WO2000072336A1

WO2000072336A1 - Transformer/reactor

Info

Publication number: WO2000072336A1
Application number: PCT/EP2000/005226
Authority: WO
Inventors: Udo Fromm; Christian Sasse; Pär Holmberg; Gunnar Russberg
Original assignee: Abb Ab
Priority date: 1999-05-25
Filing date: 2000-05-25
Publication date: 2000-11-30
Also published as: GB9912204D0; TW507222B; AU5403400A; GB2350483A; AR029360A1

Abstract

The invention relates to a method for regulating induced voltage in a transformer, alternatively for regulating reactive power in a reactor, comprising arranging a regulating winding on a regulating means rotatable around a magnetic flux carrier, transferring a variable part of said regulating winding from said regulating means to a storage means or vice versa to vary the length of the regulating winding around the magnetic flux carrier, and providing flux linking means for transferring magnetic flux from the regulating means to a main winding of the transformer or reactor. The invention also relates to a transformer/reactor for the realization of the method.

Description

Transformer/reactor The present invention relates to a transformer or reactor and a method for regulating an induced voltage in a transformer or the reactive power of a reactor.

The present invention relates both to transformers and reactors having a core, as described below, as well as air-cored transformers and reactors.

For all transmission and distribution of electric energy, transformers are used and their task is to allow exchange of electric energy between two or more electric systems . Transformers are available in all power ranges from a few VA up to the 1000 MVA region. The designation power transformers normally relates to transformers with a rated output from a few hundred kVA up to more than 1000 MVA and with a rated voltage ranging from 3 - 4 kV and up to very high transmission voltages.

A conventional power transformer includes a transformer core, referred to below as the core, made of laminated, preferably oriented sheet metal, usually of silicon steel. The core consists of a number of core legs connected by a yoke. A number of windings are provided around the core legs in the form of primary, secondary and regulating winding. In power transformers these windings are practically always arranged in concentric configuration and distributed along the core legs.

It has largely been shown possible to replace oil- cooled power transformers with dry transformers, i.e. oil- free transformers, of a new type. This new type of transformer is provided with a winding designed with high voltage insulated electric conductors, having solid insulation, of a design similar to cables used for transmitting electric power (for example so-called XLPE cables) . Consequently, dry transformers of this new type may be used at considerably higher powers than what was possible with dry transformers according to prior art.

Reactors include a core which is usually provided with only one winding per phase. The above comments about transformers are generally also applicable to reactors.

For different reasons it is often necessary to be able to regulate or adjust the voltage of a power transformer. This may for example apply to maintaining the secondary voltage constant with a varying primary voltage; variation of secondary voltage; providing a reduced voltage in order to start a rotating machine; providing a neutral point for earthing or for dealing with out of balance current m different circuits etc. For this reason transformers are provided with an adjustable winding, referred to below as a regulating winding, which may adjust the transformer ratio.

Regarding transformers for low voltages it is previously known from FR 805 544 and GB 1 341 050 to vary the effective length of the winding by means of a regulating winding drum onto which the winding is wound or unwound.

However, this application is strictly limited to lower voltages due to the completely different type of winding used in high-power transformers, where the winding is rigid, as well as insulation problems associated with such a winding.

A known technique for conventional power transformers m the higher power ranges, l. e. oil-cooled transformers, is set forth in, for example, "The J&P Transformer Book" (A. C. Franklin et al , 11^th Edition 1983), describing how regulation may take place in dif erent ways . The two most common ways are firstly the use of so-called off-load tap changers in which tapping may take place between different voltage outlets inside the transformer tank, which can only take place when the transformer is off-circuit, and secondly the use of so-called diverter switches in which tapping takes place between different voltage outlets extended to the outside of the transformer tank and which can therefore take place on-load. In order to provide a power transformer in which voltage regulation can be effected almost continuously rather than stepwise, WO 99/17315 describes a dry transformer, wound with a flexible cable which can be wound off the transformer core and on to a storage drum and vice versa.

However, axial electromagnetic forces are imposed on the winding of the transformer described in WO 99/17315, these forces being exacerbated since the winding often occupies only part of the leg of the transformer. These axial forces could cause failure by causing the winding to collapse or the end rings or clamping system to fracture.

Reactors provided with a regulating winding according to WO 99/17315, by means of which the reactive power of the reactor may be regulated, present the corresponding problem.

