WO2009144759A1

WO2009144759A1 - Optimized shielded transformer, in particular for carrying out dielectric tests

Info

Publication number: WO2009144759A1
Application number: PCT/IT2009/000199
Authority: WO
Inventors: Claudio Ceretta
Original assignee: S.E.A. Societa' Elettromeccanica Arzignanese S.P.A.
Priority date: 2008-05-28
Filing date: 2009-04-30
Publication date: 2009-12-03
Also published as: ITVI20080124A1; EP2289080B1; EP2289080A1

Abstract

An optimized shielding transformer (1), in particular for carrying out dielectric tests, comprising a containment case (2) which is placed on a support structure, a magnetic core (3) positioned inside the containment case (2), a primary winding (4), coupled with the magnetic core (3), and a secondary winding (6) magnetically coupled with the primary winding (4) through the magnetic core (3). The transformer (1 ) includes a shaped protection screen (8), facing the primary winding (4) and provided with a metallic outer surface (8a) which splits up in uniform and capacitive way on the primary winding (4) a possible voltage impulse which affects the shaped screen (8) itself.

Description

OPTIMIZED SHIELDED TRANSFORMER, IN PARTICULAR FOR CARRYING OUT DIELECTRIC TESTS

The present invention relates to an optimized shielding 5 transformer, in particular for carrying out dielectric tests.

It is known that in electrotechnis a special type of single-phase transformer is used to perform dielectric tests on an electrical apparatus, such as a transformer, for example in order to test the degree of insulation of the windings in respect to the other parts of the apparatus itself. 10 As known, a transformer is a static machine operating in alternating current suitable to convert the parameters of output voltage and electric current with reference to those input ones while keeping a high efficiency.

The single-phase transformer includes two or more inductor 15 circuits, called primary winding and secondary winding, coupled each other through a common magnetic flux, which varies over time, supplied by a high permeability magnetic circuit, called ferromagnetic core.

The primary winding and the secondary winding are also called in technical jargon high voltage winding and low voltage winding 20 respectively.

A first winding, usually the primary one, receives power at a given value of voltage and frequency from an electrical energy source, such as for example the electric network, while the second winding, usually the secondary one, provides power to a user load at the same 25 frequency value and at a voltage value usually different from that one which supplies the first winding.

Constructively, the ferromagnetic core is composed of a series of shaped laminations, packed together, and, in general, presents a so- called "columns" or "shell" type shape.

30 The stretches of the ferromagnetic core on which the primary and secondary windings are wound are called columns and are connected together by transverse portions called core clamps.

The section of these columns can be rectangular, square or joggled, in relation to the operating voltages and the chosen power of the 35 transformer.

The solution with columns having steps section is the most expensive but allows a better use of the space available within the windings and a better anchoring of the latter. The junction between columns and yokes constitutes a magnetic gap whose thickness must be reduced in order to minimize the reluctance of the magnetic circuit.

The windings, made of even enamelled copper or aluminium, consist of coils formed by several turns and coaxially mounted on one or more columns of the ferromagnetic core according to a concentric, most common, or alternate, less common, lay-out.

In the concentric lay-out, the coils forming the primary winding and the coils forming the secondary winding have different diameters; in the alternate lay-out, the coils forming the primary winding and the coils forming the secondary winding have the same diameter and are mounted one intercalated the other.

The primary winding and the secondary winding are properly insulated each other, in case of concentric lay-out for example by a tubular cylinder made of high dielectric strength material, such as prestressed paper-based material, or by a screen composed of a series of cylinders made of carton and overlapped each other.

The transformer also provides appropriate channels among the windings, suitable to facilitate the passage of a refrigerant fluid, for example mineral oil or air in natural or forced ventilation, in order to dispose the power dissipated both in the ferromagnetic core, due to hysteresis and parasitic currents, and in the windings due to Joule effect.

