BACKGROUND

The subject of invention is a high-voltage (HV) instrument transformer based on a new type of combined dry insulation system.

Currently, most high-voltage instrument transformers, both current transformers and voltage transformers are electrically insulated by oil-impregnated paper.

The current transformers are built in top-core or hair-pin technologies.

The hair-pin construction of high-voltage current transformers comprises a long HV conductor with a U shape, around which the insulation paper is wrapped and immersed in oil. The insulation typically comprises a field grading structure consisting of electrically conducting layers inserted between the layers of the paper wrapping. The secondary winding of the transformer is arranged around the insulated conductor at the bottom of the U shape. The very long conductor of the primary winding of the hair-pin type is a disadvantage, as due to the electric resistance of the conductor the heat is generated and limits significantly the current rating of the transformer that can be obtained.

The top-core construction of a high-voltage current transformer typically consist of a support insulator, a metallic head housing carried by the support insulator and a bushing enclosed in the head housing, the bushing running through the iron cores and the secondary windings of the transformer. The primary winding consists of a straight HV conductor passing through the opening of the bushing. The conductor of the primary winding is short and can easily be made in large diameter and thus small electric resistance and be characterized by small thermal losses which allow for much higher current ratings than those of the hair-pin types.

The most common electrical insulation system of the top-core current transformer is made in oil-impregnated paper technology. The insulation paper is wrapped around the corresponding high-voltage and grounded construction elements or stacked from precut sheets and inserted in the space designed to carry the electric stress. The whole insulation paper system, including that in the head housing and in the support insulator, is subsequently dried and immersed in transformer oil. It is important that the dimension and the weight of the top part of the transformer with its bushing, iron cores, windings and insulating materials can be minimized to reduce the mechanical stresses to which the relatively high support insulator is exposed when loaded dynamically, e.g. in case of strong wind or an earthquake.

One of the methods to reduce the weight of the head of the transformer is to provide the insulation system with a possibly uniform electric field, thus minimizing the insulation distances and consequently the physical size of the head. This can be done with the capacitive field grading which is typically realized using electrically conductive sheets inserted between the layers of insulating paper. An example of capacitive field grading leading to the reduction of the size of is given in the patent US41 17437. The capacitive field grading system consists of concentric cylindrical electrically conducting sheets arranged around the conductor of the primary winding of the transformer and a number of flat parallel conductive rings with annular openings arranged at both sides of the field grading structure consisting of the cylindrical sheets.

The solution presented in patent PL195878B1 discloses the insulation of the toroidal secondary winding of the current transformer, placed in the head tank of a combined current- and voltage transformer, with a layer of solid-state insulating material, epoxy resin, arranged around a metal shield of the winding. The so formed insulation system is immersed in oil. The application of the layer of epoxy insulation, with the dielectric strength larger than oil, leads to some decrease of the insulation distances. The lack of capacitive grading in the structure limits however this decrease significantly.

The construction of a high-voltage inductive voltage transformer typically consist of a column insulator mounted on top of a grounded tank and having a metallic high-voltage head electrode at its top. The HV lead is led down through the column insulator to the primary winding of the transformer placed in the grounded tank. The electrical insulation of the column insulator and the primary winding is made of paper impregnated with transformer oil.

To compensate for the thermal shrinkage and expansion of oil, both in the current transformer and in the voltage transformer, a part of the volume of oil is included in the area which is not electrically stressed and serves only as the expansion compensation volume. This expansion volume can be contained in a separate expansion tank on top of the head housing. Such solution is adopted in the transformer described in PL195878B1. Systems with moderate volume of oil can use an elastic diaphragm included in the head tank, as described e.g. in US41 17437. In each case the additional amount of oil in the head of the transformer increases its weight and in consequence leads to the higher mechanical stress exerted on the support insulator when loaded dynamically. Application of oil in the insulation system causes also a risk of leakages and environmental pollution and oil is typically not fire resistant. Those disadvantages can be eliminated by using dry, liquid-free insulation in the head and the column of the transformer.