An aim of the present invention is to provide a method solving the aforementioned problems and allowing for improved regulation of transformers and reactors, especially of the dry type, in the high power range. Another aim is to obtain such an improved transformer/reactor . These aims are achieved by means of a method, as defined in Claim 1, as well as by a transformer/reactor as defined in Claim 2.

The present invention relates thus to a method for regulating induced voltage in a transformer, alternatively for regulating reactive power in a reactor, comprising arranging a regulating winding on a regulating means rotatable around a magnetic flux carrier, transferring a variable part of said regulating winding from said regulating means to a storage means or vice versa to vary the length of the regulating winding around the magnetic flux carrier, and providing flux linking means for transferring magnetic flux from the regulating means to a main winding of the transformer or reactor. The present invention also relates to a transformer or reactor comprising a magnetic flux carrier carrying a main winding, a regulating winding arranged on a regulating means rotatable around said magnetic flux carrier, a storage means to and from which a variable part of said regulating winding can be transferred from and to said regulating means to vary the length of the regulating winding around the magnetic flux carrier, and flux linking means for transferring magnetic flux from the regulating means to a main winding of the transformer or reactor. The flux linking means preferably comprises a balance winding which is divided into two interconnected parts, each having the same number of turns. The connection is such that the main flux is almost unaffected, whilst leakage flux is redistributed. The first part links the winding on the regulating means, which is preferably a rotating drum, and the second part links the magnetic flux carrier where the mam winding is arranged. Such a balance winding can be formed from a simple twisted loop of copper.

Alternatively, the balance winding may have a plurality of parallel-connected turns arranged on the regulating means so as to cancel short circuit forces.

The magnetic flux carrier may be a transformer core or a reactor core, as described above. However, the present invention is also applicable to air-cored transformers and reactors, as also mentioned above.

The regulating winding (and preferably also the main winding) preferably comprises at least one current carrying conductor, a first semiconducting layer surrounding the at least one conductor, a solid insulating layer surrounding said first semiconducting layer and a second semiconducting layer surrounding said insulating layer. These windings may be formed from cables having solid, extruded insulation, of a type now used for power distribution, such as XLPE-cables or cables with EPR- msulation. Such cables are flexible, which is an important property in this context since the winding is formed from cable which is bent during assembly. The flexibility of an XLPE-cable normally corresponds to a radius of curvature of approximately 20 cm for a cable with a diameter of 30 mm, and a radius of curvature of approximately 65 cm for a cable with a diameter of 80 mm. In the present application the term "flexible" is used to indicate that the winding is flexible down to a radius of curvature in the order of twice the cable diameter, preferably four to eight times the cable diameter.

The flexible windings should be constructed to retain their properties even when bent and when subjected to thermal or mechanical stress during operation. The material combinations stated above should be considered only as examples. Other combinations fulfilling the conditions specified and also the condition of being semiconducting, i.e. having resistivity within the range

of 10^_1-10⁶ Ω.cm, e.g. 1-500 Ω.cm, or 10-200 Ω.cm,

naturally also fall within the 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 such as ethylene propylene rubber (EPR) or silicon rubber.

The inner and outer (first and second) semiconducting layers may be of the same basic material but with particles of conducting material such as soot or metal powder mixed in . Ethylene-vinyl-acetate copolymers/nitπle rubber

(EVA/NBR) , butyl graft polyethylene, ethylene-butyl- acrylate copolymers (EBA) and ethylene-ethyl-aerylate copolymers (EEA) may also constitute suitable polymers for the semiconducting layers. The conductivity of the two semiconducting layers is sufficient to substantially equalize the potential along each layer. The conductivity of the outer semiconducting layer is sufficiently high to enclose the electrical field within the cable, but sufficiently low not to give rise to significant losses due to currents induced in the layer.

Thus, each of the two semiconducting layers essentially constitutes one equipotential surface, and these layers will substantially enclose the electrical field between them. There is, of course, nothing to prevent one or more additional semiconducting layers being arranged in the insulating layer.

An example of an insulated conductor or cable suitable to be used in the present invention is described in more detail in WO 97/45919 and WO 97/45847. Additional descriptions of the insulated conductor or cable concerned can be found m WO 97/45918, WO 97/45930 and WO 97/45931.

