OA11364A

OA11364A - Magnetic tap changer.

Info

Publication number: OA11364A
Application number: OA1200000085A
Authority: OA
Inventors: Udo Fromm; Bengt Rydholm; Mats Leijon; Christian Sasse
Original assignee: Abb Ab
Priority date: 1997-09-30
Filing date: 2000-03-24
Publication date: 2003-12-17
Also published as: HUP0004517A2; SE9703561D0; KR20010052085A; AU733243B2; NO20001265L; NO20001265D0; AU9371298A; AP2000001763A0; WO1999017313A2; WO1999017313A3; JP2001518699A; CN1272215A; CA2305420A1; SE511961C2; EP1051716A2; HUP0004517A3; SE9703561L; PL339943A1

Abstract

An induction controlled voltage regulator, primarily for high-voltage regulation, includes a magnetic circuit based upon a core (1) having one or more flux paths or legs (2) surrounded by high and low voltage windings (3, 15). The leg (2) is divided in at least two branches (2A, 2B), at least one (2B) of which includes a regulation arrangement with a zone (5) of amendable permeability. The amendment of the permeability may be made by a magnetic rod (4) being movable into and out from the zone (5) formed as an airgap. Another embodiment for such an amendment includes the use of a regulator winding acting on the magnetic flux passing through the leg branch (2B). A compensator winding can be arranged around the leg branch (2B) involving the zone (5) of amendable permeability. The compensator winding is electrically connected in series with capacitor means. At least one of the windings is wound by a high-voltage cable comprising a conductor surrounded by an inner semiconductor, an insulator layer and an outer semiconductor.

Description

1 MAGNETIC TAP CHANGER 011364 /

Technical Field

The présent invention relates generally to induction controlled voltage re- 5 gulators and more particularly, to such an inductance régulation by an electric transformer or reactor means as defined in the preamble of Claim 1. The inventionrelates also to a regulator winding used by such an induction controlied voltageregulator as defined in Claim 10 and to a method for voltage control in an elec-trical line or for reactive power control in plants as defined in Claim 13. 10

Background of the Invention

Conventional induction controlled voltage regulators for lower voltageranges are arranged by using inductors with coils rotated or-shifted in relation toeach other as described in the literature, e.g. by I.L. la Cour and K. Faye-Hansen15 in the book "Drehtransformator und Schubtransformator, Die Wechselstromtech-nik Bd. 2, Die Transformatoren", Veriag von Julius Springer, Berlin, Germany, 1936, pages 586 - 598. Furthermore, such an induction control cannot be madefor high voltage at reasonable costs. The insulation construction would be a se-vere design limitation. 20 Another technique is known from US-A-4 206 434 where the magnetic flux between different legs of an induction controlled voltage regulator is described tobe redistributed by a variable DC-magnetization. For this purpose a variable DC-source is needed.

Thus, electric high-voltage control is mostly made by electric transformers 2 5 involving one or more windings wound on one or more legs of the transformer ironcore. The windings involve taps making possible of supplying different voltage lev-els from the transformer. The présent power transformers and distribution trans-formers as tbose mentioned above and used in voltage trunk lines involve tap-changers for the voltage régulation. They are subject to mechanical wear and 30 electrophysical érosion due to discharges between contacts. Régulation is onlypossible in steps. Thus, a stepwise voltage régulation and movable contacts arerequired for connection with the different taps. 011364

Summary of the invention

The drawbacks of prior art voltage régulation are avoid by the induction controlled voltage regulator according to the présent invention. The characteristic features are to be found in that a short length of the flux path or leg of the magne- 5 tic circuit surrounded by the high and low voltage windings is divided in at leasttwo branches, where at least one of which includes a régulation arrangement witha zone of amendable permeability. A favourable embodiment of the inventive regulator has the high-voltagewinding wound farthest out for containing ail the core flux. The low voltage winding 10 of the regulator is then divided in at least two winding parts, where one windingpart comprises the major part of tums and is wound inside the high-voltage wind-ing. The other winding part(s) comprising the minor part of turns, is/are woundaround the at least one leg branch having the zone of amendable permeability.

The number of tums of the leg branch dépends on the required voltage régulation 15 range.