From the GB742554 patent decsriprion there is known a dry instrument transformer which has current transformer windings at least partialy embeded in a closed end of a hollow insulating member of tubular shape made of synthetic thermosetting resin. The insulating member has potential gradient control means which are at least partly embedded in the walls of the insulating member. The outer surface of the closed end of the insulating member is provided with electrically conducting coating which is connected to the primary winding of the current transformer. Weathersheds are provided wich may be integral with the insulating member. The potential gradient control means comprises overlapping cylindrical condenser plates embedded in the walls of the insulating members. The insulating member is preferably manufactured by a centrifugal casting process forming the subject of Specification GB 742 413. In this solution the current transformer core is a ring embedded in the head of the insulating member. The HV primary winding is reduced to a rod passing through the transformer core. The low-votage (LV) secondary winding is wound around this core and is embedded with it in the insulating material. The radial electric field in the vicinity of the rod is very high, much higher than the average field between the rod and the secondary winding. Due to this nonuniformity of the field the distances beteween those LV and HV elements must be very large to guarantee the necessary dielectric strength of the insulation system. The potential gradient control means in the column of the transformer, which are placed only close to the external surface of the column, provide only the grading of the axial field, while the radial field in the column is ungraded. The resulting high electric field around the secondary winding leads in the top part of the apparatus causes the radial field to be highly non-uniform requiring a large radial insulation distance. All these requirements contribute to a substantial increase of the dimensions of the apparatus.

The GB742554 patent decsriprion discloses also a dry voltage transformer in which the primary winding is placed vertically along the column insulator close to its outer surface, providing in the same time the grading of the axial electric field along the column. The radial electric field in the column is ungraded and thus, especially in the area close to the head of the transformer, strongly non-uniform, resulting in a requirement of large insulation distances and large diameter of the column.

The resulting large size of the instrument transfomer is the main disadvantage of the solution described.

Apart of the large stresses when dynamically loaded, the large size of the instrument transformer causes also difficulties in manufacturing of the transformer, as due to the chemical and thermal shrinkage during the process of hardening of the synthetic resins, as well as the exothermic nature of the process and low thermal conductivity of the resins, the mechanical stresses generated in the hardening process increase substantially with increasing size of the hardened structure causing a risk of producing voids or cracks inside the insulation structure. This causes a size limitation, which, in turn limits the voltage level at which the instrument transformer can be applied.

From patent application WO 2009/146569 there is known a high-voltage transducer having a form of a current transformer or a voltage transformer. The current transformer has an elastic insulation placed in a metal encapsulation. The elastic insulation is made of a flexible and compressible silicone gel material. The current transformer is provided with a column comprising a column insulating body for electric insulation of leads of a secondary winding assembly of the transformer. The voltage transformer has an elastic insulation placed in a grounding housing. The voltage transformer is provided with a column comprising a column insulating body for electric insulation of a lead of primary winding of the transformer. The column insulating body is in contact with the elastic insulation which is made of the same material as for the current transformer. For the voltage transformer the elastic insulation tightly adheres to matching outer surfaces of the primary winding, column insulating body and to the inner surface of the grounding housing. For the current transformer the insulation tightly adheres to matching outer surfaces the column insulating body, a winding shield and to the inner surface of the metal encapsulation.

SUMMARY

The essence of a HV instrument transformer according to the invention, having a form of a current transformer or a voltage transformer, is that the current transformer has a head insulating body having a form of a bushing for electrical insulation of the secondary winding assembly from the primary winding conductor, the head insulating body being placed within a conductive encapsulation and being in contact with the insulating member, which is made of an elastic conformable material which tightly adheres to matching outer surfaces of the head insulating body, the column insulating body, a winding shield and to the inner surface of the conductive encapsulation of the current transformer, whereas for the voltage transformer the insulating member is made of the same material as for the current transformer and the insulating member tightly adheres to matching outer surfaces of the primary winding, column insulating body and to the inner surface of the conductive encapsulation. The conductive encapsulation is made of electrically conductive polymer or elastomer material.

Preferably the column of the current or the insulating column of the voltage transformer has a slit which is filled with the insulating member.

Preferably the winding shield is made of metal or electrically conductive polymer or elastomer material.

Alternatively the elastic conformable material of the insulating member is a silicone elastomer or a gel chosen from silicone, polyurethane gel or urethane modified epoxy gel.

Preferably the conductive encapsulation is made of electrically conductive elastomer, preferably conductive silicone rubber.