The method of the invention may be characterized in that, starting from a zero position in which there are no turns on the regulating winding drum, the induced voltage/reactive power of the transformer/reactor respectively is increased when the winding is wound onto the regulating winding means m the same direction as the direction of the main winding, and that the induced voltage/reactive power of a transformer/reactor respectively, is decreased when the winding is wound onto the regulating winding means in the direction opposite to the direction of the main winding, whereby the maximum

variation of the number of winding turns is H—N, where N is the number of winding turns which are available on the regulating winding means. The advantage achieved hereby is that the winding may either be varied stepless or by an optional number of turns . Winding up the regulating winding in one direction corresponds naturally to an unwinding of the regulating winding in the opposite direction. Should the whole regulating winding be rolled-on in one direction, which is assumed to be the same as the winding direction of the mam winding, obtaining therefore a maximum induced voltage/reactive power, a reduction of the voltage/power naturally takes place by unwinding the regulating winding firstly before starting to wind it up in the opposite direction.

The storage means may include a rotatable storage drum similar to the rotatable regulating winding drum forming the regulating means . The winding up and the unwinding preferably takes place by arranging the regulating winding on a rotatable means, such as said drum, but other solutions are also possible. Also other solutions regarding the storage means are conceivable, such as several drums, a reel or coil, etc. or nothing at all. According to an optional feature the regulating winding may be arranged on a magnetic flux carrier leg appertaining to one phase of a polyphase system and the mam winding may be arranged onto a magnetic flux carrier leg belonging to another phase of the polyphase system.

This has the advantage of enabling a phase shift.

According to yet another optional feature the storage means may include a second winding arranged around a magnetic flux carrier belonging to another phase of the polyphase system than the regulating winding. With this arrangement both voltage control, by means of the regulating winding, and phase shift, by means of the second winding, can be achieved. Preferably, the transferring means includes a drive means for the rotation of a regulating drum and a drive means for the rotation of a storage drum. These drive means are preferably the form of at least one motor and a device for belt driving the respective drum. It is thus possible for a common motor to drive the regulating winding drum as well as the storage drum. Each drum having its own motor is another possibility. The transformer may also be of a polyphase type. In a transformer of a three- phase type, for example, thereby having three regulating windings, which may each be independent of the other, it is conceivable that each one of the regulating windings is driven by its own motor so that m total there are three, alternatively six motors, or that all phases are regulated in the same way amounting then to one or two motors depending on whether the respective storage drum is also driven by this motor. Alternatives other than belt driving are naturally feasible.

According to another optional feature, the regulating winding drum is rotatable in two directions.

As the regulating winding drum is preferably arranged around the core it is preferable to construct it out of at least two drum parts which are joined together in the radial direction n order to form the drum. According to a particularly preferred feature the insulating electric conductor of said winding has a second layer which is connected to a predetermined potential, preferably earth potential. As mentioned, this has the advantage that the electrical field generated around the current carrying conductor is enclosed within the solid insulation layer. Since this has the result that no electrical field exists outside the winding, the further advantage is obtained that it generally will be possible to apply techniques that are previously only known from the low-voltage range and the electronics field.

The high voltage electric conductor preferably has a diameter lying m the interval of 20 - 250 mm and a conductor area lying in the interval of 80 - 3000 mm². The first layer is furthermore essentially at the same potential as the current carrying conductor. The second layer is preferably arranged such that it forms a substantially equipotential surface surrounding the current carrying conductor/conductors . The current carrying conductor may include a plurality of strands whereby at least one of the strands is not insulated from the inner, first semiconducting layer, and finally each one of the three layers may be securely connected to the adjacent layer along essentially the whole connecting surface.

Another optional characteristic is that at least one of, and possibly both, the regulating winding drum or the storage drum is provided with means for connection of the said second layer of the winding, having semiconducting properties, to a predetermined potential, preferably earth potential . These means may be designed in several ways .

The regulating winding drum is also preferably provided with a means by which to earth the conductors in the winding. This means is preferably in the form of a sliding contact, for example m two halves.

The present invention will now be described in detail, by way of example only, with particular reference to the accompanying drawings, n which: Figure 1 is a diagram showing the principle of a transformer according to the invention, but not showing the balance winding;

Figure 2 schematically shows a transformer according to one embodiment of the invention;

Figure 3 schematically shows a transformer according to an alternative embodiment; and

Figure 4 is a graph showing axial forces in the transformer shown in Figure 3. Figure 1 shows a transformer core 1 consisting of a yoke and two legs, in which a mam winding 2 is applied around the one leg and a regulating winding 3 is arranged around the other leg. The mam winding may either be formed of a primary winding or a secondary winding. The regulating winding is thus used to vary the ratio of the transformer. The regulating winding 3 is arranged in the form of winding turns 5 wound onto a rotatable drum 6. As can be seen, the drum 6 is divided into two drum halves 7, 8. Other ways of dividing the drum are also conceivable so as to facilitate the installation around the legs of the core. The drum is provided with at least one flange for belt driving by means of a motor (not shown) . The regulating winding functions thus as a variable coil . The number of winding turns on the regulating winding drum 6 is made to vary with the aid of a rotatable winding storage drum 12 for the winding 5. The storage drum 12 is likewise preferably belt driven by a motor.

