The present invention relates to an active part of a dry-type distribution transformer having liquid-hardenable transformer insulation system with electrical field control.

The issue of high electrical stress in dry insulated transformers has been addressed over the years from different angles in order to reduce the size and weight of dry transformers.

One of the areas in the insulation system of the dry transformer where the electrical stresses are particularly high is the vicinity of the HV winding edges. The high electrical stress in this area can be reduced by winding the coil into a bobbin with permittivity higher than the permittivity of the surrounding insulation. This moves the high electrical field away from the winding edges and into the solid insulation, which leads to a device of smaller size and weight and allowing for savings on construction materials. Furthermore, the composite bobbin forms a rigid and stable support for the HV winding and the winding process of both LV and HV windings can be carried out separately, shortening the manufacturing process.

The example of such active part of the dry transformer is presented in patent application WO2012/000985. In this solution the coil body comprised a wound conductor, an insulating material attached to the wound conductor and a field control layer arrangement for reducing the maximum field strength of an electric field generated in the coil. A supporting U-shaped device in the form of a ring with sidewalls for supporting the wound conductor is provided. At least one first field control layer of the field control layer arrangement is applied at edges of the wound conductor. At least one second field layer arrangement of the field control layer is provided to the supporting device.

In other known ABB solutions, for example EP2400510 a transformer coil with conductive electrical shielding is presented. In WO 2012/000983 patent application a transformer with shielding rings in windings is presented.

The essence of an active part of the dry-type transformer according to the invention is that the external facial surfaces of the insulating body, situated in the plane perpendicular to the axis A of the longitudinal limb of the core transformer, is provided with a metallic layer having such electric conductivity that the part of an electrical insulating body between an internal surface of the metallic layer and an external surface of the annular disc of the bobbin embedded in the insulating body has a thickness d3 which is sufficient for conservation electric strength of the insulation body against electric breakdown of the insulating body. A side surface of the cylinder of the bobbin is covered with an electric resistive layer being a control shield of an electric field around the limb of the core and the HV coils and the LV coils.

Preferably the internal edges of the annular discs are rounded.

Preferably the HV coils are placed in the space of the bobbin between the annular discs and the external surface of the resistive layer, and the external edges of the annular discs protrude outside the external surface of the side walls of the HV coils.

Preferably the external edges of the annular discs protruding outside the external surface of the side walls of the HV coils are rounded.

Preferably the resistive layer is tightly abutted the external surface of the cylinder of the bobbin.

Preferably the resistive layer is made from conductive paint dotted by carbon black.

Preferably between the external surface of the metallic layer placed on the frontal face of the insulating body and an internal surface of the yoke, an air gap is situated which is free of an electric field generated by LV coils and HV coils of the transformer, having a thickness d1 bigger than zero.

Preferably a thickness d4 of the cylindrical wall of the bobbin is equal or bigger than a thickness d5 of the disc of the bobbin.

Alternatively a thickness d4 of the cylindrical wall of the bobbin is lower than a thickness d5 of the disc of the bobbin.

Preferably the insulation body is made from a thermoplastic material or a thermosetting material.

Preferably the insulation body is performed as a whole body for at least two limbs of the same core of the transformer.

The dry transformer is characterized in that it has an active part according the claims 1-11.

The solution according to the invention allows for lower the electrical stress in the dry transformer insulation system design. The combination of the use of three different field control techniques including metallization of the transformer body, coating of the HV coil winding bobbin with a resistive layer and carried out the HV coil winding bobbin from a higher permittivity composite in comparison with the insulating system renders the new design of the active part of the dry transformer as more compact. Reduction of the size, the weight and the cost of the material is possible. This allows for successful targeting of the market with dry type resin insulated units of the same electrical parameters comparable to oil insulated ones and also in dry type of the distribution transformer because it allows to make it more compact than conventional dry- type transformers with air-gap between LV and HV coils.

An exemplary embodiment of the present invention is presented in the drawing where fig. 1 is a schematic cross-section of the active part of one phase of a dry transformer, fig.2 - presents the detail "a" from fig.1 in enlargement, fig.3 - presents a schematic cross-section of the active part of two adjacent phases of a dry transformer. The active part of a dry distribution transformer comprises a transformer core 1 with two yokes 2 and two core limbs 3 with a longitudinal axis A. Around the axis A LV (Low Voltage) coils 4 are wound concentrically and a winding bobbin 5 is placed concentrically. The bobbin 5 has a shape of a longitudinal cylinder 5a with annular discs 5b on the ends of the cylinder 5a. The discs 5b protrudes outside the cylinder 5b in an outward direction of the core 1 and their ends are rounded. Also the internal edges of the annular discs are rounded. Between the discs 5b HV (High Voltage) coils 6 are wound on an external surface of the cylinder 5a, which is covered by an electrical resistive layer 7. The resistive layer 7 forms a shield for control an electric fields around the limb 3 and LV and HV coils. The resistive layer 7 is tightly abutted the external and/or internal surface of the cylinder 5a and it is made from conductive paint dotted by carbon black. The resistive layer 7 is indicated on the drawing only on the external surface of the cylinder 5a but it can be placed also only on the internal surface of the cylinder 5a or on both side surfaces of the cylinder 5a. The external edges of the annular discs 5b protruding outside the external surface of the side walls of the HV coils 6 are rounded. The limb 3, the coils 4 and the bobbin 5 with the coils 6 are located in an electrical insulating body 8 surrounding the core 1 along its limb 3. The insulating body may be a thermoplastic material or a thermosetting material. A permittivity of the insulating body 8 is 2-5 times lower than the permittivity of the insulating bobbin 5. Between the insulating body 8 and the yokes 2 an air gaps 9 are left from each side of the core 1. The dielectric strength of the air gap 9 is significantly lower than that of the insulating body 8. This hampers the size reduction of dry transformers, as the thickness d1 of the air gap 9 must be sufficient to prevent electrical breakdown. In order to eliminate the electric field from the air gaps 9 and to restrict it only to the insulating body 8 a metallic layer 10 is deposited on the frontal external faces of the insulating body 8, perpendicular to the axis A of the limb 3. The metallic layer 10 is made from thermal sprayed zinc. The metallic layer has thickness d2 and electrical conductivity σ such that the electric field generated in the coils 4 and 6 of the transformer is entirely restricted to the insulating body 8, thus making it possible to minimize the thickness d1 , and in consequence to significantly reduce the size of the active part of the transformer. When the entire electric field is restricted to the insulating body 8, the electric field intensity in the electrical insulating body 8 rises, increasing the electrical stress near the edges of the HV winding 6, and also in the space between the external surface 5a of the winding bobbin 5 and the external frontal surface 8 of the electrical insulating body 8. The thickness d3 of the space between the external surface of the disc 5b and the frontal surface of the insulating body 8 must be kept sufficient to prevent electrical failure.

In other embodiment of the invention the dry insulation body 8 is carried out for more than one phase of the transformer. For such embodiment the whole insulation body 8' is made for at least two limbs 3 as one body and it is placed on the limbs by sliding over on the limbs when one of the yokes is removed from the core.

For the two embodiment of the invention the bobbin 5 may be performed in two forms. The first form is where the thickness d4 of a wall of the cylinder 5a is equal or bigger than the thickness d5 of the disc 5b. In the second form the thickness d4 of a wall of the cylinder 5a is lower than the thickness d5 of the disc 5b. The right choice of such parameters is dependent on electrical parameter of the transformer and on the condition of exploitation of it.