Preferably the winding shield is made of metal, electrically conductive polymer material or electrically conductive elastomer material, preferably conductive silicone rubber.

Preferably the head insulating body, the column insulating body and the column insulating body have a capacitive field grading system with field-grading layers and in each field grading system the field-grading layers are spaced from each other by insulating material made of paper impregnated with hardenable resin.

Alternatively the head insulating body and the column insulating body have a capacitive field grading system with field-grading layers and in each field grading system the field- grading layers are spaced from each other by insulating material made of polymer fibers impregnated with hardenable resin.

Preferably the hardenable resin comprises epoxy resin and an inorganic filler material.

Preferably the external insulator is made of a silicon elastomer directly molded on the column insulating body.

Alternatively the high-voltage instrument transformer has the external insulator which is provided with a stiff hollow cylinder, overmolded with weather sheds made of polymer or elastomer material. Preferably the high-voltage instrument transformer has the external insulator which is made of porcelain material.

ADVANTAGES

The instrument transformer in the form of the current transformer comprises two insulating elements with capacitive field grading systems providing the grading of both the radial and the axial field in those elements. The substantially uniform radial electric field in the insulating elements allows for effective exploiting of the field strength of the insulating material applied and consequently for minimizing the diameters of the elements. The space bridging the two capacity graded elements, as well as the volume in the current transformer head, is filled with an elastic insulating member, compressible or comformable, having the ability to accommodate the thermal shrinkage or expansion of the transformer structure.

The instrument transformer in the form of the voltage transformer comprises one insulating element with capacitive field grading system providing the grading of both the axial and radial field in the element. The space bridging the capacitively graded element, the dry insulation of the primary winding is filled with an elastic insulating member, compressible or conformable, having the ability to accommodate the thermal shrinkage or expansion of the transformer structure.

The small size and the simple shape of the capacitively graded elements allows for manufaturing those using hardenable resins without generating large mechanical stresses or producing voids or cracks. The material of the insulating member can be filled into the appropriate space in a separate proces so as it adheres firmly to the matching elemnets of the construction. The elasticity and/or compressibility of the insulating member allows for maintaining this adhesion in the broad span of operating temperatures of the apparatus as the insulating member can deform or change its volume without generating substantial mechanical stresses thus compensating for the differences in thermal expansion of materials used in the apparatus.

The instrument transformer is compact and light-weight. Also the new instrument transformer allows to reduce the mechanical stresses, to which the relatively high column of the current transformer or insulator column of the voltage transformer is exposed when loaded dynamically, e.g. in case of strong wind or an earthquake.

The dry, liquid-free construction of the instrument transformer prevent in the same time the risks of leakages or fire hazards. DETAILED DESCRIPTION

The HV instrument transformer according to the invention is presented on the drawing in a cross-section view, where fig.1 shows current transformer in a first embodiment, fig.2 - current transformer in a second embodiment, fig.3 - voltage transformer in a first embodiment, fig.4 - voltage transformer in a second embodiment, fig.5 - voltage transformer in a third embodiment.

The instrument transformer has a form of a current transformer 1 or a voltage transformer 21. Both current and voltage transformers can also be carried out as a combined instrument transformer what is not presented on the drawing.

