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Cryogenic Power Conversion Systems

Kaushik Rajashekara

The University of Texas at Dallas

Richardson, TX

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E-mail: K.RAJA@UTDALLAS.EDU

Contents

• Introduction to power devices and power converters

• Cryogenic power electronics: • Behavior of devices and converters at cryogenic temperatures

• Properties, advantages

• Cryogenic cooling systems

•Cryogenic Power Electronics Applications

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INTRODUCTION TO POWER ELECTRONICS SYSTEMS

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WHAT IS POWER ELECTRONICS?• Conversion and control of electrical power by power semiconductor devices

• Definition: To convert i.e. to process and control the flow of electric power by supplying voltages and currents in a form that is optimally suited for user loads.

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MODES OF CONVERSION

RECTIFICATION AC – to – DC

INVERSION DC – to – AC

CYCLOCONVERSION AC – to – AC (Frequency Changer)

AC CONTROL AC – to – AC (Same frequency)

DC CONTROL DC – to - DC (Choppers)

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Basic block diagram

•Building Blocks:– Input Power, Output Power

– Power Processor

– Controller

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Power Processor

Controller

Load

measurement

reference

POWERINPUT

POWEROUTPUT

vi , ii vo , io

Source

ADVANTAGES OF POWER ELECTRONICS SYSTEM

• To convert electrical energy from one form to another, i.e. from the source to load with:

• Highest efficiency

• Highest availability

• Highest reliability

• Lowest cost

• Smallest size

• Least weight

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APPLICATIONS

• Transportation – EV/HV, subway, locomotives, elevators

• Home appliances – blender, mixer, drill, washing machine

• Paper and textile mills

• Wind power generation

• Air conditioners and heat pumps

• Rolling and cement mills

• Machine tools and robotics

• Pumps and compressors

• Ship propulsion

• Computers and peripherals

• Solid state starter for machines

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Role of Power Electronics

Static applications- Power Supply

• Involves non-rotating or moving mechanical components• Examples:

• DC Power supply, Un-interruptible power supply, Power generation and transmission (HVDC), Electroplating, Welding, Heating, Cooling, Electronic ballast

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Static Application: DC Power Supply

FILTER LOADDC-DCCONVERTER

DIODERECTIFIER

AC voltage

AC LINEVOLTAGE(1 or 3 )F F

Vcontrol

(derived fromfeedback circuit)

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Drive applications

• Intimately contains moving or rotating components such as motors.• Examples:

• Electric trains, Electric vehicles, Air-conditioning System, Pumps, Compressor, Conveyer Belt (Factory automation)

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SystemController

PowerElectronicsConverter

Motor Airconditioner

Power Source

BuildingCooling

Desiredtemperature

Indoorsensors

Indoor temperatureand humidity

Temperature andhumidity

Desiredhumidity

Variable speed drive

Drive Application: Air-Conditioning System

POWER CONVERSION CONCEPT: EXAMPLE

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• Supply :

50Hz, 240V RMS (340V peak). Customer need DC voltage for welding purpose, say.

• Sine-wave supply gives zero DC component!

• We can use simple half-wave rectifier. A fixed DC voltage is now obtained. This is a simple PE system.

Vo

time

Vdc

+Vs

_

m

oV

V

:tageoutput vol Average

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CONVERSION CONCEPT

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+vo

_

+vs

_

igia

1

sin 1 cos2 2

mo m

VV V t d t

a

w w a

• What if customer wants variable DC voltage? More complex circuit using SCR is required

• By controlling the firing angle, a, the output DC voltage (after conversion) can be varied

• Obviously this needs a complicated electronic system to set the firing current pulses for the SCR

wt

vo

a

ig

wt

wt

v s

Circuit Diagram

WaveformAverage output voltage

POWER ELECTRONICS IN ENERGY SAVING

ENERGY SCENARIO

• Need to reduce dependence on fossil fuel

• Tap renewable energy resources

• About 60% - 65% of generated energy is consumed in electrical machines –mainly pumps and fans

• Variable speed control of electric machines can improve efficiency by 30% at light load. Light load reduced flux machine operation can further improve efficiency

• Variable speed air-conditioner/heat pump can save energy by 30%

• About 20% of generated energy is used in lighting. High frequency fluorescent lamps are 2-3 times more efficient than incandescent lamps

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GROWTH OF POWER ELECTRONICS

• The rapid growth of PE is due to:

• Advances in power (semiconductor) switches

• Advances in microelectronics (DSP, VLSI, microprocessor/microcontroller, ASIC)

• New ideas in control algorithms

• Demand for new applications

• PE is an interdisciplinary field:• Digital/analogue electronics

• Power and energy

• Microelectronics

• Control system

• Computer, simulation and software

• Solid-state physics and devices

• Packaging

• Heat transfer

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Power Semiconductor Devices

•Diode

•Thyristor

•Triac

•Gate Turn-Off Thyristor (GTO)

•Bipolar Power Junction Transistor (BJT)

•Power MOSFET

• Insulated Gate Bipolar Transistor (IGBT)

•Silicon carbide Devices

•Gallium Nitride Devices

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POWER SEMICONDUCTOR DEVICES

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• Power semiconductor devices represent the heart of modern power electronics, with two major desirable characteristics guiding their development:

Switching speed (turn-on and turn-off times) Power handling capabilities (voltage-blocking and current-carrying capabilities)

• Power devices operate in two states: Fully on i.e. switch closed: Conducting state Fully off i.e. switch opened: Blocking state

• Power switch never operates in linear mode

POWER SWITCHSWITCH ON (fully closed)

Vin

Vswitch

= 0

I

SWITCH OFF (fully opened)

Vin

Vswitch= Vin

I=0

• The power semiconductor devices are operated as high speed switches

• When a switch is turned on, it offers a very small resistance (ideally zero). When a switch is turned off, it offers a very high resistance (ideally infinity)

• These switches should have the ability to turn on and off ideally in almost zero time. Practical devices offer characteristics which differ from the ideal characteristics

• There are a number of power devices which have been developed over the years and are capable of operating at high voltages (up to 10 kV) and high currents (up to 5kA)

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POWER SEMICONDUCTOR DEVICES

UNCONTROLLED

EX: DIODE

CONTROLLED

EX: THYRISTOR, BJT, MOSFET, IGBT etc.

