EP2075806A1

EP2075806A1 - Dry-type resin-insulated transformer with shielded side-by-side primary windings

Info

Publication number: EP2075806A1
Application number: EP07425825A
Authority: EP
Inventors: Franco Marini
Original assignee: BTicino SpA; Elettromeccanica di Marnate SpA
Current assignee: BTicino SpA
Priority date: 2007-12-27
Filing date: 2007-12-27
Publication date: 2009-07-01

Abstract

A dry-type resin-insulated transformer comprising at least one primary winding (39) and one secondary winding (22, 23) arranged coaxially with each other, with the secondary internal to the primary, the windings being jointly encapsulated in a body (6) of insulating resin having the shape of a cylindrical annulus, wherein the outer cylindrical surface of the resin body (6) is covered with a first metal shield (32) in the form of a split cylinder, electrically grounded and formed of a metal mesh incorporated in the resin of the insulating body and stiffened by upper and lower inwardly folded rims and two outwardly folded axial juxtaposed edges (35,36), whereas a second grounded shield (28) is formed of a metal mesh wrapped around the outer cylindrical surface of the secondary winding (23).

Description

The present invention relates to a dry-type resin-insulated transformer with shielded side-by-side primary windings.
Medium-voltage (up to 50 kV) dry-type transformers, particularly three-phase transformers are known to be used for civil and industrial use, in which the windings are encapsulated in a resin, such as an epoxy resin, instead of being immersed in an oil bath with both insulation and thermal convection functions.
In comparison with dielectric oil immersed transformers, dry-type transformers provide a number of advantages, in that they require no maintenance, involve no pollution risk and have a very low flammability rate.
Nonetheless, they still have certain drawbacks, caused by the displacement currents inevitably generated in the dielectric material due to the variable electric fields that develop around the conductors as a result of the voltages applied to the windings or induced therein.
These displacement currents have the effect of electrically charging the surface of the insulating material that incorporates the windings (particularly, the medium-voltage primary windings) and changing its electric potential to a (variable) value very close to that of the windings.
The electric field so developed does not run out within the insulating material, but extends into the surrounding environment and develops dangerous voltage gradients, depending on the potential, shape and distance of the surrounding elements.
These voltage gradients should remain within acceptable safety limits, considering that the dielectric strength of dry air is of the order of 21 kV/cm and rapidly decreases as a function of humidity.
Furthermore, the creation of dust deposits and humidity considerably reduce the surface resistance of the insulating material, and dangerous leakage surface currents may develop as a result thereof.
Typically, a safety margin of at least three is used.
In other words, with a voltage gradient of 20 kV/cm, the safety distance in air has to be at least 3 cm.
This involves two consequences: on the one hand, these transformers ensure a minimum protection degree (IP00) and must be housed in closed, weather-proof compartments.
On the other hand, safety distances in air between the various elements of the transformer must be accounted for during design, and particularly safety distances of the medium-voltage winding from the other parts having a different electric potential, as well as from the walls of the compartment in which the transformer is designed to be housed.
Thus, for instance, when air gaps are present, the insulating bodies that incorporate the various medium-voltage windings must be spaced from each other, from the low-voltage windings and from the grounded metal parts, such as the magnetic core, by a distance from 30 to 150 mm (30 mm at least for voltages of the order of 10 kV and 150 mm for voltages of the order of 50 kV.
If this aspect involves advantages, because channels are advantageously created for ventilation and natural air circulation between the medium- and low-voltage windings, between the low voltage windings (generally disposed inside the medium-voltage windings) and the columns of the magnetic circuit and around the medium-voltage windings, it also involves a considerable shortcoming in that it requires magnetic and electric parts of larger size, larger weight, higher material costs, which involve larger magnetic and resistive losses in the materials, greater magnetic leakages and, as a result, poorer performance.
This occurs even though the power dissipated in the transformer does not require the provision of large ventilation passages, like in medium-voltage transformers (12-50 kV) with a power lower than 150 kVA and total losses, even under full-load conditions, lower than 2-3 kW.
In an attempt to obviate the above prior art drawbacks, a number of solutions have been proposed, all based on a common principle: preventing the displacement currents in the dielectric material from accumulating dangerous local charges, by discharging such charges through electrically conducting shields, connected to a predetermined potential, generally the ground potential, which shields act as a capacitor plate, provided that the dielectric material is any way subjected to voltage gradients lower than its dielectric strength, with an adequate safety margin.
The very low currents so developed, generally lower than one milliampere, do not affect performance.
Besides this common principle, well described for instance in WO 01/08175 , the above solutions are diversified to account for further needs, such as ventilation and cooling of windings, simple and inexpensive fabrication, reliability of the resulting product.
The technical problem to be solved basically consists in providing, without complicating the winding encapsulating process, a conducting shield that can be perfectly integrated in the encapsulation insulating material, while ensuring reliable adhesion thereto even under thermal stresses, with very small volume requirements.
In the absence of specific indications to this purpose in the above document, the prior art fulfils only part of these requirements.
For example, FR 2784787 and JP59207611 provide medium-voltage primary windings and low-voltage secondary windings which are coaxial with the former, internal thereto and individually (separately) incorporated in separate resin bodies.
In the former document, the outer cylindrical surface of the insulating body of the medium-voltage primary winding is coated, by painting or similar processes (obviously carried out after formation of the insulating body), with a grounded semiconducting layer.
Mention is made to the fact that the treatment can be also performed on the inner cylindrical surface and to both inner and outer cylindrical surfaces of the insulating body that encapsulates the low-voltage secondary winding.
In the latter document, conductive plating is provided instead of a semiconducting layer, to be also applied by painting or similar processes.
Adhesion of the conducting or semiconducting layer to the resin bodies is particularly problematic and unreliable and requires a burdensome process.
Thus, for example, to ensure proper adhesion, EP0923785 provides encapsulation of the medium-voltage primary winding in a thermoplastic resin and later hot application of a few millimeters thick layer of electrically conducting thermoplastic resin.
This process is also burdensome and involves degradation of the insulation class of the product.
Even if a more limited purpose is proposed (reducing the size of the air channel separating the individually resin-encapsulated primary and secondary windings), EP0061608 may be also considered, in which a grounded metal shield is attached to the inner cylindrical surface of the insulating body that encapsulates the medium-voltage primary winding. The shield may be also encapsulated.
Metal-wire gauze having the shape of a split cylinder (to avoid the formation of closed turns) is suggested as a shield.
Nonetheless, no mention is made of a shield on the outer cylindrical surface of the resin body that encapsulates the primary winding, ad no explanation is provided about how to achieve accurate placement of the shield, whose thin gauze is highly flexible and resilient in itself, when it has to be encapsulated.
The present invention eliminates the above drawbacks and provides a dry-type transformer that can be fabricated in a simple and inexpensive manner, wherein a pair of coaxial primary and secondary windings are encapsulated in a common insulating resin body and separated by a first electrically grounded metal shield, which is encapsulated in the resin body in close proximity of the secondary winding, which acts as a positioning guide therefor, whereas a second metal grounded metal shield is encapsulated in the resin body on its outer cylindrical surface.
Attachment of the two shields to the resin body is reliable with time even under temperature fluctuations and resulting size changes, the shields being formed of a woven metal mesh which is be encapsulated in the resin during the single casting step required for encapsulating the two windings.
Accurate positioning of the shields in the casting mold is ensured by their particular structure and design.
The invention is more particularly characterized by the annexed claims.
The features and advantages of the invention will be more apparent from the following description of a preferred embodiment, when taken with reference to the accompanying drawings, in which

