Patent Description:
European patent <CIT> discloses a high-voltage transducer comprising insulation, which includes a compressible silicone gel insulation. Compressible insulation permits the application of available transducer constructions for strongly varying temperature ranges, a conventionally necessary oil compensation bellows can be avoided and furthermore the high voltage measuring transducer can be cheaply produced.

European patent <CIT> discloses a HV dry instrument transformer having a form of a current transformer or a voltage transformer, wherein the column insulating body of the dry HV instrument transformer has a form of a dry capacitor bushing wound as a block of a spacer sheet. The HV current transformer has a head insulating body for electrical insulation of a secondary winding assembly from a primary winding conductor. For the HV current transformer, the head insulating body has a form of a capacitor bushing being in contact with an insulating member. The HV voltage transformer has a primary winding being in contact with an insulating member. A column insulating body has an impregnation material having substantially the same coefficient of thermal expansion as a material of the insulating member. The insulating member has a form of a hardenable resin in both types of the transformer.

European patent <CIT> discloses a high-voltage instrument transformer having a form of a current transformer or a voltage transformer. The HV instrument transformer is characterized in that the current transformer has a head insulating body in a form of a bushing for electrical insulation of a secondary winding assembly from a primary winding conductor. A head insulating body is placed within a conductive encapsulation and the head insulating body is in contact with an insulating member. The insulating member is made of an elastic conformable material which tightly adheres to matching outer surfaces of the head insulating body of the current transformer and the conductive encapsulation. Furthermore, the insulating member can be made of electrically conductive polymer or electrically conductive elastomer material.

Furthermore, European patent application <CIT> discloses a HV apparatus, in particular, a HV dry instrument transformer, which has a form of a current transformer, a voltage transformer or a combined transformer with an insulating gel. The apparatus is characterized in that it comprises at least two electrically conductive elements such as a head transformer cover, a head housing base, a core casing, a primary conductor, a bottom external housing, a bottom support flange, a core and an electric insulation material comprising an insulating gel filling enclosed space between the conductive elements. Further, at least one of the electrically conductive elements has a coating made of a solid insulating material separating the surface of the at least one electrically conductive element from the insulating gel.

Japanese patent application <CIT> discloses a transformer comprising a high voltage coil and a low voltage coil. Further the transformer comprises a first interlayer insulating material arranged between each layers of the high voltage coil, a second interlayer insulating material arranged between each layers of the low voltage coil, and a third interlayer insulating material arranged between the high voltage coil and the low voltage coil. The first interlayer insulating material, the second interlayer insulating material and the third interlayer insulating material are made of a composite sheet of a ceramic fibre sheet and a synthetic resin film. The disclosed construction is made up in one body by using mouldable and curable coating synthetic resin.

Moreover, British patent application <CIT> discloses a transformer comprising non-woven polyester fabric interposed between adjacent winding layers of a high-voltage winding and/or of a low-voltage winding of the transformer.

In the concept of dry HV current transformers known from the prior art (<FIG>), the surface of a core casing (1a) is on ground potential, while a top housing (2a) and a current track (3a) are on high potential. Dielectric insulation between these elements is realized by means of a large quantity of a compressible silicone material (4a), which is relatively expensive. On top of that, the surfaces of metallic elements need to be coated with a high dielectric strength coating (5a) to improve the dielectric strength of the disclosed insulation system.

In turn, in the concept of dry HV voltage transformers known from the prior art (<FIG>), the surface of a primary winding module (1b) and a field grading disc (2b) are on high potential. For this reason, it is necessary to provide electrical insulation between these elements and a grounded core (3b), as well as a grounded bottom tank (4b). This requires a relatively high amount of an insulating material (5b), such as e.g., a compressible microsphere filled silicone gel, to fill the entire volume of the grounded bottom tank (4b). Such approach generates additional cost and raises the probability of defects formation in the disclosed insulation system due to a larger amount of an insulating gel. Analogously to the case the dry HV current transformer, some parts of the grounded bottom tank (4b) may need to be coated with a high dielectric strength coating (6b) to improve the dielectric strength of the disclosed insulation system.