The current transformer 1 comprises a transformer head 2 supported by the column 3 of the transformer. The column 3 has a form of an elongated tube mounted on a grounded box, not shown in the drawing, which contains terminals for the secondary winding leads 4 running from at least one secondary winding assembly 5 located inside the head 2 of the transformer. The windings can be arranged, in a known manner, around annular iron cores, or can be made as a Rogowski coils, without a magnetic core. The primary winding, consisting of a primary conductor 6, runs through a head insulating body 7 in the form of a hollow cylinder constructed as a capacitor bushing and extending through the head 2. The head insulating body 7 can be built-up around a primary conductor tube 8, or can be built-up directly around the conductor 6. The head insulating body 7 has a field-grading layer system which is built-up by winding layers of a sheet-like spacer insulating material onto the tube 8, or directly onto the conductor 6, and inserting electrically conductive field-grading layers 9 of head insulating body 7 with outwardly diminishing axial length between the spacer material turns. The resulting capacitor bushing forming the head insulating body 7 is then impregnated with hardenable resin and hardened. The secondary winding assembly 5 is arranged around the central portion of the head insulating body 7. The winding assembly 5 is enclosed at the sides and at the outer surface by an electrically conductive winding shield 10 which, together with the outermost of the layers 9 in head insulating body 7, is electrically connected to ground through a current transformer lead tube 1 1 , passing centrally through the column 3. The head 2 is covered from outside by an electrically conductive encapsulation 12. The innermost of the layers 9 in the head insulating body 7 is electrically connected to the tube 8 and to the encapsulation 12. The primary conductor 6 is suitably connected to the encapsulation 12 only at one side, so as to prevent part of the primary current from flowing through the encapsulation 12. The primary winding can also be made with more turns, what is not shown in the drawing, with its conductor passing twice or more times through the head insulating body 7 and returnign back through the space external to the conducive encapsulation 12. Secondary winding leads 4 are passed through the tube 11. The tube 1 1 in the column 1 is surrounded by a column insulating body 13 with field grading layers 14 of column insulating body. The column insulating body 13 is made using the similar technique as that used to manufacture the head insulating body 7. The outermost field- grading layer is electrically connected to the encapsulation 12. The innermost field- grading layer is suitably grounded by being connected to the tube 11.

In one embodiment of the invention concerning the current transformer the space inside the encapsulation 12 and outside the head insulating body 7, the winding shield 10 and the top part of the column insulating body 13 is filled with an insulating member 15 tightly adhering to the surfaces of the latter components. The insulating member 15, the winding shield 10 and the encapsulation 12 are made using such materials or material combinations that no substantial mechanical stresses are released during thermal shrinkage or expansion of those elements. This is realized by application of elastic conformable and/or elastic compressible materials to manufacture at least one of these components and/or application for pairs of adhering components materials of substantially identical thermal expansion coefficients. The column 3 is fixed at its bottom to the support flange 16 and is coated by an external insulator 17 providing suitable creepage length for the insulator.

In other embodiment of the invention concerning the current transformer the column insulating body 13 is placed in the column 3 is such a way that between an internal surface of the external insulator 17 and an external surface of the column insulating body 13 there is a slit 18. The slit 18 is filled with an elastic compressible or elastic conformable material, for example the same material as that used for the insulating member 15.

In the first embodiment of the current transformer the external insulator 17 is provided with weather sheds and is carried out as a silicone elastomer directly molded on the column insulating body 7. In the second embodiment the external insulator 17 has a form of a hollow cylinder 19 prefabricated with flange 16 and covered by the a silicone elastomer molded on the hollow cylinder 19. Alternatively, in the second embodiment of the current transformer the external insulator 17 can be made of a ceramic material firmly mounted to the flange 16.

Alternatively, in both embodiments, especially if the conductive encapsulation 12 is made of an elastic material, the head 2 of the current transformer can be provided with an additional metal enclosure, not shown in the drawing, surrounding the conductive encapsulation 12 and suitably electrically connected to the primary winding conductor 6 on its one side.

In both the first and the second embodiment the part of of the conductive encapsulation 12 adjacent to the external insulator 17 can be made in a form of a metallic flange firmly mounted to the column insulating body 13 or to the external insulator 17.

The voltage transformer 21 comprises a magnetic core 22 with a secondary winding 23 and primary winding 24. The primary winding 24 is made as a solid block, wound in layers on a primary winding tube 25 to an elongated shape and is impregnated with a hardenable material, for example based on epoxy resin, and hardened. Alternatively the primary winding 24 can be wound on a mandrel which is withdrawn after hardening the resin which results in a winding having a form of a solid block without the tube, which embodiment is not shown in the drawing. The primary winding 24 is placed inside a conductive encapsulation 26 of voltage transformer 21. The conductive encapsulation 26 is grounded. The outer end of the primary winding 24 is connected with a primary winding lead 27 wich is placed inside the column insulating body 28 having a form of a capacitor bushing. The inner end of the primary winding 24 is grounded. The column insulating body 28 has a field-grading layer system which is built-up by winding layers of a sheet-like spacer insulating material on the lead 27 having a form of a metal tube, and inserting electrically conductive field-grading layers 29 of column insulating body with outwardly diminishing axial length between the spacer material turns. The resulting capacitor bushing forming the column insulating body 28 is then impregnated with hardenable resin and hardened. The outermost field-grading layer of the column insulating body 28 is electrically connected to the conductive encapsulation 26. The innermost field-grading layer is electrically connected to the lead 27.