Figure 1 is a front schematic view of a dry-type three-phase transformer of the present invention;
Figure 2 is a detail view, as taken along section A-A of Figure 1, of a body of insulating resin that encapsulates a pair of windings, primary and secondary winding, of the transformer and a pair of electric shields of the primary winding;
Figure 3 is a top view of the resin body shown in Figure 2;
Figure 4 is an exploded perspective view of a variant embodiment of the electric screen for the resin body of Figures 2 and 3;
Figure 5 is a top view of a variant embodiment of the resin body as shown in Figure 2.

Referring to Figure 1, a dry-type three-phase transformer typically has three parallel ferromagnetic columns 1, 2, 3 arranged with a convenient center-to-center spacing I (e.g. 350 mm).
The magnetic circuit of the columns is closed by two yokes 4, 5.
A resin body 6, 7, 8 is disposed coaxially with each of the columns and encapsulates a medium-voltage primary winding and a low-voltage secondary winding arranged coaxially one inside the other, as shown in detail with reference to Figure 2.
As shown in Figure 3, the resin bodies are essentially shaped as sections of a cylindrical annulus with an axial cylindrical opening for receiving a column of the magnetic circuit and an axial rib 9 projecting from the outer cylindrical surface, and holding projecting elements for connection to the primary windings allowing connection thereof with each other and with the mains, as is known in the art, as well as terminals for adjusting the turn ratio and adapting the output voltage of the transformer to the voltage drop along the transformer supply line.
These connecting elements are individually recognizable from the view of Figure 1, where they are designated by numerals 10 to 18.
The dash lines 19, 20, 21 in Figure 1 represent a delta connection of the three primary windings when the terminals 10, 11, 12 are used for connection to a three-phase supply system.
Figure 1 also shows that the resin bodies 6, 7, 8 are in juxtaposed relationship and substantially in contact with each other, except for a very small clearance, of the order of 2-3 mm, which is required to allow the transformer to be assembled, and prevent any mechanical interference between the insulating bodies, due to thermal expansions (the linear thermal expansion coefficient of resin is much higher than that of iron in the yokes and still higher, though to a smaller extent, than that of copper or aluminum in the windings).
This feature, i.e. the side-by-side relationship of the insulating bodies is allowed by the particular shielded structure of the insulating bodies as described below with reference to Figure 2.
Since the resin body 6 of Figure 2, as well as its contents, result from a very simple fabrication process, which consists in arranging the various elements to be resin-encapsulated in a casting mold and later filling the mold with fluid resin, preferably of epoxy type, which hardens under the action of appropriate catalysts, the structure of the body 6 is clearly described by its fabrication process.
A cylindrical core is placed at the center of a casting mold, which is known in the art to consist of a cylindrical container (preferably composed of multiple separable elements for easier demolding), with a lateral undercut or compartment corresponding to the rib 9 of the resin body, for forming the central axial channel for the passage of a column of the magnetic circuit.
A non-stick gel is spread on the inner walls of the mold and on the central core.
Now, the low-voltage (LV) secondary winding is placed in the mold.
In the illustrated preferred embodiment, this is composed of two appropriately spaced concentric windings 22, 23, each being formed, in a known and conventional manner, on a very thin cylindrical fiberglass form 24, 25.
The ends 26, 27 of the secondary winding, whose flat metal turns are insulated by polyester taping, project out of the top of the mold.
A rectangular metal gauze sheet is previously stretched around the outermost cylindrical surface of the low voltage assembly (winding 23) to act as a cylindrical shield 28 with overlapping edges.
A preferably double-sided adhesive tape is interposed between the overlapping edges of the shield, to maintain the shield in a stretched state on the winding surface, while preventing the formation of a closed electric turn.
Thus, the LV winding acts as a rigid form for accurate positioning of the shield.
The lower 29 and upper edges 30 of the shield 28 are conveniently folded outwards with relatively large radius of curvature and extension, for reasons to be explained in greater detail below.
The shield 28 is equipped with a pre-welded terminal 31 on its upper edge 30, for connection to a ground element (such as the magnetic core of the transformer or its mechanical support frame).
For better clarity, Figure 3 also shows, by dash lines, the arrangement of the shield 28 and its folded upper edge 30 in the resin body.
A second shield 32, also formed of a rectangular metal gauze sheet similar to the one described above, but conveniently calendered to assume the shape of a split truncated cylinder, with a diameter equal to or slightly larger than that of the peripheral surface of the casting mold, is placed in the mold.
Here, unlike the former gauze, the upper and lower edges 33, 34 are folded inwards and the edges in the axial (vertical) direction do not overlap but maintain a juxtaposed relationship with a convenient spacing therebetween, for the passage of the terminals of the primary winding (two power terminals and three or more intermediate terminals for turns ratio adjustment).
