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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 2 1 6/9/2016 INTRODUCTION TO POWER ELECTRONICS SYSTEMS 3 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) 4 2 6/9/2016 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 5 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 6 3 6/9/2016 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 7 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) 8 4 6/9/2016 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 9 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 10 5 6/9/2016 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 11 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 12 6 6/9/2016 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 13 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 14 7 6/9/2016 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) 15 • 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. 16 8 6/9/2016 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 17 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 18 9 6/9/2016 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 19 20 10 6/9/2016 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 21 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 22 11 6/9/2016 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. 23 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 24 12 6/9/2016 The general IV characteristic of an IGBT. 25 IGBT Features 26 13 6/9/2016 POWER SWITCHES: POWER RATINGS 1GW Thyristor 10MW GTO/IGCT 10MW 1MW IGBT 100kW 10kW MOSFET 1kW 100W 10Hz 1kHz 100kHz 1MHz 10MHz 27 28 14 6/9/2016 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 29 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. 30 15 6/9/2016 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 31 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 16 6/9/2016 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. 33 Switching loss of Si and SiC diodes at different operating temperatures 34 17 6/9/2016 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 35 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 21 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 36 18 6/9/2016 38 39 19 6/9/2016 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" 41 20 6/9/2016 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 42 Power Flow Unidirectional: input-to-output (input) Source Side (output) Load Side Power Processing circuit (Ploss) II I III IV Load 43 21 6/9/2016 Power Flow – Bi-directional (input) Source Side (output) Load Side Power Processing circuit (Ploss) II I III IV Load 44 Uninterrupted Power Supply 45 22 6/9/2016 Wind Electric Systems 46 Adjustable Speed Drives 47 23 6/9/2016 Motor Drive AC/DC DC/AC 48 Transportation System • • • • • Hybrid electric vehicles with much higher gas mileage Electric vehicles Light rail, fly-by-wire planes All-electric ships More Electric Aircraft 49 24 6/9/2016 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 50 Toyota’s Power conversion unit in HVs 51 25 6/9/2016 Block Diagram of the Power Electronics Systems Components 52 Typical Motor Control System for HEV Power-train 53 26 6/9/2016 Photovoltaic System Block Diagram DC 55 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 56 27 6/9/2016 Block Diagram of Turbine Power Conversion System 57 Power Converters Power Module A small Drive unit 58 28 6/9/2016 59 60 29 6/9/2016 61 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 62 30 6/9/2016 Tesla Electric Roadster 1. 2. 3. 4. 5. Electric Motor Transmission Power Electronics Unit Battery Pack Body and Frame 63 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 64 31 6/9/2016 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) 65 Power Electronics is the enabling Technology for Transportation Electrification 67 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 67 32 6/9/2016 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. 69 33 6/9/2016 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 71 34 6/9/2016 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 35 6/9/2016 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] 75 36 6/9/2016 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 37 6/9/2016 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. 79 38 6/9/2016 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 81 39 6/9/2016 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. 83 40 6/9/2016 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 . 85 41 6/9/2016 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. 87 42 6/9/2016 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. 89 43 6/9/2016 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) 91 44 6/9/2016 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. 93 45 6/9/2016 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. 95 46 6/9/2016 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. 97 47 6/9/2016 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 99 48 6/9/2016 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 101 49 6/9/2016 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 50 6/9/2016 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 51 6/9/2016 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. 107 52 6/9/2016 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. 109 53 6/9/2016 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. 111 54 6/9/2016 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: 113 55 6/9/2016 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 115 56 6/9/2016 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. 117 57 6/9/2016 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 REFERENCES- Cryogenic Power Electronics 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 11. 12. 13. 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