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IDEAL CHARACTERSTICS OF POWER DEVICES

• In the on-state when the switch is ON, it must have

• The ability to carry a high forward current tending to infinity

• A low on state forward voltage drop tending to zero

• A low on-state resistance tending to 0

• In the OFF state, it must have

• The ability to withstand a high forward or reverse voltage tending to infinity

• A low-state leakage current tending to zero

• A high off-state resistance tending to infinity

• During the turn-on and turn-off process, it must be completely turned on and off instantaneously so that the device can be operated at high frequencies. Thus it must have

• Low delay time tending to 0

• Low rise time tending to 0

• Low storage time tending to 0

• Low fall time tending to 0

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• For turn on and turn off it must require A low gate drive power tending to 0 A low gate drive voltage tending to 0 A low gate drive current tending to 0

• Both turn-on and turn-off must be controllable. Thus, it must turn on with a gate signal and must turn off with another gate signal

• For turn-on and turn-off, it should require a pulse signal only, that is, a small pulse with a very small width tending to 0

• It must have a high dv/dt tending to infinity (switch must be capable of handling rapid changes in voltages across it)

• It must have high di/dt tending to infinity (switch must be capable of handling rapid changes in current through it)

• Ability to sustain any fault current for a long time is needed• It requires very low thermal impedance from the internal junction to the

ambient, tending to 0 so that it can transmit heat to ambient easily

IDEAL CHARACTERSTICS OF POWER DEVICES

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POWER SWITCHES (From Powerex Inc.)

• Power Diodes

– Stud type

– “Hockey-puck” type

• IGBT

– Module type: Full bridge and three phase

• IGCT

– Integrated with its driver

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Thyristor

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• Static characteristics of thyristor: 4 layer device Blocking (off) when reverse biased,

even if there is gate current applied Conducting only when forward biased

and there is triggering current applied to the gate

Once triggered on, will be latched and continue to conduct even when the gate current is no longer applied

• If the forward break over voltage (Vbo) is exceeded, the SCR “self-triggers” into the conducting state

• The presence of gate current will reduce Vbo• “Normal” conditions for thyristors to turn on:

– the device is in forward blocking state (i.e. Vak is positive)

– a positive gate current (Ig) is applied at the gate

• Once conducting, the anode current is latched• Vak collapses to normal forward volt-drop,

typically 1.5-3V• In reverse -biased mode, the SCR behaves

like a diode

v-i characteristics

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Power MOSFET Features

• Voltage controlled majority carrier device

• Asymmetric blocking

• High conduction voltage drop

• Low switching loss

• Slow recovery time of body diode

• Easy device paralleling

• Low voltage, low power, high frequency switching applications

• Used in chopper, voltage fed inverter in SMPS, automobile power electronics, solid state relay, etc.

• The main advantage of a MOSFET over a regular transistor is that it requires very little current to turn on (less than 1mA), while delivering a much higher current to a load

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MOSFET CHARACTERISTICS

• Because of the positive temperature coefficient, the devices can be paralleled easily for higher current capability.

• Internal (dynamic) resistance between drain and source during on state, RDS(ON), limits the power handling capability of MOSFET.

• High losses especially for high voltage device due to RDS(ON) .

• Dominant in high frequency application (>100kHz). Biggest application is in switched-mode power supplies.

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Insulated Gate Bipolar Transistor (IGBT)• An insulated-gate bipolar transistor (IGBT) is a

three-terminal power semiconductor device primarily used as an electronic switch which combines high efficiency and fast switching

• The IGBT combines the simple gate-drive characteristics of MOSFETs with the high-current and low-saturation-voltage capability of bipolar transistors.

• Combination of BJT and MOSFET characteristics:

Gate behavior similar to MOSFET - easy to turn on and off.

Low losses like BJT due to low on-state Collector-Emitter voltage (2-3V)

Switching frequency up to 100KHz Typical applications: 20-50KHz

24IGBT_3300V_1200A_MITSUBISHI

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The general IV characteristic of an IGBT.

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IGBT Features

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POWER SWITCHES: POWER RATINGS

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10Hz 1kHz 1MHz100kHz 10MHz

1kW

100kW

10kW

10MW

1MW

10MW

1GW

100W

MOSFET

IGBT

GTO/IGCT

Thyristor

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HEAT REMOVAL MECHANISM

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SCR (STUD-TYPE) ON AIR-COOLED KITS

FIN-TYPE HEAT SINK SCR (HOKEY-PUCK-TYPE) ON POWER PAK KITS

ASSEMBLY OF POWER CONVERTERS

Why Silicon is not suitable for High temperature operation

• Silicon (Si) is the common and most widely used semiconductor material for power devices.

• Silicon is efficient for medium power and medium temperature applications.

• It has lower band gap energy.

Band Gap Energy: The Energy required to raise the electrons from valence band to the

conduction band, which is the primary limitation of Si-based devices during high

temperature operation.

• Maximum operating junction temperature of

Silicon power devices is less than 150oC.

• Lower Thermal conductivity.

• Lower Melting Point.

• Lower Break down field.

• Lower Electric field Strength.

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Wide Band Gap Semiconductors

• Wide band gap semiconductor - power electronic components

have high current density, faster, and more efficient than silicon

(Si)-based devices.

• They have lower on-resistances (Ron) and hence lower

conduction losses [SiC MOSFETS]

• They operate efficiently at much higher temperatures, voltages,

and switching frequencies.

