WO2018114661A1

WO2018114661A1 - Bushing with integrated voltage sensor

Info

Publication number: WO2018114661A1
Application number: PCT/EP2017/083008
Authority: WO
Inventors: Abhijeet Kshirsagar; Gayatri DHARNE; Santosh Sharma; Arend Lammers
Original assignee: Eaton Industries (Netherlands) B.V.
Priority date: 2016-12-20
Filing date: 2017-12-15
Publication date: 2018-06-28

Abstract

The invention relates to a bushing (1) comprising: - an elongate electrical conductor (2); - an insulation layer arranged around the elongate electrical conductor; - a voltage sensor (5,6,7,8) embedded in the insulation layer, wherein the voltage sensor comprises at least a first and a second capacitor (5,7;6,8), wherein each capacitor comprises a proximal (5;6) and a distal electrode (7;8) and wherein both proximal electrodes are co- planer and wherein both distal electrodes are co-planar.

Description

Eaton Industries (Netherlands) B.V. Bushing with integrated voltage sensor

The invention relates to a bushing comprising:

- an elongate electrical conductor;

- an insulation layer arranged around the elongate electrical conductor;

- a voltage sensor embedded in the insulation layer. A bushing is known in the electrical field of medium and high voltages as an insulated device that allows an electrical conductor to pass safely through a typically earthed conducting barrier such as the wall of a transformer or circuit breaker.

Such a device is for example known from US 2006202671. This document describes a bushing having a voltage sensor embedded in the insulation layer. The voltage sensor is embodied by a ceramic sleeve having on the inside a conducting layer and on the outside a conducting layer. This sleeve is arranged around the elongate electrical conductor and is embedded in a insulation layer, which is typically an epoxy material. The conductor and the inner conducting layer form a first capacitor, while the outer conducting layer together with a conductive layer arranged on the outside of the

insulation layer provides a second capacitor. The difference between both capacitors provides a signal which is indicative for the voltage of the electrical conductor.

During use, the bushing typically is subjected to temperature changes. Due to these temperature changes, the insulation layer, which is generally an epoxy material, will expand and shrink, which will change the distance between the electrical conductor and the different conducting layers of the voltage sensor and thus will influence the reading of the voltage sensor. US 2006202671 provides a ceramic sleeve on which at least two conducting layers are arranged. The ceramic sleeve will not expand as much as the epoxy material, such that at least the distance of two (out of the three) conducting layers is more stable. However, the distance of the third conducting layer, being arranged on the outside of the insulation layer, will still be subjected to the expansion and shrinking of the insulation layer influencing the reading of the voltage sensor .

Furthermore, the provision of a separate ceramic sleeve in the bushing increases cost and due to the difference of expansion between the insulation layer and the ceramic sleeve, cracks between the sleeve and insulation layer could occur, which will disturb the electrical field and which could result in high field concentrations.

It is an object of the invention to reduce or even remove the above mentioned disadvantages.

This object is achieved according to the invention by a bushing according to the preamble, which bushing is characterized in that the voltage sensor comprises at least a first and a second capacitor, wherein each capacitor comprises a proximal and a distal electrode and wherein both proximal electrodes are co-planer and wherein both distal electrodes are co-planar.

As both capacitors are molded in the same insulation material and because both proximal electrodes are arranged in the same plane, as well as both distal electrodes are arranged in the same plane, a better accuracy will be obtained over a wide temperature range.

When the insulation layer expands, the distance between the electrical conductor and the proximal electrodes will change as well as the distance between the distal

electrodes and the electrical conductor, but the change in distance will be the same for both capacitors, such that when the difference between both capacitors is measured to derive a voltage signal, this change in distance is canceled. As a result, a low cost voltage sensor is provided, which is less or even not susceptible for temperature changes.

Preferably, the proximal and distal electrodes are parallel to the elongate electrical conductor. Such an

arrangement would provide the best cancellation of temperature influences. However, depending on the overall design of the bushing, a not fully parallel arrangement of the electrodes relative to the electrical conductor could also prove

workable, as long as both the proximal and distal electrodes are coplanar, such that they are subjected to the same

expansion and shrink when temperature of the insulation layer changes .

In another embodiment of the bushing according to the invention the distance of the proximal electrodes to the elongate conductor is smaller than the distance of the distal electrodes to the elongate conductor.

Although it should be clear to a skilled person, proximal means closest to and distal means further away. By having the distance of the proximal electrode to the elongate conductor smaller than the distance of the distal electrode to the elongate conductor, this is further elucidated.

In a preferred embodiment of the bushing according to the invention the proximal and distal electrodes are cylinders arranged concentrically around the elongate

electrical conductor.

Cylinders can be easily positioned around the elongate conductor when the insulation layer is casted around the electrical conductor. The cylinders could for example be positioned with non-conducting spaces. Furthermore, the arrangement of concentric cylinders provides symmetry around said conductor for the electrical field.

A further preferred embodiment of the bushing according to the invention further comprises a cylindrical screen electrically earthed and enveloping the voltage sensor at least in radial direction from the elongate electrical conductor .

The electrical shield basically shields

electromagnetic radiation coming from elongated conductor inside. The electrical shield further will shield the voltage sensor from external disturbances, for example from the electrical field of a neighboring bushing.

In yet a further preferred embodiment of the bushing according to the invention the proximal electrode of the first capacitor is electrically earthed. By earthing the proximal electrode of the first capacitor, the electrical field

surrounding the distal electrode of the first capacitor is changed. As result, the first capacitor will have a

capacitance different from the second capacitor, which will enable one to derive a voltage signal out of this difference.