In the absence of a balance winding, when the regulating winding 3 is wound and unwound onto and from the regulating winding drum 6, the local ampere-turn balance is distorted, which greatly increases the short circuit forces.

Figure 2 shows a transformer including a mam winding package 2 having high voltage HV and low voltage LV turns. A regulating winding drum 6 is provided. A balance winding 13 comprises simple connected coils of copper, one coil having at least one turn being arranged in or around the mam winding 2 and the other coil , having the same number of turns, arranged to be linked by the flux on the regulating drum 6. This can decrease the short circuit forces by a factor of about 10.

Figure 3 shows an alternative transformer which a balance winding 14 is wound mside the regulating winding drum 6. The balance winding consists of a number of coils, each having a number of turns, the coils being connected in parallel. As shown in Figure 4, the axial forces on the balance winding are approximately equal and opposite to those on the part of the regulating winding 3 on the regulating drum 6. In other words , when the balance winding is mechanically connected to the regulating winding drum, these forces effectively cancel each other out.

Claims

1. A method for regulating induced voltage in a transformer, alternatively for regulating reactive power in a reactor, comprising arranging a regulating winding on a regulating means rotatable around a magnetic flux carrier, transferring a variable part of said regulating winding from said regulating means to a storage means or vice versa to vary the length of the regulating winding around the magnetic flux carrier, and providing flux linking means for transferring magnetic flux from the regulating means to a mam winding of the transformer or reactor .

2. A transformer or reactor comprising a magnetic flux carrier carrying a main winding, a regulating winding arranged on a regulating means rotatable around said magnetic flux carrier, a storage means to and from which a variable part of said regulating winding can be transferred from and to said regulating means to vary the length of the regulating winding around the magnetic flux carrier, and flux linking means for transferring magnetic flux from the regulating means to a mam winding of the transformer or reactor.

3. A transformer/reactor according to claim 2, wherein the flux linking means comprises a balance winding which is divided into a first part and an interconnected second part, the first and second parts each having at least one turn and both having the same number of turns, the first part linking the winding on the regulating means, and the second part linking the magnetic flux carrier where the mam winding is arranged.

4. A transformer/reactor according to claim 2 or 3, wherein the regulating means comprises a rotatable regulating winding drum.

5. A transformer/reactor according to claim 4, wherein the balance winding has a plurality of parallel-connected turns arranged on and mechanically connected to the regulating winding drum.

6. A transformer/reactor according to claim 4 or 5, wherein the regulating winding drum consists of at least two drum parts, which are joined together in the radial direction in order to form a drum.

7. A transformer/reactor according to any one of claims 2 to 6, wherein the storage means comprises a rotatable storage drum.

8. A transformer/reactor according to claims 4 and 7, comprising transferring means including drive means for the rotation of the regulating winding drum and drive means for the rotation of the storage drum.

9. A transformer/reactor according to claim 8, wherein said drive means are in the form of at least one motor and a device for belt driving the respective drum.

10. A transformer/reactor according to claim 8 or 9, wherein the regulating winding drum is rotatable in two directions .

11. A transformer/reactor according to any one of claims 2 to 10, wherein the storage means includes a second winding arranged around a magnetic flux carrier.

12. A transformer/reactor according to any one of claims 2 to 11, wherein the regulating winding is arranged on a magnetic flux carrier leg belonging to one phase of a _ - polyphase system and that the storage means includes a winding, arranged on a magnetic flux carrier leg belonging to another phase of the polyphase system.

13. A transformer/reactor according to any one of claims 2 to 12, wherein the regulating winding is provided with means for electric connection to the main winding.

14. A transformer/reactor according to any one of claims 2 to 13, wherein the regulating winding comprises at least one current carrying conductor, a first semiconducting layer surrounding the at least one conductor, a solid insulating layer surrounding said first semiconducting layer and a second semiconducting layer surrounding said insulating layer.

15. A transformer/reactor according to claim 14 , wherein the second semiconducting layer is connected to a predetermined potential .

16. A transformer/reactor according to claim 15, wherein the predetermined potential is earth potential.

17. A transformer/reactor according to any one of claims 2 to 16, characterized m that it is a dry transformer/reactor .