Cryogenic conceptions for full superconducting generators: realization of superconducting armature cryostat

Cryogenic conceptions for full superconducting generators: realization of superconducting armature cryostat Y Brunet, P T i x a d o r and H. N i t h a r t * CNRS CRTBT, Laboratomre d'Electrotechmque, 25, Avenue des Martyrs, 38042 Grenoble C6dex, France *Alsthom DEA, 3, Avenue des trois chines, 90018 Belfort C6dex, France Recetved 7 July 1987, rewsed 4 November 1987

The construction of fully superconducting (SC) a c machines ms now possible using multffdamentary SC wires with low losses under a c magnetmc fields From a cryogenic point of wew, these machines introduce new desmgn concepts and new materials statmonary and rotating hehum vessels combined desmgn, fibreglass reinforced epoxy structure, low-loss hormzontal cryostat These general consmderatmons are discussed and a cryostat for a three-phase SC winding ms presented Two specmal features of this cryostat are easy d~smanthng through the use of slhcone joints, thermal shields w~th a layer of copper w~res Tests on the cryostat are reported

Keywords superconductors, cryostats, superconducting generators

Recent progress in the field of a c SC wires has made possible the design of a c rotating machinery using SC coils for both field and armature windings This concept is of interest for both the electrical and mechanical properties 1 Nevertheless, the cryogemc design of this kind of machine ~s complicated by the presence of the variable magnetic field throughout the machine and the existence of two hqmd He containers, one being stationary and the other rotating As a first step towards the construction of a 20 kVA synchronous prototype, a static armature He cryostat has been built and tested

Room temperature air gap The armature and the rotor cryostats are completely independent (see F~gute lb) A room temperature gap allows the rotation of the inner cryostat in the outer

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Schematic descrmptmonof a full SC generator

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/ ~ _ 1 : , 1 ~ ~i - 1--S C field w,ndmg The electrical parts of a full a c SC generator are a SC field winding on a rotating shaft, a three-phase stationary SC armature winding, and an external magnetic core In our design, the damping is provided by an external controlled SC storage coil 2 Each SC winding ~s enclosed m a He dewar (see Ftgure la), which has to be nonconducting to avoid eddy current losses the stator receives permanent rotating magnetic fields and the rotor, as no electromagnetic shield reduces varying fields during transient and unbalanced sequences, supports strong magneUc variations The cryogemc reqmrements are vacuum insulation, thermal shielding between liquid He temperature and ambient temperature, low thermal conduct]wty materials To satmsfy these requirements various designs are possible They differ essentially by the thermal state of the gap between the static and the rotating parts (called the air gap below)

0011 2275/88/110751-05 $03 O0 ,( 1988 Butterworth & Co (Publishers)Ltd ,

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Fmgure I Schematm diagram showing the axial view of a fully SC generator (a) Electrical structure, (b) cryogemc structure (room temperature air gap)

Cryogenics 1988 Vol 28 November

751

Cryogenic conceptions for full superconducting generators Y Brunet et a l armature vessel bore Each cryostat has its own thermal shields and vacuum chambers

Cold temperature atr gap As the two cold parts of the He cryostats are facing each other it is not necessary to insert room temperature walls between them Nevertheless, the air gap cannot be at normal pressure as forced convection will appear due to the rotation and the axial temperature gradient This turbulence would be the cause of huge thermal loads from the extremities at 300 K Moreover, friction loss will be significant In this case The best solution would be a perfect vacuum gap ( ~ 10 -4 Pa) Efficient rotating seals 3 are then required this quite complicated solution becomes impossible for large peripheral speeds ( > 30 m s - 1) even when the rotation can be used as a pumping system near each bearing 4 The stability of the flow around a rotating cyhnder is governed by the pressure of the fluid in the gap for a given speed For high pressures, turbulent convection arises, but stable Couette flow IS possible when the Taylor number remains lower than the critical value, T~ (see Reference 5) The corresponding critical speed IS 41 2v