• These materials are significantly more powerful and energy

efficient than those made from conventional semiconductor

materials

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Wide Band Gap Semiconductors

• Handle voltages 10 times higher than Silicon.

• Operates at temperatures over 300°C.

• Operates at frequencies 10 times higher than silicon.

• Higher breakdown voltages.

• Large band gap

• High carrier mobility

• High electrical conductivity

• High thermal conductivity

Result:

• High power capability

• High frequency

• Low conduction drop

• High junction temperature

• Good radiation hardness

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High temperature Power Electronics applications

• Down hole drilling in Oil & Gas exploration.

• Aerospace Applications

Jet engine starter - generator system.

Brake actuators.

Electric and plug-in hybrid vehicles.

• Power Systems Applications

HVDC, FACTS

• Geothermal drilling operations.

• Military applications.

• Under sea cabling, etc.

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Switching loss of Si and SiC diodes at different operating temperatures

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GALLIUM NITRIDE (GaN)

• GaN devices have direct bandgap and high-frequency performance.

• Suitable for optoelectronics and radio frequency application.

• GaN Schottky diode has negligible reverse recovery current and

consequently lower switching loss that is independent of the

operating temperature.

• Thermal conductivity is almost one fourth that of SiC.

• Growing GaN on SiC wafers increases the overall thermal conductivity.

• GaN wafers generally come in two forms:

GaN on SiC or GaN on sapphire

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Comparision

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Si 4H-SiC GaN Diamond

Band gap energy (eV)1.1

(Indirect)3.0

(Indirect)3.4

(Direct)5.5

(Indirect)

Dielectric constant 12 10 9.0 5.5

Mobility (cm2/Vs) 1500 1000 1500 3800

Electric field strength (MV/cm)

0.25 2.0 3.3 1-10

Thermal conductivity (W/cmK)

1.5 4.5 1.3 21

Wafer size@2010 12-inch 4-inch6-inch

(on Si sub.)1-inch

(Research level)

4H-SiC – 4 hexagonal polytype crystal structure - Silicon Carbide

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Summary of Devices

• Replacement of SiC power devices for Si devices will result in

Reduced switching and Conduction losses

Increased efficiency and reduced size and volume

Increased Switching frequency

Reduced passive components

Very high temperature operation.

• SiC power device technology is much more advanced than GaN technology and is leading in research and commercialization efforts

• GaN on SiC is suitable for power device applications and GaN on sapphire

is for LEDs and other optical applications.

• No pure GaN wafer based commercial products are available yet.

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Simplified Block Diagram of a Power Electronics System

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Power Electronic"Power" Circuit

Feedback"Control Circuit"

Load1

Load2

Loadn

ElectricalInputs

"Sources"

Electrical orMechanical

Output "Loads"

x1

x2

xm

y1

y2

yn

f1 f2 fk

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Detailed Block Diagram of

Power Electronics System

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Filter&

Rectify

PowerConverter

LoadFilter

&Rectify

ControlCircuit

Mechanical VariableFeedback

Electrical VariableFeedback

Input

Form ofelectricalenergy

Electrical

Mechanical

Pre-stagePower processing

stage Post stageOutput

Form of electric ormechanical energy

Sw

itch

Dri

ves

Process feedbacksignals and decide

on control

Could generateundesirablewaveforms

Mostly ac linevoltage (singleor three phase)

Mostly

unregulateddc

voltage

Interface betweencontrol and power

circuits

Power Flow Unidirectional: input-to-output

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PowerProcessing

circuit(P

loss)

(input)Source Side

(output)Load Side

Load

I

III IV

II

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Power Flow – Bi-directional

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PowerProcessing

circuit(Ploss)

(input)Source Side

(output)Load Side

Load

I

III IV

II

Uninterrupted Power Supply

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Wind Electric Systems

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Adjustable Speed Drives

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Motor Drive

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AC/DC DC/AC

Transportation System

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• Hybrid electric vehicles with much higher gas mileage• Electric vehicles• Light rail, fly-by-wire planes• All-electric ships• More Electric Aircraft

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ISG System in a Vehicle

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ECMElectric

Machine

Po

wert

rain

Rad

iato

r

Integrated Power Electronics

Controller/Inverter/DC-DC Converter

Engine

14VLoads

VehicleInterfaces

12V Aux.Battery

48VBattery

Transmission

Toyota’s Power conversion unit in HVs

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Block Diagram of the Power ElectronicsSystems Components

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Typical Motor Control Systemfor HEV Power-train

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Photovoltaic System Block Diagram

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DC

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DC/DCCONVERTER INVERTER MOTOR TRANSMISSION

CONTROL ELECTRONICSFOR DC/DC CONVERTER, INVERTER

AND MOTOR CONTROL

BATTERY

VEHICLE SYSTEM CONTROL

Fuel CellStack

FUELSUPPLY

FUELPROCESSOR

Controller

Typical Fuel Cell Vehicle System

H2

VEHICLE

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Block Diagram of Turbine Power Conversion System

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Power Converters

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A small Drive unit

Power Module

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60

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Tesla Roadster• Top Speed – 125 mph

• 2 Speed Transmission

• Range – 220 miles

• Full charge in 3.5 hrs (with 70 amp home charging station)

• Shaft Drive

• Weight – 2690 lbs

• 6,831 Lithium Ion batteries (laptop)

• Each cell is independent

• 100,000 mile life expectancy

• 3-phase, 4-pole electric induction motor, 215 kW

• Weighs 115 lbs - size of a watermelon

• Propels car 0 – 60 mph in under 4 seconds

• 85% – 95% efficient

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Tesla Electric Roadster

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1. Electric Motor2. Transmission3. Power Electronics Unit4. Battery Pack5. Body and Frame

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Chevrolet VOLT Concept (PHEV)

• Global Compact Vehicle Based

• Electric Drive Motor

• 120 kW peak power• 320 Nm peak torque

(236 lb-ft)

• Li-ion Battery Pack

• 136 kW peak power• 16 kWh energy content• Home plug-in charging

• Generator

• 53 kW

• Internal Combustion Engine

• 1.0L 3-cylinder turbo

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Bombardier ZEFIRO Very High Speed Trains

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.