As it can be seen from the cited examples, electrical insulation in dry HV transformers needs to be provided as follows.

One of the most critical issues in high voltage instrument transformers when changing from oil-impregnated paper insulation to dry insulation is to keep the reliability of the dry insulation on at least a similar level as the reliability of the oil-impregnated paper insulation. To achieve it solutions from the prior art require a large volume of insulating material, such as a compressible silicone gel, which is relatively expensive and significantly influences the final production costs of a high voltage instrument transformers, a HV current transformers and a HV voltage transformers as well.

The object of the invention is to provide a high voltage instrument transformer a with reduced amount of used insulation material while still providing high reliability of insulation in dry HV transformers.

The first object of the invention is a dry high voltage instrument transformer in the form of a HV current transformer comprising a core casing provided with a secondary winding, a top housing, a primary winding, and a dry bushing, wherein the core casing comprises an insulation for its electrical insulation from the primary winding and from the top housing, and the HV current transformer further comprises an insulation made of compressible silicone material, arranged between the connection point of the dry bushing and the core casing and the top housing. The insulation of the core casing comprises an insulating composite containing an electrically insulating material made of PES nonwoven impregnated and cured with low viscosity epoxy.

The application of the insulation of the core casing comprising an insulating composite containing an electrically insulating material made of PES nonwoven impregnated and cured with low viscosity epoxy provides an increase of reliability of insulation in the HV current transformer. Insulation according to the invention made of PES nonwoven impregnated and cured with low viscosity epoxy forms void free and crack free insulation and is characterized by very high dielectric strength.

Furthermore, the application of the insulation and the high voltage screen ensures that it is not necessary to insulate the inside surface of the top housing and/or the surface of the primary winding of the HV current transformer. This allows for limiting the amount of a used compressible silicone insulation material thereby reducing the material costs for production costs of the HV current transformer compared to solutions known from the prior art.

Further the application of the mentioned insulation ensures significant reduction of the dimensions and weight of the top housing.

Preferably, the insulation of the core casing is equipped with a high voltage screen which is located on the side of the insulation opposite to the core casing.

Preferably, the high voltage screen is made of an easily impregnatable semi-conductive nonwoven.

Preferably, the high voltage screen is terminated with a field-shaping ring.

Preferably, the internal surface of the top housing is provided with a high dielectric strength coating.

The second object of the invention is a dry high voltage instrument transformer in the form of a HV voltage transformer comprising a bottom tank housing, a cast of a primary winding module with an embedded screen surrounding the primary winding, a field grading disc, a core, and a compressible silicone insulation material arranged between the field grading disc, the internal surface of the bottom tank housing, and the screen. The cast of the primary winding module is made of an insulating composite containing an electrically insulating material made of PES nonwoven impregnated and cured with low viscosity epoxy.

The application of the cast of the primary winding module made of the insulation comprising an insulating composite containing an electrically insulating material made of PES nonwoven impregnated and cured with low viscosity epoxy provides an increase of reliability of insulation in the HV voltage transformer. The composite insulation has higher dielectric strength than a compressible silicone material, so it can be made much thinner, causing that the amount of used compressible silicone insulation can be significantly reduced. Limitation of the amount of a used compressible silicone insulation material leads to reduction of the material costs for production costs of the HV current transformer compared to solutions known from the prior art. It allows for significantly reducing dimensions and weight of the bottom tank as well.

Preferably, the HV voltage transformer further comprises a primary winding high voltage screen, wherein the primary winding high voltage screen is insulated from the primary winding screen by the cast of the primary winding module and a distance between the primary winding high voltage screen and the primary winding screen is based on the dielectric strength of the insulating composite and the adopted margin of safety.

Preferably, the screen is made of an easily impregnable semi-conductive material, such as semiconductive PES nonwoven.

Preferably, the screen is terminated with a field grading ring, wherein the field grading ring is insulated from the primary winding screen by the cast of the primary winding module. Termination of the screen with the field grading ring prevents occurring harmful stresses at the edge of the screen.

