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Title:
CURING INVESTIGATING ARRANGEMENT AND METHOD FOR CONTROLLING THE CURING OF EPOXY RESIN IN THE PRODUCTION OF A HIGH-VOLTAGE LEAD-THROUGH DEVICE
Document Type and Number:
WIPO Patent Application WO/2021/063689
Kind Code:
A1
Abstract:
An curing investigating arrangement (10) comprises a conductor (12), insulation (21) surrounding the conductor (12), the insulation (21) comprising epoxy resin, where the insulation (21) and conductor (12) are provided for forming a high-voltage lead-through device, a curing vessel (22) having an enclosure through which the conductor (12) stretches and in which the insulation (21) is placed, and a group of sensors comprising at least one passive wireless sensor (14, 16, 18, 20) in the insulation (20) in contact with the epoxy resin. The at least one passive wireless sensor (14, 16, 18, 20) is configured to measure a property of the epoxy resin during production of the high-voltage lead-through device and provide it to the first active communication unit (24) using wireless short-range communication wherein the curing investigating arrangement (10) further comprises a number of conductive foils (F1, F2, F3, F4) placed in the insulation (21) surrounding the conductor (12), wherein at least one sensor (14, 16, 18) in the group is placed in a region of the insulation that is electrically stress-free during operation of the lead-through device (34). There is also a method for controlling the curing of epoxy resin.

More Like This:
WO/2013/157428TIRE-CURING PRESS
Inventors:
MARTINI HARALD (SE)
FORSSÉN CECILIA (SE)
SCHIESSLING JOACHIM (SE)
LOGAKIS EMMANUEL (CH)
HO CHAU HON (DE)
Application Number:
PCT/EP2020/075982
Publication Date:
April 08, 2021
Filing Date:
September 17, 2020
Export Citation:
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Assignee:
ABB POWER GRIDS SWITZERLAND AG (CH)
International Classes:
B29C35/02; H01B17/00; H01B17/28
Foreign References:
EP3521786A12019-08-07
US20080175753A12008-07-24
US20160329134A12016-11-10
Attorney, Agent or Firm:
VALEA AB (SE)
Download PDF:
Claims:
CLAIMS

1. A curing investigating arrangement (io) comprising: a conductor (12), insulation (21) surrounding the conductor (12), the insulation (21) comprising epoxy resin, where the insulation (21) and conductor (12) are provided for forming a high-voltage lead-through device (34), a curing vessel (22) having an enclosure through which the conductor (12) stretches and in which the insulation (21) is placed, a first active communication unit (24) as a part of the curing vessel and placed outside of the insulation (21), and a group of sensors comprising at least one passive wireless sensor (14, 16, 18, 20) in the insulation (21) in contact with the epoxy resin, the at least one passive wireless sensor (14, 16, 18, 20) being configured to measure a property of the epoxy resin during production of the high-voltage lead- through device (34) and provide the measured property to the first active communication unit (24; 40) using wireless short-range communication, wherein the curing investigating arrangement (10) further comprises a number of conductive foils (Fi, F2, F3, F4) placed in the insulation (21) surrounding the conductor (12), wherein at least one sensor (14, 16, 18) in the group is placed in a region of the insulation that is electrically stress- free during operation of the lead-through device (34).

2. The curing investigating arrangement (10) according to claim 1, wherein the first active communication unit (24) comprises transceiving circuitry (24A) and an antenna (24B), wherein at least the antenna of the first active communication unit (24) is placed in the enclosure of the curing vessel. 3. The curing investigating arrangement (10) according to claim 1, wherein a closest neighbouring foil (Fi) to the conductor (12) is also electrically connected to the conductor (12).

4. The curing investigating arrangement (10) according to claim 3, wherein at least one sensor (14, 16) in the group is placed between the conductor (12) and the closest neighboring foil (Fi).

