MXPA00005158A - Transformer - Google Patents

Abstract

A power transformer comprising at least one high voltage winding (32) and one low voltage winding (30). Each of the windings includes at least one current-carrying conductor, a first layer having semi-conducting properties provided around said conductor, a solid insulating layer provided around said first layer, and a second layer having semi-conducting properties provided around said insulating layer. The windings are intermixed such that turns of the high voltage winding are mixed with turns of the low voltage winding.

Description

TRANSFORMER FIELD OF THE INVENTION The present invention consists of an electrical power transformer comprising at least one high voltage winding and a low voltage winding.

BACKGROUND OF THE INVENTION The term "power transformer", as used herein, denotes a transformer having a rated output power of between a few hundred kVA and more than 1000 MVA, and a nominal voltage comprised between 3-4 kV and very high transmission voltage values, in the order of 400-800 kV or higher.

DESCRIPTION OF THE INVENTION Conventional power transformers are described, for example, in "The J &P Transformer Bock, A Practical Technology of the REF. : 120085 Power Transformer ", by AC Franklin and DP Franklin, published by Butterworths, 11th edition, 1990. Problems related to internal electrical insulation and related topics are addressed, for example, in" Transormerboard, Die Verwendung von Transformerboard in Grossleistungstransformatoren ", of HP Moser, published by H. Weidman AG, Rapperswil mit Gesamtherstellung: Birkhauser AG, Basel, Switzerland.

In the transmission and distribution of electrical energy transformers are used exclusively to enable the exchange of electrical power between two or more electrical systems. There are transformers for powers of the order of 1 MVA up to magnitudes around 1000 MVA and for voltages that comprise the highest transmission voltages that are currently used.

Conventional power transformers contain a transformer core, consisting of laminated sheet with common orientation, usually made of silicon steel. The core consists of several columns connected by cylinder heads that together form one or more core windows. Transformers fitted with a core of such characteristics are usually referred to as column transformers. Several windings are arranged around the core columns. In power transformers these windings almost always adopt a concentric configuration and run along the core column.

However, other types of core structures are known, for example, so-called armored transformer structures, which usually have rectangular windings and rectangular column sections located outside the windings.

Conventional air-cooled power transformers are known for lower power ranges. To shield these transformers, an external housing is often placed, which also reduces the external magnetic fields of the transformers.

- The majority of the power transformers are, however, cooled by oil, the oil also serving as an insulating medium. The conventional oil-cooled and oil-insulated transformer is encapsulated in an external casing that has to meet stringent requirements. The construction of such a transformer, with its circuit couplers, circuit breaker elements and associated insulators, is therefore complicated. The use of oil for cooling and insulation also complicates the repairs of the transformer and constitutes a danger to the environment.

A transformer called "dry" without insulation by oil or oil cooling and adapted for nominal powers of up to 1000 MVA, with nominal voltages from 3-4 kV and capable of withstanding very high transmission voltages, comprises windings formed from conductors as those shown in figure 1. The conductor contains a central conductor medium composed of non-insulated wires ( and optionally some isolates) . Around the conductive means is an internal semiconductor housing 6 which is in contact with at least several of the uninsulated wires 5. This semiconductive housing 6 is in turn surrounded by the main insulation of the cable in the form of a solid extruded insulating layer. 7. This insulating layer 7 is surrounded ppr by an outer semiconductive housing 8. The conductive zone of the cable can oscillate between 80 and 3000 mm2 and the external diameter of the cable between 20 and 250 mm. At least two adjacent layers have substantially equal thermal expansion coefficients.

Although the casings 6 and 8 are presented as "semiconductors", in practice they are constituted by a base polymer mixed with carbon black or metallic particles and have an electrical resistance comprised between 1 and 105 Ocm, preferably between 10 and 500 Ocm . Suitable base polymers for carcasses 6 and 8 (and for insulating layer 7) include ethylene vinyl acetate / nitrile rubber copolymer, polyethylene with grafted butyl, ethylene-butyl acrylate copolymer, ethylene-ethylacrylate copolymer, ethylene-propane rubber and polyethylenes. low density, polybutylene, polymethylpentene and ethylene acrylate copolymer.

The internal semiconductor housing 6 is rigidly connected to the insulating layer 7 along the entire contact surface between them. In the same way, the outer semiconductive casing 8 is rigidly connected to the insulating layer 7 along the entire contact surface between both. The casings 6 and 8 and the layer 7 constitute a solid insulating system and are conveniently extruded around the threads 5.

