124175857 Surge Current Protection Using Superconductors

8/12/2019 124175857 Surge Current Protection Using Superconductors

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Surge current protection using superconductors Seminar Report

stray impedance of transformers and generators or splitting power&grids

artificially to lower the number of power sources that could feed a fault current.

2ut such measures are inconsistent with today"s demand for higher power

!uality, which implies increased #oltage stiffness and strongly interconnected

grids with low impedance.

3hat is need is a de#ice that normally would hardly affect a power system bit

during a fault would hold surge current close to nominal #alue that is a fault

current limiter. 4ntil recently most fault current limiter concepts depend on

mechanical means, on the detuning of 567 resonance circuit or use of strongly

non&linear materials other than igh )emperature super conditions $)*(. 2one

is without drawbacks.

TRADITIONAL WAY OF FIXING FAULT CURRENT LIMITERS

Device Advantages Disadvantages

7ircuit 0reaker -ro#en 2eeds zero current to break

8eliable -erformances limited to 1A

as limited life time

igh&impedance widely used 0reeds inefficiency in system

$high losses(

9use simple 0reaks too soon $ha#e too

low a with&standable fault current( -

:ust be replaced by hand

Air&core reactor pro#en ;ntails large #oltage drops

)raditional 7auses substantial power

5oss during normal operation

*ystem pro#en 8educe system reliability

8econfiguration preferred for reduces operating flexibility

$bus&splitting( fast&growing areas Incurs high

cost of adding line. Adds cost

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opening circuit breakers

0efore examining super conducting fault current limiters some

characteristics f non&linear material deser#e a closer clock.

*uper conductors because of their sharp transition from zero resistance at

normal currents to finite resistance at higher current densities are tailor made for

use in fault current limiters. ;!uipped with proper power controlled electronics, a

super conducting limiter can rapidly detect a surge and taken and can also

immediately reco#er to normal operation after a fault is cleared.*uperconductors lose their electrical resistance below certain critical

#alues of temperature, magnetic field and current density. A simplified phase

diagram of a super conductor defines three regions.

In the innermost, where #alues for temperature, field, and current density

are low enough, the material is in its true superconducting state and has zero

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resistance. In a region surrounding that area, resisti#ely rises steeply as #alues for

three #ariables so higher.


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MATERIAL ISSUES

5ow&temperature superconducting $5)*( wire has been a#ailable for

se#eral decades. Its ac losses ha#e been reduced by the de#elopment of multi

filament wire. )he diameter of the filament is of the order of .1@m and they are

decouples by a highly resisti#e, normal conducting matrix which also ser#es as

thermal stabilization. *ince any magnetic field interacts only with the #ery thin

and decoupled filaments, the ac losses in the materials are tolerable e#en at

extremely low temperatures $for 5)* application, usually ?.% >, boiling point of

li!uid helium(.

>ept this cold, the specific heat of 5)* is #ery low, but the current

carrying capacity is #ery high $greater than 1 A7m%(. 7onse!uently any

concei#able *7975 based on 5)* would exceed its critical temperature within

se#eral hundred microseconds of a fault. 0y the same token the material is prone

to hot spots, which some tiny disturbance can trigger e#en at sub critical current

#alues.0ecause of such properties 5)* material is predestined for the fast heating

resistor design. A fast homogenous transmission into the normal conducting state

is supported by excellent thermal conducti#ity which together with the low

specific heat, leads to rapid propagation of hot spots.

3hile there is only one large program left in the low temperature type of

*795, more than 1 maor proects are under way worldwide on high

temperature type of de#ice. )he main reason in the lower )* cooling cost.

;ssentially ust three types of )* materials are a#ailableB all made from

bismuth $0*77


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RESISTIVE LIMITERS

In the resisti#e *7975, the super conductor is directly connected in

series with the line to be protected. )o keep it superconducting, it is usually

immersed in a coolant that is chilled by a refrigerator. 7urrent leads are designed

to transfer as little heat as possible from the outside to the coolant.

In normal operation, the current and its magnetic field can #ary but

temperature is held constant. )he cross section of super conductor is such as to

let it stay below critical current density, since its recepti#ity is zero in this

regimeB the impedance of the *7975 is negligible and does not interfere with the

network. All the same the superconductor"s impedance is truly zero only for dc

currents. )he more common as applications are affected by two factors. 9irst, the

finite length of the conductor produces a finite reactance which howe#er can be

kept low by special conductor architecture. *econd a superconductor is not loss

free in ac operation, the magnetic as field generated by the current produces so

called ac losses basically, ust eddy current losses. )hese are hea#ily influencedby the geometry of the conductor and can be reduced by decreasing the

conductor dimension trans#erse to direction of local magnetic field. )hey barely

contribute to total *7975 impedance but dissipate energy in superconductor,

thus raising cooling costs.