In still a further preferred embodiment of the bushing according to the invention the proximal electrode of the second capacitor is electrically floating.

Although an electrically floating electrode has no electrical influence on the voltage measurement, it ensures that the structural embodiment of both first and second capacitors is identical. This will ensure that the electrodes of both capacitors will displace identically relative to the elongate conductor when the insulation layer expands or shrinks .

Another embodiment of the bushing according to the invention further comprises a first terminal electrically connected to the distal electrode of the first capacitor and a second terminal electrically connected to the distal electrode of the second capacitor, wherein the first and second terminal extend out of the insulation layer.

The first and second terminal allow to connect processing electronics to the voltage sensor. These processing electronics will process the signal from the voltage sensor, in particular the values of the first and second distal electrode, into a signal which is usable for other equipment, for example a voltage display.

Yet another embodiment of the bushing according to the invention further comprises a differential amplifier having a first input connected to the distal electrode of the first capacitor and having the second input connected to the distal electrode of the second capacitor. With the

differential amplifier, the difference between the first and second distal electrode is converted in a voltage signal usable for further processing. By using the difference between the two distal electrodes, the changes in the each capacitor due to the temperature influences are eliminated.

These and other features of the invention will be elucidated in conjunction with the accompanying drawings.

Figure 1 shows a schematic cross sectional view of a first embodiment of a bushing according to the invention.

Figure 2 shows a partial schematic cross sectional view of a second embodiment of a bushing according to the invention .

Figure 1 shows a bushing 1 with an electrical conductor 2 with a copper core 3. An insulation layer 4 is arranged around the electrical conductor 2. Embedded in the insulation layer 4 is a voltage sensor having a first and second proximal electrode 5, 6 and first and second distal electrodes 7, 8. The electrodes 5, 6, 7, 8 are cylindrical and co-axially arranged with the electrical conductor 2.

The voltage sensor 5, 6, 7, 8 is enveloped by a cylindrical screen 9, which is electrically earthed. The first proximal electrode 5 is connected to the screen 9, such that this electrode 5 is also earthed. The second proximal

electrode 6 is not electrically connected such that the electrode 6 is electrically floating. This ensures that the structural embodiment of the bushing 1 is symmetrical, which ensures that by expansion of the insulation layer 4, the proximal electrodes 5, 6 will displace relative to the

conductor 2 equally and that the distal electrodes 7, 8 will displace relative to the conductor equally.

The distal electrodes 7, 8 are electrically connected via lines 10, 11 to the terminals 12, 13. A

differential amplifier 14 is connected to the terminals 12, 13 to convert the difference between the two distal electrodes 5, 6 into a voltage signal 15.

Figure 2 shows a second embodiment of a bushing 20 according to the invention. The bushing 20 has, similar to the bushing 1 of figure 1, an electrical conductor 21 with a copper core 22. An insulation layer 23, typically of an epoxy material, is arranged around the electrical conductor 21.

Embedded in the insulation layer 23 is a voltage sensor having a first proximal electrode plate 24, a second proximal electrode plate 25, a first distal electrode plate 26 and a second distal electrode plate 27.

A screen 28 is arranged between the outside of the bushing 20 and the voltage sensor 24, 25, 26, 27. The screen 28 is electrically earthed and shields the sensor 24, 25, 26, 27 from external electrical fields. Preferably, the screen 28 is more or less cylindrical and co-axially arranged around the conductor 21.

Similar to the embodiment of figure 1, the first proximal electrode plate 24 is connected to the screen 28 and earthed and is the second electrode plate 25 electrically floating.

The distal electrode plates 26, 27 are connected via lines 29, 30 to external processing electronics, such as a differential amplifier, to provide a voltage signal.

Claims

1. Bushing comprising:

- an elongate electrical conductor;

- an insulation layer arranged around the elongate electrical conductor;

- a voltage sensor embedded in the insulation layer, characterized in that

the voltage sensor comprises at least a first and a second capacitor, wherein each capacitor comprises a proximal and a distal electrode and wherein both proximal electrodes are co-planer and wherein both distal electrodes are co- planar .

2. Bushing according to claim 1, wherein the

proximal and distal electrodes are parallel to the elongate electrical conductor.

3. Bushing according to claim 1 or 2, wherein the distance of the proximal electrodes to the elongate conductor is smaller than the distance of the distal electrodes to the elongate conductor.

4. Bushing according to any of the preceding claims, wherein the proximal and distal electrodes are cylinders arranged concentrically around the elongate electrical

conductor .

5. Bushing according to any of the preceding claims, further comprising a cylindrical screen electrically earthed and enveloping the voltage sensor at least in radial direction from the elongate electrical conductor.

6. Bushing according to any of the preceding claims, wherein the proximal electrode of the first capacitor is electrically earthed.

7. Bushing according to any of the preceding claims, wherein the proximal electrode of the second capacitor is electrically floating.

8. Bushing according to any of the preceding claims, further comprising a first terminal electrically connected to the distal electrode of the first capacitor and a second terminal electrically connected to the distal electrode of the second capacitor, wherein the first and second terminal extend out of the insulation layer.

9. Bushing according to any of the preceding claims, further comprising a differential amplifier having a first input connected to the distal electrode of the first capacitor and having the second input connected to the distal electrode of the second capacitor.