EP3460810B1

EP3460810B1 - Advanced high voltage capacitance graded bushing

Info

Publication number: EP3460810B1
Application number: EP18193147.8A
Authority: EP
Inventors: Giovanni Testin; Fabrice Andre Jean PERROT
Original assignee: General Electric Technology GmbH
Current assignee: General Electric Technology GmbH
Priority date: 2017-09-21
Filing date: 2018-09-07
Publication date: 2020-08-19
Anticipated expiration: 2038-09-07
Also published as: EP3460810A1; IT201700105778A1

Description

TECHNICAL FIELD

The present invention relates to electrical bushings and in particular to High Voltage (HV) bushings suitable for use with power transformers for electrical power transmission and distribution (T&D) applications, the bushings having a capacitance graded core.
By high voltage is meant a voltage in excess of about 1 kV; the present invention finds particular application to voltages above 24 kV.

STATE OF THE PRIOR ART

High voltage bushings for electrical power transmission and distribution applications are usually capacitance graded type bushings with a capacitance graded core arranged around a main elongated conductor (tube or rod). The graded capacitance core (also called condenser core or graded capacitor core) is obtained by winding around the main elongated conductor a sheet of insulating material which is inter-spaced with several screens layers, the layers being made of a conductive or semi conductive material and forming the electrodes of the capacitance graded core.
Such high-voltage bushings are known from e.g. WO 2015/117823 .
Three technologies are mainly available for the manufacture of a capacitance graded core by the winding technique:

the Oil Impregnated Paper technology (OIP technology);
the Epoxy Resin Impregnated Paper technology (RIP technology);
the Epoxy Resin Impregnated Synthetic fabric technology (RIS or RIF technology).

The OIP technology is the older technology on the market. From a production point of view, the OIP technology requires a long drying process and a complex impregnation process of the capacitance graded core to achieve good electrical performances. As a consequence, the production of an OIP technology bushing is time consuming and its delivery time is long.
Moreover, since the active part of the OIP resulting capacitance graded core is based on paper and mineral oil, the long-term performances of the OIP resulting capacitance graded core are dependent on the maximum temperature that the active part can reach during service operation. Currently, the limit set by the standards is 105°C.
This technology is also not particularly environmentally friendly because of the use of mineral oil as impregnating fluid.
Another risk linked with this technology is the risk of fire in the event of an electrical fault.
Furthermore, it requires significant investments for the equipment needed respectively for the winding manufacturing, the paper drying, the oil processing and the oil impregnation.
RIP technology is presently a growing technology in the high voltage bushing market. It is based on a complex and critical manufacturing process based on a lengthy epoxy resin impregnation phase under vacuum of a wound crepe paper and screens core, followed by a lengthy curing of the impregnated core to obtain a cast core. The cast core is then machined to obtain the finished capacitance graded core.
The RIP technology demands large investments for the winding machine, the resin mixing and dosing equipment, the casting vessels, the autoclaves and the machine tool for the machining of the cast core.
The chemical process of the epoxy resin is very critical and the scrap rate is usually comprised between 3 to 10% for long/large cores.
After the machining operation, the surface of the capacitance graded core becomes slightly hygroscopic, so special care is necessary to prevent moisture absorption inside the core, which would result in poor electrical performances of the finished capacitance graded core.
The epoxy resin formulation is critical from an Environmental, Health and Safety (EHS) point of view as some of its components are classed as toxic and suspected carcinogens.
The production time is long and prevents fast delivery times.
Scrapping of failed cores, of the residual materials resulting from machining or of the residual resin is very expensive as they are considered special wastes that cannot be recycled.
Accordingly, due to the large amount of investment necessary, the relatively high scrap rate and corresponding high materials costs, the RIP bushing technology is much more expensive compared to an OIP technology.
Concerning the RIS or RIF technology, their production processes are based on two different methodologies. The first methodology is based on a rapid high pressure epoxy resin casting and impregnation of a wound capacitance graded core produced with a fabric made of hydrophilic synthetic fibers; the second methodology is based on a winding manufactured with a fiber glass filaments pre impregnated with epoxy resin.
The disadvantages of both methodologies for the impregnation of a wound condenser core are:

the use of an epoxy resin as for RIP technology with the same EHS and environmental issues already mentioned;
the casting with epoxy resin can be performed under pressure in specially built and expensive mold tools which are size specific or with a casting process in an autoclave. In the latter case, the delivery time of the bushing is comparable with the RIP technology;
the equipment for mixing and dosing, for casting under pressure and for machining are very expensive and it has a large impact on the industrial cost of the bushing.

