OA11417A - Electromagnetic device. - Google Patents

Abstract

An electromagnetic device comprises at least one magnetic circuit (1) and at least one electric circuit (2, 3) comprising at least one winding (4, 5). The magnetic and electric circuits are inductively coupled to each other. The device comprises a control arrangement (7) to control operation of the device. This control arrangement is adapted to control frequency, amplitude and/or phase as concerns electric power to/from the device by the control arrangement comprising means (9) controlling the magnetic flux in the magnetic circuit.

Description

I G1141 7

Electromagnetic device 5

FIELD OF THE INVENTION AND PRIOR ART

This invention is related to an electromagnetic device com-prising at least one magnetic circuit and at least one electric 10 circuit comprising at least one winding, the magnetic and elec-tric circuits being inductiveiy connected to each other and thedevice comprising a control arrangement to control operationof the device. 15 This electromagnetic device may be used in any electrotechni-cal connection. The power range may be from VA up to the1000-MVA range. High voltage applications are primarily In-tended, up to the highest transmission voltages used today. 20 Accordîng to a first aspect of the invention a rotating electricmachine is contemplated. Such electric machines comprisesynchronous machines which are mainly used as generatorsfor connection to distribution and transmission networks, com-monly referred to below as power networks. The synchronous 25 machines are also used as motors and for phase compensationand voltage control, in that case as mechanically idling ma-chines. The technical field also comprises double-fed ma-chines, asynchronous converter cascades, external pôle ma-chines, synchronous flux machines and asynchronous ma- 30 chines.

Accordîng to another aspect of the invention, said electromag-netic device is formed by a power transformer or reactor. For alitransmission and distribution of electric energy, transformers are 35 used and their task is to ailow exchange of electric energy be- 01141 7 tween two or more electric Systems and for this, electromagneticinduction is utilized in a well-known manner. The transformersprimarily intended with the présent invention belong to the so-called power transformers with a rated power of from a few hun-dred kVA up to more than 1000 MV/A with a rated voltage of from 3-4 kV and up to very high transmission voltages, 400 kV to 800kV or higher.

Although the following description of the prior art with respect tothe second aspect mainly refers to power transformers, the pré-sent invention is also applicable to reactors, both single-phaseand three-phase reactors. As regards insulation and coolingthere are, in principle, the same embodiments as for transform-ers. Thus, air-insulated and oil-insulated, self-cooled, pressure-oii cooled, etc., reactors are available. Although reactors hâveone winding (per phase) and may be designed both with andwithout a magnetic core, the description of the background art isto a large extent relevant also to reactors.

The at least one winding of the electric circuit may in some em-bodiments be air-wound but comprises as a rule a magnetic coreof laminated, normal or oriented, sheet or other, for exampleamorphous or powder-based, material, or any other action forthe purpose of allowing an alternating flux, and a winding. Thecircuit often comprises some kind of cooling System etc. In thecase of a rotaiing electric machine, the winding may be disposedin the stator or the rotor of the machine, or in both. A problem with known embodiments of electromagnetical de-vices of the above indicated nature is that it is either relativelydifficult to achieve efficient control within a certain spectrum ofparameters or that the control arrangements tend to be rela-tively costly. It is in this connection pointed out that it is knownwithin the aenerator art to execute control of operation pa-rameters via the field winding. If the rotor comprises electro- 011417 magnets, this field winding is provided on the rotor with thedisadvantages this involves in the form of a more expensiveand more difficult-to-control embodiment. In the case of a per-manent magnet rotor, the problem arises that field control is 5 not practically possible. This makes it, of course, more difficultto carry out control in general and in especially délicate controlsituations in particular. A further problem with prior art is thatthe conventional winding technique makes it expensive toobtain the windings. The known embodiments aiso cause sub- 10 stantial energy losses and involve restrictions as far as the lo-cation of the windings in the magnetic circuit is concerned.

SUMMARY OF THE INVENTION 15 The object of the présent invention is to devise ways to simplifyand improve the possibilities to control operation of eiectro-magnetic devices according to the precharacterizing part of theenclosed claim 1, better conditions for rational winding produc-tion and mounting also being aimed al. 20

The basic object of the présent invention is achieved by ar-ranging the control arrangement to control frequency, ampli-tude and/or phase with respect to electric power to/from thedevice by the control arrangement comprising means to control 25 the magnetic flux in the magnetic circuit.

Thus, the présent invention is based upon the idea to directlyaffect, by flux control, the magnetic flux in the magnetic circuitin a desired regard so as to be able to control the operation of 30 the device. This provides a very rational and cost efficient em-bodiment, and besides increased possibilities to control. so asto achieve an optimized operation.

According to a particularly preferred embodiment of the inven- 35 tion, the control means comprises at least one control winding 4 1417 inductively connected to the magnetic circuit. The control ar-rangement is, accordingly, capable of effecting, via the controlwinding, required control of the magnetic flux in the magneticcircuit by applying, via the control winding, such control pa-rameters that the magnetic flux in the magnetic circuit is af-fected in the required extent. The control winding could evenbe short-circuited. The magnetic flux may then in certain em-bodiments not pass the control winding. Depending upon thedesign of the magnetic circuit, partial or total blocking of themagnetic flux may occur.

Examples of control functions which may be achieved with thesolution according to the invention are voltage change andvoltage stabilization, élimination of transients, damping of os-cillations in the power network, filtering-off of overtones, fre-quency adjustments and phase adjustments (in case separatecontrol for the phases is provided for). It is pointed out that thecontrol arrangement according to the invention may be adaptedto add a magnetic flux addition to the magnetic flux in the mag-netic circuit, i.e. the control arrangement could operate withdirect energy supply.

The control according to the invention of the magnetic flux inthe magnetic circuit means, for instance in a transformer, that agood control can be executed over the secondary winding volt-age so that it fulfils the requirements imposed despite trouble-some fluctuations regarding the primary voltage or the loadconnected to the secondary winding.

Further details and advantages with the flux control accordingto the invention in the magnetic circuit will appear from the fol-lowing detailed description.

It is within the scope of the invention that at least one of thewindings of the electromagnetic device or at least a part of this 01141 7 winding comprises ai least one flexible electrical conductorhaving a casing, which is magnetically perméable but capableof substantially enclosing the electric field occurring around theconductor. Expressed in other words, this means that the flexi- 5 ble electrical conductor and the casing thereof (in the form ofan insulation System) are formed by means of a flexible cable.This involves substantial advantages with respect to manufac-turing and mounting compared to the rigid windings in prefab-ricated shape which hâve been conventional up to now. The 10 insulation System according to the invention results, in addition,in absence of gaseous and liquid insulation materials.