Q¢- -

e(eR)l/2

where v IS kinematic viscosity, e is gap width, R is internal rotor radius As the kinematic viscosity vanes in inverse ratio to the density, the corresponding critical speed, Q¢, defines the critical pressure, Pc K p¢ - e(eR)l/2f~ s where f2s is the rotating speed (synchronous speed) and K is a constant for a given fluid In our case the fluid in the air gap must be He to avoid any condensation on the cold parts For a two pole 50 Hz a c generator with a rotor of 0 8 m diameter and a gap of 10 m m width, P c = 1 65 Pa at 4 2 K , 10Pa at 10K, and 2700Pa at 300 K However, this theory does not take into account axial thermal gradient and considers the fluid as a continuous medium The temperature profiles along the gap's tubes can be identical to suppress radial thermal gradients but an axial thermal gradient remains As the lengths of thermal transition are important compared to the gap e, the turbulence developments will remain small At room temperature the mean free path, Lp, is of the same order as e (for He gas Lp = 4 m m for T = 300 K and p = 5 Pa) and the hypothesis of continuous medium is no longer true However, in the molecular mode the risk of convection is weaker and at low temperature Lp is smaller than e (for He gas Lp = 0 08 m m for T = 10 K and p = 5 Pa) The result of a low pressure gap is the removal of thermal shields between the helium vessels and the use of quite simple rotating seals as the pressure remains relatively high (1 10 Pa) For small peripheral speeds, mechanical face seals allow this level of pressure to be easily maintained, whereas labynnth seals can be envisaged for higher surface speed Figures 2 and 3 illustrate several possiblhtles using a low pressure gap In Figure 2 the gap ts independent of

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F,gure 2 Schematm dmgram of the low pressure gap with separated rotor and stator vacuum 1, Magnetic core, 2, armature

winding, 3, thermal shield, 4, cold point, 5, field winding, 6, rotating seal, 7, bearing

the stator and the rotor which have their own vacuum chamber The gap is delimited by the cylinders which support the two He vessels so that the coupling between the windings is very good We can imagine other solutions with additional tubes between the gap and the He vessels to limit direct thermal conduction to the cold vessels through the gap Figure 3 presents a dtfferent solution the gap pressure is c o m m o n with a part of the stator However, a vacuum chamber surrounds it to avoid radial gas conduction from the outside To limit thermal losses by axial conduction to the static He vessel, materials with weak thermal conductivity can fill the space around it

Realizat=on

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Although extensive theoretical studies are necessary for large units, small scale experiments are useful to confirm the feasibility of this new type of machine and to check some assumptions Thus, a fully superconducting 2 pole, 50 Hz, 380 V, 20 kVA machine has been designed and the construction of the SC stator is achieved It is indeed the most original part of the machine as a lot of superconducting rotating field windings have already been built and

F=gure 3 Schemat=c diagram of the low pressure gap with the gap common w=th a part of the stator 1, Magnetm core, 2, armature

winding, 3, thermal shmld, 4, cold point, 5, field winding, 6, rotating seal, 7, bearing

Cryogemc conceptions for full superconductmg generators Y Brunet et tested throughout the world 6 The first electrical tests have been made with a classical iron rotor The construcUon of a superconducting rotor will be the second step of this work The overall dimensions are length 1000 m m and external diameter 350 m m The geometric characterlStlCS are summarized in Tabh" 1 and the main elecmcal characteristics are given in Table 2 assuming a fully SC synchronous conception

Cryogemc armature design Genera/prmciples Except for small stainless steel bolts and tubes m which eddy currents are limited, all the central parts of the armature cryostat are non-metallic because of the rotating field The horizontal cryostat must have a rmg-shaped structure to enable the installation of the rotor (external diameter 175 mm) The He vessel has to be sufficiently rigid to transmit electrical torques and is dimensioned for short-circuit torques Special cold joints have been installed to make dlsmanthng it simple Fibre-glass epoxy has been chosen for its mechanical and thermal p r o p e m e s This materml can be easily worked and bonded to other materials, such as stainless steel, with epoxy casting resin Differential contraction ts such that many cool-downs can be carried out without fissures appearing However a lot of disadvantages are linked to its use m a cryogemc environment and a lot of care must be taken Flbreglass epoxy is not vacuum tight for He gas at normal temperatures This phenomenon ts characterized by the porosity defined by the quantity of gas passing through a wall per second and per surface unit for a pressure gra&ent The ratio between epoxy and alummlum is 106 at room temperature 7 This figure increases further if the thickness of the He vessels is less than about 2 m m A good surface coating plays a prominent part In addition to its porosity, fibreglass epoxy outgasses a lot at room temperature But these properties are rapidly decreasing functions of the temperature Thus, iffibreglass epoxy use is limited to the parts where He ~s near its condensation temperature (In the He vessel, for example), some charcoal IS sufficient to maintain high vacuum ( < 10 -4 Pa) once the cool-down to near 3 0 K has been achieved