• The ZEFIRO features sustainable technologies and an aerodynamic design that generates 20% energy savings. It requires the lowest energy consumption per seat in its segment. It also offers the highest service speed among the ZEFIRO class of trains

• Power:

– Voltage/frequency nom.: 25 kV-50 Hz; min. 17.5 kV; Max 30 kV,

– Asynchronous motors, forced cooling

– Distributed drives

– 20 MW (16 cars, 380 kph)

The ZEFIRO is the latest class of very high speed (VHS) trains from Bombardier. It is one of the fastest sleeper trains in the world and is currently being operated in China. Operating speed of 250kmph to 380kmph

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Power Electronics is the enabling Technology for Transportation

Electrification

POWERELECTRONICS(Si, SiC, FUTURE)

Semiconductorswitches

CapacitorsCircuitry

• OtherComponents

ENERGY STORAGE

•Batteries

Ultracaps

MaintainanceFree

Lithium ion

THERMALMANAGEMENT

Natural or liquid Spray coolingHeat sinks

POWER GENERATION /UTILIZATION

Motor technologies

Fuel cells

POWER DISTRIBUTION /

SYSTEM INTEGRATION

AC or DC Dist.Voltage levels, frequency StabilityEMI

PMSRInduction

Starter/Generators

ModelingControllersHardware

Electricactuation

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Summarizing the Role of Power Electronics

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Block diagram of power electronic interface.

or

DC Source

REFERENCES- Power Electronics

1. Bose, Bimal K.

• Modern power electronics and AC drives, Prentice Hall, 2002.

2. Mohan, Ned

• First Course on Power Electronics and Drives, Minneapolis, MN: MNPERE, 2003.

3. Mohan, Ned, Tore M. Undeland, William P. Robbins,

1. Power electronics : converters, applications, and design, 3rd ed. John Wiley & Sons, 2003.

4. Rashid, Muhammad H.,

• Power Electronics, Circuits, Devices and Applications, 3rd ed., Pearson Education, 2003. (also Prentice Hall lists under same ISBN)

5. Erickson, Robert W. and Dragan Maksimovic

• Fundamentals of Power Electronics, 2nd ed, Kluwer Academic, 2001.

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Cryogenic Power Electronics

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What is Cryogenic Temperature

• Cryogenics is the study of the production and behavior of materials at very low temperatures.

• It is not well-defined at what point on the temperature scale refrigeration ends and cryogenics begins.

• The cryogenic temperature range has been generally defined as from −150 °C (123K) to absolute zero (−273 °C or 0K), the temperature at which molecular motion comes as close as theoretically possible to ceasing completely.

• Cryogenic temperatures are usually described in the absolute or Kelvin scale, in which absolute zero is written as 0 K, without a degree sign. Conversion from the Celsius to the Kelvin scale can be done by adding 273 to the Celsius scale.

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http://www.britannica.com/science/cryogenics

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Cryogenic Power electronics

• Silicon based power devices are generally designed to operate in the range between –40 ◦C and +150 ◦C

• Commercially available power devices are not specifically designed for operation at cryogenic temperatures.

• Use of cryogenically cooled power converters opens up numerous opportunities to change the way we design and manufacture lightweight, low cost high power converters for the global markets.

• A number of system level benefits; lower power losses, low-current feed through connections and overall increased power density.

• Obtain characteristics close to that of ideal power devices

• Collocation of the power converter that converts the superconducting generator output to required power within the cryogenic environment

• The cryogenic power converter provides extremely high levels of controlled generator excitation with extremely low losses.

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• Significant performance improvements have been reported for many power devices when operated at cryogenic temperatures

• The on-state resistance of power MOSFETs falls by about five times

• It is also reported that the MOSFET threshold voltage and

transconductance increase at low temperatures.

• At 77 K (temp. of liquid Nitrogen), the threshold voltage has been found to increase by one volt due to carrier concentration reduction when compared to room temperature

• The breakdown voltage of the power MOSFETs reduced by up to 23%. The drain current capability increased three times from 300K to 77K for that particular device. This is due to the higher carrier mobility at lower temperatures

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Power Devices and Cryogenic Behavior of MOSFETs

Singh, R., Baliga, B.J. “Cryogenic operation of silicon power devices”(KluwerAcademic Publishers, MA, USA, 1998) [1]

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On-State Resistance of Power MOSFETs

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On-State Resistance of Power MOSFETs

• Three power MOSFETs of different voltages were tested from as temperatures of 300K to 77K. All three power MOSFETs exhibited decreased on-state resistances as the temperature was reduced from 300K to 77K.

• For a 1000 V rated power MOSFET, for a drain current of 2 A, the on-state resistance decreased by a factor of 14 between room temperature and 77 K. The device also appears to be able to handle at least twice the rated drain current at 77 K without serious degradation on the on-state resistance

On resistance is the total electrical resistance between the source and drain during the on-state of the device.

Gate Threshold voltages of Power MOSFETs(VGTh)

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Three devices of different voltages were selected. The gate threshold voltagesfor all three MOSFETs were found to increase with decreasing temperature. They each exhibited a ∼0.7 V higher threshold voltage at 77 K than at 300 K. [1,35]

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Breakdown voltage of power MOSFETs[1]

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The breakdown voltage of devices decrease with decrease in temperature . At lower temperatures, the free path of carriers increases giving them more energy for a given electric field.