Since the electric field occurring about the electric conductor inthe cable in the invention is substantially enclosed in the însu- 15 lation System, the invention reduces occurring losses such thatthe device accordingly may operate with a higher degree of ef-ficiency. The réduction of losses results, in its turn, in a lowertempérature in the device, which reduces the need for coolingand allows possibly occurring cooling devices to be designed in 20 a more simple way than without this aspect of the invention.

As to the aspect of the Invention as a rotating electric machine itis thus possible to operate the machine with such a high voltagethat the conventional step-up transformers can be omitted. That 25 is, the machine can be operated with a considerably higher volt-age than machines according to the State of the art to be able toperform direct connection to power networks. This means con-siderably lower investment costs for Systems with a rotatingelectric machine and the total efficiency of the System can be in- 30 creased. The invention éliminâtes the need for particular fieldcontrol measures at certain areas of the winding, such field con-trol measures having been necessary according to the prior art.A further advantage is that the invention makes it more simple toobtain under- and overmagnetization for the purpose of reducing 01 141 7 reactive effects as a resuit of voltage and current being out ofphase with each other.

As to the aspect of the invention as a power transformer/ re- 5 actor, the invention, first of ail, éliminâtes the need for oil fillingof the power transformers and the problems and disadvantagesassociated thereto.

The design of the winding so that it comprises, along at ieast a 10 part of its length, an insulation formed by a solid insulating mate-rial, inwardly of this insulation an inner layer and outwardly ofthe insulation an outer layer with these layers made of a semiconducting matériel makes it possible to enclose the electricfield in the entire device within the winding. The term "solid in- 15 sulating material" used herein means that the winding is to lackliquid or gaseous insulation, for instance in the form of oil. In-stead the insulation is intended to be formed by a polymericmaterial. Also the inner and outer layers are formed by a poly-meric material, though a semiconducting such. 20

The inner layer and the solid insulation are rigidly connected toeach other over substantially the entire interface. Also the outerlayer and the solid insulation are rigidly connected to each otherover substantially the entire interface therebetween. The inner 25 layer opérâtes equalizing with respect to potential and, accord-ingly, equalizing with respect to the electrical field outwardly ofthe inner layer as a conséquence of the semiconducting proper-ties thereof. The outer layer is aiso intended to be made of asemiconducting material and it has at Ieast an electrical conduc- 30 tivity being higher than that of the insulation so as to cause theouter layer, by connection to earth or otherwise a relatively lowpotential, to function equalizing with regard to potential and tosubstantially enclose the electrical field resulting due to saidelectrical conductor inwardly of the outer layer. On the other C11417 hand, the outer layer should hâve a resistivity which is sufficientto minimize electrical losses in said outer layer.

The rigid interconnection between the insulating material and the 5 inner and outer semiconducting layers should be uniform oversubstantially the entire interface such that no cavities, pores orsimilar occur. With the high voltage levels contemplated ac-cording to the invention, the electrical and thermal loads which.may arise wil! impose extreme demands on the insulation mate- 10 rial. It is known that so-called partial discharges, PD, generallyconstitute a serious problem for the insulating material in high-voltage installations, if cavities. pores or the iike arise, internaicorona discharges may arise at high electric voltages, wherebythe insulating material is gradually degraded and the resuit could 15 be electric breakdown through the insulation. This may tead toserious breakdown of the electromagnetic device. Thus, theinsulation should be homogenous.

The inner layer inwardly of the insulation should hâve an electri- 20 cal conductivity which is lower than that of the electrical con-ductor but sufficient for the inner layer to function equalizing withregard to potential and, accordingly, equalizing with respect tothe electrical field externally of the inner layer, This in combina-tion with the rigid interconnection of the inner layer and the 25 -etectrical insulation over substantially the entire interface, i.e.the absence of cavities etc, means a substantially uniform elec-trical field externally of the inner layer and a minimum of risk forPD. 30 It is preferred that the inner layer and the solid electricai insula-tion are formed by materials having substantially equal thermalcoefficients of expansion, The same is preferred as far as theouter layer and the solid insulation are concerned. This meansthat the inner and outer layers and the solid electricai insulation 35 will form an insulation System which on température changes 011417 expands and contracts uniformly as a monolithic part withoutthose température changes giving rise to any destruction ordisintegration in the interfaces. Thus, intimacy in the contactsurface between the inner and outer layers and the soiid insula- 5 tion is ensured and conditions are created to maintain this inti-macy during prolonged operation periods. The adhérence shouldbe of such a nature that adhérence between at least the innerlayer and the solid insulation and preferably also the outer layerand the solid insulation is ensured also in connection with such 10 bending that the electric conductor and the insulation System willbe subjected to. It is pointed out here that the cable, in order tobe able to carry out threading of the winding, should be bend-able or flexible in a radius of curvature which is less than 25times the cable diameter, preferably less than 15 times the. cable 15 diameter. The most preferred is that the cable is flexible down toa radius of curvature which is less than or substantially similar to8 times the cable diameter.

It is essential that the insulation System consists of materiais 20 having a good elasticity. The E-modulus of the materiais shouldbe comparatively low, i.e. the résistance to deformation of thematerial should be relatively low. In order to avoid that hazard-ous shear tensions occur in the border zone between differentlayers contained in the insulation System, it is preferred that the 25 electricity (E-modulus) of the layers contained in the insulationSystem is substantially equal.

The electrical load on the insulation System decreases as a con-séquence of the fact that the inner and the outer layers of semi- 30 conducting material around the insuiation will tend to form sub-stantially equipotential surfaces and in this way the electricalfield in the insulation properly will be distributed retatively uni-formly over the thickness of the insulation. C1 1 41 7

It is known, per se, in connection with transmission cables forhigh-voitage and for transmission of electric energy, to designconductors with an insulation of a soiid insulation material withinner and outer layers of semiconducting material. In transmis- 5 sion of electric energy, it has since long been realised thaï theinsulation should be free from defects. However, in high voltagecables for transmission, the electric potential does not changealong the length of the cable but the potential is basically at the.same level. However, also in high voltage cables for transmis- 10 sion purposes, instantaneous potential différences may occurdue to transient occurrences, such as lightning. According to theprésent Invention a flexible cable according to the encloseddaims is used as a winding in the electromagnetic device. 15 An additional improvement may be achieved by constructing theelectric conductor in the winding from smaller, so-called strands,at least sortie of which are insulated from each other. By makingthese strands to hâve a relatively small cross section, preferablyapproximately circular, the magnetic field across the strands will 20 exhibit a constant geometry in relation to the field and the occur-rence of eddy currents are minimized.