In case of a fault the inrush of current and magnetic field take the super

conductor into the transition region, between zero resistance and normal

recepti#ity. )he fast rising resistance limits the fault current to a #alue some

where between the nominal current and what e#er fault current otherwise would

ensure. After some time, perhaps a tenth of seconds, a breaker will interrupt the

current.

)he beha#ior of resistic fault current limiter is largely determined by the

length of the superconductor and the type of material used for it.


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SUPERCONDUCTORS AS VARIALE RESISTORS AND

SWITC!ES*e#eral anisotropic high temperature superconductor show critical current

densities which are strongly dependent on direction of an applied external

magnetic field. )he resistance of a sample can change by se#eral orders of

magnitude by applying a magnetic field.

)he current carrying capability of both low temperature and high

temperature super conductors decreases with the application of a magnetic field.

*ome anisotropic high temperature superconductors in particular the bismuth and

thallium based super conductors show a resistance that is highly dependent on

the amplitude and direction of applied field.E1,%F

Resistance Fie"d De#endence O$ !TS Wi%e

Anisotropic )* materials show a dependence of the critical current

density and therefore the resisti#ity, on the direction of applied magnetic field.owe#er, if the magnetic field is perpendicular to the ab plane, a steep

exponential reduction with field in the critical current density is obser#ed.

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0y rotating a )* wire sample along a&axis in a constant magnetic field,

the #oltage #aries as a function of angle as shown in figure below.

Goltage drop in a 0*77< sample as a function of external magnetic field angle.

)he measured #oltage drop is directly proportional to the resistance of the

samples because the current is constant. )he resistance of the samples shows, to

the first approximation a sinusoidal dependence on angle, which is formed by

the c axis and the direction of external field. )he sample resisti#ity is the highest,

when the field is parallel to the c axis $H (.

3hile the #oltage drop and resistance #alues of the samples shown in

figure abo#e $measurements made at k. )est sample was 1 cm long 0*77awasaki, Mapan

together with )okyo utility )epco has built a 1'.% :GA $>GN%A( single

phase prototype a %&? GA de#ice is under de#elopment.

;7 Alstom along with ;lectrocute de 9rance $;D9( de#eloped and

tested a . :GA $'>GN%1A( single phase de#ice.

!TS

American superconductor corp. $A*7( and sumitomo electric Industries

ltd ha#e produced ling lengths of sil#er sheathed wire based on 0i %%%' with a

critical current at > on the order of >Acm&%. )his wire might suit cable,

motor and transformer applications but is poorly suited for *7975 because its

high sil#er content gi#es it a low normal resistance. At this stage of material

de#elopment the sil#er sheath must be rather thick if it is not to leak 0i%%%'

during processing. )hus #ery ling lengths are needed to build up the resistance

for fault limitation. *o used only for constant temperature resisti#e type of

*7975. )he situation will change if resist#ity of sil#er matrix can be raised. A

proect along these lines led by A00 in partnership with A*7 and ;lectricite de

9rance has been launched to de#elop a current timely transfer.

*iemens ha#e demonstrated a resisti#e 1 >GA model base?d on C07Acm&% at >(. )he de#ice operated for one year under

actual conditions in *wiss hydroplant

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CONCLUSION

)he purpose of this paper was the study of surge current protection using

superconductors. )he *7975 offers efficient ad#antages to power system and

opens up a maor application for super conducting materials.

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REFERENCES

+, www.superconductors.com

-, >; ray, D; flower&Osuperconducting fault current limiterP

., I;;; transaction on Applied superconducti#ity #ol.', march 1==

/, www. I;;;.org

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ASTRACT

)he recent growth of power circuit capacities has caused fault currents to

increase. *ince the protection of power systems from the fault currents is #eryimportant, it is needed to de#elop a fault current limiter. A fault current limiter is

re!uired to assure $1( rapid reaction to fault currents, $%( how impedance in

normal operation and $'( large impedance during fault conditions. A super

conducting fault current limiter $*7975( can meet these re!uirements

superconductors, because of their sharp transition from zero resistance at normal

current to finite resistance at higher current densities, are tailor&made for use in

975s.

*uper conductors are of two types&high temperature superconductors

$I)*( and low temperature superconductor $5)*(. )he )* are substances that

lose all resistance below temperature main tamable by li!uid nitrogen. 5)* are

substances that lose all recepti#ity close to ?k, a temperature attainable only

using by using li!uid helium. 7ost of cooling 5)* $which are mostly metals,

alloys and intermettalics( makes their use in many applications commercially

impractical. )* material a#ailable are all made of bismuth $0*77

124175857 Surge Current Protection Using Superconductors

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