Table 1

Geometnc characteristics of the fully SC machine

Mean radius of field winding (mm) Mean radius of armature winding (mm) Inner radius of magnetic shield (mm) Outer radius of magnetic shield (mm) Actwe length (mm)

70 121 165 175 200

al

For the same reason, all the He recovery tubes linked to the He vessel are stainless steel tubes Epoxy is quite a black body and effective thermal shielding is necessary to limit radiation losses Its low thermal dlffUSlVlty and its high enthalpy lead to slow thermal balance during cool-down, leading to mechamcal strains in the materials

CoM Iomts The use of cold joints is dictated by the frequent dismantling which is necessary in the case of a test cryostat The problem is comphcated by the ringshaped structure (see Figure 4) In this shape one of the two seals, (1) and (2) in Fzgure 4, cannot be perfectly crushed Only 'elastic' jomts can assure tightness in such conditions Elastomer seals have been used for the last few years a but rarely with large diameters (266 mm) and never with a double bearing Usually, silicone films, with a thickness less than 0 1 mm, guarantee a good tightness even at room temperature Preliminary tests have shown that the tightness of a thick silicone seal (e > 0 6 mm) is insufficient at room temperature but becomes excellent at low temperature ( < 100 K) These seals are very reliable (more than 10 dlsmantllngs without problems) and do not require careful preparation of the mating surfaces Cryostat description The He vessel 1S bounded by tubes (6) and (7) (see Figure 5) enclosed between the dismountable flange (5) and the flange (12) glued onto the cylinder (6) The vessel is mamly supported by the 'torque tube' (18) which transmits the torque from the armature winding It is also supported on the other side by six steel rods (3) to limit vibration displacements It hangs inside a vacuum chamber between cyhnders (1) and (2) sealed to the outer metalhc flanges Charcoal (13) ensures a good vacuum at low temperature A large part of the cold He vapour passes through the heat exchanger (15) which recovers the enthalpy The heat exchanger is made of a stacking of bronze balls which define very thin channels through which the gas circulates the large exchange surface gives compactness and small pressure drop This heat exchanger cools both a cold point (l 7) on the 'torque tube' and one extremity of both the thermal shields, (8) and (9), surrounding the He vessel These shields are made of epoxy fibreglass tubes covered on one side by a layer of small axial copper wires to ensure a good axial thermal conductivity so that temperatures are not very different at the two extremities of the shields However, they must not add high eddy current losses Figure 6 gives the scheme of thermal transfers Under a time varying (angular frequency t~) magnetic field B(BrBoBz) a copper wire (diameter D) dissipates a power density, Pj 1 aco2(B2 + BE)DE

1 6o~2B2D2

where a ts the electrtcal conducttvlty of copper As the Table 2

Electncal specifications for the fully SC machme

Rated output (kVA) Speed of rotation (rev mm 1) Frequency (Hz) Stator voltage (Vrms) Stator current (Arms) Synchronous reactance (p u ) Transient reactance (p u )

18 5 3000 50 220/380 28 28 2 16

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Figure 4 Schematic diagram dlustratmg the ring-shaped structure of the He vessel

Cryogenics 1988 Vol 28 November

753

Cryogemc conceptmns for full superconducting generators Y Brunet et al where em IS the mean emissivity, ostef lS the Stefan constant, Do and De are the vessel diameters at temperatures To and T~, respectively As the shield temperature, T, is lower than 120 K, some terms can be neglected and Pr becomes Pr = O'stef/~mTo4 i

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If the temperature does not vary very much along the shields, total power density P t = Pr + PJ IS constant and the temperature text, on the uncooled extremity is given by the following relation, neglecting thermal contacts