Preferably, the dry HV voltage transformer further comprises an additional layer of high dielectric strength coating.

Insulation comprising insulating composite containing an electrically insulating material made of PES nonwoven impregnated and cured with low viscosity epoxy provides an increase of reliability of insulation in dry HV transformers, both in HV current transformers and HV voltage transformers.

The application of the above-mentioned insulation allows limiting the amount of a used compressible silicone insulation material thereby reducing the material costs in the production of dry HV transformers compared to solutions known from the prior art.

Furthermore, the application of the insulation comprising insulating composite containing an electrically insulating material made of PES nonwoven impregnated and cured with low viscosity epoxy ensures significant reduction of the dimensions and weight of a top housing in a HV current transformer and a bottom tank housing in a HV voltage transformer as an amount of a required compressible insulation material in both types of said HV transformers is limited by confining the majority of the electric stress to the first mentioned insulation.

The application of the high voltage screen located on the side of the PES composite insulation opposite to the core casing of the HV current transformer provides an increase of dielectric strength of the PES composite insulation.

Termination of the screen of the HV voltage transformer with the field grading ring prevents occurring harmful stresses at the edge of the screen.

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which:.

A dry high voltage instrument transformer in the form of a HV current transformer <NUM> according to the invention as shown in <FIG> comprises a core casing <NUM> provided with a secondary winding <NUM>. The core casing is filled with a resilient filler material <NUM>, such as a PUR foam which is light and inexpensive.

The HV current transformer <NUM> further comprises a current track constituting a primary winding <NUM>, a dry bushing <NUM> and a top housing <NUM>. The core casing <NUM> is insulated from the top housing <NUM> and from the current track <NUM> by insulation <NUM>.

The insulation <NUM> comprises an insulating composite containing an electrically insulating material, which is made of PES nonwoven impregnated and cured with low viscosity epoxy.

The insulating material made of the PES nonwoven forms the backbone of the insulation similar to traditional crepe paper. The difference is that the PES nonwoven can be easily impregnated with low viscosity epoxy in all directions, which is very difficult to achieve in the case of crepe paper. As a result the application of the insulation <NUM> provides an increase of insulation reliability in the HV current transformer <NUM> and allows to limit the amount of a used compressible silicone insulation material thereby reducing the material costs in the production of the HV current transformer <NUM> compared to solutions known from the prior art.

Furthermore, the insulation <NUM> is equipped with a high voltage screen <NUM> which is located on the side of the insulation <NUM> opposite to the core casing <NUM>. What is more, the high voltage screen <NUM> is terminated with a field-shaping ring <NUM>. The high voltage screen <NUM> is made of an easily impregnatable semi-conductive nonwoven. The semi-conductive nonwoven can be made of polyester, cotton and viscose wherein for instance a surface of non-woven is impregnated with carbon. Such insulation structure can be impregnated with low viscosity epoxy and cured. The application of the high voltage screen <NUM> provides increased dielectric strength of the insulation <NUM>. Further, the application of the high voltage screen <NUM> ensures that the insulation is free from voids and cracks, which provides operation of the HV current transformer <NUM> free of partial discharges. Furthermore, the insulated and screened core casing <NUM> does not require any additional insulation from the top housing <NUM> or the current track <NUM> made of a compressible silicone material.

The internal surface of the top housing <NUM> is provided with a high dielectric strength coating <NUM> what further improve the dielectric strength of the insulation of the HV current transformer <NUM>.

The HV current transformer <NUM> further comprises insulation <NUM> made of a compressible silicone material. The insulation <NUM> is located between the connection point of the dry bushing <NUM> and the core casing <NUM> and the top housing <NUM>, wherein the connection point of the dry bushing <NUM> and the core casing <NUM> are on ground potential, and the top housing <NUM> is on HV potential. The compressible silicone material can be for instance a silicone elastomer, such as Liquid Silicon Rubber LSR, or silicone gel, polyurethane gel, utherane modified epoxy gel can be used. As an exemplary material one of the following material can be chosen: Silicone gel Q-Gel <NUM> from ACC Silicones LTD, Polyurethane gel MPP-V37A from Northstar Polymers LLC, Urethane modified epoxy gel-Polymer system super gel <NUM> from Master Bond In.