• The commercially available MOSFETs in plastic or metal packages have been found to work well if immersed in a bath of liquid nitrogen despite the fact that they have not been designed for such a cold application.

• They can be operated at much higher current levels and hence high efficiency switching power converters could be designed.

• It was also proven that heatsinks other than the liquid nitrogen are not required. This permits the design of extremely small, lightweight and low-cost power conversion circuits for many applications.

• The on-resistance of commercially available high-voltage MOSFETs (500-1000V) decreases by a factor 10-30 or more depending on the drain current if cooled down to 77 K .

• Power MOSFETs with higher voltage ratings showed more significant improvements compared to lower voltage devices. For example, the on-state resistance of a 1500 V Power device reduced by a factor of 14.7. The on-resistance of 500 V HEXFET devices reduced by a factor of 8.4 and the 200 V HEXFET device reduced by a factor of 3.6.

77

Commercial Power MOSFETs at low temperatures

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The average on-state resistances of the three MOSFETs against temperature [35]

.

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The average on resistance of three different types of MOSFETS exhibited global minimum between 50K and 100K. Below this range, the decreased electron mobility and carrier freeze-out effect dominates and the average on-resistance increases.

SiC and GaN Devices at Cryogenic Temperatures [Leong]

• The measured SiC power MOSFET exhibited no improvements at 20K comparedto room temperature. Therefore it would not be a good device for cryogenicoperation

• For the measured normally-off GaN HEMT, the on-state resistance improves fromroom temperature down to 20 K and exhibits no carrier freeze-out effects.

• The turn-on voltage of the reverse body diode also reduced with decreasingtemperatures which is the opposite of the power MOSFET. This would reducethe voltage drop across the diode.

• In terms of operating with low power losses at temperatures below 50 K, the GaNHEMT appears to be the most optimized device for the application.

• GaN HEMTs have very good potential in cryogenic applications but still needsfurther investigation.

• The characteristics of the recent silicon carbide devices need to be furtherinvestigated to understand their operation at very low temperatures.

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Summary [35]• The characteristics of various power devices up to 20K have been investigated in

[Leong -35] and the observed on-state behavior for all the measured devices at their known temperature range.

• For n-channel power MOSFETs, there is an optimum temperature range where the device experiences a minimum on-state resistance. This is between 60 K and 90 K, centered at around 75 K. At this range the electron mobility is highest and the temperature is not low enough to cause significant carrier freeze-out effects.

• From room temperature to the optimum temperature range, power MOSFETs with higher voltage ratings show more significant improvements compared to lower voltage devices. For example, the on-state resistance of 500 V HEXFET device reduced by a factor of 8.4 and the 200 V HEXFET device reduced by a factor of 3.6.

• Below this optimum temperature range, most devices experience degradation in on-state resistance. This could be due to a combination factors including reduced electron mobility and carrier freeze-out effects.

• It is concluded that among the commercially available power devices, silicon n-channel power MOSFETs are the most optimized for cryogenic applications. They can achieve extremely low on-state resistance and reasonable breakdown voltages

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Summary of findings of the on-state behavior for all the measured devices

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Leong, “Utilising Power Devices Below 100K to Achieve Ultra-low Power Losses,” PhD Thesis, University of Warwick, UK, August 2011

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• The cryogenic performance of the IGBT devices has shown that IGBTs could work more efficiently at low temperatures, with the decrease of on-state voltage and turn-off time, despite the decrease of breakdown levels .

• Insulated gate bipolar transistor tail current effects are reduce by approximately an order of magnitude .

• The reductions in on-state voltage drop were found to be about 20-30 %, and the turn-off time reduction was by a factor of approximately three or four over the temperature range from room temperature down to 50 K.

• Similar to the MOSFETs, the gate threshold voltage for IGBTs was found to increase approximately one volt due to the intrinsic carrier concentration at 77K and the transconductance increase twice at the same temperature.

• It is also reported that most IGBTs exhibit low forward voltage drop at lower temperatures till 100K and slightly increase after that due to carrier freeze out. Also, the switching performance of the IGBT improves at low temperatures.

82

IGBTs at Cryogenic Temperatures

Temperature dependence of IGBTs

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(a) Forward voltage of an asymmetric n-channel IGBT, showing the temperature dependence of the junction voltage (VJ ) and the voltage drop across the drift and channel region (Vd + Vch), measured by Singh and Baliga b) the temperature dependent forward voltage of three different IGBTs, measured by Forsyth et al.

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Temperature dependence of IGBTs

The cryogenic behavior of IGBTs have been measured in a number of studies

down to 77 K. Further studies measured IGBTs down to 4 K and for more

advanced IGBT structures, such as trench gate IGBTs down to 50 K. The

temperature dependence of the forward voltage of an n-channel IGBT is

presented in Fig (a)

The total forward voltage (VF ) is shown to decrease at lower temperatures.

However, The voltage drop across the PN junction (VJ ) was shown to increase

at lower temperatures, limiting the reduction in the achievable forward voltage

at cryogenic temperatures.

IGBTs have shown a reduction in the forward voltage, down to ∼100 K . Below

this temperature, the forward voltage increases again.

The temperature dependence of the NPT and PT IGBTs are similar to the

power MOSFETs and reduce linearly by approximately 20-25 % from 300 K

down to 77 K.

In general, the stored in a minority carrier device [IGBT, Thyristor, etc.] reduces

dramatically with a reduction in temperature, thus increasing the switching

frequency capability.

84

• Many passive components and off-the shelf integrated circuits have been shown to operate satisfactorily at temperatures down to 50K

• The low temperature impact on the capacitors depends on the dielectric medium such as polypropylene, polycarbonate, mica, film and ceramic. These capacitors function properly up to 77K and the leakage current and dissipation factor shown to be decreasing at cryogenic temperature .