According to the invention, the winding is thus preferably madein the form of a cable comprising the electric conductor and the 25 previously described insulation System, the inner layer of whichextends about the strands of the conductor. Outside of this innersemiconducting layer is the main insulation of the cable in theform of a soiid insulation material. 30 The outer semiconducting layer shaii according to the inventionexhibit such electrical properties that a potential equalizationalong the conductor is ensured. The outer layer may, however,not exhibit such conductivity properties that an induced currentwill flow along the surface, which could cause losses which in 35 turn.may create an unwanted thermal load. For the inner and 10 011417 outer layers the résistance statements (at 2O°C) defined in theenclosed daims 22 and 23 are valid. With respect to the innersemiconducting layer, it must hâve a sufficient electrical con-ductivity to ensure potential equalization for the electrical field 5 but at the same time this layer must hâve such a resistivity thatthe enclosing of the electric field is ensured.

It is important that the inner layer equalizes irregularities in thesurface of the conductor and forms an equipotential surface with 10 a high surface finish at the interface with the solid insuiation.The inner layer may be formed with a varying thickness but toensure an even surface with respect to the conductor and thesolid insuiation, the thickness is suitably between 0.5 and 1 mm. 15 Such a flexible winding cable which is used according to theinvention in the electromagnetic device thereof is an improve-ment of a XLPE (cross-linked poly ethylene) cable or a cablewith EP (ethylene-propylene) rubber insuiation. The improve-ment comprises, inter aiia, a new design both as regards the 20 strands of the conductor and in that the cable, at least in someembodimënts, has no outer casing for mechanical protection ofthe cable. However, it is possible according to the invention toarrange a conducting métal shield and an outer mantle exter-nally of the outer semiconducting layer. The métal shield will 25 then hâve the character of an outer mechanical and electricalprotection, for instance to lightning. It is preferred that the innersemiconducting layer will lie on the potential of the electricalconductor. For this purpose at least one of the strands of theelectrical conductor will be uninsulated and arranged so that a 30 good electrical contact is obtained to the inner semiconductinglayer. Alternativeiy, different strands may be alternatinglybrought into electrical contact with the inner semiconductinglayer.

Il 011 417

Manufacturing transformer or reactor windings of a cable ac-cording to the above entails drastic différences as regards theelectric field distribution between conventional power transform-ers/reactors and a power transformer/reactor according to the 5 invention. The décisive advantage with a cable-formed windingaccording to the invention is that the electric field is enclosed inthe winding and that there is thus no electric field outside theouter semiconducting layer. The electric field achieved by thecurrent-carrying conductor occurs only in the solid main insula- 10 tion. Both from the design point of view and the manufacturingpoint of view this means considérable advantages; - the windings of the transformer may be formed without havingto consider any electric field distribution and the transposition of 15 strands, mentioned underthe background art, is omitted; - the core design of the transformer may be formed without hav-ing to consider any electric field distribution; 20 - no oil is needed for electrical insulation of the winding, that is, the medium surrounding the winding may be air; - no spécial connections are required for electrical connectionbetween the outer connections of the transformer and the inrime- 25 diately connected coils/windings, since the electrical connection,contrary to conventional plants, is integrated with the winding; - the manufacturing and testing technoiogy which is needed for apower transformer according to the invention is considerably 30 simpler than for s conventional power transformer/reactor sincethe imprégnation, drying and vacuum treatments described un-der the description of the background art are not needed. in application of the invention as a rotating electric machine a 35 substantially reduced thermal load on the stator is obtained. 12 01141 7

Temporary overloads of the machine will, thus. be less criticaland it will be possible to drive the machine at overload for alonger period of time without running the risk of damage arising.This means considérable advantages for owners of power gen-erating plants who are forced today, in case of operational dis-turbances, to rapidly switch to other equipment in order to en-sure the delivery requirements laid down by law.

With a rotating electric machine according to the invention, themaintenance costs can be significantly reduced because trans-formers and circuit breakers do not hâve to be included in theSystem for connecting the machine to the power network.

Above it has aiready been described that the outer semicon-ducting layer of the winding cable is intended to be connected toground potential. The purpose is that the layer should be keptsubstantially on ground potential along the entire length of thewinding cable. It is possible to divide the outer semiconductinglayer by cutting the same into a number of parts distributedalong the length of the winding cable, each individual layer partbeing connectable directly to ground potential. In this way abetter uniformity aiong the length of the winding cable isachieved.

Above it has been mentioned that the solid insulation and theinner and outer layers may be achieved by, for instance, extru-sion. Other techniques are, however, also weil possible, for in-stance formation of these inner and outer layers and the insula-tion respectively by means of spraying of the material in ques-tion onto the conductor/winding.

It is preferred that the winding cable is designed with a circularcross section. However, also other cross sections may be usedin cases where it is desired to achieve a better packing density. 13 011417 Το build up a voltage in the rotating electric machine, the cableis disposed in several consecutive turns in slots in the magneticcore. The winding can be designed as a multi-layer concentriccable winding to reduce the number of coil-end crossings. The 5 cable may be made with tapered insulation to utilize the mag-netic core in a better way, in which case the shape of the slotsmay be adapted to the tapered insulation of the winding. A significant advantage with a rotating electric machine accord- 10 ing to the invention is that the E field is near zéro in the coil-endrégion outside the outer semiconductor and that with the outercasing at ground potential, the electric field need not be con-trolled. This means that no field concentrations can be obtained,neither within sheets, in coil-end régions nor in the transition 15 therebetween.