In one of the embodiments of the voltage transformer the space inside the encapsulation 26 and outside the primary winding 24 and the bottom part of the column insulating body 28 is filled with an insulating member 30 tightly adhering to the surfaces of the latter components. The insulating member 30 and the conductive encapsulation 26 are made using such materials or material combinations that no substantial mechanical stresses are released during thermal shrinkage or expansion of those elements. This is realized by application of elastic comformable and/or elastic compressible materials to manufacture at least one of those components,

The column insulating body 28 is fixed at its top to the top flange 31 which is closed by the top cover 32. The lead 27 is electrically connected to the flange 31 and to the cover 32, which together form the HV electrode of the voltage transformer 21. Alternatively the HV electrode can be made in many different ways, for example being formed by the top end of the lead 27 directly protruding from the column insulating body 28, which is not shown in the drawing.

The column insulating body 28 is placed in an external insulator 33 providing suitable creepage length for the insulator.

In the second embodiment of the voltage transformer 21 the column insulating body 28 is placed inside the column 34 of voltage transformer in such a way that between the internal walls of the external insulator 33 and the external surface of the insulating body 28 there is a slit 35 which is filled with an elastic compressible material, for example the same material as that being used for the insulating member 30. The lower part of the conductive encapsulation 26 is placed in an additional container 36 which forms a base support of the voltage transformer.

The upper part of the conductive encapsulation 26, adjacent to the external insulator

33, can be made in a form of a metallic flange, which is not shown in the drawing, firmly attached to the column insulating body 28 or to the external insulator 33 from its top side and to the container 36 in its bottom part.

In the third embodiment of the invention the voltage transformer 21 is built-up similarly as the voltage transformer from fig.4 but with the secondary winding 23' and the core 22 being embedded in the insulating member 30 within the conductive encapsulation 26. The conductive encapsulation 26 in this embodiment is formed by a flange 37 and the container 36, both made of metal.

In the first embodiment of the voltage transformer the external insulator 33 is provided with weather sheds and carried out as a silicone elastomer directly molded on the column insulating body 28. In the second and the third embodiment the external insulator 33 is provided in a form of a hollow cylinder 38 prefabricated with the flange 31 and/or 37 and covered by the a silicone elastomer molded on the hollow cylinder 38. Alternatively, in the second and third embodiment of the voltage transformer shown in fig. 4 and fig. 5 the external insulator 33 can be made of a ceramic material firmly mounted to the flanges 31 and/or 37.

In all of the embodiments of the invention where elastic compressible electrically insulating material is used for the insulating member 15 or 30, a silicone gel filled with hollow compressible microsferes, such as for example Powersil Gel C 670 from Wacker Chemie AG or microcellular gel, such as microcellular gel elastomer CG-1 from Northstar Polymers LLC can be applied. The hollow compressible microspheres have a diameter of 10μιη - ΙΟΌμιη and are filled with gas or a mixture of liquid and gas.

In such cases the conductive encapsulation 12 or 26 and the winding shield 10 of the current transformer should be made of rigid or elastic electrically conductive materials. The conductive encapsulation can be made as one integral part or consist of more parts connected together, possibly also made of different materials. The rigid materials applicable for such encapsulation are metal or electrically conductive polymers, for example aluminium alloy or a polymer material with an electrically conductive filler, e.g. carbon-black filled polycarbonate, polyester, polyamide or semi-aromatic polyamide. The applicable elastic material is for example an electrically conductive elastomer material, e.g. silicone elastomer filled with carbon black.

The winding shield 10 can also be an integral part of the winding assembly 5, which is not shown in the drawing, being for example formed by the external layer of tightly wound wires of the secondary winding of the current transformer.

Alternatively in all of the embodiments of the invention where elastic conformable electrically insulating material is used for the insulating member 15 or 30, a silicone elastomer, such as Liquid Silicon Rubber LSR, or silicone gel, polyurethane gel, utherane modified epoxy gel can be used. As an exemplary matrial one of the following material can be chosen: Silicone gel Q-Gel 331 from ACC Silicones LTD, Polyurethane gel MPP-V37A from Northstar Polymers LLC, Urethane modified epoxy gel-Polymer system super gel 9 from Master Bond Inc.