Furthermore, the transformer includes a containment case, made of metallic material, especially for outside installations, or insulating material, and an expansion reservoir of the refrigerant fluid, commonly called conservator.

The member organs described above, such as the magnetic core, the windings and the core clamps are placed inside the containment case, while the conservator is externally coupled with the containment case.

A transformer for dielectric tests provides the application of a prefixed voltage at industrial frequencies for a given time interval, in order to determine for an electrical apparatus, usually another transformer, the insulation of the windings towards the other parts. At the light of this, a specific classification of the transformers for carrying out dielectric tests has been made ready in the field: there are, in fact, transformers with insulation class up to 130 kV, up to 170 kV, up to 400 kV and so forth.

During dielectric tests, the transformer supplies an electric current only to feed its own capacities and those ones of the test apparatus, included between fractions of amperes and 1/2 amperes. The present invention relates to transformers, in particular for carrying out dielectric tests, provided with the so-called "cigar" winding, made of several layers facing each other whose height gradually decreases, according to a predetermined rule, starting from the column on which the winding is wound towards the containment case, in direction orthogonal to the column itself.

As already mentioned, this type of construction of the transformer allows a better use of the space available inside the windings and favours the limitation of the overall sizes of the containment case.

In these transformers of known type, however, a drawback remains related to possible voltage impulses, consisting of overvoltages, short circuit voltages, voltage drops or so forth which, for a play of parasitic capacities, are distributed on the various sections of the winding.

Indeed, these phenomena, which can be easily found in the practice, generate a series of voltages which, in the known transformers, do not split up in controlled and homogeneous way on the winding sections.

This is extremely dangerous since the electric potential differences which are created among the sections of the winding due to voltage impulses sometimes irreparably damage the winding, with the obvious consequent disadvantages, such as the need for repair or replacement interventions always unprofitable in terms of times and costs. The present invention intends to overcome the drawbacks of the prior art just mentioned.

Specifically, it is main purpose of the present invention to develop an optimized shielding transformer, in particular for carrying out dielectric tests, which operates a voltage impulses distribution on the sections of at least one of the windings more uniform than the known transformers.

Within such a purpose, it is task of the invention to reduce, compared to the known art, the risks of even irreparable damages suffered by one or more of the windings of a transformer due to uncontrolled voltage impulses. It is another task of the present invention to limit with respect to the currently known technique the interventions of maintenance, repair and/or replacement of the windings of a transformer, eliminating or greatly reducing the costs related to such operations compared to the prior art. The aforesaid purposes are achieved by an optimized shielding transformer, in particular for carrying out dielectric tests, as the attached claim 1 , to which they refer for the sake of brevity.

Other features of detail of the optimized shielding transformer of the invention are described in the relative dependent claims. Advantageously, the transformer of the invention, thanks to the shaped screen having the metallic or metalled outer surface, preserves the integrity status of the windings to a greater extent than the equivalent transformers of the known type.

In case of transformers with winding consisting of more layers, the shaped screen, after all, allows to configure the winding itself as a capacitive potential divider in which two mutually adjacent and facing layers form a condenser.

In such an occurrence, indeed, a voltage impulse does not directly interfere with the winding but hits the shaped screen which splits up in capacitive way the voltage on the various layers of the winding.

This allows a distribution and, consequently, a control of the voltage impulse which is generated inside the containment case more uniform than in the equivalent prior art.

Equally advantageously, the metallic and equipotential outer surface of the shaped screen does not give constraints to the connection of the insulator, another classic component of a transformer, thus simplifying the fitting operations and the constructive choices.

Still advantageously, in case of cigar transformers, the shaped screen belonging to the transformer of the invention removes the need, which can be found, instead, in the current technology, to isolate the last layer of the primary winding.