In a method for manufacturing a device according to the inven-tion, a flexible cable, which is threaded into openings in slots ina magnetic core of the rotating electrical machine, is used as a 20 winding. Since the cable is flexible, it can be bent and this per-mits a cable length to be disposed in several turns in a coil, Thecoil ends will then consist of bending zones in the cables. Thecable may also be joined in such a way that its properties remainconstant overthe cable length. This method entails considérable 25 simplifications compared with the state of the art. The so-calledRoebel bars are not flexible but must be preformed into the de-sired shape. Winding of insulation and imprégnation of the coilsis also an exceedingly complicated and expensive techniquewhen manufacturing rotating electric machines today. 30

To sum up, thus, an electromagneiic device in the form of arotating electric machine according to the invention means aconsidérable number of important advantages in relation tocorresponding prior art machines. First of ail, the machine 35 according to the invention can be connected directly to a power 14 tn w? network at ait types of high voltage. Another importantadvantage is that ground potential has been consistentlyconducted along at least a part of and preferably along thewhole winding, which means that the coil-end région can be 5 made compact and that bracing means at the coil-end régioncan be applied at practically ground potential. Still anotherimportant advantage is that oil-based insulation and cooling Sys-tems disappear also in rotating electric machines as aiready hasbeen pointed out above with regard to power 10 transformers/reactors. This means that no sealing problems mayarise and that the dielectric ring previously mentioned is notneeded. Important is also that ail forced cooling can be made atground potential.

15 BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the enclosed drawings, a more spécifie de-scription of embodiment examples of the invention will followhereinafter. 20

In the drawings:

Fig 1 is a diagrammatical view illustrating the device ac-cording to the invention in the form of a transformer; 25 Fig 2 is a diagrammatical view of a transformer variant; Fig 3 is a diagrammatical view of a further transformervariant; Fig 4 is a view of an embodiment similar to the one in Fig 3but concerns a reactor; 30 Fig 5 is a diagrammatical view illustrating a generator em-bodiment; Fig 6 is a partly eut view showing the parts included in thecurrent modified standard cable; Fig 7 is an axial end view of a sector/pole pitch of a mag- 35 netic circuit according to the invention: 15 011417

Fig 8 is a view showing the electric field distribution arounda winding of a conventional power trans-former/reactor;

Fig 9 is a perspective view showing an embodiment of a5 power transformer according to the invention;

Fig 10 is a cross section illustrating a cable structure modi-fied relative to Fig 1 and having several electricalconductors; and

Fig 11 is a cross section of a further cable structure com-10 prising several electric conductors but in another ar- rangement than that in Fig 10.

DESCRIPTION OF PREFERRED EMBODIMENTS 15 The electromagnetic device illustrated in Fig 1 has the nature ofa transformer. It comprises a magnetic circuit 1 and two electriccircuits 2, 3, each comprising at least one coil shaped winding 4and 5 respectively. 20 It is illustrated in the example that the transformer comprises acore 6 of a magnetic material. The core consists suitably of apackage of magnetic sheets to reduce eddy-current losses.However, it is pointed out that it is not a prerequisite for applica-tion of the invention that a core is really présent. Air wound em- 25 bodiments etc are, thus, well possible within the scope of theinvention, it follows from this that the term magnetic circuit is tobe interpreted in a wide sense. The term in question means,accordingly, not more than that the magnetic field generated bythe windings 4; 5 occurring should be capable of generating s 30 magnetic flux.

The device according to the invention comprises an arrangementgenerally denoted 7 to control operation of the transformer. Thiscontrol arrangement 7 is adapted to control frequency, amplitude 35 and/or phase as concerns electric power exiting the transformer. 16 011417

In the example the electric circuit 2 forms the primary side of thetransformer whereas the electric circuit 3 forms the secondaryside of the transformer. Power from the device exits, accord-ingly, via the secondary circuit 3, to which a load diagrammati- 5 cally indicated with 8 is coupled. This load may be of an arbi-trary nature, e.g. consumers proper but also distribution andtransmission networks.

The control arrangement 7 comprises means 9 for controlling the 10 magnetic flux in the magnetic circuit 1. The control means 9includes, in the example, at least one control winding inductivelyconnected to the magnetic circuit 1. In the example this controlwinding 9 is wound about a portion of the core 6. In a corelesstransformer embodiment the control winding 9 must be co-ordi- 15 nated such with the primary and secondary windings 4 and 5respectively that the magnetic flux induced in the coreless mag-netic circuit is inductively coupled to the control winding 9.

The control arrangement 7 is, according to a preferred embodi- 20 ment of the invention, conceived to be of an active type, i.e. thecontrol arrangement 7 should be adapted to actively control, viathe control winding 9, the magnetic flux in the magnetic circuit 9to obtain the desired character. It is then preferred that thecontrol arrangement 7 comprises an externai power source such 25 that the control arrangement 7 is capable of controlling themagnetic flux through the magnetic circuit 1 by causing a currentto flow through the winding 9. The invention is particularly préf-érable in connection with high voltage applications. This means,accordingly, that a comparatively high voltage is normally in- 30 tended to be associated to the electric circuits 2 and 3. in such acase, it is, however, sufficient for control purposes that the con-trol arrangement 7 causes a relatively high current to flow in thewinding 9 with a relatively low voltage. The control arrangement7 may be adapted to, for control purposes. add a magnetic flux 35 addition to the magnetic flux in the magnetic circuit 1. This flux 17 011417 addition will be added to the flux otherwise occurring and bysuitable control of this flux addition, the desired parameters withregard to the power exiting through the secondary circuit 3 maybe achieved. The arrangement 7 may be adapted to receive, as 5 basis for its control function, voltage information from a voltagemeasuring device 10 with respect to the voltage in the secon-dary circuit and/or over the load 8. A current measuring member11 serves for current measurement in the secondary circuit 3.The flux addition generated via the control arrangement 7 may, 10 as mentioned before, be used to control frequency, amplitudeand/or phase as concerns the power exiting via the secondarycircuit 3.

It is pointed out thaï the control arrangement 7 may be adapted 15 to obtain external control instructions via an input 12.