5. The curing investigating arrangement (10) according to any of claims 1 - 4, wherein at least one sensor (18) in the group is placed in a region of the insulation that is located between an outermost foil (F4) and a location for a flange (36) of the finalized lead-through device (34) that is to be electrically connected to the outermost foil (F4).

6. The curing investigating arrangement (10) according to any previous claim, wherein at least one sensor (20) in the group is placed in an area of the insulation (21) that is removed before the lead-through device is finalized.

7. The curing investigating arrangement (10) according to any previous claim, further comprising a curing control unit (26) configured to communicate with the first active communication unit (24) for obtaining a curing degree of the epoxy based on the property sensed by the at least one sensor (14, 16, 18, 20) and control the curing process based on the obtained curing degree.

8. The curing investigating arrangement (10) according to claim 7, wherein the curing control unit (26) is configured to control the curing through controlling process parameters e.g. temperature in the vessel and/or the time duration of the curing.

9. The curing investigating arrangement (10) according to any previous claim, wherein passive sensors (14, 16, 18) in the group that remain in the insulation (21) after the lead-through device (34) has been finalized are configured to provide measurements to a second active communication unit (40) during operation of the high-voltage lead- through device (34).

10. The curing investigating arrangement (10) according to any previous claim, wherein the property being measured is the permittivity of the epoxy resin.

11. The curing investigating arrangement (10) according to any of claims 1 - 9, wherein the property being measured is the temperature.

12. The curing investigating arrangement (10) according to any previous claim, wherein the high-voltage lead through device (34) is a bushing, a cable termination or an instrument transformer. 13. A method for controlling the curing of epoxy resin in the production of a high-voltage lead-through device (34) where insulation (21) with the epoxy resin is placed surrounding a conductor (12) in a curing vessel (22), the method comprising detecting (30), during production of the production of the high-voltage lead-through device, the curing degree of the epoxy resin via wireless short-range communication between a group of sensors comprising at least one passive wireless sensor (14, 16, 18, 20) and a first active communication unit (24), which sensors are provided in the insulation in contact with the epoxy resin and the first active communication unit is a part of the curing vessel and placed outside the insulation, and controlling (32) the curing process based on the detected curing degree.

14. The method according to claim 13, wherein the controlling comprises controlling (32A; 32B) process parameters, such as the temperature, in the interior of the curing vessel (22) and/ or the time duration of the curing.

Description:
CURING INVESTIGATING ARRANGEMENT AND METHOD FOR CONTROLLING THE CURING OF EPOXY RESIN IN THE PRODUCTION OF A HIGH-VOLTAGE LEAD-THROUGH DEVICE FIELD OF INVENTION

The present invention generally relates to high-voltage lead-through devices. The invention more particularly relates to a curing investigating arrangement and a method for controlling the curing of epoxy resin in the production of a high-voltage lead-through device.

BACKGROUND

Lead-through devices, such as bushings, cable terminations and instrument transformers, are high-voltage devices that are used for leading a high voltage conductor through a medium which has a different electric potential than the high voltage conductor. Lead-through devices may be used in high-voltage power applications such as power transmission systems.

It is known to measure properties of a high-voltage device during its operation.

JP 2009053025 does for instance disclose the use of an active radio- frequency identification (RFID) sensor in contact with a conductor connection pipe for measuring the temperature of a connection portion of a high-voltage power cable. The document also discusses the use of a passive wireless temperature sensor. EP3521786 discloses a wound electrical component, such as a high-voltage lead-through device, e.g. a bushing for a transformer arrangement. The component comprises at least one printed electronics sensor between two of the wound layers. The sensors are configured to monitor properties such as temperature, moisture and/or acoustics in the component.

US2008/175753 discloses sensors embedded in resin for wirelessly monitoring process parameters, such as temperature, during manufacturing of a cured component. The sensors are arranged not to interfere with the production process and they remain in the produced part for the life of the part without causing structural degradation of the part.