The aforesaid purposes and advantages will appear better evident from the description that follows, relating to a preferred embodiment of the invention, given as indicative but not limitative way, with reference to the attached drawings in which: figure 1 is the view in longitudinal section of the transformer of the invention; figure 2 is the view in cross section of the transformer of figure 1 ; figure 3 is an enlargement of a portion of the transformer of figure 1 ; - figure 4 is an enlarged detail of figure 3.

The single-phase optimized shielding transformer of the invention, mostly suitable for carrying out dielectric tests, is shown sectioned in figures 1 and 2, where it is globally indicated with 1.

As it is noted, the optimized shielding transformer 1 comprises: - a containment case 2 which is arranged on a support structure S, for example the floor of an industrial plant; a magnetic core, as a whole numbered with 3, placed inside the containment case 2; a primary winding, overall indicated with 4, associated with the magnetic core 3 and which in this case, at the output 5 of the containment case 2, is electrically connected with a user load, not illustrated and constituted, for example, by an electric apparatus to be tested; a secondary winding, as a whole reported with 6, magnetically coupled with the primary winding 4 through the magnetic core 3 and which, in this case, at the connection terminals 7 is electrically connected with electricity supply means, such as an electricity generator, not shown.

In accordance with the invention, the transformer 1 comprises a shaped screen, as a whole indicated with 8, facing the primary winding 4 and provided with a metallic outer surface 8a which, in case of need, splits up in a uniform and capacitive way on the primary winding 4 a possible voltage impulse which succeeds to hit the shaped screen 8.

Preferably but not necessarily, the shaped screen 8 includes a metalled coating 9, coupled with the main core 10 of the shaped screen 8 itself through junction means, not represented and of the type in itself known to the person skilled in the art.

The metalled coating 9, thus, constitutes the metallic outer surface 8a, making it equipotential. The shaped screen 8 is also provided with a first curved and convex end 8b and in longitudinal section presents a substantially rectangular shape profile which defines a longitudinal direction Y-i. As far as the containment case 2 is concerned, it is of the type in itself known in the field in question, for example made of metallic material.

Even the magnetic core 3 presents shape and composition of the type in itself known in the electrotechnical industry: in particular, the magnetic core 3 is of the so-called column type and is formed by a plurality of laminations packed each other.

The magnetic core 3 has in longitudinal section a rectangular profile, comprising two opposite columns 31, 32, on the second of which the windings 4 and 6 are arranged, the columns 31 , 32 being reciprocally connected by two yokes 33, 34 which are compressed through core clamps 11 , 12 made of metallic material, one of which connected with the containment case 2 through anchorage means, overall indicated with 13 and the other one constrained to a reference plane 14 of the case 2. Figures 1 and 2 show moreover some of the other traditional member organs of a transformer, in particular for carrying out dielectric tests, also provided by the optimized transformer 1 of the invention, such as the high voltage insulators 15 connected with the primary winding 4, the low voltage insulators 16 connected with the secondary winding 6. Figures 1 and 2 show, in addition, the expansion conservator

17 containing hot oil used as refrigerant fluid and insulator responsible for the disposal of heat generated inside the containment case 2 of the transformer 1.

In a preferred manner, the primary winding 4 comprises a plurality of layers 18 facing each other, defining linear directions Y substantially parallel each other and to the longitudinal direction Yi of the shaped screen 8.

Each of the layers 18 includes in turn a plurality of elementary coils one independent from another, electrically connected each other. In detail, the coils are reciprocally connected in series and arranged one consecutively the other along the tortuous path defined by the layers 18 of the primary winding 4.

The aforesaid coils include the same number of turns, each of them producing, preferably but not necessarily, a voltage of 20 kV. From figures 1 and 2 it is derived that, in the case at issue, the primary winding 4 comprises four layers 18 facing each other.

For pure sake of exposition, the various coils which compose the primary winding 4 are respectively indicated with:

• A, if belonging to the first layer, that one closest to the magnetic core 3;

• B, if belonging to the second layer; • C, if belonging to the third layer;

• D, if belonging to the fourth layer, that one farthest from the magnetic core 3.