Although the conductivity of the internal semiconductor housing 6 is lower than that of the electroconductive wires 5, it is still sufficient to equalize the potential present on its surface. Consequently, the electric field is evenly distributed around the circumference of the insulating layer 7 and the risk of localized field increases and partial discharges is minimized.

The potential present in the outer semiconductive housing 8, which is conveniently zeroed, grounded or in contact with any other controlled potential, is equalized at this value by the conductivity of the housing. At the same time, the semiconductor housing 8 has sufficient electrical resistivity to close the electric field. In view of this electrical resistivity, it is desirable to connect the conductive polymeric housing to ground, or to any other controlled potential, at intervals.

The transformer according to the invention can be a single-phase, three-phase or multiphase transformer, and the core can have any design. Figure 3 shows a three-phase laminated core transformer. The core has a conventional design and comprises three core columns 9, 10 and 11 and the connecting heads 12 and 13.

The windings are wrapped concentrically around the columns of the. core. In the transformer of Figure 2 there are three concentric winding layers 14, 15 and 16. The innermost winding layer 14 can represent the primary winding, and the other two winding layers, 15 and 16, the secondary winding. To facilitate the understanding of the figure, details such as the winding connections have been omitted. At certain points around the windings, the separating bars 17 and 18 have been placed. These bars 17 and 18 can be manufactured with insulating material to delimit a certain space between the winding layers 14, 15 and 16 for the purpose of cooling, retention, etc., • or manufactured with an electroconductive material to form part of a grounding system of windings 14, 15 and 16.

The mechanical design of the individual coils of a transformer must be such that they can withstand the forces resulting from short circuit currents. Since these forces can be very high in a power transformer, the coils must be distributed and proportioned to offer a generous margin of error, which is why the coils can not be designed to optimize performance in normal operation.

The main objective of the present invention is to remedy the aforementioned problems related to the forces resulting from short circuits in a dry transformer.

- This objective is achieved by a transformer as indicated in claim 1.

When manufacturing transformer windings from a conductor which is magnetically permeable, but practically lacks electric fields outside the outer semiconductor housing that covers it, the high and low voltage windings can be easily mixed arbitrarily to minimize the forces resulting from short circuits. Such mixing would be unfeasible in the absence of the semiconductor housing or other means of containing the electric fields, and would therefore be considered impossible in a conventional power transformer. oil, since the insulation of the windings would not support the electric field between the high and low voltage windings.

It is also possible to reduce the distributed inductance and design the transformer core for optimum adaptation of the size of the window to the core mass.

According to one embodiment of the invention, at least several of the layers of the low voltage winding are individually divided into a number of sublayers connected in parallel to reduce the difference between the number of high voltage winding layers and the total of layers of low voltage winding, so that the mixing between high voltage winding layers and low voltage winding layers is as uniform as possible, preferably, each layer of the low voltage winding is divided into such a number of connected sublayers in parallel that the total of low voltage winding layers is equal to the number of high voltage winding layers. The high and low voltage winding layers can then be mixed in a uniform manner, so that the magnetic field generated by the low voltage winding substantially cancels the magnetic field of the high voltage winding layers.

According to another advantageous embodiment, the layers of the high-voltage winding and the layers of the low-voltage winding are arranged symmetrically in the form of a chessboard, as seen in the cross section of the windings. This is an optimum arrangement to achieve reciprocal cancellation of the magnetic fields of the low and high voltage windings, and therefore an optimum arrangement to reduce the forces of short circuits in the coils.

According to yet another advantageous embodiment, at least two contiguous layers have substantially equal thermal expansion coefficients. In this way damage of thermal character in the winding is avoided.

Another aspect of the invention offers a winding method for the transformer that is set forth in claim 18.

In order to explain the invention in greater detail, the following embodiments of the transformer according to the invention will be described only as an example, with reference to the figures: Figure 1 gives an example of the cable used in the windings of the transformer according to the invention.

Figure 2 shows a conventional three-phase transformer.

Figures 3 and 4 show in cross section different examples of arrangement of the low and high voltage windings of the transformer according to the invention.

Figure 5 shows a winding system for the transformer.

Figure 3 is a cross section of the winding part of a power transformer according to the invention located inside the core 22 of the transformer. A layer of a low voltage winding 26 is located between two layers of a high voltage winding 28. In this embodiment the transformation ratio is 1: 2.

The direction of the current in the low voltage winding 26 is opposite to the direction of the current in the high voltage winding 28 and consequently the resulting forces of the currents in the low and high voltage winding cancel each other out. This possibility of reducing the effect of the forces induced by the currents is of great importance, especially in the case of a short circuit.