Furthermore, it is pointed out that the control arrangement 7 maybe adapted to effect a passive control via the control winding 9.A passive control in this regard means that power from some 20 external source is not used for control. In this connection it ispointed out that the control arrangement 7 may be capable ofcoupling one or more passive éléments, such as resistors, ca-pacitors or inductances coupled in sériés or paraliei, over thecontrol winding 9. Such passive éléments coupled to the control 25 winding 9 in a manner adapted to the purpose enable, accord-ingly, different influences on the magnetic flux, said influences intheir turn resulting in conséquences with respect to frequency,amplitude and/or phase as concerns the electric power from thedevice. 30

It also appears from Fig 1 that the device on its primary sidecomprises a voltage measuring device 13 and a current meas-uring device 14 similar to what is occurring on the secondaryside. 35 18 011417

Fig 2 illustrâtes a transformer embodiment differing from the onejust described in Fig 1 only in the regard that the magnetic cir-cuit 1 here comprises a core 6 compristng a further leg 16 inaddition to the one occurring on the secondary side in Fig 1 and 5 denoted 15 and the one occurring on the primary side and de-noted 17. Thus, this means that the core 6 according to Fig 2will form two different flux paths diagrammatically indicated with18 and 19 respectively. The control winding 9a is arranged inthis case around the central leg 16, i.e, at the flux path 18, which 10 passes the primary winding 4 of the transformer. The secondflux path 19, on the contrary, passes around the control winding9a via the secondary winding 5. It is now possible via the controlarrangement 7, to affect the magnetic flux in the leg 16 bymeans of the control winding 9a, which in its turn will affect the 15 magnetic flux in the leg 15 through the winding 5 of the secon-dary side. Expressed in other words, the control winding 9a ishere only associated to one of the two flux paths.

The variant in Fig 3 means addition of a further controi winding 20 9b2 to the one already occurring 9b1. These two control wind- ings are arranged around its own of the legs 16b, 15b, i.e. thesecontrol windings 9b1 and 9b2 will belong to its own of the fluxpaths 13, 19. The control arrangement 7b comprises a controlunit 20, which in its turn Controls control éléments 21 and 22 25 respectively coordinated with the control windings 9b1 and 9b2respectively. By actively or passively controlling the control élé-ments 21, 22 via the control unit 20, an adjustment may bemade so that the magnetic flux either passes through only one ofthe flux paths 18, 19 or is divided on the same. 30

In connection with Fig 3 it should also be mentioned that thesecondary winding 4b of the transformer comprises at least twowinding parts 23 and 24 respectively coupled in sériés. Themagnetic flux in both flux paths 18, 19 passes through the main 35 winding part 23 whereas only the flux in the flux path 19 passes 19 01 141 7 through the winding part 24. Thus, this means that when themagnetic flux is allowed to pass only through the leg 16b bymeans of the control windings 9b1 and 9b2, no magnetic fluxpasses through the winding part 24. Thus, this means lower 5 output voltage than that which is due for the operation casewhere the magnetic flux passes entirely through the flux path 19than when the total magnetic flux passes through both secon-dary winding parts 23 and 24. Thus, the control winding 9b1 isintended, in such an operation case, to hâve interrupted mag- 10 netic flux through the leg 16b entirely or at least partially.

Fig 4 illustrâtes a reactor embodiment somewhat reminding ofthe transformer according to Fig 3. The différence consists inthat the reactor does not hâve any secondary side so that in- 15 stead its power winding is divided into two winding parts 25, 26.As in the preceding embodiment, there are two control windings9c1 and 9c2, by means of which the magnetic flux may be con-trolled so that it passes through the winding part 26 in a desireddegree. The entire flux always passes through the winding part 20 25.

Fig 5 illustrâtes a very simplified generator embodiment, therotor of which is denoted 26. This rotor is in the exampie con-ceived to be a permanent magnet rotor. It would, however, also 25 be possible to design the rotor with field windings. The magneticcircuit 1d comprises here an eiectric output circuit 5d inductivelycoupled to the magnetic flux in the core 6d. The core 6d hasportions located adjacent to the rotor 26 such that the perma-nent magnets will generate a magnetic flux in the core during 30 rotation of the rotor. This flux passes through the output winding5d and generates an output effect therein. The control arrange-ment 7d comprises as before a control winding 9d inductivelycoupled to the magnetic circuit 1d. Measuring devices 10d and11d respectively for voltage and current occur also here to su- 35 pervise the output power. By means of the control arrangement 20 011417 7d the control winding 9d may now be subjected to a functional-ity, required for the control purpose, passively or actively, forimparting the output power from the generator desired propertieswith regard to frequency, amplitude and/or phase. 5 lt is pointed out that very simplified embodiments are presentedin the figures and this more specifically only with one phase. Inreality the embodiments may be much more complicated, inparticular multiphase embodiments. The number of windings and 10 winding parts may be much higher than what has beendiscussed, not only as far as the primary and secondary wind-ings are concerned but also with respect to the number of côn-troi windings. Also the magnetic circuits may hâve a varyingdesign depending upon functional requirements. 15 lt is particularly pointed out that the circumstance that accordingto the invention at least one of the occurring windings comprisesan electric conductor surrounded by two mutually spaced equi-potential layers and a solid insulation placed between these 20 layers means that the electric field around the conductor will besubstantially enclosed in the cable such that the primary andsecondary windings may be placed anywhere on the magneticcircuit with a very great freedom. £ven interposition of thewindings is possible, lt is in this connection pointed out that the 25 control arrangement is useful for transformers both of the typewith a core and a shell.

In particular in high voltage applications the just described de-sign of the winding is suitable. lt is pointed out that normally the 30 control winding/control windings S will be at a lower potentielthan the power windings, for what reason the control wind-ing/control windings do not necessarily hâve to be provided withsuch an insulation System as at least one of the power windings. 2] 011417

An important aspect for being able to provide an electromagneticdevice in accordance with the invention, is to use for at leastone of the winding a conductor cable with a solid eiectrical in-sulation with an inner semiconducting layer or casing between 5 the insulation and one. or more eiectrical conductors locatedinwardly thereof and with an outer semiconducting layer or cas-ing located outwardly of the insulation. Such cables are avail-able as standard cables for other power engineering fields ofuse, namely power transmission. To be able to describe an em- 10 bodiment, initially a short description of a standard cable will bemade. The inner current-carrying conductor comprises a numberof strands. Around the strands there is a semiconducting innerlayer or casing. Around this semiconducting inner layer, there isan insulatina layer of solid insulation. The solid insulation is 15 formed by a polymeric material with low eiectrical losses and ahigh breakthroûgh strength. As concrète examples polyethyiene(PE) and then particularly cross-linked polyethyiene (XLPE) andethylene-propylene (EP) may be mentioned. Around the outersemiconducting layer a métal shield and an outer insulation 20 casing may be provided. The semiconducting layers consist of apolymeric material, for example ethylene-copolymer, with anelectrically conducting constituent, e. g. conductive soot or car-bon black. Such a cable will be referred to hereunder as a power.cable. 25 A preferred embodiment of a cable intended for a winding in arotating eiectrical machine appears from Fig 6. The cable 41 isdescribed in the figure as comprising a current-carrying con-ductor 42 whicn comprises transposed both non-insulated and 30 insulated strands. Electromechanically transposed, solidly insu-lated strands are also possible. These strands may bestranded/transposed in a plurality of layers. Around the conduc-tor there is an inner semiconducting layer 43 which, in turn, issurrounded by a homogenous layer of a solid insulation material. 35 The insulation 44 is entirely without insulation material of liquid 22 01141 7 or gaseous type. This layer 44 is surrounded by an outer semi-conducting layer 45. The cable used as a winding in the pre-ferred embodiment may be provided with métal shield and exter-nat sheath but must not be so. To avoid induced currents and 5 losses associated therewith in the outer semiconducting layer45, this is eut off, preferably in the coil end, that is, in the tran-sitions from the sheet stack to the end windings. The cut-off iscarried such that the outer semiconducting layer 45 will be di-vided into several parts distributed along the cable and being 10 electrically entirely or partly separated from each other. Eachcut-off part is then connected to ground, whereby the outersemiconducting layer 45 will be maintained ât, or near, groundpotential in the whole cable length. This means that, around thesolidly insulated winding at the coil ends, the contactable sur- 15 faces, and the surfaces which are dirty after some time of use,only hâve negligible potentials to ground, and they also causenegligible electric fields.