A lead-through device comprising an insulator body formed using epoxy resin maybe a bushing. A bushing that is formed using epoxy resin may as an example be a so-called resin impregnated paper (RIP) bushing. There is a problem in the production of lead-through devices that include epoxy resin in that the curing process is hard to monitor and control.

Today typically a “dummy” is produced with integrated sensors to follow temperature variation during the curing process. The data from this “dummy” is then used in the production of actual lead-through devices.

The “dummy” itself cannot be used as a product and is discarded.

There is therefore a need for allowing a more efficient production to be made.

SUMMARY OF THE INVENTION

One object of the present invention is to enable a more efficient production of a high-voltage lead-through device.

This object is according to a first aspect obtained through a curing investigating arrangement comprising: a conductor, insulation surrounding the conductor, the insulation comprising epoxy resin, where the insulation and conductor are provided for forming a high- voltage lead-through device, a curing vessel having an enclosure through which the conductor stretches and in which the insulation is placed, a first active communication unit as a part of the curing vessel and placed outside of the insulation, and a group of sensors comprising at least one passive wireless sensor in the insulation in contact with the epoxy resin, the at least one passive wireless sensor being configured to measure a property of the epoxy resin during production of the high-voltage lead-through device and provide the measured property to the first active communication unit using wireless short-range communication.

The provision of a measured property maybe triggered by the first active communication unit. The first active communication unit may more particularly perform reading of the sensor. It may for instance read a tag of a sensor providing a property measurement.

The first active communication unit may comprise transceiving circuitry and an antenna, where at least the antenna of the first active communication unit is placed in the enclosure of the curing vessel. The transceiving circuitry may also be placed in the enclosure. Alternatively, it may be placed outside of the enclosure.

The curing investigating arrangement may furthermore comprise a number of conductive foils placed in the insulation surrounding the conductor. In this case at least one sensor in the group may be placed in a region of the insulation that is electrically stress-free during operation of the lead-through device. In one variation a closest neighbouring foil to the conductor is also electrically connected to the conductor. In this case at least one sensor in the group may be placed between the conductor and the closest neighbouring foil.

In another variation at least one sensor in the group is placed in a region of the insulation that is located between an outermost foil and a location for a flange of the finalized lead-through device that is to be electrically connected to the outermost foil.

According to yet another variation at least one sensor in the group is placed in an area of the insulation that is removed before the lead-through device is finalized.

The curing investigating arrangement may also comprise a curing control unit configured to communicate with the first active communication unit for obtaining a curing degree of the epoxy based on the property sensed by the at least one sensor and control the curing process based on the obtained curing degree. The curing control unit may more particularly be configured to control the curing process through controlling process parameters, such as the temperature in the vessel and/or the time duration of the curing. Passive sensors in the group may remain in the insulation after the lead- through device has been finalized. These passive sensors may additionally be configured to provide measurements to a second active communication unit during operation of the high-voltage lead-through device. These measurements may be indicative of moisture or used for obtaining the moisture of the insulation. The moisture may be used to determine the ageing of the high-voltage lead-through device. It may also be used for scheduling maintenance. The property being measured may additionally be the permittivity of the epoxy resin. Alternatively or additionally, the measurements may be temperature measurements.

According to another aspect there is provided a method for controlling the curing of epoxy resin in the production of a high-voltage lead-through device, where insulation with the epoxy resin is placed surrounding a conductor in a curing vessel.

The method comprises detecting, during production of the high-voltage lead-through device, the curing degree of the epoxy resin via wireless short-range communication between a group of sensors comprising at least one passive wireless sensor and a first active communication unit, which sensors are provided in the insulation in contact with the epoxy resin and the first active communication unit is a part of the curing vessel and placed outside the insulation, and controlling the curing process based on the detected curing degree. The controlling may comprise controlling a process parameter, such as the temperature in the interior of the curing vessel. Additionally, or instead, the controlling may comprise controlling another process parameter, such as the time duration of the curing.

