HV 1.7kV SiC JFETs * Avalanche Rated Diodes
Ultra Fast Switching Low Losses Parasitics
* Passive Elements for Simultaneous Turn-on and Turn-off * Stabilization of Turn-off State Voltage Distribution
Synchronous Switching
MOSFET
JFETs
C / R
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Selected Multi-Cell Converter Topology
Pi = 6.25kW Vth,1,i = 2kV
► MV-Side Series-Connection / LV-Side Parallel-Connection
■ Winding Arrangement & Efficiency / Weight Optimization of Transformer
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Transformer Design
► MV-Winding Arranged Around Inductor Cores ► Cooling Provided by Heatpipes ► Stacked Cores - Scalable Arrangement
■ Optimization - Weight / Efficiency Trade-off
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Transformer Optimization
► Degrees of Freedom / Parameter Ranges
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Transformer η-γ-PARETO Front
► Selected Design
η = 97% γ = 4.5 kW/kg
■ Transformer Volt-Second Balancing - Series Capacitor or “Magnetic Ear” Control
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Transformer Volt-Second Balancing – “Magnetic Ear”
► Magnetic Ear Magnetized with 50% Duty Cycle Rectangular Voltage Winding ► Measured Aux. Current iaux / Voltage vm Indicates Flux Level ► Enables Closed-Loop Flux Control
N27 E55 Ferrite
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System Overview
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Overall System Consideration Total Weight
Overall Efficiency η-γ-PARETO Front
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Determination of Overall System Performance
► Consideration of the η-γ-Characteristics of the Partial Systems
■ Efficiencies of the Partial Systems Need to be Taken into Account ■ PD/PR = Overrating Ratio (8x16kW/100kW)
► Overall η-γ-Characteristic outP
m
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Overall System Performance
■ Final Step: System Control Consideration
►
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Electric System Control Stability
Reference Response Disturbance Response
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System Control
► Control of Flight Trajectory / Max. Energy Generation ► Generator (Motor) Speed / Torque Control ► etc. ► Control of DC Voltage Levels is Mandatory !
■ Simplified Control-Oriented Block Diagram of the Electric System
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Control Block Diagram
► Ground Station Controls the Tether Voltage ► Control Objectives: LV DC Bus 650…750V; MV (Tether) < 8kV
■ Only Tether Voltage at Ground Station is Measured (ITh Feedforward) ■ Motor AND Generator Operation Must be Considered
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Tether Voltage Control Plant ►
►
Motor Operation (100kW)
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Voltage Control Reference Step Response
■ Overshoot Could be Avoided with Reference Form Filter
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Voltage Control Disturbance Response
■ Motor Operation 100kW 0 ■ Gen. Operation ─100kW 0
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► AWTs are Basically Technically Feasible ► AWTs Realization Combines Numerous Challenges - Aircraft Design - MVDC Transmission - MV/HF Power Electronics - etc. ► AWTs are a Highly Interesting Example for η-γ Trade-off Studies ► AWTs are Examples for Smart Pico Grids or MEA Power System Analysis ► AWTs is a Clear Example of Thinking “Out-of-the-Box” !
Conclusions
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Questions ?