• It is shown that the magnetic losses generally increase with cooling unlike the reduction in copper losses, and the power dissipation is not too much different than at the room temperature.

• If superconducting windings are substituted with the copper windings, then the loss comparison between core and windings becomes more promising.

• Another study showed that most powder cores maintain a constant inductance value and exhibit dependency, with varying degrees, in their quality factor and resistance on test-frequency and temperature. Also most cores exhibited good stability with changing temperature as well as frequency .

• A more comprehensive research should be conducted to characterize the cryogenic behavior of passive components.

• The longer term effects of low-temperature operation and the repeated cyclic operation at low temperatures is less understood .

85

Passive Components

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• A 175 W buck dc-dc converter operating at 50 kHz was tested at 77K. Full-load efficiency increased from 95.8% at room temperature to 97% at 77K

• A similar test was conducted using three level 60 W dc-dc buck converters which reports a fully functional converter at 77K with slight efficiency degradation .

• Another comparative study reported the testing of dc-dc converters such as synchronous rectifier, zero voltage switching (ZVS) and multilevel topologies operating from 120V to 500V down to 20K. In this study, the on-state resistance reduced by a factor of six at low temperatures whereas switching losses and speed found to be insensitive to temperature.

• Among these converters, zero voltage switching (ZVS) has been suggested as the most efficient option; the overall losses reduced 18% of the room temperature.

• In another study, a 50 kW three phase inverter with soft switching was tested at 77K, where the total inverter loss was about 1% of the input power.

86

Power Electronics: Converters

• Reduced size and higher power density

• Higher switching speed of devices due to reduced carrier lifetimes

• Lower conduction and switching losses

• Higher efficiency

• Reduced package volume and higher operating current densities due to an increase in the thermal conductivity of silicon and packaging material. No additional heat sinks

• Reduced device leakage currents because of lower temperatures. Also higher reliability for the same reason.

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Summary of Cryogenic Power Advantages

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Cryogenic Cooling Systems

88

Cryogenic Cooling

The cryogenic systems need to be low cost, high reliable, high efficiency, and smaller size. The choice of the cryogenic plant is determined by several factors:

1. Steady-state cooling, temperature uniformity, and control.

2. Transient response.

3. Recovery requirements.

4. Power requirements in steady-state and recovery.

5. Availability of cryogens.

6. Reliability/ Safety.

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The cryogenic challenge

• Factors affecting cooling requirements• Operating temperature• Electric current dissipation (DC/AC)• Leaks from the outside world• Geometrical proportions

• Applications vary hugely, thus leading to requirement for many cooler types

• Several immature technologies are available

• Not enough demand “right now” for any single application

The primary power requirement of the cryogenic plant is dictated by the Carnot efficiency.

• The removal of 1W at 77K requires 10W of refrigeration• The removal of 1W at 30K requires 30W of refrigeration• The removal of 1W at 4K requires 1000W of refrigeration

90

Cryogenics

91

Typically the unit cost of achieving the required refrigeration is about $150/W, based on commercially available Gifford McMahon cryogenic-coolers(Regenerative Cycle Cryocoolers)

– Liquid Helium (LHe) ≈ $5/L (LTS)

– Liquid Neon (LNe) ≈ $150/L

– Liquid Nitrogen (LN2) ≈ $1.20/L (HTS)

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Cryogenic Coolers

There are three types of cryogenic refrigeration systems:

• Recuperative (steady flow),

• Regenerative (oscillating flow) or

• a hybrid of the two.

92

Recuperative coolersRecuperative systems are steady flow liquefaction plants primarily utilising heatexchangers to transfer heat between a working fluid and a transportation fluid.

The common types of recuperative cycle based systems are:• Joule- Thomson• Brayton

• The Joule-Thomson cycle is steady vibration-free flow and can transport coldfluids long distances. The absence of moving parts in the cold end is anotheradvantage. However, it requires high pressure and that typically means oil-flooded compressors (reduced lifetime) and the possibility of cold-headcontamination. It can be scaled easily for microsizes.

• The Turbo-Brayton cycle has a major advantage is that the transport fluid canbe carried long distances, and this allows the cryogenics to be placed out of theway in tight configurations. Another advantage is that the operating lifetime islong, because it uses gas bearings.

• The disadvantages of the Turbo- Brayton cycle are that it requires a large heatexchanger and this is expensive to build (the smallest units available cost$800,000 for 11.5kW at 80K). Most important, these systems cannot beminiaturized as the cost hits a plateau and doesn't go any lower when the sizedeclines further.

• For example in the 77 K temperature range the plateau comes at about 1000watts of refrigeration power that is too big for most superconductive devices.

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Regenerative Crycoolers

• There are basically three types of regenerative coolers.

• Stirling Cycle

• Gifford McMahon, and

• Pulse Tube refrigerators.

• The regenerative cryocoolers operate with oscillating flows and oscillating pressures, analogous to AC electrical systems, and almost always use high-pressure helium as the working fluid.

• In these regenerative cryocoolers, heating occurs as the pressure is increasing, and cooling occurs as the pressure is decreasing.

• These systems operate at frequencies below 60 Hz at 77 K, and as slow as 1 Hz when cooling to below the 4 K range.

• All three work by having a transport fluid (a gas) pass cyclically through a regenerator and a displacer. The displacer moves back and forth at lower temperatures such that the gas expands when heating and cools when compressed.

• The flow of the gas is controlled such that one end of the displacement tube forms a coldhead while the other forms a hot end.

94

Gifford-McMahon cryocooler

• The Gifford-McMahon cryocooler is the most popular type.• It isolates the compressor from the regenerator and displacer,

which allows a modified air-conditioning compressor to be used. This keeps the cost down to typically $10,000-$20,000 for 150W at 65K.

• The efficiency is much lower than in the Stirling cycle (nominally 85% for Stirling cycles and 50% for GM cycles) expressly because an external AC compressor is used. There is still inherent vibration from the moving displacer.

