Overvoltage Protection for HV Bushing Test Tap

Abstract

The present disclosure relates to a protection device (7) configured to be electrically connected to a test tap (9) of an HV bushing for protecting the bushing from transient overvoltages. The protection device includes at least two protection branches (20) connected in parallel between the test tap and a ground connector (24) configured to be grounded. Each protection branch includes a plurality of gas discharge tubes (21) connected in parallel, a transient voltage suppression (TVS) diode (22) connected in series with the gas discharge tubes, and a resistor (23) connected in series with the gas discharge tubes and connected across the TVS diode.

Description

Technical Field

[0001] The present disclosure relates to overvoltage protection of test taps of high voltage (HV) bushings. Background Art

[0002] A test tap is included in a high voltage (HV) bushing for connecting an external device that monitors internal characteristics of the bushing, such as electric potential, partial discharge, or insulation loss. During a fast transient, due to the additional inductance between the test tap and the grounded flange of the bushing, external devices connected to the HV bushing test tap typically cause high overvoltages. These overvoltages may damage the insulation between the field-grading foil connected to the test tap inside the bushing and the flange. These failures can spread to the main insulation of the capacitor core of the bushing, resulting in bushing failure. Some external devices have built-in overvoltage protection to protect the external devices, but they do not have low enough inductance to protect the bushing.

[0003] US9,557,349 discloses a three-stage protection circuit with a low-pass filter for a measurement system for monitoring HV bushings, where each stage is activated at a corresponding different overvoltage. US6226166B1 relates to an overvoltage protection circuit for protecting electrical equipment from transients on a power line. The high energy absorption module includes a plurality of metal oxide varistors (MOVs) connected in parallel, and a switching device in the form of a gas arrester is connected in series with the MOVs. US2004 / 070913A1 relates to a lightning arrester device for protecting an electrical circuit connected to a low voltage network from transient overvoltages, where the lightning arrester device includes a plurality of gas type spark gaps connected in parallel. WO2013 / 178168A1 relates to a protection device for a gas discharge tube. Summary of the Invention

[0004] An object of the present invention is to provide improved protection for an HV bushing during transient overvoltages (also referred to as "transients").

[0005] According to one aspect of the present invention, there is provided a protection device configured to be electrically connected to a test tap of an HV bushing for protecting the bushing from transient overvoltages. The protection device includes at least two protection branches connected in parallel between the test tap and a ground connector configured to be grounded. Each protection branch includes a plurality of gas discharge tubes (GDTs) connected in parallel, a transient voltage suppression (TVS) diode connected in series with the gas discharge tubes, and a resistor connected in series with the gas discharge tubes and connected across the TVS diode.

[0006] According to another aspect of the present invention, there is provided an HV bushing including a test tap and an embodiment of the protection device of the present disclosure connected to the test tap.

[0007] According to another aspect of the present invention, there is provided an electrical system including an electrical device and an embodiment of the bushing of the present disclosure arranged to pass through a wall of a housing of the electrical device.

[0008] By using multiple parallel branches of a voltage clamping assembly, a lower inductance of the overall circuit can be obtained. By using a GDT in series with a TVS diode, a lower parallel capacitance can be achieved, which can allow high-frequency signals to pass through without increasing the capacitance. It is preferable to use a TVS diode rather than a varistor because the TVS diode ages and varies less with respect to the number of transients. A discharge resistor is used to ensure a damping effect and prevent accidental ignition of the GDT when the TVS is charged by an earlier transient.

[0009] It should be noted that, where appropriate, any feature of any aspect can be applied to any other aspect. Similarly, any advantage of any aspect can be applied to any other aspect. Other objects, features, and advantages of the accompanying embodiments will become apparent from the following detailed disclosure, the appended dependent claims, and the drawings.

[0010] In general, unless otherwise explicitly defined herein, all terms used in the claims should be interpreted according to their ordinary meaning in the technical field. Unless otherwise explicitly stated, all references to "a / an / element, device, component, method, step, etc." should be interpreted openly as referring to at least one example of the element, device, component, method, step, etc. Unless explicitly stated, the steps of any method disclosed herein need not be performed in the determined order disclosed. The use of "first", "second", etc. for different features / components of the present disclosure is only intended to distinguish the features / components from other similar features / components, and is not intended to give any order or hierarchical relationship to the features / components. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] With reference to the drawings, embodiments are described by way of example, in which:

[0012] Figure 1 is a schematic cross-sectional view of an electrical system according to some embodiments of the present invention.