The high-voltage lead-through device may be a bushing, such as a transformer bushing or a wall bushing, a cable termination or an instrument transformer. The present invention has a number of advantages. It provides a non destructive way of in-situ monitoring of the epoxy curing of the insulator body of the high-voltage lead-through device. This makes it possible to monitor each produced high-voltage lead-through-device. Thereby a more efficient production of high-voltage lead-through devices is possible. The concept can be used to monitor every produced high-voltage lead-through device and allows for optimization of the process control resulting in improved product quality/reliability and saving of production time. Sensors may additionally be used later in operation of the finalized high- voltage lead-through device.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will in the following be described with reference being made to the accompanying drawings, where fig. l schematically shows a curing investigating arrangement comprising insulation around a conductor in a curing vessel, where the curing vessel is equipped with a first active communication unit and the insulation comprises passive sensors, fig. 2A shows a first localization of the first active communication unit, fig. 2B shows a second localization of the first active communication unit, fig. 3 schematically shows a curing control unit communicating with the active communicating unit, which in turn communicates with the passive sensors, fig. 4 schematically shows a number of method steps in a method of controlling the curing of a lead-through device formed by the curing investigating arrangement, and fig. 5 schematically shows a finalized lead-through device that is in operation and communicating with a second active communicating unit.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention are discussed in detail below. In describing embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. While specific exemplary embodiments are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations can be used without parting from the spirit and scope of the invention.

The present invention generally concerns high-voltage lead-through devices.

A high-voltage lead-through device typically comprises an insulator body with insulation, for instance in the form of a condenser core, and a main conductor at a high voltage passing through the insulator body. The insulator body also often comprises epoxy resin which is used to solidify it during the production of the high-voltage lead-through device. The high- voltage lead-through device may be a bushing, like a wall bushing or a transformer bushing. The high-voltage lead-through device may also be an instrument transformer or a cable termination.

The invention more particularly concerns a curing investigating arrangement, which arrangement comprises a conductor and insulation used for forming a high-voltage lead-through device as well as comprises at least one passive sensor in contact with epoxy resin in the insulation. The arrangement additionally comprises a curing vessel and a first active communication unit that is a part of the curing vessel. It is furthermore possible that the arrangement also comprises a curing control unit.

As was discussed above, a high-voltage lead-through device may be a bushing. The invention will in the following be described in relation to a first type of high-voltage lead-through device in the form of a transformer bushing comprising a condenser core, i.e. a bushing with a main conductor surrounded by insulation and conductive foils. The bushing may additionally be a resin impregnated paper (RIP) bushing. It should here be realized that it is applicable also to other types of high-voltage lead- through devices comprising epoxy resin. Fig. l schematically shows a curing investigating arrangement io used in the production of a bushing. The curing investigating arrangement comprises a conductor 12 surrounded by insulation 21. Moreover, the curing investigating arrangement 10 of fig. 1 also comprises a curing vessel 22 having an enclosure. Through this enclosure there is placed a conductor 12, which conductor 12 runs through a first wall of the vessel 22, then continues through the interior of the vessel i.e. through the enclosure, and through a second opposite wall of the vessel 22. The vessel 22 may fit tightly around the conductor 12 such that the entrance and exit of the conductor 12 into and out from the enclosure is sealed. In the enclosure there is also insulation 21 as well as conductive foils Fi, F2, F3 and F4 surrounding the conductor 12. The insulation 21 typically comprises insulation made of insulating sheets, such as cellulose or paper sheets or sheets of synthetic material such as thermoplastic sheets. Between these sheets, foils of electrically conducting material, such as metal, are provided for voltage control or field grading purposes. These foils and insulation encircle the main conductor 22. There is also epoxy resin in the insulation, which epoxy resin may initially be in liquid form. The epoxy resin may then be solidified through curing in order to obtain a solid insulator body comprising the foils.

As can be seen in fig. 1, the conductor 12 is radially surrounded in the enclosure of the curing vessel 22 by the conductive foils Fi, F2, F3 and F4.