To optimize a rotating electric machine, the design of the mag- 20 netic circuit as regards the slots and the teeth, respectively, areof décisive importance. As mentioned above, the slots shouidconnect as ciosely as possible to the casing of the coil sides. Itis also désirable that the teeth at each radial level are as wideas possible. This is important to minimize the losses, the mag- 25 netization requirement, etc., of the machine.

With access to a conductor for the winding such as for example,the cable described above, there are great possibilities of beingable to optimize the magnetic core from several points of view. 30 In the foliowing, a magnetic circuit in the stator of the rotatingelectric machine is referred to. Figure 7 shows an embodimentof an axial end view of a sector/pole pitch 46 of a machineaccording to the invention. The rotor with the rotor pôle isdesignated 47. In conventional manner, the stator is composed 35 of a laminated core of electric sheets successively composed of 23 011417 sector-shaped sheets. From a back portion 48 of the core, lo-cated at the radially outermost end, a number of teeth 49 extendradialiy inwards towards the rotor. Between the teeth there are acorresponding number of siots 50. The use of cabies 51 ac- 5 cording to the above among other things permits the depth ofthe siots for high-voltage machines to be made larger than whatis possible according to the State of the art. The siots hâve across section tapering towards the rotor since the need of cableinsuiation becomes lower for each winding layer towards the 10 rotor. As is clear from the figure, the Slot substantially consistsof a circular cross section 52 around each layer of the windingwith narrower waist portions 53 between the iayers. With somejustification, such a slot cross section may be referred to as a"cycle chain slot". Since there will be required, in such a high 15 voltage machine, a relatively large number of Iayers and theavailability of cabies in relevant dimensions and having relevantinsuiation and external semiconductors is restricted it may inpractice be difficult to achieve a desired continuous tapering ofthe cable insuiation and the stator siot respectively. In the 20 embodiment shown in Figure 7, cables with three different di-mensions of the cable insuiation are used, arranged in threecorrespondingly dimensioned sections 54, 55 and 56, that is, inpractice a modified cycle chain slot will be obtained. The figurealso shows that the stator tooth 49 can be shaped with a 25 practically constant radial width along the depth of the wholeslot.

It is again pointed out that the winding sections denoted 54, 55and 56 in Fig 7 correspond to the winding denoted 5d in Fig 5. In 30 Fig 7 on the contrary, one or more windings corresponding to thecontrol winding 9 in Fig 5 are denoted with the reference 40.These control windings 40 are in the embodiment located radi-ally outermost from the rotor. It is pointed out that it is not nec-essary to locate the control winding 9 on the location denoted 40 35 in Fig 7. 24 01 1417

In an alternative embodiment, the cable which is used as awinding may be a conventional power cable as the one de-scribed above. The grounding of the outer semiconducting Jayer 5 45 then takes place by stripping the métal shield and the sheath of the cable at suitable locations.

The scope of the invention accommodâtes a large number of al-ternative embodiments, depending on the available cable di- 10 mensions as far as insuiation and the outer semiconductor layeretc. are concerned. Also embodiments with so-called cycle chainslots can be modified in excess of what has been describedhere. 15 As mentioned above, the magnetic circuit may be located in thestator and/or the rotor of the rotating electric machine. However,the design of the magnetic circuit wiil largeiy correspond to theabove description independently of whether the magnetic circuitis located in the stator and/or the rotor. 20

As winding, a winding is preferably used which may be de-scribed as a multilayer, concentrée cable winding. Such a wind-ing means that the number of crossings at the coil ends hasbeen minimized by placing ail the coils within the same group 25 radially outside one another. This also permits a simpler methodfor the manufacture and the threading of the stator winding inthe different slots. Since the cable used according to the inven-tion is relatively easily flexible, the winding may be obtained by acomparatively- simple threading operation, in which the flexible 30 cable is threaded into the openings 52 présent in the slots 50.

Figure 8 shows a simplified and fundamental view of the electricfield distribution around a winding of a conventional powertransformer/reactor, where 57 is a winding and 58 a core and 59 35 illustrâtes equipotential lines, that is, lines where the electric 25 011417 field has the same magnitude. The lower part of the winding isassumed to be at ground potential.

The potential distribution détermines the composition of the in- 5 sulation System since it is necessary to hâve sufficîent insulationboth between adjacent turns of the winding and between eachturn and ground. The figure thus shows that the upper part of thewinding is subjected to the highest insulation loads. The designand location of a winding relative to the core are in this way 10 determined substantially by the electric field distribution in thecore window.

The cable which can be used in the windings contained in thedry power transformers/reactors according to the invention hâve 15 been described with assistance of Fig 1. The cable may, asstated before, be provided with other, additional outer layers forspécial purposes, for instance to prevent excessive electricalstrains on other areas of the transformer/reactor. From the pointof view of geometrical dimension, the cables in question will 20 hâve a conductor area which is between 2 and 3000 mm2 andan outer cable diameter which is between 20 and 250 mm.