Moreover, the optimized shielding transformer 1 includes a plurality of spacer gates 21, better highlighted in figure 3, which separate one from another the coils in each of the layers 18.

The secondary or low voltage winding 6 is, instead, divided into two secondary sub-windings 19, 20 of the conductive sheets type, each side-by-side to the other along a direction parallel to the longitudinal direction Yi and both wound on the column 32 of the magnetic core 3. In further embodiments of the transformer of the invention, not illustrated in the appended drawings, the number of layers present in the primary winding might be different from that one indicated above, depending on design choices and voltage values to be achieved.

There might also exist other embodiments of the invention, not shown, in which only the secondary winding includes the layers or both the primary winding and the secondary winding include such layers.

The windings 4 and 6 are magnetically coupled with the column 32 of the magnetic core 3, with the primary winding 4 which surrounds the secondary winding 6 through the interposition of an insulating screen 16 and, consequently, the column 32 of the magnetic core 3 in respect of which it is coaxial.

In particular, the linear directions Y defined by the various layers 18 of the primary winding 4 are parallel not only between themselves but also to the columns 31 and 32 of the magnetic core 3. The height of each of the layers 18 decreases in gradual and prearranged way, on the basis of the constructive choices, starting from the column 32 of the magnetic core 3 and according to a longitudinal direction X orthogonal to the linear directions Y defined by the layers 18.

The last coil of a layer 18 is electrically connected with the first coil of the immediately following layer 18 through an intermediate connecting terminal 22 which splits up the voltage in order to avoid peaks thereof. The intermediate connecting terminal 22 faces the ends of each of the layers 18 in order to interpose between any layer 18 and the core clamps 11 , 12 arranged opposite each other inside the containment case 2. In this case, through an intermediate connecting terminal 22 the last coil A of the first layer 18 is connected with the first coil B of the second adjacent layer 18, the last coil B of the second layer 18 is connected with the first coil C of the third layer 18 and the last coil C of the third layer 18 is connected with the first coil D of the fourth layer 18. The intermediate connecting terminal 22 has a substantially semi-elliptical shape profile 22' which includes a flat surface 22a facing the coils of two layers 18 contiguous and consecutive each other, as it can be seen for example in figure 4 for the coils marked with C and the coils marked with D. The intermediate connecting terminal 22 also includes a metalled coating 23, for example a laminar paper, suitable to make equipotential the outer wall 22b of the intermediate connecting terminal 22. The optimized shielding transformer 1 also comprises: a low voltage terminal 24, well visible in figures 3 and 4, which splits the voltage up, facing the first coil A of the highest height layer 18 closest to the column 32 of the magnetic core 3; a full voltage terminal 25, well visible in figures 3 and 4, which splits the voltage up, facing the coil D of the lowest height layer 18 and farthest from the column 32 of the magnetic core 3. The low voltage terminal 24 and the full voltage terminal 25 are interposed between one of the layers 18 and one of the core clamps 11 , 12.

Even the low voltage terminal 24 and the full voltage terminal 25 include a metalled coating 26 which makes them equipotential. By virtue of these constructive projects, in the case at issue, for example, the gradual decrease of the height of the layers 18 along the longitudinal direction X allows to achieve voltage values V divided as follows along the primary winding 4, starting from a value V = 0 V at the low voltage terminal 24: • V = 160 kV at the first intermediate connecting terminal 22 included between the first and second layer 18;

• V = 300 kV at the second intermediate connecting terminal 22 included between the second and third layer 18;

• V = 420 kV at the third intermediate connecting terminal 22 included between the third and fourth layer 18;

• V = 500 kV at the full voltage terminal 25. Figure 3 shows that the shaped screen 8 is arranged between the containment case 2 and the lowest height layer 18 of the primary winding 4 and has a height substantially equal to that one of the lowest height layer 18.