There is thus a first innermost foil Fi, which surrounds the conductor 12. A second foil F2 in turn surrounds the first foil Fi. Thereafter follows a third foil F3 that surrounds the second foil F2. A fourth foil F4 finally surrounds the third foil F3. It should be realized that the number of foils shown is merely an example and that more or fewer could be used. The purpose of the foils inside the insulation is to grade an electric field from a high potential at the conductor to a second significantly lower potential. The center of the conductor 12 may define a longitudinal axis through the vessel 22, i.e. between the first and second walls. The conductive foils may in this case be wound around and be coaxial with the conductor 12. The foils are also spaced from each other in a direction that is perpendicular to the longitudinal axis of the conductor 12. This direction that is perpendicular to the longitudinal axis is in this embodiment a radial direction. Each foil, which as an example maybe an aluminum or copper foil, is as an example formed as a hollow cylinder, with each foil having an increased diameter in relation to the previous foil closer to the conductor 12 in the radial direction. A foil that is closer than a neighboring foil to the conductor 12 in the radial direction furthermore has a longer extension along the longitudinal axis than this neighboring foil. The diameter and length may be selected so that each foil should cover the same area. Therefore, the lengths of the foils along the longitudinal axis may decrease with the distance, here radial, to this axis. It can thus be seen that the foil length of any outer foil is lower than any inner foil that it surrounds. Put differently, the foil length in the longitudinal direction of any foil on a radial distance from the conductor 12 is shorter than the foil length of any neighboring foil on a lower radial distance from the conductor. The first foil Fi is here also a closest neighbouring foil to the conductor 12.

There is also a group of sensors comprising at least one passive sensor in the insulation 21. Every sensor in the group is more particularly in contact with the epoxy resin in the insulation 21. As an example, there is a first and second passive sensor 14 and 16 adjacent the conductor 12 and more particularly placed between the conductor 12 and the first foil Fi. There is also a third passive sensor 18 placed radially outside of the fourth foil F4 as well as a fourth passive sensor 20 placed in an area where there are no foils. The first, second and third sensors 14, 16 and 18 are here all placed in electrically stress-free regions of the insulation 21, i.e. in regions of the insulation where there is no electrical stress during operation of the finalized bushing. This is due to the fact that in these regions there are no electric fields, because the electrically stress-free regions lie between elements which are connected to the same electric potential, as described below. The regions are electric field-free regions.

In the shown embodiment, the first foil Fi is electrically connected to the conductor 12. This means that when the bushing has been finalized and put in operation the conductor 12 and the first foil Fi have the same electric potential. Thereby the first and second sensors 14 and 16 are placed in an electric field-free region of the solid insulator body. The electrical field experienced by the third sensor 18 will likewise be limited. In this case the outermost foil F4 is to be surrounded by and electrically connected to a flange having an electric potential that may be a ground potential. Thereby the third sensor 18 is placed in a region of the insulation located between the outermost foil and a location intended to be filled by the flange in the finalized bushing. It can be seen that the region is placed radially outside of the outermost foil. The fourth sensor 20 may on the other hand be placed in a region where the electrical stress is of no concern for the finalized bushing.

The placing of the sensors is such that the functionality of the bushing when in operation will not be affected. The placing of the first, second and third sensor 14, 16 and 18 is such that they are placed in electrically stress- free regions of the bushing that do not disturb the functionality. The area in which the fourth sensor 20 is placed is on the other hand an area that is removed before the bushing is finalized i.e. after the curing is finished. Thereby it will have no influence on the bushing performance. The removal may as an example be performed through milling, The sensors are all wireless and use a suitable short-range communication technology such as near field communication (NFC) or radio frequency identification (RFID). As they are passive they do not need any own power source for their operation, but they operate triggered and powered by the wireless transmissions of an active communication unit. They may more particularly be tags delivering data when being read by a reader.