The windings of a power transformer/reactor manufactured fromthe cable described under the summary of the invention may be 25 used both for single-phase, three-phase and polyphasé trans-formers/reactors independently of how the cote is shaped. Oneembodiment is shown in Figure 8 which shows a thrëe-phaselaminated core transformer. The core comprises, in conventionatmanner, three’core limbs 60. 61 and 62 and the retaining yokes 30 83 and 64. In the embodiment shown, both the core limbs and the yokes hâve a tapering cross section.

Concentrically around the core limbs, the windings formed withthe cable are disposed. As is clear, the embodiment shown in 35 Figure 9 has three concentric winding turns 65. 66 and 67. The 26 011417 innermost winding turn 65 may represent the primary windingand the other two winding turns 63 and 64 may represent secon-dary windings. In order not to overload the figure with too manydetails, the connections of the windings are not shown. Other- 5 wise the figure shows that, in the embodiment shown, spacingbars 68 and 69 with several different functions are disposed atcertain points around the windings. The spacing bars may beformed of insulating material intended to provide a certain spacebetween the concentric winding turns for cooling, bracing, etc. 10 They may also be formed of electrically conducting material inorder to form part of the grounding System of the windings.

No control windings 9 are drawn in Fig 9. 15 Alternative cable designs

In the cable variant illustrated in Fig 10, the same referencecharacters as before are used, only with the addition of the lettera characteristic for the embodiment. In this embodiment the 20 cable comprises several electric conductors 42a, which are mu-tually separated by means of insulation 44a. Expressed in otherwords, the insulation 44a serves both for insulation betweenindividual adjacent electrical conductors 42a and between thesame and the surrounding. The different electrical conductors 25 42a may be disposed in different manners, which may provide for varying cross-sectional shapes of the cable in its entirety. Inthe embodiment according to Fig 10 it is illustrated that the con-ductors 42a are disposed on a straight line, which involves arelatively fiat cross-sectional shape of the cable. From this it can 30 be concluded that the cross-sectional shape of the cable mayvary within wide iimits.

In Fig 10 there is supposed to exist, between adjacent electricalconductors, a voltage smaller than phase voltage. More specifi- 35 caliy, the electrical conductors 42a in Fig 10 are supposed to be 011417 formed by different révolutions in the winding, which means thatthe voltage between these adjacent conductors is comparativelylow. 5 As before, there is a semiconducting outer layer 45a exteriorly ofthe insulation 44a obtained by a solid electrical insulation mate-rial. An inner layer 43a of a semiconducting material is arrangedabout each of said electrical conductors 42a, i.e. each of theseconductors has a surrounding inner semiconducting layer 43a of 10 its own. This layer 43a will, accordingly, serve potential equaliz-ing as far as the individual electrical conductor is concerned.

The variant in Fig 11 uses the same reference characters asbefore only with addition of the letter b spécifie for the embodi- 15 ment. Also in this case there are several, more specifically three,electrical conductors 42b. Phase voltage is supposed to be pré-sent between these conductors, i.e. a substantially higher volt-age than the one occurring between conductors 42a in the em-bodiment according to Fig 10. In Fig 11 there is an inner semi- 20 conducting layer 43b inwardly of which the electrical conductors42b are arranged. Each of the electrical conductors 42b is, how-ever, enclosed by a further layer 70 of its own, with propertiescorresponding to the properties discussed hereinabove withregard to the inner layer 43b, Between each further layer 70 and 25 the layer 43b arranged thereabout, there is insulation material.Accordingly, the layer 43b will occur as a potential equaiizinglayer outside the further layers 60 of semiconducting materialbelonging to the electrical conductors, said further layers 70being connected to the respective electrical conductor 42b to be 30 placed on the same potential as the conductor.

Possible modifications

It is évident that the invention is not only limited to the embodi- 35 ments discussed above. Thus, the man skilled within this art will 011417 réalisé that a number of detailed modifications are possiblewhen the basic concept of the invention has been presentedwithout deviating from this concept as it is defined in the en-closed daims. As an example, it is pointed oui that the invention 5 is not only restricted to the spécifie material sélections exempli-fied above. Functionally equal materials may, accordingly, beused instead. As far as the manufacturing of the insulation Sys-tem according to the invention is concerned, it is pointed out thatalso other techniques than extrusion and spraying are possible 10 as long as intimacy between the various layers is achieved.Furthermore, it is pointed out that additional equipotential layerscould be arranged. For example, one or more equipotential la-yers of semiconducting material could be placed in the insulationbetween those layers designated as "inner" and "outer" here- 15 inabove. It is again pointed out that it is normally not supposedto be necessary according to the invention to form the controlwindings 9 by means of such a flexible cable as the one dis-cussed hereinabove as a conséquence of the tact that the con-trol winding or control windings are normally at a lower voltage 20 than the rest of the windings of the electromagnetic device inquestion. More specifically, the rest of the windings may be truehïgh voltage windings. For the rest it is pointed out that the exactcontrol principle on execution of the method according to theinvention may be varied in a variety of ways within the scope of 25 control functions aimed at.

Claims (29)