The shield 32 is also equipped with a pre-welded terminal 37 on its upper edge 33, for connection to a ground element (such as the magnetic core of the transformer or its mechanical support frame).
Figure 3 shows, by dash lines, the arrangement of the shield 32 and its folded upper edge 33 in the resin body.
It also shows that the juxtaposed edges 35, 36 of the shield 32, providing a passage for the terminals of the primary winding, are conveniently folded outwards and are received within the rib 9 of the insulating body.
This is an essential aspect because the folded edges prevent local generation of high voltage gradients and also act as mechanical stiffening elements.
It shall be noted that proper mechanical adhesion between the shield and the resin requires the shield to feature relatively high plasticity and resilience properties.
This is achieved using a mesh structure having woven metal wires with a diameter ranging from 0.1 to 0.5 mm.
The plasticity and resilience requirement facilitates accurate positioning of the shield on a convex surface (such as a outer cylindrical surface, namely the outer surface of the secondary winding 23 of Fig. 2), but is not compatible with the need of also accurately placing the shield on a concave surface (such as the inner cylindrical surface of the mold).
Prior calendering does not ensure maintenance of the cylindrical shape during later handling, possibly due to the shield weight, though little.
Nevertheless, inward folding of the upper and lower edges and especially folding of the edges 34 and 35 allow in most cases the structure to be stiff enough for later handling and accurate positioning in the mold.
Assuming a resin thickness of the order of 10 mm, as is required outside the primary winding to ensure dielectric insulation thereof, any displacement of the shield relative to the resin surface, even of a few millimeters, significantly reduces the safety margin of the insulation so formed.
For further stiffening of the shield, particularly if the metal mesh is formed of very thin wires (having a diameter of 0.1-0.2 mm), as shown in the exploded view of Figure 4, a cylindrical glassfiber element 38 is conveniently provided in the form of a split elastic band with a diameter equal to or slightly greater than the peripheral wall of the mold.
The element 38, conveniently stiffened by resin spraying which does not reduce porosity, acts as a core or form for application of the shield 32.
Then, the assembly so formed is introduced in the casting mold, so that the shield 32 perfectly adheres to the peripheral wall of the mold.
The assembly step is completed by inserting in the mold the medium-voltage winding 39, preassembled and insulated, in a known and conventional manner, on a fiberglass form 40.
Once the winding has been inserted, the terminals of the primary winding are connected to their respective through connectors/ insulators 41, 42 and to a terminal block 16 disposed on the mold wall at the rib 9.
It shall be noted that, for the primary winding to be inserted in the mold, at least the upper folded edges 30 and 33 of the inner 28 and outer 32 edges respectively shall have a diameter smaller than the diameter of the form 40 and larger than the outer diameter of the winding 39.
This problem does not apply to the lower edges 34 and 29 which can extend to mutual connection.
Obviously, once the winding 39 has been inserted in the casting mold, the upper edges 30 and 33 may be further folded, thereby almost completely shielding the upper end of the winding.
Now, the casting mold may be filled with fluid epoxy resin which penetrates all the free cavities in the mold, impregnates the various fiberglass-reinforced forms or cores therein and, after hardening, firmly incorporates the windings and the shields disposed therein, in a single casting step.
As a complement to the above description, it can be noted, with reference to Figure 3, that forms may be provided that can be removed from the mold to form gaps designed to act as ventilation passages.
Advantageously, thanks to the provision that the low-voltage secondary winding is formed of two spaced concentric windings 22, 24 (Fig. 2), the ventilation passages 41, 42, 43, 44 as shown in Figure 3 may be interposed between the two windings, thereby ensuring effective heat dissipation for the secondary winding, wherein resistive losses, due to the high currents being involved, generally require a much higher heat dissipation than for the primary winding.
While the above disclosure concerns a preferred embodiment, it shall be understood that a number of changes may be applied thereto.
Particularly, the cylindrical shape of the windings and the shields, that is shown in Figures 2 to 4 with a forcibly circular section, may also have a section other than a circular section, such as a quadrangular section with chamfered corners, as schematically shown in Figure 5, where the peripheral walls 47, 48, 49, 50 of the resin body are advantageously slightly convex.
Furthermore, concerning the fabrication process, while the order in which the various elements are introduced in the mold is preferable, it only has to be intended by way of illustration.
For instance, the medium-voltage primary winding may be introduced in the mold before the outer shield and before the secondary winding.
Furthermore, it will be appreciated that the above construction principles are also applicable to single-phase transformers, either with concentric windings distributed on two adjacent columns, or in so-called shielded transformers, wherein the primary and secondary windings are placed in concentric arrangement on a single column, with the magnetic circuit enclosing the windings on both sides.