Therefore, the curing investigating arrangement may also comprise at least one active communication unit. The curing investigating arrangement in fig. l comprises a first active communication unit 24 in the vessel 22, which first active communication unit 24 is placed outside of the insulation 21. In the case of NFC or RFID this first active communication unit 24 maybe a reader configured to read the tags of the passive sensors 14, 16, 18 and 20. The first active communication unit 24 maybe integrated in the vessel 22 and may additionally extend into the enclosure in order to communicate with the sensors 14, 16, 18 and 20. The first active communication unit 24 may comprise a transceiving circuitry and an antenna connected to the transceiving circuitry. The antenna may be placed on an inner surface of a wall facing the enclosure, e.g. on a surface of a wall of the vessel, such as the previously mentioned first or second wall, that faces the enclosure with the insulation. The antenna may additionally be casted in a material able to withstand high temperatures, for instance of up to about 170 °C.

Fig. 2A schematically shows a first placement of transceiving circuitry 24A and antenna 24B. In this placement the transceiving circuitry is placed on an outer surface of a wall W of the vessel, i.e. on a surface that faces away from the enclosure, while the antenna 24B is placed on an inner surface of the wall W. A conductor in this case runs through the wall W and interconnects the transceiving circuitry 24A with the antenna 24B. Fig. 2B schematically shows a second placement of the transceiving circuitry 24A and antenna 24B. In this placement both the transceiving circuitry 24B and the antenna 24B are placed on the inner surface of the wall W of the vessel. Also the interconnecting conductor, if needed, is placed there. In this case the transceiving circuitry 24A is connected to a conductor running through the wall to the exterior of the vessel for connection of the first active communication unit with other units, such as with a curing control unit.

Fig. 3 schematically shows the communication of the first active communication unit 24 with the passive sensors 14, 16, 18 and 20 as well as with the curing control unit 26 that may optionally also be a part of the curing investigating arrangement. The connection between the first active communication unit 24 and the curing control unit 26 maybe wired or wireless. Also, optical communication is an option. As was stated above and as can also be seen in fig. 3, the communication between the first active communication unit 24 and the passive sensors 14, 16, 18 and 20 is wireless.

As was also mentioned above a sensor senses a property of the epoxy resin in the insulation. This property may be the permittivity. The permittivity may easily be converted into a curing degree which is of interest in the forming of a bushing. However, the permittivity may additionally be of use in a live and operational bushing.

As was mentioned earlier, the curing process in the forming of a bushing is hard to control. It has traditionally been based on measurements made on a “dummy” and using data obtained in the production of this “dummy” as input in the production of actual or real bushings. However, this data is also often not reliable enough so that the production may not be efficient enough and the rate of acceptable bushings being produced may be low. There is therefore a need for allowing a better controllability in the production of bushings and especially in the curing of the insulator body like the curing of a condenser core. How this may be achieved will now be described also with reference being made to fig. 4 which shows a number of methods steps in a method of controlling the curing of a condenser core in the production of a bushing, which steps are performed by a variation of the curing investigating arrangement comprising the elements of fig. 1 and the curing control unit 26.

Before the method is started the conductor 12, foils Fi, F2, F3 and F4 and insulation 21 including epoxy resin have been placed in the enclosure of the curing vessel 22, which is then sealed. Thereafter curing is started, which may involve applying heat in the interior of the curing vessel for a certain amount of time. It is possible that further epoxy resin is added during the curing. This maybe needed because the epoxy resin shrinks during the curing. During curing the curing control unit 26 then detects the curing degree of the epoxy resin via wireless short-range communication between the passive wireless sensors 14, 16, 18 and 20 in the insulation 21 that are in contact with the epoxy resin and the first active communication unit 24 that is placed outside the insulation, step 30. This maybe performed through the curing control unit 26 ordering the first active communication unit 24 to obtain measurements of the desired property of the epoxy resin or the first active communication unit 24 being set to obtain this property at regular time intervals. The first active communication unit 24 then obtains the measurements from the sensors 14, 16, 18 and 20. This maybe done by the first active communication unit 24 acting as a reader reading the tags of the sensors, which tags comprise the property measurement. The signals from the sensors will then be transferred wirelessly through the insulation to the first active communication unit 24.

