Download Cryogenic Power Electronics

6/9/2016
Cryogenic Power Conversion
Systems
Kaushik Rajashekara
The University of Texas at Dallas
Richardson, TX
E-mail: [email protected]
1
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.
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
POWER
INPUT
vi , ii
POWER
OUTPUT
Power
Processor
Source
vo , io
Load
Controller
measurement
reference
•Building Blocks:
– Input Power, Output Power
– Power Processor
– Controller
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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
Static Application: DC Power Supply
AC voltage
AC LINE
VOLTAGE
(1F or 3F )
DIODE
RECTIFIER
FILTER
DC-DC
CONVERTER
LOAD
V control
(derived from
feedback 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)
Drive Application: Air-Conditioning System
Power Source
Power
Electronics
Converter
Desired
temperature
Desired
humidity
System
Controller
Temperature and
humidity
Variable speed drive
Motor
Air
conditioner
Indoor temperature
and humidity
Building
Cooling
Indoor
sensors
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POWER CONVERSION CONCEPT: EXAMPLE
•
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.
+
Vs
_
Vo
Average output voltage :
V
Vo  m

Vdc
time
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CONVERSION CONCEPT
•
What if customer wants variable DC voltage?
 More complex circuit using SCR is required
vs
ig
ia
wt
+
vs
_
+
vo
_
vo
wt
ig
Circuit Diagram
a
Average output voltage
Vo 
1
2

V
m
sin w t d w t 
a
wt
Waveform
Vm
1  cos a 
2
• 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
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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
• 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
I=0
I
V switch = V in
V switch= 0
V in
POWER SWITCH
V in
SWITCH ON (fully closed)
SW ITCH OFF (fully opened)
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• 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)
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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IDEAL CHARACTERSTICS OF POWER
DEVICES
• 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
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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
•
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
•
•
•
•
•
•
v-i characteristics
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
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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
IGBT_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
1GW
Thyristor
10MW
GTO/IGCT
10MW
1MW
IGBT
100kW
10kW
MOSFET
1kW
100W
10Hz
1kHz
100kHz
1MHz
10MHz
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HEAT REMOVAL MECHANISM
FIN-TYPE HEAT SINK
SCR (STUD-TYPE) ON AIR-COOLED KITS
SCR (HOKEY-PUCK-TYPE) ON POWER
PAK KITS
ASSEMBLY OF POWER CONVERTERS
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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
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
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Wafer size@2010
12-inch
4-inch
6-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.
40
Simplified Block Diagram of
a Power Electronics System
x1
x2
Power Electronic
"Power" Circuit
Load1
y2
Load2
yn
xm
Electrical
Inputs
"Sources"
y1
f1 f2
fk
Loadn
Electrical or
Mechanical
Output "Loads"
Feedback
"Control Circuit"
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Detailed Block Diagram of
Power Electronics System
Input
Form of
electrical
energy
Filter
&
Rectify
Mostly
unregulated
dc
voltage
Mostly ac line
voltage (single
or three phase)
Power processing
stage
Power
Converter
Post stage
Filter
&
Rectify
Form of electric or
mechanical energy
Output
Electrical
Mechanical
Load
Could generate
undesirable
waveforms
Switch
Drives
Pre-stage
Control
Circuit
Electrical Variable
Feedback
Mechanical Variable
Feedback
Interface between
control and power
circuits
Process feedback
signals and decide
on control
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Power Flow
Unidirectional: input-to-output
(input)
Source Side
(output)
Load Side
Power
Processing
circuit
(Ploss)
II
I
III
IV
Load
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Power Flow – Bi-directional
(input)
Source Side
(output)
Load Side
Power
Processing
circuit
(Ploss)
II
I
III
IV
Load
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Uninterrupted Power Supply
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Wind Electric Systems
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Adjustable Speed Drives
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Motor Drive
AC/DC
DC/AC
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Transportation System
•
•
•
•
•
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
Integrated
Power Electronics
Powertrain Radiator
Controller/Inverter
/DC-DC Converter
48V
Battery
14V
Loads
Engine
Transmission
12V Aux.
Battery
Vehicle
Interfaces
Electric
Machine
ECM
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Toyota’s Power conversion unit in HVs
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Block Diagram of the Power Electronics
Systems Components
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Typical Motor Control System
for HEV Power-train
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Photovoltaic System Block Diagram
DC
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Typical Fuel Cell Vehicle System
FUEL
SUPPLY
FUEL
PROCESSOR
H2
Fuel Cell
Stack
DC/DC
CONVERTER
INVERTER
MOTOR
TRANSMISSION
BATTERY
Controller
CONTROL ELECTRONICS
FOR DC/DC CONVERTER, INVERTER
AND MOTOR CONTROL
VEHICLE
VEHICLE SYSTEM CONTROL
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Block Diagram of Turbine
Power Conversion System
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Power Converters
Power Module
A small Drive unit
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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
1.
2.
3.
4.
5.
Electric Motor
Transmission
Power Electronics Unit
Battery Pack
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
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
•
•
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)
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Power Electronics is the enabling
Technology for Transportation
Electrification
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POWER GENERATION /
UTILIZATION
POWER
ELECTRONICS
(Si, SiC, FUTURE)
ENERGY STORAGE
Motor technologies
PM
SR
Induction
Starter/
Generators
Fuel cells
Electric
actuation
•Batteries
Maintainance
Free
Lithium ion
POWER DISTRIBUTION /
SYSTEM INTEGRATION
Semiconductor
switches
Capacitors
Circuitry
• Other
Components
Ultracaps
THERMAL
MANAGEMENT
Natural or liquid
Spray cooling
Heat sinks
AC or DC Dist.
Voltage levels,
frequency
Stability
EMI
Modeling
Controllers
Hardware
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Summarizing the Role of Power Electronics
or
DC Source
Block diagram of power electronic interface.
68
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
70
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.
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.
72
Power Devices and Cryogenic Behavior
of MOSFETs
• 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
Singh, R., Baliga, B.J. “Cryogenic operation of silicon power devices”(Kluwer
Academic Publishers, MA, USA, 1998) [1]
73
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On-State Resistance of Power MOSFETs
On resistance is the total electrical
resistance between the source and
drain during the on-state of the
device.
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
74
Gate Threshold voltages of Power
MOSFETs(VGTh)
Three devices of different
voltages were selected. The
gate threshold voltages
for 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]
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.
76
Commercial Power MOSFETs
at low temperatures
• 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 (5001000V) 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 onresistance 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
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The average on-state resistances of the three
MOSFETs against temperature [35]
.
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.
78
SiC and GaN Devices at Cryogenic
Temperatures [Leong]
• The measured SiC power MOSFET exhibited no improvements at 20K compared
to room temperature. Therefore it would not be a good device for cryogenic
operation
• For the measured normally-off GaN HEMT, the on-state resistance improves from
room temperature down to 20 K and exhibits no carrier freeze-out effects.
• The turn-on voltage of the reverse body diode also reduced with decreasing
temperatures which is the opposite of the power MOSFET. This would reduce
the voltage drop across the diode.
• In terms of operating with low power losses at temperatures below 50 K, the GaN
HEMT appears to be the most optimized device for the application.
• GaN HEMTs have very good potential in cryogenic applications but still needs
further investigation.
• The characteristics of the recent silicon carbide devices need to be further
investigated 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 nchannel power MOSFETs are the most optimized for cryogenic applications. They
can achieve extremely low on-state resistance and reasonable breakdown voltages
80
Summary of findings of the on-state
behavior for all the measured devices
Leong, “Utilising Power Devices Below 100K to Achieve Ultra-low Power Losses,” PhD Thesis, University of
Warwick, UK, August 2011
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IGBTs at Cryogenic Temperatures
• 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
Temperature dependence of IGBTs
(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
Passive Components
• 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 testfrequency 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 .
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Power Electronics: Converters
• A 175 W buck dc-dc converter operating at 50 kHz was tested at 77K. Fullload 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
Summary of Cryogenic Power Advantages
• 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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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
Typically the unit cost of achieving the required refrigeration is about $150/W,
based on commercially available Gifford McMahon cryogeniccoolers(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 coolers
Recuperative systems are steady flow liquefaction plants primarily utilising heat
exchangers 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 cold
fluids long distances. The absence of moving parts in the cold end is another
advantage. However, it requires high pressure and that typically means oilflooded compressors (reduced lifetime) and the possibility of cold-head
contamination. It can be scaled easily for microsizes.
• The Turbo-Brayton cycle has a major advantage is that the transport fluid can
be carried long distances, and this allows the cryogenics to be placed out of the
way in tight configurations. Another advantage is that the operating lifetime is
long, because it uses gas bearings.
• The disadvantages of the Turbo- Brayton cycle are that it requires a large heat
exchanger and this is expensive to build (the smallest units available cost
$800,000 for 11.5kW at 80K). Most important, these systems cannot be
miniaturized as the cost hits a plateau and doesn't go any lower when the size
declines further.
• For example in the 77 K temperature range the plateau comes at about 1000
watts 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 highpressure 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]
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.
Cryogenic System Configuration:
. The cooling system and cryostat.
. The vacuum pump system.
. The temperature controller
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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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N3-X Turboelectric Distributed
Propulsion (TeDP) Vehicle Concept
Aircraft Attributes
Range
7500nm
Payload
118100 lbm
Mcruise
>0.8
Cruise alt
35,000 ft
Turboelectric Distributed Propulsion in a Hybrid Wing Body Aircraft – (AIAA)
ISABE-2011-1340
Takeoff
Cruise
Thrust/Engine
54888 lbf
Empty Weight
(Baseline B777200LR)
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)