• A Gifford-McMahon unit is larger and heavy than a Stirling cycle unit but this problem is often mitigated because the compressor can be placed some distance away from the place where cooling must occur.

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Stirling coolers

• A stirling cooler works by repeated heating and cooling of a sealed amount of working gas, usually helium for cryogenic temperatures

• A piston varies the working gas volume , and a displacer shuttles the gas within the cooler between the warmer components and the cooler components

• Stirling coolers are available in a wide range of sizes- from mW, where they can be very small to hyndreds of watts of cooling capaxity.

• Temperatures down to 20K are possible with two stage units

• The Stirling Cycle has several advantages, notably high efficiency, small size and weight, and moderate cost ($400,000 for 4200W at 80K)

• There is considerable manufacturing experience with such units; over 100,000 have been made already

96

Pulse-Tube Coolers

• The advantage of the Pulse Tube device is that the displacer is made out of a column of gas, not solid material. It is a gas plug. This eliminates a crucial moving part at low temperatures and enhances reliability and reduces vibration

• Most pulse tube cryocoolers built to date have had small cooling capacities (50 W or less). Recent advances have demonstrated the feasibility of systems with up to 1kW of cooling capacity at 77K and much larger capacities are expected in the future.

• Pulse tube cold heads have also been used with thermoacoustic engines. Such systems offer the possibility of high reliability due to the lack of moving parts in either the driver or the cold head, however the current technology results in a physically large cryocooler with limited efficiency.

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Selection of Cryocooler• For small and medium refrigeration plants, regenerative systems are preferred.

• For larger sized systems a recuperative system may be preferable due to increased

efficiency.

• Ideal regenerators cannot accept heat so either they must remove all the heat at the

lowest temperature or must be constructed as multi-stage devices, both reducing

efficiency. Within the small and medium scale requirements regenerative systems

provide the preferred solution.

• For smaller scale requirements the GM system is preferred as although it is less

efficient, larger and heavier than the other regenerative system options, the

advantages of using modified air-conditioning compressors that can be placed some

distance away provides the most commercially available, lowest cost and most

flexible system solution.

• For medium scale requirements the Stirling cycle system becomes more attractive

due to its increased efficiency and thus effective capital and through-life cost per

watt removed.

• The different refrigerators have various advantages and disadvantages, which trade off against one another in choosing the best cryogenic system for a particular application.

98

Cryocooler for a power Electronic System [Leong Thesis]

99

Cryogenic System Configuration:. The cooling system and cryostat.. The vacuum pump system.. The temperature controller

The cryogenic cooling system is a closed cycle helium cryostat with a modified aluminium outer and inner casing.The cooling system consists of the expander module which expands helium vapour inside a chamber. The cooling process is based on the Joule-Thomson effect.

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Cryogenic Power Conversion Applications

100

Applications

• Magnetic resonance imaging (MRI)

• HVDC system based on cryogenic cooled cables

• Deep space and terrestrial applications

• Magnetic levitation transportation systems

• Military all-electric vehicles

• Medical diagnostics

• Cryogenic instrumentation

• Super conducting magnetic energy storage systems

• Propulsion motors for aircrafts and ships

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Turboelectric Distributed Propulsion Engine Cycle Analysis for Hybrid Wing Body Aircraft – (AIAA) 2009-1132

Aircraft Attributes

Range 7500nm

Payload 118100 lbm

Mcruise >0.8

Cruise alt 35,000 ft

Takeoff Cruise

Thrust/Engine 54888 lbf 19293 lbf

Empty Weight

(Baseline B777-200LR)

267400 lbm

(Δ73,400)

Number of Propulsors 14 or 15 (function of aircraft width, FPR, and net thrust)

Generator/engine 30,000 hp (22.4 MW)

Motor/propulsor 4000 hp (3 MW)

N3-X Turboelectric Distributed Propulsion (TeDP) Vehicle Concept

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Turboelectric Distributed Propulsion in a Hybrid Wing Body Aircraft – (AIAA) ISABE-2011-1340

Increased Aerodynamic Efficiency

◦ Hybrid Wing Body Concept Aircraft

◦ Blended wing body (BWB) aircraft have higher aerodynamic efficiency

◦ Additional 3-7% fuel burn reduction

Increased Propulsive Efficiency

◦ Decouple fan and engine speeds

◦ Operation at optimal fan speed

◦ Effective bypass ratio > 30

Cryogenically Cooled Superconducting Electrical System

◦ Tasked with providing aircraft propulsion and some level of differential thrust for yaw control

102

Power Generation and Distribution Technology

103

103

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Hybrid Electric Distributed Propulsion (HEDP) Aircraft

30 MW Superconductor Power Transmission

6 MW Superconductor Electrical Motor Turbo Fans

Worldwide $700B

US Domestic $40B

US DOD $3.6 B

USAF $4.2 B

Fuel Efficiency + 70%Potential World-Market Pull : $400/yr saving

Superconductor

Applications Needed Class

Generators 30-40 MW

Motors 4-6 MW

Power Transmission Cables 5-70 MW

DC ±270

Power Inverters 1-30 MW

Power Electronics 30-40 MW

30 MW Superconductor Electrical Generator

104T.J. Haugan, “Design of SMES Devices for Air and Space Applications,” http://www.cvent.com/events/tenth-epri-superconductivity-conference/custom-18-0ac856fa88e84a97ac2058094d0a4629.aspx, October 2011

Superconducting Electric Machine Technology

105

Limited number of fully cryogenic electric machine designs have been developed

Long Engineering and GE for AFRL – armatures are cryogenic metal, not superconducting

Rolls-Royce Strategic Research Center

Superconducting rotor machine prototype –98.8 Nm/kg

Superconducting rotor and stator machine design – 92 Nm/kg

NASA Schematic Drawing of a Fully Superconducting Electric Machine from “Turboelectric Distributed Propulsion in a Hybrid Wing Body Aircraft” – (AIAA) ISABE-2011-1340

105

Cryogenic converter prototypesCryogenically-cooled converter at 40 kW/kg (Liquid-cooled converter for hybrid vehicle application at 26 kW/kg)

Electric Machines: 100 Nm/kg

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Applications in Wind power

106

Superconducting wind turbine Generators

• The ability of superconductors to increase current density allows for high magnetic fields, leading to a significant reduction in mass and size for superconducting machines.