More specifically, the shaped screen 8 presents: - a second end 8c, opposite the first end 8b, directly facing the bottom 25a of the full voltage terminal 25; a first flat side face 8d oriented towards the primary winding 4; a second flat side face 8e oriented towards the containment case 2, spaced apart by a linear stretch from the first side face 8d.

The optimized shielding transformer 1 includes, moreover, a plurality of insulating elements 27 suitable to limit the electric field in the containment case 2. These insulating elements 27 are arranged alongside each other between one layer 18 and the other of the primary winding 4, between the secondary winding 6 and the highest height layer 18 of the primary winding 4.

Furthermore, the insulating elements 27 are located, in preferred but not binding way, between the first side face 8d of the shaped screen 8 and the lowest height layer 18 of the primary winding 4, as well as between the second side face 8e of the shaped screen 8 and the containment case 2.

Each of the insulating element 27 includes a laminar cylinder made of cardboard, paper or other suitable cellulosed-based material, sized so as to endure the mechanical and electrical stresses to which the optimized shielding transformer 1 is subjected.

Advantageously, the transformer 1 comprises a pressing ring 28 positioned close to the low voltage terminal 24 and interposed between the latter and the core clamp 11.

Such a pressing ring 28 performs also the function of laterally protecting the primary winding 4 at the first layer 18. According to the preferred embodiment here described of the invention, the optimized shielding transformer 1 includes a series of piles

29 of pressing rings 30, interposed between each of the core clamps 11 , 12 and the primary winding 4 and positioned close to the intermediate connecting terminals 22 and the full voltage terminal 25.

The height of these piles 29 increases gradually as the height of the layers 18 of the primary winding 4 decreases along the longitudinal direction X and, therefore, as the lateral distance between the primary winding 4 and the core clamps 11, 12 increases. In essence, the height of the pile 29 is inversely proportional to the height of the layers 18 of the primary winding 4.

Since to the decrease of the height of the layers 18 along the longitudinal direction X corresponds also an increase of the voltage in the primary winding 4, the height of the pile 29 is, therefore, directly proportional to the voltage produced in the primary winding 4 passing from the first layer 18, near the magnetic core 3, to the fourth layer 18, that one farthest from the magnetic core 3.

More specifically, as figures 3 and 4 show, the pressing rings

30 belonging to the same pile 29 has variable thickness: in any case, the pressing rings 30 of smallest thickness are arranged in the vicinity of the intermediate connecting terminals 22 and the full voltage terminal 25.

At preferential title, the optimized shielding transformer 1 comprises shielding means, indicated as a whole with 35, interposed among some insulating elements 27 and among some pressing rings 30 in correspondence of the intermediate terminals 22 and the full voltage terminal 25.

The shielding means 35 increase the protection of the primary winding 4 against possible electrical discharges, voltage drops, overvoltages and so on since they create among the insulating elements 27 channels which compel such anomalous effects in order to make a forced and tortuous path which causes their dissipation, actually preventing that they reach the primary winding 4.

In the example described, the shielding means 35 comprise, preferably but not necessarily, a plurality of laminar rings 36 and a plurality of laminar caps 37 opposite and facing the laminar rings 36.

In particular, a stretch of the laminar rings 36 and the laminar caps 37 is interposed among the lowest thickness pressing rings 30, placed at the intermediate terminals 22 and the full voltage terminal 25.

Each of the laminar rings 36 and each of the laminar caps 37 present, preferably, a substantially L-shaped profile.

In addition, both the laminar rings 36 and the laminar caps 37 are made of cardboard, paper or other material derived from cellulose.

The functioning of the single-phase optimized shielding transformer 1 of the invention is essentially equivalent to that one of the common transformers currently available on the market.

One of the main innovations of the optimized shielding transformer 1 is, then, represented by the arrangement of the shaped screen 8 suitable to split up in a uniform manner on the various layers 18 of the primary winding 4 a possible voltage impulse, thus limiting the likely adverse effects on the structural integrity of the winding 4 itself.