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
19293 lbf

Turboelectric Distributed Propulsion Engine Cycle Analysis for
10
Hybrid Wing Body Aircraft – (AIAA) 2009-1132
2
102
Power Generation and Distribution
Technology
10
3
103
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Hybrid Electric Distributed Propulsion (HEDP)
Aircraft
6 MW Superconductor
30 MW Superconductor Electrical Motor Turbo Fans
Power Transmission
30 MW
Superconductor
Electrical Generator
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
T.J. Haugan, “Design of SMES Devices for Air and Space Applications,” http://www.cvent.com/events/tenth-eprisuperconductivity-conference/custom-18-0ac856fa88e84a97ac2058094d0a4629.aspx, October 2011
104
Superconducting Electric Machine
Technology
Electric Machines: 100 Nm/kg
Cryogenic converter prototypes
NASA Schematic Drawing of a Fully
Superconducting Electric Machine from
“Turboelectric Distributed Propulsion in a Hybrid
Wing Body Aircraft” – (AIAA) ISABE-2011-1340
Cryogenically-cooled converter at 40 kW/kg
(Liquid-cooled converter for hybrid vehicle
application at 26 kW/kg)

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
105
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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 partload 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
Output at rated load 10MW
Outer diameter
10 meters
Weight
300 metric tons
SCDD
10MW
4.5 – 5 meters
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.
•
•
•
•
112
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
Critical Current (A)
Below the surface graph the
material is superconducting
Above the surface graph
the material reverts to its
normal resistive state
with zero resistance
 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.
Critical Temperature (K)
Critical Field (T)
Superconductor
Temperature
FCL
Resistance
()
I2R
Temperature
Rise
Critical
Temperature
Superconductor
Performance
I Rated
Critical I
114
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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