Between the windings 26 and 28 are located uprights 27 of laminated magnetic material, comprising spacers 29 that provide air pockets, in order to improve the efficiency of the transformer.

The cancellation of the forces resulting from the short-circuits can be further improved by dividing the layers of the low-voltage winding into several sub-layers connected in parallel, preferably in such a way that the total of low-voltage layers is equal to the number of high-winding layers. tension. Thus, if the transformation ratio is equivalent, for example, to 1: 3, each layer of the low voltage winding is divided into three sublayers. It is then possible to mix the low and high voltage windings more evenly. An optimal arrangement of the windings is shown in Figure 4, where the low and high voltage winding layers 30 and 32 respectively are arranged symmetrically in the form of a chessboard. In this embodiment the magnetic fields of each layer of the low and high voltage windings 30 and 32 cancel each other substantially and the short-circuit forces are almost completely canceled out.

By dividing a winding layer into several sublayers the conductive area of each sublayer can be proportionally reduced, since the sum of the current intensities of the sub-layers remains equal to the current intensity of the original winding layer. In this way, no more copper conductor material is normally required when dividing the winding layers, provided that the other factors do not change.

Figure 5 shows schematically how the transformer of the invention can be wound up. A first drum 40 carries a high voltage conductor 42 and a second drum 44 carries a low voltage conductor 46. The conductors 42 and 46 are unwound from the drums 46 and 44 and wound on the drum 48 of the transformer, simultaneously rotating the three Drums 40, 44 and 48. In this way the high and low voltage conductors can be intermixed easily. Joins can be made ben different winding layers.

In the transformer of the invention the magnetic energy and consequently the scattered magnetic field of the windings is reduced. A wide range of impedances can be chosen.

The electrical insulation systems of the windings of a transformer according to the invention are designed to cope with very high voltages and the consequent electrical and thermal loads that may arise operating at these voltages. By way of example, the power transformers according to the invention can have nominal currents higher than 0.5 MVA, preferably higher than 10 MVA and even more preferably higher than 30 MVA, up to 1000 MVA, and nominal voltages from 3-4 kV, in particular greater than 36 kV and preferably more than 72.5 kV, up to very high transmission voltages of 400-800 kV or higher. Operating at high voltages, partial discharges, or DP, are a serious problem for known insulation systems. If the insulator has cavities or pores, an internal corona discharge can occur, whereby the insulating material experiences a gradual degradation that eventually leads to the breakage of the insulator. The electrical charge received by the electrical insulator using a transformer according to the present invention is reduced by ensuring that the first inner layer of the insulating system having semiconductor properties is substantially at the same electrical potential as the conductors of the central electroconductive environment it surrounds, and that the second outer layer of the insulating system having semiconductor properties is at a controlled potential, for example connected to earth. Thus, the electric field of the solid electrical insulating layer located ben these internal and external layers is distributed substantially uniformly along the thickness of the intermediate layer. By having materials with similar thermal properties and few defects in these layers of the insulating system, operating at certain voltages reduces the possibility of partial discharges. The transformer windings can then be designed to withstand very high service voltages, typically up to 800 kV or higher.

Although it is preferable that the electrical insulation is extruded in its position, it is possible to manufacture an electrical insulating system from superposed and closely wound layers of sheet or sheet material. Both the semiconducting layers and the electrical insulating layer can be constructed in this way. An insulating system can be made from a fully synthetic sheet with. inner and outer semiconducting layers or parts made of thin polymeric sheet of, for example, PP, PET, LDPE or HDPE, with embedded conductive particles, such as carbon black or metallic particles, and with an insulating layer or part ben the layers or semiconductor parts.

As for the concept of insulating coating, a sufficiently thin sheet would have smaller joint gaps than the so-called minimal Paschen, which would make the impregnation of liquid unnecessary. A thin film insulation of several rolled dry layers also has good thermal properties.

Another example of an electrical insulator system is similar to a conventional cellulose-based cable, in which a thin paper with cellulose or synthetic base or non-woven material is wound looped around a conductor. In this case, the semiconductive layers, on either side of an insulating layer, can be made with cellulose paper or nonwoven material made of fibers of insulating material and with embedded conductive particles. The insulating layer can be made of the same base material or another material can be used.

Another example of an insulating system is achieved by combining sheet and fibrous insulating material, either as laminate or as co-coated insulator. An example of this insulating system is the so-called paper polypropylene laminate, PPLP, which is sold on the market, but several other combinations of foil and fibrous parts are also feasible. In these systems, various impregnations can be used, such as mineral oil or liquid nitrogen.

