FEASIBILITY OF LARGE A.C. SUPERCONDUCTING EQUIPMENT

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FEASIBILITY OF LARGE A.C.

SUPERCONDUCTING EQUIPMENT

J.-L. Sabrié

To cite this version:

J.-L. Sabrié. FEASIBILITY OF LARGE A.C. SUPERCONDUCTING EQUIPMENT. Journal de

Physique Colloques, 1984, 45 (C1), pp.C1-717-C1-720. �10.1051/jphyscol:19841146�. �jpa-00223618�

JOURNAL DE PHYSIQUE

Colloque C I , supplkment a u no 1, Tome 45, janvier 1984 page Cl-717

F E A S I B I L I T Y O F LARGE A a C * SUPERCONDUCTING EQUIPMENT

J..L. Sabri6

EZectricitE de France, 1 Avenue du Ge'n6raZ de GauZZe, 92141 Clamart, Frame

RCsumC : Certains brins supraconducteurs existants peuvent fonctionner aux frkquences industrielles, aussi leur emploi dans l e matdriel dldctrique doit l t r e 6tudi6.

A v e c l e s c a r a c t i r i s t i q u e s des brins d d j i obtenues, d e s alternateurs e t transformateurs e n t i i r e m e n t supraconducteurs s o n t dconomiquement envisageables mais un t e l groupe d e production a u r a i t des rdactances t r i s dlevdes e t s e r a i t instable. L e s groupes devraient donc & t r e associgs

i

des s y s t i m e s stabilisateurs t e l s que l e stockage supraconducteur e n service

i

l a Bonneville Power Administration. Une t e l l e combinaison laisse augurer des i c o n o m i e s d e masses (66 %) et d e p e r t e s (75 %) qui justif ient un approfondissement des Ctudes.

A b s t r a c t : Some commercially available superconducting wires c a n b e operated a t industrial frequencies which leads t o t h e study of their possible consequences in t h e electrical equipment. With t h e to-day s t r a n d performances, all superconducting generators and transformers a r e economically imaginable but such groups would have very high r e a c t a n c e s and would b e unstable. I t t h e r e f o r e should b e operated with some stabilizing device like f o r instance t h e SMES of t h e BPA at T a c o m a substation. Such combination of t h r e e machines would probably s a v e weight (66 %) and losses (75 %) so t h a t f u r t h e r studies should b e made.

INTRODUCTION

Superconducting wires with 50160 h e r t z losses low enough t o permit economical use in electrical equipment /1,2/ a r e now commercially available 131. This paper deals, in t h e example of l a r g e e l e c t r i c generating units, with some technical and operational consequences of very l a r g e A C current densities in not t o o l a r g e magnetic fields.

PECULIARITIES O F ALL S.C.A.C. GENERATOR

In t h e domain of A C generators, t h e performances of t h e b e s t existing superconducting products /3/ prompt t h e designer t o choose low m a g n e t i c flux density in order t o obtain large c u r r e n t densities a n d small A C losses.

O n t h e o t h e r hand, i t is not sure whether, with developed superconductors, flux density larger than those of t h e existing cryogenerator programs can b e reached.

T h a t is why in a f i r s t comparison of conventional, superconducting field and all superconducting generators, w e varied f o r t h e last one t h e radial component of t h e flux density (Br) at t h e a r m a t u r e mean radius from .1 t o .9 T. T h e r e f e r e n c e machine i s our regular (all copper) 1650 MVA generator "Paluel" t y p e now under in-situ tests. T h e value of .9 T is t h a t of t h e 1650 MVA cryogenerator with SC field only.

We impose also t h a t powers and main dimensions a r e constant when Br varies. T h e superconducting a r m a t u r e and i t s dewar i s assumed not t o t a k e m o r e volume than t h e copper a r m a t u r e in t h e S C field only generator. Main results a r e summarized figure I ( a t o h).

a )

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Reactances. They a r e proportional t o t h e square of Br and can reach very l a r g e values (the flux due t o t h e a r m a t u r e alone c a n b e much higher than t h e flux resulting from a r m a t u r e and field).

b)

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Overcurrents and overtorques. T o e s t i m a t e these over values due t o f a u l t conditions, we have multiplied r a t e d values by t h e f a c t o r 1

+

l/x" represented figure 1

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b

-.