• It is estimated that the superconducting technology could achieve an efficiency improvement of 1% in large electrical machines, which offers substantial savings to utilities and end-users.

• Another distinct feature of superconducting machines is their higher part-load efficiency advantages. This is particularly relevant to wind power generation since the wind turbines operate mostly at part-load conditions.

• HTS wind turbine generators can extract more wind energy than other types of machines even though they have the same nominal efficiency on the nameplates.

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Schematic of a multi-MW, low speed, direct drive HTSWTG system

(Courtesy of Converteam)

Comparison 10MW PMGDD vs 10MW SCDD(Snitchler Gregory, Gamble, B. ‘10MW Class Superconductor Wind Turbine Generators. IEEE

transaction on applied superconductivity, Vol.21, No. 3, June 2011.)

PMG SCDD

Output at rated load 10MW 10MW

Outer diameter 10 meters 4.5 – 5 meters

Weight 300 metric tons 150-180 metric tons 108

Superconducting Generator for Wind Power

• DOE has identified superconducting wind turbine generators as a technology that can enable low cost offshore wind turbine energy by enabling 10-20 MW wind turbine generators

Superconducting generators have

• less initial generator cost,

• lower transportation costs,

• lower tower cost,

• lower foundation costs,

• lower installation costs,

• lower maintenance costs,

• less forced outages,

• less lost revenue due to un expected maintenance and forced outage costs.

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SUPERCONDUCTING FAULT LIMITERs

110

Fault current

• Electrical systems suffer faults due to various causes.

• Faults cause damage at the point of fault.

• The electrical disturbance threatens the stability of other equipment.

• Fault currents impose heat and electromagnetic stress on equipment:

– Electrical equipment must be braced against electromagnetic forces.

– Cables must be rated for fault current

• Therefore, fault current must be interrupted quickly and safely and this imposes severe duties on switchgear.

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DC vs AC: fault current interruption. • AC current falls to zero twice per cycle.

• Zero current helps switching.

• DC has no such feature and so switching is more difficult.

• AC reactance can be used to limit fault current.

Therefore:

• DC circuit breakers are heavier, larger, more difficult to install, and more expensive than equivalent AC circuit breakers. Hence, a real barrier to the adoption of DC.

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Superconductivity: a solution.

• The ability to pass electrical current WITHOUT loss.

• Property possessed by certain materials when cooled to cryogenic temperatures.

• Superconductivity is maintained provided:

– Temperature is below the critical temperature.

– Magnetic field is below the critical magnetic field.

– Current is below the critical current:

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Resistive SFCL

FCLResistance

()

I Rated Critical I

SuperconductorPerformance

SuperconductorTemperature

CriticalTemperature

I2RTemperature

Rise

FCLResistance

()

I Rated Critical I

SuperconductorPerformance

SuperconductorTemperature

CriticalTemperature

I2RTemperature

Rise

Critical Temperature (K)

Critical Field (T)

Critical Current (A)

Critical Temperature (K)

Critical Field (T)

Critical Current (A)

A superconductor has zero dc resistance while temperature, magnetic flux and current density are below the critical values.

Exceeding a critical value causes the superconductor to quench to its normal state where a finite resistance occurs.

A large impedance ratio can therefore be achieved by quenching a superconductor.

Above the surface graph the material reverts to its normal resistive state

Below the surface graph the material is superconducting

with zero resistance

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Resistive Superconducting fault current limiter: general principle.

• A length of superconductor is included in the protected circuit.

• The superconductor passes load current without loss.• If the current rises above a critical current,

superconductivity is lost. • The former superconductor introduces electrical resistance

into the circuit, reducing the current.• Process takes place in less than 1 ms, fast enough to

significantly ease the switchgear breaking duty.

• Failsafe –does not rely on control circuits• Circuit no longer “sees” peak fault current

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HTS Superconducting Materials• The best known materials are complex CuO ceramics

known as BSCCO and YBCO• These are very difficult and fragile materials to make and

therefore expensive.

• Primarily only available in tape form• But they have good performance at T<60K

• MgB2 is a relatively new class of inter-metallic materials.• This is a very cheap material with costs~ similar to copper

• These are an inter-metallic with mechanical properties similar to metals available in wire form.

• They have sufficient performance with Tc< 39K

116

Conclusions

• Several studies have concluded that there is a great potential for cryogenic power conversion in applications such as wind energy, propulsion motors and power generators for on-board the ships, future military applications, and aircraft where size and weight are the primary design considerations.

• Most of the research results related to power converters are on the characterization of the operating behavior of the devices instead of the whole converter/inverter system.

• Although some research related to converters is presented, it is mostly at the dc-dc converter level for powering the field windings of a synchronous generator.

• Also relatively little has been written on the design and performance of complete cryogenic converter systems, consisting of both active and passive devices, though much has been written about the superconducting generators.

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Conclusions

• The selection and integration of the right cryogenic system for a given application contributes to the overall performance, efficiency, power density, and cost of the overall system

• A significant advancements in cryogenic converter/inverter technology is required for its application in wind energy, NASA distributed propulsion system based aircraft, ship propulsion, and other high power applications.

• Cryogenic power electronics technology is the next step in the evolution of power electronics technology to obtain high power density, high efficiency, and superior performance for various applications

118

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