It is noted that in relation to this date, the best method known to the applicant, to put into practice the aforementioned invention is that which is clear from the manufacture of the objects to which it relates.

Having described the invention as above, the content of the following is claimed as property.

Claims (21)

1. A power or energy transformer comprising at least one high voltage winding and a low voltage winding, characterized in that each of said windings comprises a flexible conductor provided with means for containing electric fields, but which is magnetically permeable, and characterized in that the windings are intermixed in such a way that layers of the high voltage winding are mixed with layers of the low voltage winding.

2. A transformer according to claim 1, characterized in that said low voltage winding is wound up as a low voltage winding layer located between the two adjacent layers of corresponding high voltage winding.

3. A transformer according to claim 1 or 2, characterized in that said windings are arranged according to a periodic repeated pattern consisting of a high voltage winding layer, followed by a low voltage winding layer, followed by two winding layers of winding. high voltage, followed by a low voltage winding layer, followed by two layers of high voltage winding, etc.

4. A transformer according to any of claims 1 to 3, characterized in that each of at least several of the layers of the low voltage winding is divided into a certain number of sublayers connected in parallel to reduce the difference between the number of layers of High voltage winding and the total of low voltage winding layers.

5. A transformer according to claim 4, characterized in that each layer of the low voltage winding is divided into a certain number of sublayers connected in parallel equal to the number of high voltage winding layers.

6. A transformer according to claim 5, characterized in that the layers of the high-voltage winding and the layers of the low-voltage winding are arranged symmetrically in the form of a chessboard, as can be seen in a cross section of the windings.

7. A transformer according to any of the preceding claims, characterized in that the conductor comprises a central electroconductive means, a first layer provided with semiconducting properties disposed around the said conductive means, a solid insulating layer disposed around said first layer and a containment means of electric fields comprising a second layer provided with semiconductive properties arranged around said insulating layer.

8. A transformer according to claim 7, characterized in that the potential of said first layer is substantially equal to the potential of the conductor.

9. A transformer according to claim 7 or 8, characterized in that said second layer is arranged to substantially constitute an equipotential surface around said conductor.

10. A transformer according to claim 9, characterized in that said second layer is connected to a predetermined potential.

11. A transformer according to claim 10, characterized in that said predetermined potential is mass potential.

12. A transformer according to any of claims 7 to 11, characterized in that at least two contiguous layers have substantially equal thermal expansion coefficients.

13. A transformer according to any of claims 7 to 12, characterized in that said central conductor means comprises a set of wires, only a minority of said wires being in electrical contact with each other.

14. A transformer according to any of claims 7 to 13, characterized in that each of the three mentioned layers are permanently connected to the adjacent layers along practically all of the connection surface.

15. A transformer according to any of claims 7 to 14, characterized in that the cross-sectional area of the central conductor means measures between 80 and 3000 mm2.

16. A transformer according to any of the preceding claims, characterized in that the outer diameter of the conductor measures between 20 and 250 mm.

17. A transformer according to any of the preceding claims, characterized by the presence of uprights of laminated magnetic material between the windings.

18. A transformer according to any of the preceding claims, characterized in that the means for containing electric fields is designed for high voltage, being suitable that exceeds 10 kV, in particular that is greater than 36 kV, and preferably greater than 72.5 kV until reaching Very high transmission voltages, such as between 400 and 800 kV or more.

19. A transformer according to any of the preceding claims, characterized in that the electric field containing means is designed for a power range greater than 0.5 MVA, preferably higher than 10 MVA and more preferably higher than 30 MVA, up to 1000 MVA.

20. A winding system for a power transformer, which comprises simultaneously winding high voltage and low voltage flexible conductors provided with a means for containing electric fields but which are magnetically permeable, so that layers of high voltage winding are intermixed with layers of the low voltage winding.

21. A system according to claim 20, characterized in that the high voltage and low voltage conductors are simultaneously unwound from their respective drums and wound around a drum of the transformer. TRANSFORMER SUMMARY OF THE INVENTION A power transformer comprising at least one high voltage winding and a low voltage winding. Each of the windings contains at least one current-guided conductor, a first layer provided with semiconductive properties disposed around said conductor, a solid insulating layer arranged around said first layer and a second layer provided with semiconductive properties disposed around the cited insulating layer. The windings are intermixed in such a way that layers of the high voltage winding are mixed with layers of the low voltage winding.