DISCLOSURE OF THE INVENTION

The invention aims to overcome some of the known disadvantages of the prior art solutions.
The present invention provides a method for manufacturing a capacitance graded core for a high voltage electrical bushing, the capacitance graded core comprising:

an elongated electrical conductor;
an electrically insulating body disposed around the elongated electrical conductor; and
a plurality of electrodes disposed coaxially to the elongated electrical in the electrically insulating body;

a) providing an insulating pre-impregnated fabric made of insulating fibers impregnated with an insulating thermoplastic material, the pre-impregnated fabric having a surface;
b) heating the pre-impregnated fabric to melt the thermoplastic material of the pre-impregnated fabric;
c) winding the melted pre-impregnated fabric around the elongated conductor while pressing the melted pre-impregnated fabric on the elongated conductor;

At step b), the pre-impregnated fabric is melted. Preferably, the heating is a localized heating, in order to obtain a localized melting.
Preferably, the pressing of the melted pre-impregnated fabric on the elongated conductor is realized with the use of a roller.
The pre-impregnated fabric is a fabric made of fibers which are impregnated with an insulating thermoplastic material. The pre-impregnated fabric is preferably in a tape or sheet form.
In the present invention, what is meant by "insulating" is "electrically insulating". An insulating fiber will thus mean an electrically insulating fiber.
According to a variant of the invention, the step of forming an electrically conductive layer on the surface of the insulating pre-impregnated fabric is obtained by applying an electrically conductive layer on the surface of the insulating pre-impregnated fabric. The electrically conductive layer could be, for example, a thin aluminum foil, or a thin layer of a semi conductive material, for example a polyester film filled or coated with metal or graphite.
According to another variant, the step of forming an electrically conductive layer on the surface of the insulating pre-impregnated fabric is obtained by spraying a solution containing electrically conductive particles directly on the surface of the insulating pre-impregnated fabric.
According to another variant, the step of forming an electrically conductive layer on the surface of the insulating pre-impregnated fabric is obtained by direct metal or semiconductor vaporization on the surface of the insulating pre-impregnated fabric.
The formation of the electrically conductive layers may be done before step c), for example by providing an insulating pre-impregnated fabric already incorporating electrically conductive layers formed therein. Preferably, the formation of the electrically conductive layers is done during step c) .
The present invention also provides a capacitance graded core for a high voltage electrical bushing, manufactured by the method of the invention, wherein the insulating pre-impregnated fabric is a fabric made of fibers which are impregnated with an insulating thermoplastic material and the fabric is in a tape form.
The elongated conductor is a longitudinal electrically conductive element, for example a metallic rod or tube. The invention provides a capacitance graded core with a body made of a continuous insulating composite material directly integrating the inter-laced conducting screens and based on a thermoplastic polymer, the capacitance graded core being obtained directly as a net shape, not requiring further machining.
Solutions of the known prior art which use resins use thermosetting resins, such as, for example, polyesters, epoxies, polyurethanes, etc.
In the present invention, a thermoplastic material can be chosen, for example, amongst polyolefins, such as polyethylene (PE), High Density PE (HDPE), polyethylene terephthalate (PET) and polypropylene (PP), poly (vinyl chloride) (PVC), polystyrene (PS), polyamide (PA), polyphenylene sulfide (PPS), polyether ether ketone (PEEK), polyetherimide (PEI), polyether sulfone (PES), polyphenylsulfone (PPSF or PPSU) and the likes.
The steps of the method according to the invention may be carried out with a tape, a foil or a sheet winding platform equipped with a concentrated heating system, such as a robotic arm-guided laser, an electric heater or an air jet. With the robotization and guided heating, this platform allows the rapid realization of large thermoplastic composite parts.
The present invention has many unique advantages:

there is low scrap and residuals are fully recyclable;
the insulation materials used are thermoplastic polymers (which are more eco-friendly and fully recyclable materials);
the insulating material is supplied as a solid pre-impregnated fabric (preferably in a tape or sheet form) that can be handled and managed without particular health and safety restrictions, since the solid tape or sheet is made of an inert thermoplastic material;
the production of the core winding is a dry and rapid process and does not require complex chemical liquid substances and processes;
a fully programmable robotized arm system can be used for the manufacture of the capacitance graded core for a high voltage bushing according to the invention;
with this technology, it is possible to manufacture customized capacitance graded cores;
the delivery time is extremely fast since the production cycle for the cores ranges from a few minutes to one hour depending on the size of the core;
the level of investments is reasonable and is mostly related to the winding machine;
the mechanical properties of the insulating material can be improved with fiber-reinforcement and appropriate winding patterns to match current mechanical performances of the RIP technology;
the low level of investments needed and the reduced production time will lead to a very competitive price for the capacitance graded cores manufactured with the method according to the invention compared with RIP and RIS/RIF technologies;
the processed capacitance graded core will have good mechanical properties and a limited mass.

Additional features and advantages of the present invention are described in, and will be apparent from, the detailed description of the preferred embodiments, the figures and the claims.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a schematic side view of a capacitance graded core according to an embodiment of the invention.
Figure 2 is a schematic sectional side view of the capacitance graded core illustrated in FIG. 1.
Figure 3 is a schematic perspective side view of the capacitance graded core during its manufacturing according to a first embodiment of the invention.
Figure 4 is a schematic perspective side view of the capacitance graded core during its manufacturing according to a second embodiment of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Figure 1 illustrates a capacitance graded core manufactured according to an embodiment of the invention. The capacitance graded core 1 comprises an elongated electrical conductor 2 and an insulating body 3 arranged axially around the elongated conductor 2. The insulating body 3 is obtained by winding a pre-impregnated fabric 5 (layers of fibers impregnated with a thermoplastic resin) around the elongated conductor and inserting conducting layers 4 between the windings (figure 2).
For carrying out the process according to the invention, we can use a manufacture station comprising a first support axle for supporting the elongated conductor 2 (here, the elongated conductor 2 is the first support axle) of the capacitance graded core 1, a second support axle 8 for supporting the pre-impregnated fabric 5, at least one pressure roller 6, to press the pre-impregnated fabric on the elongated conductor, and one or more heating elements 7, for example a laser, an electrical heater, an air jet or any other focused heating system (figures 3 and 4). The pre-impregnated fabric 5 can be in a sheet form (figure 3) or a tape form (figure 4). The first support axle can be rotating. The second support axle 8 can be rotating (figure 3); it can also be rotating and be moved in translation with respect to the first support axle (figure 4). The at least one pressure roller 6 and the heating elements 7 can also be moved in translation and in rotation with respect to the first support axle.
During the rapid thermoplastic tape dry winding operation, the tape is heated to melt the thermoplastic material. This is a dry winding operation since the heated thermoplastic tape is not in a molten and liquid state; the heat is applied in order to melt the thermoplastic material, thus allowing the adhesion of the melted tape on the body of the winding support (i.e. the elongated electrical conductor or the tape already wound around the conductor). Preferably, the heat is applied locally near or in the zone where the tape is wound on the winding support. The heating of the thermoplastic tape may be obtained by using one or more heating systems such as, for example, a high power laser beam, an electrical heather or a high temperature air stream.
The adhesion of the melted tape on the body of the capacitance graded core is ensured by applying pressure on the body of the capacitance graded core during the winding, in order to consolidate the tape on the rotating surface of the cylindrical capacitance graded core. The pressure may be obtained by using a suitable roller.
The capacitance graded core is obtained by integrating within the core an adequate number of correctly shaped and positioned concentric electrodes in the form of electrically conducting layers 4 at different concentric diameters during the winding operation. The integration during the winding operation of the grading electrodes, necessary to obtain the grading of the electric field, can be done by the insertion during the winding operation at the prescribed radial and axial positions of electrically conductive or semi-conductive layers, by using one of the following methods:

insertion of a thin electrically conductive layer, for example an aluminum foil;
insertion of a thin layer of a semi conductive material, for example a polyester film filled or coated with metal or graphite;
application of a thin semi conductive layer fused to the thermoplastic tape on the surface of the capacitance graded core;
direct spraying or varnishing with a semi conductive enamel on the winding surface;
direct metal vaporization on the winding surface.

To illustrate the method according to the invention, we could operate a wound fiber-reinforced thermoplastic tape or sheet processed with a high-speed laser-melting robotic platform, the fiber-reinforced thermoplastic being a pre-impregnated fabric in tape or sheet form, in order to produce a wound cylindrical condenser body around an aluminum rod or tube serving as a mechanical support and an electrical conductor for the capacitance graded core.
With the method according to the invention, the final net-shape capacitance graded core can be directly obtained by controlling the application of the thermoplastic tape along the axis of the capacitance graded core.
The condenser core final net-shape can be achieve without additional machining or thermal treatment so that it can directly be integrated and assembled within a bushing structure.
Please note that the details such as the number and diameter of fibers of the pre-impregnated fabric and the thermoplastic material used for the impregnation, the dimensions of the tissue, the number of conductive layers, etc. are chosen by the one skilled in the art according to the condenser core to be obtained. It is the same with the choice of winding of complex geometries and tape trajectories during the winding.

Claims

A method for manufacturing a capacitance graded core (1) for a high voltage electrical bushing, the capacitance graded core (1) comprising:
- an elongated electrical conductor (2);

- an electrically insulating body (3) disposed around the elongated electrical conductor (2); and

- a plurality of electrodes disposed coaxially to the elongated electrical conductor in the electrically insulating body;
the method comprising:
a) providing an insulating pre-impregnated fabric (5) made of insulating fibers impregnated with an insulating thermoplastic material, the pre-impregnated fabric having a surface;

b) heating the pre-impregnated fabric (5) to melt the thermoplastic material of the pre-impregnated fabric;

c) winding the melted pre-impregnated fabric around the elongated conductor (2) while pressing the melted pre-impregnated fabric on the elongated conductor;

the method further comprising forming an electrically conductive layer (4) on the surface of the insulating pre-impregnated fabric, this step being repeated intermittently, thereby obtaining a plurality of electrically conductive layers inserted between the windings of pre-impregnated fabric and thus forming the plurality of electrodes.
The method according to claim 1, wherein the step of forming an electrically conductive layer (4) on the surface of the insulating pre-impregnated fabric is obtained by applying an electrically conductive layer on the surface of the insulating pre-impregnated fabric.
The method according to claim 1, wherein the step of forming an electrically conductive layer on the surface of the insulating pre-impregnated fabric is obtained by spraying a solution containing electrically conductive particles directly on the surface of the insulating pre-impregnated fabric.
The method according to claim 1, wherein the step of forming an electrically conductive layer on the surface of the insulating pre-impregnated fabric is obtained by direct metal or semiconductor vaporization on the surface of the insulating pre-impregnated fabric.
The method according to any one of claims 1 to 4, wherein the step of forming an electrically conductive layer (4) is done before step c).
The method according to any one of claims 1 to 4, wherein the step of forming an electrically conductive layer (4) is done during step c).
A capacitance graded core (1) for a high voltage electrical bushing manufactured by the method according to claim 1, wherein the insulating pre-impregnated fabric (5) is a fabric made of fibers which are impregnated with an insulating thermoplastic material and the pre-impregnated fabric is in a tape form.