c) - Maximum value of flux density on t h e a r m a t u r e wires is t h a t of t h e tangent component f o r Br less than -45 T and t h a t of t h e radial component other wise.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841146

C1-718 JOURNAL DE PHYSIQUE

d)

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Tangential flux density on t h e field winding decreases a s t h e radial component increases in t h e armature.

e )

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Critical current density is maximum when t h e maximum flux density is minimum.

f )

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R a t e d current density results of b and e. I t is calculated s o t h a t t h e a r m a t u r e is a t t h e limit of quenching f o r a t h r e e phase short-circuit a t t h e terminals.

g) - Comparing t h e r e f e r e n c e current density and a r m a t u r e volume t o r a t e d c u r r e n t density, one then obtains t h e wire volume.

h)

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Then, this volume and loss density versus maximum 50 h e r t z field gives t o t a l A.C. losses a t 4K.

With some f o r m e r values of A.C loss density t h e encouraging result was t h a t , f o r Br between .3 and .7 T, t h e necessary power of refrigeration (with 500 w a t t s per w a t t coefficient) was less o r equal t o RI2 losses in r e f e r e n c e armature. To compare with /I/, t h e r a t i o of refrigeration power (including dewars) t o t h e RI2 losses of t h e a r m a t u r e is about 0.75 when Br is .45 T. This b e t t e r result is essentially due t o t h e very large value of x" (in this c a s e x = 3.75, x' = 2.69, x" = 1.52). Also t h e mass of superconducting wires is about 350 kg instead of 23 Mg of copper in t h e r e f e r e n c e armature.

250 MW ALL S.C.A.C. GENERATOR

T o confirm this encouraging f i r s t study, a 250 MW a l l S.C.A.C. g e n e r a t o r has been designed in t h e main following hypothesis:

a )

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T h e r e is no damper a t all.

b)

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A t any t i m e during a three-phase short-circuit t h e peak value of a r m a t u r e c u r r e n t is less than 80 % of t h e critical c u r r e n t a t t h a t time.

c )

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Critical current density at .96 T is 1490 A/mm2 (131 and CGE measures).

d)

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A C loss density a r e f r o m /3/ figure 4, C/14496 curve. I t has been integrated on t h e winding thickness.

e )

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T h e a r m a t u r e is connected t o a superconducting transformer with no room t e m p e r a t u r e medium voltage terminals.

f )

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T h e refrigeration f a c t o r is 500 w a t t s per watt.

g)

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T h e field winding technology is t h a t of t h e Alsthom Atlantique model rotor /4/.

h)

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T h e r e i s no c r y o s t a t between a r m a t u r e and field winding and regular oil seals allow vacuum in t h a t place.

T h e main results a r e in t a b l e I. I t is t o b e noted t h a t reactances a r e 300 % (synchronous) and 220 % (transient) and t h a t , a s compared with existing 250 MW generators, weight is about 3 3 % and losses about 25 %.

These figures mean financial savings on construction and operation cost.

F o r t h e superconducting transformer, preliminary studies indicate that, with laminated iron, t h e savings could b e 50 % in weight and 75 % in losses.

SUPERCONDUCTING SPEED AND VOLTAGE REGULATOR

Of course, t h e 250 MW damperless generator has no natural damping possibilities and a n external device should b e provided t o stabilize t h e speed. This can be done using small SMES unit such a s t h e LANL-BPA one 151. Within c e r t a i n limits, this device can absorb or supply a c t i v e power and absorb an adjustable reactive power. L e t us consider t h e association of a 250 MW all S.C.A.C. generator and a 30 MVA-90 MJ SMES; t h e following function can b e reached:

1'

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Voltage Regulation. T h e SMES t a k e s in charge instantly variations of reactive power up t o 10 % of generator rated MVA. Then t h e field current c a n be adjusted so t h a t t h e SMES comes back t o i t s normal operation i n a f e w seconds.

2

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S t a t i c stability. According t o t h e results of t h e BPA tests, t h e stabilisation of small oscillations is obviously possible.

3

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Torque Regulation. T h e SMES t a k e s in charge instantly variations of a c t i v e power up t o 10 % of t h e generator rated MW. T h e mechanical torque must then b e adjusted in roughly one second and t h a t 5an b e done without s h a f t oscillations.