Owing to the disclosed solution an amount of the compressible silicone insulation material <NUM> is greatly reduced, as it is largely replaced with the much less expensive PES nonwoven composite insulation <NUM> to which most electrical stresses are confined. The composite insulation <NUM> has higher dielectric strength than the compressible silicone insulation material <NUM>, so it can be made much thinner. This renders it possible to significantly reduce the dimensions and weight of the top housing <NUM> as well.

A dry high voltage instrument transformer in the form of a HV voltage transformer <NUM> according to the invention as shown in <FIG> comprises a bottom tank housing <NUM> and a cast of a primary winding module <NUM> with an embedded screen <NUM> surrounding the primary winding <NUM>. The cast of the primary winding module <NUM> is made of the insulating composite <NUM> containing an electrically insulating material made of PES nonwoven impregnated and cured with low viscosity epoxy. The PES nonwoven is easily impregnable in all directions and not mostly parallel to its surface, as it is in the case of crepe paper.

The application of the insulation <NUM> comprising an insulating composite containing an electrically insulating material made of PES nonwoven impregnated and cured with low viscosity epoxy provides an increase of reliability of insulation in the HV voltage transformer <NUM> and allows to limit the amount of used compressible silicone insulation material thereby reducing the material costs in the production of the HV voltage transformer <NUM> comparing to solutions known from the prior art.

The screen <NUM> is made of an easily impregnable semi-conductive material, such as semi-conductive PES nonwoven. Further, the screen <NUM> is terminated with a field grading ring <NUM> to prevent occurring harmful stresses at the edge of the screen <NUM>. The field grading ring <NUM> is insulated from the primary winding screen <NUM> by the cast of the primary winding module <NUM>.

The HV voltage transformer <NUM> also comprises a field grading disc <NUM> and a core <NUM>.

Screen <NUM> and the core <NUM> are grounded while in use.

The HV voltage transformer <NUM> further comprises a primary winding HV screen <NUM>, wherein the primary winding HV screen <NUM> is insulated from the primary winding screen <NUM> by the cast of the primary winding module <NUM> and a distance between the primary winding HV screen <NUM> and the primary winding screen <NUM> is determined based on the dielectric strength of the insulating composite <NUM> and the adopted margin of safety. The distance is such as to avoid the risk of electrical breakdown caused by the electrical stress during routine tests, for example the power frequency withstand voltage tests and lightning impulse tests and/or subsequent long term operation of the dry HV voltage transformer <NUM>.

The HV voltage transformer further comprises a compressible silicone insulation material <NUM> between the field grading disc <NUM>, the internal surface of the bottom tank housing <NUM>, and the screen <NUM>. Owing to the disclosed solution an amount of the compressible silicone insulation material <NUM> is greatly reduced, as it is largely replaced with the much less expensive PES nonwoven composite insulation <NUM> to which most electrical stresses are confined. The composite insulation <NUM> has higher dielectric strength than the compressible silicone material <NUM>, so it can be made much thinner. This renders it possible to significantly reduce the dimensions and weight of the bottom tank housing <NUM> as well.

Claim 1:
A dry high voltage instrument transformer in the form of a HV current transformer (<NUM>) comprising:
- a core casing (<NUM>) provided with a secondary winding (<NUM>),
- a top housing (<NUM>), a primary winding (<NUM>), and
- a dry bushing (<NUM>),
wherein
- the core casing (<NUM>) comprises an insulation (<NUM>) for its electrical insulation from the primary winding (<NUM>) and from the top housing (<NUM>), and
- the HV current transformer (<NUM>) further comprises an insulation (<NUM>) made of a compressible silicone material, arranged between the connection point of the dry bushing (<NUM>) and the core casing (<NUM>) and the top housing (<NUM>),
characterized in that:
the insulation (<NUM>) of the core casing (<NUM>) comprises an insulating composite containing an electrically insulating material made of PES nonwoven impregnated and cured with low viscosity epoxy.