On the basis of what expressed, it is understood, therefore, that the optimized shielding transformer of the invention, in particular for carrying out dielectric tests, reaches the purposes and achieves the advantages previously mentioned.

In implementation phase, changes may be made to the optimized shielding transformer object of the present invention, consisting, for example, in a shape of each of the primary and secondary windings different from that one indicated during the previous pages of the description.

The containment case could have any size, as well as being made of any of the materials traditionally used for a transformer in the electrotechnic field.

In addition, there could be other embodiments of invention in which the optimized shielding transformer includes more then one shaped screen facing one of the windings.

Furthermore, such a shaped screen could be positioned directly close to the winding, whether primary or secondary, without interposing the insulating elements previously indicated for the preferred embodiment of the invention described in the present patent text.

It is, finally, clear that many other variations may be made to the optimized shielding transformer in question, without for this reason going out of the novelty principles inherent to the inventive idea here expressed, as it is clear that, in the practical implementation of the invention, materials, shapes and sizes of the illustrated details may be any, depending on the needs, and replaced with others technically equivalent.

Claims

1. Optimized shielding transformer (1), in particular for carrying out dielectric tests, comprising: a containment case (2) suitable to be placed on a support structure; at least one magnetic core (3) positioned inside said containment case (2); at least one primary winding (4), coupled with said magnetic core (3); - at least one secondary winding (6), magnetically coupled with said primary winding (4) through said magnetic core (3), characterized in that it comprises at least one shaped screen

(8), facing at least one of said primary (4) and secondary windings (6) and provided with a metallic outer surface (8a) suitable to uniformly split up on said primary (4) or secondary winding (6) a voltage impulse which hits said shaped screen (8).

2. Transformer (1) as claim 1) characterized in that said shaped screen (8) comprises at least a metalled coating (9), coupled through junction means with a main core (10) of said shaped screen (8), suitable to constitute said metallic outer surface (8a) making it equipotential.

3. Transformer (1) as claim 1) or 2) characterized in that said shaped screen (8) is provided with at least one first curved and convex end (8a), and in longitudinal section presents a substantially rectangular shape profile which defines a longitudinal direction (Yi).

4. Transformer (1) as claim 3) characterized in that at least one between said primary winding (4) and said secondary winding (6) comprises two or more layers (18) facing each other, defining linear directions (Y) substantially parallel each other and to said longitudinal direction (Yi) of said shaped screen (8), each of said layers (18) including a plurality of elementary coils one independent from another, electrically connected each other.

5. Transformer (1) as claim 4) characterized in that said primary winding (4) includes two or more layers (18) facing each other.

6. Transformer as claim 4) characterized in that said secondary winding includes two or more layers facing each other.

7. Transformer as claim 4) characterized in that said primary winding and said secondary winding each comprise two or more layers facing each other.

8. Transformer (1) as claim 4) characterized in that said coils comprise the same number of turns, each of them being suitable to produce a voltage of 20 kV.

9. Transformer (1) as claim 4) or 8) characterized in that said coils are reciprocally connected in series and arranged one consecutively the other.

10. Transformer (1) as claim 4) or 8) characterized in that it comprises a plurality of spacer gates (21) which separate one from another said coils in each of said layers (18).

11. Transformer (1) as claim 4) characterized in that said magnetic core (3) is of the columns type and said windings (4, 6) are concentric and symmetrically arranged with respect to the central axis defined by one of said columns (31 , 32) of said magnetic core (3) with which are magnetically coupled, with said primary winding (4) coaxial and external to said secondary winding (6).

12. Transformer (1) as claim 11) characterized in that the height of each of said layers (18) of said winding decreases in gradual and prearranged way starting from said column (32) of said magnetic core (3) according to a longitudinal direction (X) orthogonal to said linear directions (Y) defined by said layers (18).