1. 29 011417 Claims

   1. Electromagnetic device comprising at least one magneticcircuit (1) and at least one electric circuit (2, 3) comprising at 5 least one winding (4, 5), the magnetic and electric circuits be-ing inductively connected to each other and the device com-prising a controi arrangement (7) to control operation of the de-vice, characterized in that the control arrangement (7) isadapted to control frequency, amplitude and/or phase as con- 10 cerns electric power to/from the device by the control arrange-ment comprising means (9) for controlling the magnetic flux inthe magnetic circuit, and that said at least one winding (4, 5) orat least a part thereof comprises at least one electric conductor(42) having an insulation System comprising an electric insula- 15 tion (44) formed by a solid insulation material and interioriythereof an inner layer (43), that said at least one electric con-ductor (42) is arranged interioriy of the inner layer (43) and thatthe inner layer has an electrical conductivity which is lowerthan the conductivity of the electric conductor but sufficient to 20 cause the inner layer (43) to operate for equalization as con-cerns the electrical field exteriorly of the inner layer.
2. 2. A device according to claim 1, characterized in that the con-trol means comprises at least one control winding (9) induc- 25 tiveiy connected to the magnetic circuit.
3. 3. A device according to claim 1 or 2, characterized in that thecontrol arrangement (7) is adapted to control the réluctance inthe magnetic circuit. 30
4. 4. A device according to any preceding claim, characterized inthat the control arrangement is adapted to add a magnetic fluxaddition to the magnetic flux in the magnetic circuit. 011417 30
5. 5. A device according to claim 3, characterized in that materialhaving a permeability greater than 1 is included in the magneticcircuit and that the control arrangement (7) is adapted to con-trol the réluctance in the magnetic circuit by varying the per- 5 meabiiity of one or more such zones of the magnetic circuitwhich hâve variable permeability.
6. 6. A device according to claim 5, characterized in that thezone or zones having a variable permeability comprise one or 10 more gaps in the magnetic circuit.
7. 7. A device according to any preceding claim, characterized inthat the magnetic circuit is without magnetic core. 15 8. A device according to any of daims 1-6, characterized in that the winding is wound about a magnetic core (6).
8. 9. A device according to claim 2 or one or more of the otherdaims, characterized in that the control winding (9) and the 20 winding (4, 5) of the electric circuit are arranged to be passedby substantially the same magnetic flux.
9. 10. A device according to any preceding claim, characterizedin that the device forms a reactor adapted to control, by means 25 of said at least one control winding, frequency, amplitudeand/or phase as concerns the electric power flowing in thewinding (4, 5) of the electric circuit.
10. 11. A device according to any of daims 1-8 or 10. character- 30 ized in that the electric circuit (2) comprises at least two wind- ings (23, 24) coupled in sériés, that the magnetic circuit com-prises at least two alternative flux paths (18, 19), that said atleast one control winding is adapted to control the magneticflux to pass in any of or both of these flux paths and that the 35 two windings of the electric circuit are located such that one of 31 01141 7 them is capable of being switched off from magnetic flux bymeans of said at least one control winding.
11. 12. A device according to any of daims 1-9 or 11, character- 5 ized in thaï the magnetic circuit is arranged in the stator or ro-tor of a rotating electric machine.
12. 13. A device according to any of daims 1-9, characterized inthat the magnetic circuit (1) belongs to a transformer having 10 primary and secondary windings (4, 5) and that the primary andsecondary windings and the control winding (9) are arranged tobe passed by the same magnetic flux.
13. 14. A device according to any of daims 1-8 in a transformer, 15 characterized in that the secondary winding of the transformer comprises at least two winding parts coupled in sériés, that themagnetic circuit comprises at least two alternative flux paths(18, 19), that at least two occurring control windings (9b1, 9b2,9c1, 9c2) are adapted to control the magnetic flux to pass in 20 one or both of these paths and that the two winding parts of hesecondary winding are placed such that one of them is capableof being switched off from magnetic flux by means of thecontrol windings. 25 15. A device according to any of daims 11 and 14, character- ized in that it comprises a magnetic core having at least threelegs coupled in parallel and that two of these legs belong todifferent flux paths whereas the third is common to the two fluxpaths. 30
14. 15. A device according to any preceding daim, characterizedin that the insulation System exteriorly of the insulation com-prises an outer layer (45) which has an electrical conductivitywhich is higher than that of the insulation to make the outer 32 011 4 I 7 layer capable, by connection to earth or otherwise a relativelylow potential, of operating to equaüze potential.
15. 17. A device according to any preceding claim, characterized 5 in that the outer layer is arranged to substantially enclose the electric field, arising as a conséquence of said electrical con-ductor (42), inwardly of the outer layer (45).
16. 18. A device according to any preceding claim, characterized 10 in that the inner layer (43) and the solid insulation présent substantially equal thermal properties.
17. 19. A device according to any preceding claim, characterizedin that the outer layer (45) and the solid insulation présent sub- 15 stantially equal thermal properties.
18. 20. A device according to any preceding claim, characterizedin that said at least one conductor (42) forms at least one in-duction turn. 20
19. 21. A device according to any preceding claim, characterizedin that the inner and/or outer layer (43, 45) comprises a semi-conducting material. 25 22. A device according to any preceding claim, characterized in that the inner layer (43) and/or the outer layer (45) has a re-sistivity in the range 10-6 ncm-100 kOcm, suitably 10'3-1000Ûcm, preferably 1-500 Ωαη. 30 23. A device according to any preceding claim, characterized in that the inner layer (43) and/or the outer layer (55) has a ré-sistance which per length meter of the conductor/insulationsystem is in the range 50 μΩ - 5 ΜΩ. 33 011417
20. 24. A device according to any preceding claim, characterizedin that the solid insulation (44) and the inner layer (43) and/orthe outer layer (45) are formed by polymeric materials. 5 25. A device according to any preceding claim, characterized in that the inner layer (43) and/or the outer layer (45) and thesolid insulation (44) are rigidly connected to each other oversubstantially the entire interface to ensure adhérence also onflexing and température change. 10
21. 26. A device according to any preceding claim, characterizedin that the solid insulation and the inner layer and/or the outerlayer are formed by materials having a high elasticity to main-tain mutual adhérence on strains during operation. 15
22. 27. A device according to any preceding claim, characterizedin that the solid insulation and the inner layer and/or the outerlayer are formed by materials having substantially equal E-modulus. 20
23. 28. A device according to any preceding claim, characterizedin that the inner layer (43) and/or the outer layer (45) and thesolid insulation (44) are formed by materials presenting sub-stantially equal thermal coefficients of expansion. 25
24. 29. A device according to any preceding claim, characterizedin that the conductor (42) and its insulation system constitutesa winding formed by means of a flexible cable (41). 30
25. 30. A device according to any preceding claim, characterizedin that the inner layer (43) is in electric contact with the at leastone electric conductor (42). 34 011417
26. 31. A device according to claim 30, characterized in that saidat least one electric conductor (42) comprises a number ofstrands and that at least one strand of the electric conductor(42) is at least in part uninsulated and arranged in electric 5 contact with the internai layer (43).
27. 32. A device according to any preceding claim, characterized inthat the conductor (42) and its insuiation System is designed forhigh voltage, suitably in excess of 10 kV, in particular in excess 10 of 36 kV and preferably more than 72,5 kV.
28. 33. A machine according to claim 12, characterized in that themagnetic circuit comprises one or more magnetic cores (48)having slots (50) for the winding (41). 15
29. 34. A device according to any of daims 12 and 32-33, charac-terized in that it is constituted of a generator, motor or syn-chronous compensator. 20 35. A device according to any of daims 12 and 33-34, charac- terized in that it is directly connected to a power network forhigh voltage, suitably 36 kV and more, without intermediatetransformer. 25 36. A device according to any of daims 1-11 and 13-32, char- acterized in that it is constituted by a power trans-former/reactor.