ppt on cryogenics

8/10/2019 ppt on cryogenics

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Cryogenic Power Conversion

Systems

Submitted by: Guided by:

M.Jeevan Sankar Prof. M. A. Mulla[U11EE090]


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What is cryogenics?

Cryogenics, the production of very low temperatures and the study ofphenomena at those temperatures. The range of temperatures with

which cryogenics is concerned is not precisely defined, but it

generally includes temperatures from -238F. (- 150C.) down to

absolute zero (- 459.67F., or -273.15C.). The coldest

temperatures that have been obtained are within a fraction of adegree above absolute zero.

At very low temperatures, certain materials take on peculiar

properties. Liquid helium acts as though it has no viscosity

(resistance to flowing), and its thermal conductivity is several

hundred times that of copper or silver at room temperature. Somemetals become superconductive-they lose all resistance to electric

current, and current will continue to flow through them even after the

power is cut off. These and other unique properties created by cold

give cryogenics its practical importance.


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Superconductivity

Superconductivity is a phenomenon of exactly zero electrical resistance

and expulsion of magnetic fields occurring in certain materials

when cooled below characteristic critical temperature.

Properties of super conductors

zero electrical resistance

The main feature of superconductivity is zero electrical resistance. Asshown in the figure on the right, in the case of metals such as copper,when temperature is lowered, resistance will gradually become lower,but it will never become a zero. However, in the case ofsuperconductivity, electrical resistance will become zero whentemperature is below critical temperature.


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Meissner effectWhen magnetic field is applied to a super

conductor, electric current passes through

the superconductor so as to cancel out themagnetic field, thus preventing the magnetic

field from entering. This phenomenon is called

the Meissner effect. Because no magnetic field

enters, strong repulsion is generated between the superconductor and

magnet.

Josephson effect

When a thin layer of insulator is sandwiched between two

superconductors, until the current becomes certain volume,

electrons pass through the insulator as if it does not exists.

This phenomenon can be applied to the switching devices

that conduct on-off operation at high speed.


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Semiconductors at 77k

Higher operational speed due to increased carrier mobility andsaturation velocity.

Lower power dissipation due to reduced voltage supplies

because of improved turn-on and turn-off characteristics.

Shorter signal transmission time because of reduced

interconnect resistance and also because of the possibility ofusingsuperconducting thin-film asinterconnections.

The thermal conductivities of the device and substrate materials

improve significantly at lower temperatures, leading to simpler

thermal management, lower on-state power loss, and improved

reliability.

Reduction in the switching losses.

Improved digital and analog circuit performance such asnoise

margins, gain-bandwidth products or slew rates.


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Cryogenic power electronics

Cryogenic performance of the IGBT devices show that these devices

couldwork more efficiently at low temperatures with the decrease of on-

state

Voltage and turn-off time despite the decrease of breakdown levels.

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 two

to

Three over the temperature range from room temperature to as low

as

50 K. Similar to the MOSFETs, the gate threshold voltage of the

IGBTs was

found to increase approximately 1 V because of the intrinsic carrier

concentration at 77 K, and the transconductance increase was

observed

to increase two fold for the same temperature.


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For the diode, the reverse recovery is reduced by an order of

magnitude

and for insulated-gate bipolar transistors (IGBTs), the tail current effectsare reduced.


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Superconducting rotating machinesThe main property of superconductivity that makes it so

interesting for power applications is that dc-current can be

transported loss-free together with a high current density. This

allows to build devices with large B-fields produced by the rotor

with nearly no losses. These large rotor-B-fields lead to several

consequences:

Machinery can be built with a much higher power density thanmachines with conventional copper windings in the rotor,

leading to smaller and lighter machines.

Due to the high magnetic field produced by the rotor the stator

winding cannot have iron-teeth. This leads on one hand to a

further reduction in weight and on the other hand losses due toeddy-currents in the stator iron are avoided.

Synchronous reactance is smaller as there is a large magnetic

air gap between rotor and back-iron. A small Xd in turn has

some interesting consequences for the electrical behavior of

such a machine.


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Firstly there is a small load angle which allows a high overload capability

Secondly a small results in a large short circuit current which enables

an easy detection of a short circuit situation and to switch off selectively

the fault location in the grid.

Thirdly a small Xd gives small voltage drops with changing loads and so

leading to a rather stiff behavior in the grid.


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Advantages and Disadvantages of

superconducting electrical machines

The rotor electromagnet is

subject to less resistive loss. The size and weight per power

capacity is also decreasedregardless of the coolingequipment.

The cooling system has greater

cost, size, weight and alsocomplications.

Once the superconductors exittheir superconductive state thegenerator at once stops working.

Either the motor bearings shouldbe separate from the cold rotoror it should be able to toleratethe decreased temperature.

To operate a synchronousmachine such as the

superconductor generatorpractically, it is important to haveaccess to electronic control. Thiselectronic control leads toharmonic loss in the supercooled rotor of the generator to

great extent.


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ApplicationsThe interest in applying superconductivity to power applications is motivated b

expectations of improved performance in terms of size, weight and efficiency o

conventional room-temperature devices. Use of superconducting wire or tape ipower generators or large magnets, for example, provides the ability to transp

large DC currents with no measurable resistive losses.

The early low temperature superconductors (LTS) that operated in liquid heliu

spurred the development of many prototypes for generators, motors, transmis

lines, and energy storage magnets. However, all of these demonstrations wer

compromised by the costly and technologically complicated requirement for liq

helium, and were not easily accepted by utilities or end-users.

In 1986, the discovery of high temperature superconductors provided new imp

for pursuing cryogenic power applications because of the prospect for higherTemperature operation at liquid nitrogen (77 K) temperatures or above. With hi

temperatures come not only reduced refrigeration costs but also enhanced reli

There have been many activities in the field of developing high temperature

superconductor wires and cables.


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THANK YOU

ppt on cryogenics

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