4

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Constant speed when dropping load. T h e turbine admission valves must b e closed at once. If w e assume t h a t mechanical power remains constant (250 MW) during .I s, and then goes down linearly t o 0 in .4 s, t h e corresponding energy is 75 MJ. This means t h a t t h e speed variation

c a n b e negligible if t h e SMES c a n absorb this 75 M 3 with a n e l e c t r i c power of 250 MW f o r less than .5 second.

5 - Constant speed during short-circuit. As in 4 above, this result c a n b e obtained if t h e SMES is electrically in serial with t h e generator instead of in parallel a s at Tacoma. In this case, i t is even possible t o consider t h a t t h e SMES A.C. voltage replaces a l m o s t instantly t h e vanishing system voltage: this means t h a t subtransient torque and c u r r e n t s c a n also become very low.

With any kind of A C generator, such a S C regulator may t h e r e f o r e help t h e speed governors, improve t h e voltage regulation and reduce t h e overtorques and overcurrents. This also could mean savings on turbine, on generator and on transformer.

CONCLUSIONS

T h e A.C performances now reached by some superconducting wires make economically and operationally feasible most of t h e imaginable S.C.A.C. windings.

F o r example, a 250 MW all S.C.A.C. generator would save 213 of t h e weight and 314 of t h e losses (including power of refrigeration) in comparison with a regular all copper generator.

A 250 MW SC transformer would also s a v e weight and losses and i t must b e noted t h a t these results a r e obtained f r o m measured characteristics of commercially available A C superconductors.

S.C.A.C. generators would b e unstable so t h a t they should b e associated with small SMES a s SC regulators. In this c a s e overtorques and overcurrents could b e strongly limited in a m o r e or less integrated machine and this means probably other savings.

Feasibility of SMES is not questionable since o n e is in operation in t h e BPA system. As f o r a s superconducting A.C. windings in general a r e concerned, t h e next s t e p of feasibility demonstration should b e t h e development of high current superconducting A.C. conductors with mechanical supports and cooling systems.

This could be a difficult task but, considering t h e foreseable savings in weight and losses, t h e r e f o r e money, i t is probably worth going f a r t h e r in t h a t way.

References

/ I / HLASNIK I., SEIBT E.W.: 50 H z A C losses in inductive coils with mixed m a t r i x f i n e filament Nb-Ti composites a t field up t o 1 Tesla

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T o be published in J.A.P. 1983.

/2/ OGASARAWA T. et al.: Low loss Nb-Ti multifilamentary composite conductor f o r A.C. use. IEEE, vol. MAG.19, n3, 1983.

/3/ DUBOTS P., FEVRIER A. e t al.: Behaviour of multifilamentary Nb-Ti Conductors with very fine filaments under A.C. magnetic fields. This MT-8 conference.

4 GILLET R. e t al.: Electricit6 d e France-Alsthom Atlantique superconducting turbogenerator development program. IEEE, vol. MAG. 17, n l , pp 890-893.

/5/ BOENIG M.J. et al.: T e s t s of t h e 30 MJ superconducting magnetic energy s t o r a g e unit.

This MT-8 conference.

TABLE I

Main characteristics of 250 MW all superconducting A.C. generator

R a t e d MVA : 300 kV : 7,55 kA : 23 RPM : 3000 R e a c t a n c e s xd: 300 % xd' : 220 %

R a t e d flux density B radial : .20 T Btg : .68 T

3 phase short circuit B t max : .96 T Jc : 1490 A/mm2 J = 580 A/mm2 S C a r m a t u r e AC losses : 855 W

Lengths between bearings 6 m. straight p a r t s 2.42 m.

Radii Mean field w. -45 m mean a r m a t u r e 1.1 m.

inner iron 1.2 m o u t e r iron 1.32 m.

Weights (Mg) : support 20

-

laminated 42

-

a r m a t u r e 15

-

^{rotor 15}

t o t a l 92 Mg

-

^{(regular : 270)}

Losses (kW) : mechanical 100

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iron 80 - exciter 60 refrigerator 575 - t o t a l 815

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(regular 3500) Efficiency : 99,7 % (regular 98,6)

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Radial flux d e n s i t y (teslas)

Figure 1 : Parametric .study of a .I650 MVA a l l S.C.A.C. generator