The division of the magnetic flux division necessary between the differentleg branches is obtained by changing the réluctance in the different leg branches,which is implemented by the zone of amendable permeability. Different embodi-ments are possible within the field of the invention. Most préférable embodiments 20 at the moment include either a magnetic rod movable into and out from the zoneformed as an airgap, or a regulator winding wound onto a separate magnetic coreforming a transverse path to the leg branch involving the zone. The regulator wind-ing is supplied with a control voltage affecting the magnetic fiux passing throughthe leg branch. 25 The higher réluctance at the airgaps can be compensated for by a com- pensator winding surrounding the area of said zone and electrically connected insériés with a capacitor as a separate closed circuit.

Such a compensator winding loaded with a capacitor forms a négative ré-luctance Rc = -η2 ω2 C. The number of winding turns n and the régulation of the 30 capacitor capacitance C may be chosen in such a way to correspond to the airgappositive réluctance RL = VA μο, where I is the (effective) length of the airgap, 3 011364 A is the cross section area of the magnetic core, andμο is the permeability of air.

Typical values of the capacitance C are in the order of from some microfarads tosome millifarads for voltages in the order of 1 kV. 5 An important condition to make it possible for obtaining such a voltage ré- gulation of high voltages, i.e. 36 kV up to 800 kV, is that at least a part of anyonéof the windings mentioned above is constructed of a high-voltage cable which in-clude a conductor, an inner semiconductor, an insulator and an outer semiconduc-tor. Thus, the transformer/ reactor will be of a so called ”dry” type. The use of such10 a designed high-voltage cable makes it possible to "capture" the electric field in-side the cable insulation. This means that it is possible to design induction control-led voltage regulators for high-voltage applications.

An additional advantage is that said layers are arranged to adhéré to oneanother even when the cable is bent. Hereby, good contact is achieved between15 the layers during the cable's entire life.

Different applications lie within the field for the man skilled in the art. Thus,e.g. it is possible to apply the présent invention to one-phase induction controlledvoltage regulators. Also on-load-tap-changer devices, i.e. a one-phase inductioncontrolled voltage regulator integrated in a transformer, are possible to implement.20 Furthermore, multi-phase induction controlled voltage regulators can be made withindividual phase control as well as with common phase control.

Brief Description of the Drawings

These and other features and advantaaes of the présent invention will25 become more apparent from the following detailed description of exemplaryembodiments thereof, as illustrated in the accompanying drawings, in which:

Fig. 1 is a cross section view of a part of a transformer core according to the in- vention,

Fig. 2 is a side view of a short part of the transformer core, partly shown in cross30 section, according to one embodiment of the invention,

Fig. 3 is a side view similar to the one of Fig. 2 but showing another embodimentof the invention, and 4 011 364,

Fig. 4 is a cross-section view of a high-voltage cable being used in the régulationwindings according to the présent invention.

Detailed Description of the Invention 5 The invention will now be described in detail with reference to some pre- ferred embodiments, the principle of which is shown in the drawing figures en-closed. Like référencé numbers used in the different drawings refer to similar orother equipment having a corresponding function. Figs. 1 and 2 show the part ofthe voltage regulator only being' important to the présent invention. 10 Fig. 1 shows a view of a transformer core 1 of the core flux path or leg 2, half of which being split in two branches 2A and 2B. A high-voltage winding 3 sur-rounds the unsplit half of the leg 2 in an exterior position, inside which a first part15A of a low voltage winding is wound around the leg 2. The windings are shownin a cross section view. The first part 15A of the low voltage winding comprises the 15 major part of winding turns and is electrically connected in sériés with a secondpart 15B of the low voltage winding. The second part 15B, having a minor part ofwinding tums, surrounds one 2B of the leg branches.

The voltage régulation of the transformer is based on the principle ofchanging the magnetic flux Φ linkage in the transformer windings by controlling the 20 réluctance in the different core leg branches 2A, 2B. For that purpose one or bothof the core leg branches may include area(s) 4 the permeabii'ity of which isdecreased and/or increased by a control means.