[0013] Figure 2 is a schematic circuit diagram of overvoltage protection according to some embodiments of the present invention. DETAILED DESCRIPTION

[0014] Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which specific embodiments are shown. However, many different forms of other embodiments are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Throughout the specification, the same reference numerals refer to the same elements.

[0015] Figure 1 An electrical system 1 is shown including an electrical device 3 and an HV bushing 2, the HV bushing 2 being arranged to pass through a wall 5 of a housing 4 of the electrical device 3. The electrical device 3 can be, for example, a transformer or include a transformer (e.g., a liquid-filled transformer), in which case the housing 4 can be a transformer tank or include a transformer tank, or the electrical device 3 can be a power electronic converter or include a power electronic converter, in which case the housing 4 can be a valve hall or include a valve hall.

[0016] The bushing 2 includes an insulator defining a longitudinal through-hole for an HV conductor 8 to pass through the bushing and thus through the wall 5. The insulator of the bushing 2 can, for example, include a capacitor core having a plurality of interleaved field grading layers with conductive foils (e.g., aluminum foils). The bushing 2 is configured for HV applications, for example at voltages of at least 1 kV, for example at least 3 or 10 kV, for example up to 100 kV or up to 1000 kV.

[0017] A test tap 9 is included in the bushing 2, for example for measuring the electric potential inside the bushing. The test tap 9 can, for example, be connected to the outer (usually the outermost) field grading layer of the capacitor core of the bushing. A measuring device 10 outside the bushing 2 can be connected to the test tap 9 via an electrical conductor 6. The measuring device connected to the test tap can measure any signal in a variety of signals from low frequency to high frequency, and is sometimes sensitive to additional capacitance when measuring voltage by capacitive voltage division or measuring high frequency.

[0018] The connection of the measuring device 10 may increase the risk of inductance that will increase overvoltage in transient situations, which may damage the bushing. Specifically, the external measuring device 10 connected to the test tap 9 may often cause high overvoltages during fast transients due to the additional inductance between the test tap and the flange (ground) of the bushing, the additional impedance inside the measuring device, and the above-mentioned flange being arranged for mounting the bushing to the wall 5. These overvoltages may damage the insulation between the field grading layer connected to the test tap inside the bushing insulator and the flange. These faults may spread to the main insulator and cause the bushing to fail. Some external devices have built-in overvoltage protection, but they may not have a low enough internal inductance to protect the bushing itself. Instead, the overvoltage protection device 7 according to the present invention is configured to protect the bushing rather than the external measuring device, and the overvoltage protection device 7 is generally connected to the test tap 9 via an electrical conductor 6.

[0019] Preferably, the protection device 7 is arranged close to the test tap 9, for example directly on the test tap, in order to reduce the inductance.

[0020] Figure 2 An embodiment of the protection device 7 connected to the test tap 9 is shown. The protection device includes a plurality of protection branches 20 connected in parallel between the test tap and ground (e.g., via a ground connector 24). In the embodiment of the drawing, four branches 20 are used, which may be preferred in some embodiments. By using at least two, preferably at least four, e.g., four or eight, parallel protection branches 20, the inductance can be reduced. In addition, the plurality of parallel branches 20 provides redundancy. Doubling the number of branches 20 generally results in halving the inductance.

[0021] Each branch 20 includes a plurality of GDTs 21 connected in parallel. Using a plurality of parallel GDTs provides redundancy, but also increases the speed of branch activation and cut-off transients (reduced turn-on time), because statistically, one of the plurality of GDTs is more likely to ignite faster than only one GDT. The plurality of parallel GDTs can also reduce the inductance, especially for fast transients. The GDT can be a conventional GDT, typically filled with an inert gas. In the drawing, three parallel GDTs 21 are used, which may be preferred in some embodiments, but more than three parallel GDTs can be used in other embodiments. Generally, all the GDTs 21 in the protection device 7 have the same characteristics, especially the same nominal ignition voltage. By using a GTD in series with other components, a very low parallel capacitance can be achieved, which can allow high-frequency signals to pass through without increasing the so-called C2 capacitance, i.e., the capacitance formed between the field grading layer connected to the test tap 9 and the bushing flange.