13. Transformer (1) as claim 11) characterized in that the last coils of a layer (18) is electrically connected with the first coil of the subsequent layer (18) through an intermediate connecting terminal (22), suitable to split the voltage up, facing the ends of each of said layers (18) in order to interpose between each of said layers (18) and a pair of reciprocally opposite core clamps (11 , 12) positioned inside said containment case (2).

14. Transformer (1) as claim 13) characterized in that said intermediate terminal (22) includes a metalled coating (23) suitable to make equipotential the outer wall (22b) of said intermediate terminal (22).

15. Transformer (1) as claim 13) characterized in that said intermediate terminal (22) has a substantially semi-elliptical shape profile (22¹) which includes a flat surface (22a) facing said coils of two of said layers (18) contiguous each other.

16. Transformer (1) as claim 13) characterized in that it comprises: a low voltage terminal (24), suitable to split the voltage up, facing one end of the first coil of the highest height layer (18) closest to said column (32) of said magnetic core (3); a full voltage terminal (25), suitable to split the voltage up, facing one end of the last coil of the lowest height layer (18) farthest from said column (32) of said magnetic core (3), said low voltage (24) and full voltage terminals (25) being interposed between said layer (18) and one of said core clamps (11 , 12).

17. Transformer (1) as claim 16) characterized in that said shaped screen (8) is located between said containment case (2) and said lowest height layer (18) of said primary winding (4) and has a height substantially equal to that one of said lowest height layer (18).

18. Transformer (1) as claim 16) characterized in that said shaped screen (8) presents: a second end (8c), opposite to said first end (8b), directly facing the bottom (25a) of said full voltage terminal (25); a first flat side face (8d) oriented towards said primary winding (4); a second flat side face (8e) oriented towards said containment case (2), separated from said first face (8d) by a linear stretch.

19. Transformer (1) as claim 18) characterized in that it comprises a plurality of insulating elements (27) suitable to limit the electric field inside said containment case (2), placed one facing the other between: a layer (18) and the other of said primary winding (4); said secondary winding (6) and said highest height layer (18) of said primary winding (4); said first face (8d) of said shaped screen (8) and said lowest height layer (18) of said primary winding (4); said second face (8e) of said shaped screen (8) and said containment case (2).

20. Transformer (1) as claim 19) characterized in that each of said insulating elements (27) includes a laminar cylinder made of paper, cardboard or other cellulose-based material.

21. Transformer (1) as claim 16) characterized in that it comprises at least one pressing ring (28) positioned close to said low voltage terminal (24) and interposed between said low voltage terminal (24) and one of said core clamps (11 , 12).

22. Transformer (1) as claim 19) characterized in that it includes a plurality of piles (29) of pressing rings (30) interposed between each of said core clamps (11 , 12) and said primary winding (4) and positioned close to each of said intermediate terminals (22) and said full voltage terminal (25).

23. Transformer (1) as claim 22) characterized in that the height of said pile (29) is inversely proportional to said height of said layers (18) of said primary winding (4).

24. Transformer (1) as claim 22) characterized in that it comprises shielding means (35), interposed between two or more of said insulating elements (27) and between two or more of said pressing rings (30) in correspondence of said intermediate terminals (22) and said full voltage terminal (25), suitable to increase the protection of said primary winding (4) against overvoltages, short circuit voltages, voltage drops or whatever.

25. Transformer (1) as claim 24) characterized in that said shielding means (35) include one or more laminar rings (36) and one or more laminar caps (37), facing and opposite to said laminar rings (36).

26. Transformer (1) as claim 25) characterized in that each of said laminar rings (36) and each of said laminar caps (37) have a substantially L-shaped profile.

27. Transformer (1) as claim 25) characterized in that each of said laminar rings (36) and each of said laminar caps (37) are made of paper, cardboard or other material derived from cellulose.