Fig. 2 shows a favourable embodiment of the invention where a leg 2 ofthe transformer core 1 is surrounded by the high-voltage winding 3 and the (main) 25 first part 15A of the low voltage winding above, i.e. upstreams of the point wherethe leg is divided in the two branches 2A and 2B. Just as in the embodimentshown in Fig. Tthe second part 15B of the low voltage winding is wound aroundthe leg branch 2B. This leg branch 2B includes an airgap 5 within which a magne-tic rod 4 is arranged. The rod 4 is movable out from and into the airgap 5 as indi- 30 cated by the double arrow A. The movement of the magnetic rod 4 can be control-led by suitable mechanical means either manually or by any electrical drivenmeans. The resuit of moving the magnetic rod 4 will be that the flux inside the 5 011364 second part 15B of the iow voitage winding couia be changea between zéro andfull core branch flux. The number of winding turns in the second part 15B of theIow voltage winding will iimit the régulation area.

As indicated by broken lines in Fig. 2 also the other leg branch 2A may5 include an airgap 5 with a movable rod 4. Using such movable rods 4 in the twoleg branches 2A, 2B may give a more fine régulation of the réluctance in the coré 1. The movements of the rods 4 can be made in combination with and directedtowards or from each other. The initial position of the rods 4 in the airgaps 5 maybe different, e.g. the rod in the. leg branch 2B completely fills the airgap (as shown10 in Fig. 2) while the rod in the lég branch 2A at the same time is in a position moreor less outside of the corresponding airgap.

Another embodiment of the invention is shown in FigT3* where the mecha-nical rod and airgap hâve been substituted by an external magnetic field. Such anexternal magnetic field may be obtained by a separate magnetic core 9, the axis15 of which lies in the transverse direction to the leg branch 2B axis. The core 9 canbe a premagnetized section or, preferably, a core surrounded by a regulator wind-ing 6. The external magnetic field generated by the regulator winding 6 and/ or bythe separate magnetic core 9 counteracts more or less the flux through the legbranch according to the control voltage supplied to the regulator winding 6. The20 control voltage may be an AC-voltage being in phase with the high voltaae to be

V regulated or, preferably, a DC-voltage. The same induction controlled voltage ofthe transformer or reactor means is obtained by this embodiment as by thoseshown in Figs. 1 and 2.

As shown in Fig. 2, a compensator winding 7 is wound around the flux2 5 path or leg 2 of the transformer core 1. The compensator winding 7 is forming sclose circuit including a capacitor means 8.

The compensator circuit now described can be implemented also in theembodiment shown in Fig. 3.

To make rt possible to obtain a régulation of high voltages, i.e. in the field30 of about 36 through 800 kV, at least one of the windings 3, 6, 7, 15A and 15B, or apart of anyone thereof, is wound by using a high-voltage cable 61 of a type shownin Fig. 4 as an example. 6 011364

The cabie used in the présent invention is flexible and of a kind which isdescribed in more detail in WO 97/45919 and WO 97/45847. Additional descrip-tions of the cable concerned can be found in WO 97/45918, WO 97/45930 andWO 97/45931. 5 Accordingly, the windings, in the arrangement according to the invention, are preferably of a type corresponding to cables having solid, extruded insulation;of a type now used for power distribution, such as XLPE-cables or cables withEPR-insulation. Such a cable comprises an inner conductor composed of one ormore strand parts, an inner semiconducting layer surrounding the conductor, a 10 solid insulating layer surrounding this and an outer semiconducting layer surround-ing the insulating layer. Such cables are flexible, which is an important property inthis context since the technology for the arrangement according to the invention isbased primarily on winding Systems in which the winding is formed from cablewhich is bent during assembly. The flexibility of an XLPE-cable normally corre- 15 sponds to a radius of curvature of approximately 20 cm for a cable with a diameterof 30 mm, and a radius of curvature of approximately 65 cm for a cable with a di-ameter of 80 mm. In the présent application the term “flexible" is used to indicatethat the winding is flexible down to a radius of curvature in the order of four timesthe cable diameter, preferably eight to twelve times the cable diameter. 20 The winding should be constructed to retain its properties even when it is bent and when it is subjected to thermal or mechanical stress during operation. Itis vital that the layers retain their adhesion to each other in this context. The mate-rial properties of the layers are décisive here, particularly their elasticrty and rela-tive coefficients of thermal expansion. In an XLPE-cable, for instance, the insulat- 25 ing layer consists of cross-linked, low-density polyethyiene, and the semiconduct-ing layers consist of polyethyiene with soot and métal particles mixed in. Changesin volume as a-result of température fluctuations are completely absorbed aschanges in radius in the cable and, thanks to the comparatively slight différencebetween the coefficients of thermal expansion in the layers in relation to the elas- 30 ticity of these materials, the radial expansion can take place without the adhesionbetween the layers being lost. 7 011364

The material combinations stated above should be considered only as ex-amples. Other combinations fulfilling the conditions specified and also the condi-tion of being semiconducting, i.e. having resistivity within the range of 10_1-10θohm-cm, e.g. 1-500 ohm-cm, or 10-200 ohm-cm, naturally also fall within the5 scope of the invention.