In such cases the conductive encapsulation 12 or 26 is made of an elastic electrically conductive material, for example a conductive elastomer material, e.g. silicone elastomer filled with carbon black.

In all the embodiments of the invention where the elastic comformable electrically insulating material is used for the insulating member 15 or 30, the winding shield 10 can be made of a rigid or elastic electrically conductive material, such as those used in the embodiment employing the elastic compressible material for the insulating member 12 or 26.

In the all embodiments of the invention the head insulation body 7 and the column insulating body 13 or 28 can me made in two techonologies known as RIP (Resin Impregnated Paper) and RIS (Resin Impregnated Synthetics) using as an imprgnating material a hardenable resin, e.g. epoxy resin, hardened after impregnation. In the RIP techology the primary conductor tube 8, the current transformer lead tube 1 1 or the primary winding lead 27 is wrapped with creped paper, further referred to as spacer, not presented on the drawing. Metallic layers 9, 14 or 29 are inserted inbetween the layers of that spacer at appropriate radial distances from the tube 8 or 11 or from the primary winding lead 27. The spacer has the function to hold the metallic layers in appropriate distance around the conductive tubes 8 and 11 or lead 27. The metallic layers have the function to control the electric field - radial in the space between the layers and axial at the outer surfaces of the head insulation body 7, the column insulating body 13 or 28. Afterwards, when the winding of the crepe paper is accomplished, the body is dried and subsequently impregnated under vacuum with a curable polymer resin. The polymer resin flows through the porous paper layers to impregnate the whole insulating body. Low-viscosity, unfilled polymer resins can be used for the impregnation, typically heat curable, epoxy resins. After impregnation, the resin is cured and machined to the final shape of the insulating body.

In the RIS technology the primary conductor tube 8, the current transformer lead tube 11 or the primary winding lead 27 is wrapped with a spacer made of polymer fabric. Metallic layers 9, 14 or 29 are inserted at the appropriate positions similarly as in the RIP technolgy. Depending on the porosity of the spacer material, particle filled impregnation resins can also be used for the impregnation process. The preferred fillers are typically AI203, but also other kinds other fillers, as for instance Si02, Ti02, AIN or chopped glass fibres can be used. The use of a filled impregnation resin improves the thermal conductivity of the material and reduces heat release and srhinkage during the curing process so that the insulating bodies can be cast directly to their final shape.

Alternatively, the column insulating body 13 or 28 can be wound on a tube made of an electrically insulating material and the lead 27 can be made as a wire running through the tube, which is not shown in the drawing. The innermost of the field grading layers 14 or 29 is then suitably connected to one of the leads 4 which is grounded or to the lead 27, respectively. The column insulating body 13 or 28 can also be wound on a mandrel which is withdrawn after the hardening of the resin resulting in the insulating body without the tube which is not shown in the drawing. Similar constructions can also be used for the head insulating body 7 with the appropriate electrical connection of the innermost of the field grading layers 9 to the primary winding conductor 6.

KEY TO THE SYMBOLS IN THE DRAWINGS: - current transformer

- transformer head

- column of current transformer

- secondary winding leads

- winding assembly

- primary conductor

- head insulating body

- primary conductor tube

-field-grading layers of head insulating body

0 - winding shield

1 - current transformer lead tube

2 - conductive encapsulation of current transformer

3 - column insulating body of current transformer

4 - field-grading layers of column insulating body of current transformer5 - insulating member of current transformer

6 - support flange

7 - external insulator of current transformer

8 - slit in column of current transformer

9 - hollow cylinder of current transformer 1 - voltage transformer

2 -core of voltage transformer

3 - secondary winding outside the conductive encapsulation

3' - secondary winding inside the conductive encapsulation

4 - primary winding of voltage transfomrer

5 - primary winding tube

6 - conductive encapsulation of voltage transformer

7 - primary winding lead - column insulating body of voltage transformer

-field-grading layers of column insulating body of voltage transformer -insulating member of voltage transformer

- top conductive flange

- top cover

- external insulator of voltage transformer

-column of voltage transformer

- slit in column of voltage transformer

- container

- bottom flange of voltage transformer

- hollow cylinder of voltage transformer