A property measurement may then be converted to a curing degree by the first active communication unit 24. Thereafter the property measurement, unconverted or converted, is forwarded from the first active communication unit 24 to the curing control unit 26. If not already converted, the curing control unit 26 then converts the measured permittivity to a curing degree. The curing control unit 26 thereafter uses the curing degree in the control of the curing process. The curing control unit 26 thereby controls the curing based on the detected curing degree, step 32. The control may in this case involve controlling the process parameter temperature in the interior of the curing vessel 22, step 32 A, and/or the process parameter duration of the curing, step 32B.

Thereby a non-destructive way of in-situ monitoring of the epoxy curing of the insulator body of the bushing is obtained. This makes it possible to monitor each produced bushing and not only once for process qualification of a new bushing design. The concept can be used to monitor every produced bushing and allows for optimization of the process control resulting in improved product quality/ reliability and saving of production time.

Cure monitoring of epoxy resin can thus assure a better production quality and may also reduce the processing time.

The invention is not limited to the production of a bushing. It is possible to use the sensors that remain in the solid insulator body also after the bushing has been finalized. It is for instance possible to use the sensors to obtain measurements when the bushing is in operation. The first, second and third sensors 14, 16 and 18 may thereby be used in the finalized bushing. The fourth sensor 20 may however not, as it has been removed from the finalized bushing.

A finalized bushing 34 is schematically shown in fig. 5, where now the insulation 21 has been solidified, i.e. the epoxy has been cured, and areas removed in order to adapt to a desired bushing shape of a particular use case. It can for instance be seen that the area with the fourth sensor has been removed. A flange 36 is also placed around the fourth outermost foil F4 and is also electrically connected to it as well as to the previously mentioned second electric potential, which may be ground potential. The remaining sensors 14, 16 and 18 may now be used to communicate with a second active communication unit 40, which may be movable or stationary. The second active communication unit 40 is typically of the same type as the first active communication unit and may therefore read the tags of the sensors 14, 16 and 18 and convert the measured property, also here the permittivity, to a desired bushing property, for instance moisture, to be investigated. The measured property, converted or unconverted, may then be forwarded to a property evaluating unit, which then evaluates the bushing property with regard to various activities such as the determining of maintenance intervals.

It can be seen that it is possible to collect data of the insulation during operation of the bushing, where such data may as an example comprise data about the moisture of the insulation. The moisture data may as an example be used for determining the ageing of the bushing. The ageing determination may in turn be used for scheduling maintenance. This may be performed in a maintenance control unit that communicates with the second active communication unit. Due to the placing of the first, second and third sensors 14, 16 and 18 in field free regions, these do not interfere with the electric field design of the condenser core. The sensors that remain in the finalized bushing do therefore not jeopardize the bushing function.

The big data generated from the sensors is expected to have a great impact on quality related issues. Inter-correlations among critical parameters recorded during manufacturing and operation and experienced failures during testing or even in the field will be established. Thus, scrape rate is reduced, premature failures of tested bushings are diminished (associated with high costs and liabilities) and, last but not least, predictive instead of scheduled maintenance becomes viable.

The various properties being measured and/ or used described earlier were permittivity, curing degree and moisture. It should be realized that also other properties are possible to measure, such as temperature. It may also be possible to measure electrical properties.

The curing control unit and property evaluating unit described earlier may both be implemented using software running on a processor. They may as an alternative be realized through dedicated integrated circuits such as Application-Specific Integrated Circuits (ASICs) or Field-Programmable Gate Arrays (FPGAs).

From the foregoing discussion it is evident that the present invention can be varied in a multitude of ways. It shall consequently be realized that the present invention is only to be limited by the following claims.