The insulating layer may consist, for example, of a solid thermoplastic 'material such as low-density polyethylene (LDPE), high-density polyethylene(HDPE), polypropylene (PP), polybutylene (PB), polymethyl pentene ("TPX"),cross-linked materials such as cross-linked polyethylene (XLPE), or rubber such10 as ethylene propylene rubber (EPR) or Silicon rubber.

The inner and outer semiconducting layers may be of the same basicmaterial but with particles of conducting material such as soofdr métal powdermixed in.

The mechanical properties of these materials, particulariy their coeffi-15 cients of thermal expansion, are affected relatively l'rttle by whether soot or métalpowder is mixed in or not - at least in the proportions required to achieve the con-ductivity necessary according to the invention. The insulating layer and the semi-conducting layers thus hâve substantially the same coefficients of thermal expan-sion. 20 Ethylene-vinyl-acetate copolymers/nrtrile rubber (EVA/NBR), butyl graft polyethylene, ethylene-butyl-acrylate copolymers (EBA) and ethylene-ethyl-acryla-te copolymers (EEA) may also constitute suitable poiymers for the semiconductinglayers.

Even when different types of material are used as base in the various lay- 2 5 ers, it is désirable for their coefficients of thermal expansion to be substantially thesame. This is the case with the combination of the materials listed above.

The materials listed above hâve relatively good elasticity, with an E-modu-lus of E<500 MPa, preferably <200 MPa. The elasticity is sufncient for any minordifférences between the coefficients of thermal expansion for the materials in the 30 layers to be absorbed in the radial direction of the elasticity so that no.cracks ap-pear, or any other damage, and so that the layers are not released from each 011364 other. The material in the layers is elastic, and the adhesion between the layers is at least of the same magnitude as in the weakest of the materials.

The conductivity of the two semiconducting layers is sufficient to substan- tially equalize the potential along each layer. The conductivity of the outer semi- 5 conducting layer is sufficientiy high to enclose the electrical field within the cable,but sufficientiy low not to give rise to significant losses due to currents induced in'the longitudinal direction of the layer.

Thus, each of the two semiconducting layers essentially constitutes oneequipotential surface, and these layers will substantially enclose the electrical field 10 between them.

There is, of course, nothing to prevent one or more additional semico.nduct-ing layers being arranged in the insulating layer.

Such a high-voltage cable 61 may include one or more electrical conduc-tors 631. The cable embodiment shown in Fig. 4 includes an insulation and the 15 conductor 631 is in direct connection with a first layer 632 having semiconductingproperties. The first layer 632 is in tum surrounded by a solid insulating layer 633,which then is surrounded by a second layer 634 having semiconducting proper-ties.

In fig. 4 showing the detail of the invention relating to the cable, the three 20 layers 632, 633, 634 are arranged to adhéré to each other even when the cable isbent. The cable shown is flexible, and this property is maintained during the entirelife of the cable.

Favourably, the layers 632, 633, 634 are made from the same plastic ma-terial or other materials having the same coefficient of expansion. By that, the im- 2 5 portant advantage is obtained in that deficiencies, cracks, etc. are avoided at ther- mal movement in the winding. The plastic material of the first and second layers632, 634 has an electric conductive material added thereto.

Though the présent invention has been described above with référencé totransformers or reactors shown in the drawings as a single-phase version it is ob- 30 vious that it also can be applied to multiphase transformers and similar apparatus-es, e.g. in autotransformers and in booster transformers.