Claims

A dry-type resin-insulated transformer comprising:
- at least one medium-voltage primary winding (39) and one low-voltage secondary winding (23) arranged coaxially within the primary, said windings being jointly encapsulated in a body (6) of insulating resin having the shape of a section of a cylindrical annulus, with an axial rib (9) projecting out of its outer cylindrical surface, and elements for connection to said primary winding (3) being received in said rib (9),

- a first grounded metal shield (28) encapsulated in said insulating resin and formed of a diamagnetic metal mesh wrapped around the outer cylindrical surface of said secondary winding (23) and

- a second grounded metal shield (32), encapsulated in said insulating resin, on its external cylindrical surface, and formed of a diamagnetic metal mesh in the form of a split cylinder with juxtaposed edges (35, 36), with the upper (33) and lower (34) edges folded towards the inside of the cylinder and the juxtaposed edges (35, 36) folded outwards to form two rounded axial stiffening cords, said cords being received in the sides of said axial rib (9) of the body of insulating resin.
A dry-type transformer as claimed in claim 1, wherein the lower (29) and upper (30) edges of said first shield (28) are folded outwards.
A dry-type transformer as claimed in claim 1 or 2, wherein said metal mesh is formed of aluminum or diamagnetic stainless steel wires having a diameter in a range from 0.1 to 0.5 mm, and woven to form meshes with sides of 2 to 5 mm.
A dry-type transformer as claimed in any one of the preceding claims, comprising three magnetic columns, each being coaxial with a body of insulating resin, said magnetic columns being arranged with center-to-center spacings I of not more than D+5 mm, where D is the diameter dimension of said bodies of insulating resin at said center-to-center spacings.
A dry-type transformer as claimed in any one of the preceding claims, wherein said second shield (32) is wrapped around a fiberglass core (38) having the shape of a split cylindrical elastic band.
A transformer as claimed in any one of the preceding claims, wherein said secondary winding is formed of two coaxial and spaced winding sections (22, 23) and said insulating body has a plurality of ventilation passages (43, 44, 45, 46) extending therethrough between said winding sections.