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where L is the length of the shield and k is the thermal conductivity of copper These relations led to the choice of copper wire with diameters of 0 5ram for (9) and 0 8 mm for (8) as the magnetic field is weaker The He vessel is continuously fed with two-phase hquld He to compensate losses One other gas pipe assures the exhaust of vaporized He it contains measurement wires and cools the current leads which are optimized by classical methods The four SC wires (three-phases and star pomt) are in liquid He up to the box (23) where they are connected to copper wires (classical current leads) or to a cryogenic hne In the case of SC generator equipment 9

Cryogen/c test Figure 5 Cross-sectmn of the armature cryostat 1, 2, Vacuum vessel, 3, steel rods supports, 4, He gas recovery tube, 5, 6, 7, He vessel, 8, 9, thermal sh=elds, 10, armature windings, 11, magnet=c sh=eld, 12, He vessel, 13, charcoal box, 14, thermal leads, 15, 16, heat exchanger, 17, cold point, 18, torque tube, 19-23, current leads and connect=on box a, b, c, d, Pt sensors

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Coohng down proceeds in two steps 1, He, cooled to liquid N2 temperature, circulates through the vessel, 2, after 6 h, when the He vessel temperature decreases slowly, hquld He is transferred Ftgure 7 shows the dtfferent temperature evolutions the Pt sensors correspond to those indicated in Ftgure 5 Liquid He has been transferred for 1 5 h Even when the cryostat is isolated for 5 h, it is still sufficiently cold for liquid He to be transferred directly and electrical tests can be started less than 2 h after Only the external shield temperature is relatively high, certamly due to a bad thermal contact with the heat exchanger The peak at the 18th hour corresponds to the He storage dewar being changed Ftgure 8 illustrates properties of outgassing, charcoal absorption and fibreglass permeablhty, all these terms being a function of temperature Immediately after the beginning of He circulation, the charcoal in the He supply tube absorbs air He is absorbed when hquld He is injected While pumping IS necessary during cool-down, it is completely useless when the vessel contains liquid

Cryogemcs 1988 Vol 28 November

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He as the pressure is at a very low level ( < 10- 5 Pa) The overall cryostat losses amounted to only 3 6 W

Conclusions The cryogenic conception of fully SC a c machines is very different from that of 'classical' superconducting generators with only a SC field winding with the armature winding remaining at room temperature The rotatmg part is simplified as several vessels and shields can be removed The problems of rotating seals are solved with the use of a low pressure gap Materials in the central parts static and rotating - of the machine have to be non-conducting as they endure strong magnettc vartatlons

A fibreglass epoxy hortzontal cryostat which can be dismantled easily has been butlt and successfully tested It has proved that elastomer seals are very efficient at low temperatures even m bad mechanical condtttons The first tests o f a SC armature winding have also been made successfully This generator has produced, with very low losses, the first 50 Hz a c SC current in the world The next step is the construction of a SC field rotating winding to constitute a fully SC a c generator

Acknowledgements The authors would like to thank H Reynaud, J Pllon and J P Leggert, technicians at the C R T B T who built th~s cryostat

References 1 Brunet, Y, Tlxador, P , Lecomte, T and Sabne J L First conclusions on the advantages of full superconducting synchronous machines Ele( trw Machme~ and Power Systems (1986) 11 511-521 2 Mitam, Y, Tsuji, K, Ise, T and Murakaml, Y Fundamental study on electrical power system stabdlzatlon using SMES Proc MT9 (1985) 353-356 3 Gamble, B B Development of a hehum transfer couphng for a superconducting generator rotor Adv Cr~og Eng (1977) 23 125 131 4 French Patent no 2 372 535 5 Taylor, G I. Stability of viscous hqmd contained between two rotating cyhnders Phd Trans Roy Soc London (1923) A223 289-343 6 Lambrecht, D Overview of the development of superconducting generators Proc MT9 (1985) 273-276 7 Testard, O A. and Locatelh, M Cryostats for SQUID magnetometers Cryogemcs (1983) 23 299-302 8 Chaussy, J , Escalon, G , Glanese, P and Roux, J P Easdy demountable elastomer seal for cryogemc apphcatlons Cryogemcs (1978) 18 501 9 Sabne, J L. Feasibility of large a c superconducting eqmpment J Phystque (1985) 45 717-720

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