Claims

1. An induction controlled voltage regulator, particularly a transformer or areactor means, comprising a magnetic circuit involving a core (1) with at least one 5 flux path or leg (2), being surrounded by high and low voltage windings (3, 15),said regulator being characterized in that a short length of said at least one leg '(2) being divided in at least two branches (2A, 2B), where at least one of whichhas a régulation arrangement (4 or 6) including a zone (5) of amendable perme-ability, and in that at least one of the tums of the high-voltage winding (3) is wound10 by a high-voltage cable (61) including a conductor (631), an inner semiconductor(632), an insulator (633) and an outer semiconductor (634).

2. A regulator according to claim 1, and further characterized in that saidthe high-voltage winding (3) is wound farthest out containing ail the core flux and 15 in that the low voltage winding (15) is divided in at least two winding parts (15A, 15B), where one winding part (15A) comprises the major part of tums and iswound inside the high-voltage winding (3) and the other winding part (15B), com-prising the minor part of tums, is wound around said at least one leg branch (2B)having the said zone (5) of amendable permeabiiity. 20 v

3. A regulator according to claim 1 or claim 2, and further characterized inthat said zone (5) of amendable permeabiiity inciudes a magnetic rod (4) beingmovable into and out from the zone (5) formed as an airgap. 25

4. A regulator according to claim 1 or claim 2, and further characterized in that said zone (5) of amendable permeabiiity inciudes a regulator winding (6)wound onto a~separate magnetic core (9) forming a transverse path to the said atleast one leg branch (2B) and being supplied with a control voltage for controlledamendment of the magnetic flux passing through the said at least one leg branch 30 (2B). 10 011364

5. A regulator according to any preceding claim, and further charaeterizedby a compensâtes winding (7) surrounding the area of said zone (5) with amend-able permeability, and electrically connected in sériés with a capacitor means (8). 5

6. A regulator according to any preceding claim, and further charaeterized in that also at least one of the further windings (6, 7,15A, 15B) is wound by a high- 'voltage cable (61) including a conductor (631), an inner serhiconductor (632), aninsulator (633) and an outer semiconductor (634). 10

7. A regulator according to any preceding claim, and further charaeterized in that said regulator is a multiphase transformer, the said at least one leg branch(2B) of each phase includes a régulation arrangement (4 or‘6j for independentrégulation of each phase. 15

8. A regulator according to any of the daims 1-6, and further charaeterized in that said regulator is a multiphase transformer, the said at least leg branch (2B)of each phase includes a régulation arrangement (4 or 6) being connected forhaving a joint régulation. 20 9. A regulator according to any of the daims 1 - 6, and further charaeterized in that said regulator is an autotransformer or a booster transformer.

9 011364 CLAIMS

10. A regulator according to any preceding claim, and further charaeterized inthat said layers (632, 633, 634) are arranged to adhéré to one another even when 2 5 the cable is bent.

11. A regulator winding (6) for an induction controlled voltage regulator, parti-cularly a transformer or a reactor means comprising a high-voltage winding (3)and low voltage windings (15A, 15B), according to any of the preceding daims, 30 charaeterized in that at least one of said windings (3, 6,15A, 15B), or a part of anyone thereof, is wound by a high-voltage cable (61) including a conductor (631), 11 011364 • an inner semiconductor (632V an insuiator (633) and an outer semiconductor(634).

12. A winding according to claim 11, further characterized in that said 5 semiconductors (632, 634) and said insuiator (633) hâve the same coefficient ofexpansion.

13. A winding according to claim 11 or 12, further characterized in that saidsemiconductors (632, 634) and said insuiator (633) are made from the same 10 plastic material, the semiconductors plastic material having electric conductionmaterial added.

14. A method for voltage control in an electrical line and/ or for reactive powercontrol in plants comprising at least a transformer or a reactor having at least one 15 of its windings, or a part of anyone thereof, being of a high-voltage cable type ac-cording to any of the preceding daims, where the voltage control is effected by aninduction régulation changing the magnetic flux linkage in the windings in such away that the réluctance of different leg branches of said transformer/reactor ischanged. 20

15. A method according to claim 14, characterized in that said inductionrégulation is obtained by the moving a magnetic rod out from or into an airgapwithin at least one of said different leg branches. 25

16. A method according to claim 14, characterized in that said induction régulation is obtained by variation of a régulation voltage supplied to a windingwound arouncLa régulation leg of said transformer/reactor.

17. A method according to claim 16, characterized in that said variation of 30 the régulation voltage is obtained by control of a capacitor having a controllablecapacitance.