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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 ?