[0022] In each branch 20, a TVS diode 22 is connected in series with the GDT 21. For example, the TVS diode 22 can be connected in series between the GDT 21 and the ground connector 24 (as shown) or between the GDT 21 and the test tap 9. The TVS diode 22 has a capacitance that can hold the charging voltage at an earlier transient and then deviate in the conduction voltage of the GDT. This is avoided by a resistor 23 connected across the TVS diode, typically providing a low-ohm parallel resistance on each TVS diode. The resistor 23 can also help by damping oscillations, since its amplitude can be lower than the TVS diode voltage. Generally, the breakdown voltage of the TVS diode 22 is higher than the ignition voltage of the GDT 21, which will ensure that all branches 20 at least ignite one of their plurality of GDTs 21 and connect them all in parallel to obtain a low inductance.

[0023] The protection device 7 is typically transparent to all frequencies until it is transiently activated and thus does not interfere with any measurement signals. Nevertheless, the protection device 7 can act quickly, for example within a few nanoseconds, e.g., in the range of 1 - 10 or 2 - 5 ns. Additionally, the protection device can be configured to handle high transient currents, such as at least 40 kA. The low inductance prevents a higher voltage from being added to the test tap 9 during a fault transient. The protection device 7 can also suppress resonance during voltage clipping by absorbing energy. The protection device can reduce the risk of overvoltage shorting to the flange, which could generate another transient pulse. This means that the protection device can clamp the voltage to a specific level, such as approximately a few hundred volts. Typically, the protection device does not change its impedance (capacitance or resistance) according to temperature. Typically, when the test equipment is connected to the test tap 9, the protection device can reduce the risk of failure of the bushing 2 due to transients in the bushing.

[0024] In some embodiments of the present invention, the TVS diode 22 of each branch 20 has a breakdown voltage higher than the firing voltage of the GDT 21 connected in series with the TVS diode, ensuring that at least one GDT of each branch is ignited.

[0025] In some embodiments of the present invention, the protection device 7 includes four protection branches 20 connected in parallel.

[0026] In some embodiments of the present invention, each branch 20 includes three GDTs 21 connected in parallel.

[0027] The present disclosure has been mainly described with reference to some embodiments. However, as will be readily understood by those skilled in the art, other embodiments are equally possible within the scope of the present disclosure as defined by the appended claims, in addition to the embodiments disclosed above.

Claims

1. A protection device (7), configured to be electrically connected to a test tap (9) of a high voltage HV bushing (2) for protecting the bushing from transient overvoltages, the protection device comprising: At least two protection branches (20) connected in parallel between the test tap (9) and a ground connector (24) for grounding; Each of the at least two protection branches (20) comprises: A plurality of gas discharge tubes GDT (21) connected in parallel; A transient voltage suppression TVS diode (22) connected in series with the gas discharge tube (21); and A resistor (23) connected in series with the gas discharge tube (21) and connected across the TVS diode (22), wherein the TVS diode (22) has a breakdown voltage higher than the ignition voltage of the GDT (21).

2. The protection device according to claim 1, wherein, The at least two protection branches (20) connected in parallel comprise four protection branches connected in parallel.

3. The protection device according to claim 1 or 2, wherein, The plurality of GDTs (21) connected in parallel comprise three GDTs connected in parallel.

4. An HV bushing (2), comprising: A test tap (9); And The protection device according to any one of the preceding claims, wherein the protection device is connected to the test tap.

5. An electrical system (1), comprising: An electrical device (3); And The HV bushing according to claim 4, wherein the HV bushing is arranged to pass through a wall (5) of a housing (4) of the electrical device.

6. The electrical system according to claim 5, further comprising a measuring device (10) connected to the test tap (9) by an electrical conductor (6).

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