Potential transformers

Abstract

This disclosure relates to SF 6 The present invention relates to a potential transformer of a type designed to use an insulating medium containing 3 to 5 mole percent heptafluoroisobutyronitrile, 4 to 11 mole percent oxygen (O 2 ), and 84-93 mole % nitrogen (N 2 ) in a gaseous mixture.

Description

[Technical Field]

[0001] The invention relates to a potential transformer according to the preamble of claim 1. [Background technology]

[0002] Potential transformers are well known in the art. Specifically, high-voltage potential transformers are designed to convert high current and voltage levels into low current and voltage outputs in known and precise proportions specified by the end user. Apart from high-voltage potential transformers, potential transformers also include current transformers and substation transformers.

[0003] Sulfur hexafluoride (SF6) is traditionally used to insulate the conductive components contained in instrument transformers.

[0004] SF6 is a well-established insulating gas due to its excellent dielectric properties and its chemical inertness. Due to the excellent properties of SF6 in terms of dielectric strength, existing voltage transformers have relatively compact dimensions. Especially compact designs are feasible when SF6 is used at high pressures above 6 bar. This is due to the very high dielectric performance achievable by SF6 at high gas densities, which ultimately allows for very small clearances within the voltage transformer.

[0005] Despite these properties, efforts are intensifying to find alternative insulating gases, particularly considering alternatives with a lower global warming potential (GWP) than SF6.

[0006] In an effort to provide a non-SF6 alternative, the use of organic fluorine compounds in dielectric insulating media has been proposed. Specifically, International Patent Application Publication No. 2010 / 142346 proposes a dielectric insulating medium containing fluoroketones containing 4 to 12 carbon atoms.

[0007] Fluoroketones have been shown to have high dielectric strength. At the same time, they have very low GWP and very low toxicity. This combination of features makes fluoroketones a viable alternative to SF6.

[0008] A further development in this regard is reflected in WO 2012 / 080246, which suggests a dielectric insulating gas comprising a fluoroketone containing exactly five carbon atoms, in particular 1,1,1,3,4,4,4-heptafluoro-3-(trifluoromethyl)-butan-2-one, in a mixture with a carrier gas, which together with the fluoroketone provides a non-linear increase in the dielectric strength of the insulating medium beyond the sum of the dielectric strengths of the gas components of the insulating medium.

[0009] U.S. Patent Application Publication No. 2018 / 197656A1 relates to a medium or high voltage gas-insulated switchgear having an arc control mechanism for extinguishing an electric arc formed between two contacts of the switchgear during movement, the arc control mechanism being of the rotating arc type. The switchgear housing is filled with a dielectric gas containing at least fluoronitrile in a volume fraction ranging from 0% to 20%.

[0010] Despite their favorable environmental characteristics, the dielectric strengths of the alternative insulating media described above are lower than that of SF6 at operating conditions. In the case of at least some of the proposed "non-SF6" alternatives, this is due to their relatively high boiling points. Therefore, these alternative insulating media cannot reach the same performance levels as SF6 at the high pressure levels described above (i.e., above 6 bar).

[0011] In theory, improved performance levels can be achieved by increasing the pressure of alternative insulating media, but at higher pressures, this significantly increases the minimum operating temperature of the transformer, as undesirable condensation of the insulating medium or its components can occur at higher temperatures. Conversely, when using alternative insulating media, reaching the same minimum operating temperature as is achievable when using SF6 results in a significant reduction in dielectric performance.

[0012] In addition, increasing the insulating medium pressure can lead to subsequent leakage of the insulating medium through traditionally used gas sealing materials such as EPDM. This is particularly the case for insulating media containing carbon dioxide, which is generally recognized as a suitable carrier gas due to its relatively high dielectric strength and, in particular, its good arc-quenching properties. For such insulating media containing carbon dioxide, dedicated gas sealants (or other sophisticated measures) must be applied to avoid leakage; otherwise, safe operation of the potential transformer cannot be guaranteed.

[0013] In particular, this problem can occur when using a gas mixture containing heptafluoroisobutyronitrile and carbon dioxide, as proposed in WO 2015 / 040069, which has been shown to permeate sealing parts made of EPDM.

[0014] Given the increasing need to minimize the use of SF6, it is desirable to replace existing SF6-tuned instrument transformers with those that have improved environmental friendliness, particularly in view of reduced GWP. However, for the reasons stated above, it is currently not possible to replace SF6 without compromising the safe operation of the transformer and / or requiring substantial changes to the overall design of the transformer. Summary of the Invention [Problem to be solved by the invention]

[0015] The problem to be solved by the present invention is therefore to provide a potential transformer of a type designed to use an insulating medium containing SF6 that meets the requirements of improved environmental friendliness, in particular reduced GWP, but without compromising the safety of the transformer. In other words, the combined effect of improved environmental friendliness and similar insulating performance should be achieved in the case of an "SF6-fit" potential transformer without requiring significant changes to its design. [Means for solving the problem]

[0016] The problem is solved by a potential transformer according to claim 1. Preferred embodiments of the invention are defined in the dependent claims.

[0017] According to claim 1, the potential transformer of the present invention is a transformer of the type designed to use an insulating medium containing SF6.

[0018] Specifically, the potential transformer is a type of transformer designed to use SF6 at pressures above 5 bar absolute, preferably above 6 bar absolute. As mentioned above, this particular type of potential transformer has a very compact design due to the small clearances that can be achieved.

[0019] More specifically, the potential transformer is one of a high voltage potential transformer, a current transformer, and a substation transformer, the latter having a rated output greater than 200 VA per phase, preferably between 200 VA and 333 kVA per phase.

[0020] Most preferably, the potential transformer of the present invention is a high-voltage potential transformer. In the context of the present invention, "high voltage" refers to a voltage level range above 52 kV (as distinguished from "medium voltage", which refers to a voltage level range from 1 kV to 52 kV).

[0021] If the transformer is of a type designed for use with an insulating medium containing SF6, this means that the design and dimensions of the transformer are adapted to the dielectric properties of SF6, and its components and their arrangement (including clearance distances between components) are tailored for use with SF6. This type of transformer is well known to those skilled in the art. They are gas-tight to the SF6 present in the transformer's insulating space.

[0022] In the types of potential transformers described above, the insulating space containing SF6 is typically sealed using EPDM rubber (ethylene propylene diene monomer rubber). This type of potential transformer also has the ability to efficiently dissipate heat from electrical components. The SF6-containing medium used in these potential transformers can be pure SF6, but also includes media containing impurities other than SF6. Alternatively, the SF6-containing medium can refer to a mixture containing SF6, for example, in combination with a carrier gas or additional dielectric compounds. In the specific case of potential transformers designed to use an insulating medium containing or consisting of SF6 at a pressure of at least 5 bar, this is reflected by the device's very compact design, as described above and known to those skilled in the art.

[0023] The potential transformer comprises a housing enclosing an insulating space and further comprises an electrically active part disposed within the insulating space, said insulating space containing a dielectric insulating medium.

[0024] According to the present invention, the dielectric insulating medium of the present invention contains a gaseous mixture comprising 3-5 mol % heptafluoroisobutyronitrile, 4-11 mol % oxygen (O2), and 84-93 mol % nitrogen (N2).

[0025] Surprisingly, it has been found that by using the mixture defined in claim 1, improved environmental friendliness can be achieved without compromising the safety and performance of the potential transformer. In particular, it has been found that no significant changes are necessary to the overall design of the potential transformer and to the selection of components and materials used. This also applies to existing potential transformers designed to use SF6 at a pressure of 6 bar absolute at 20°C; in the case of these potential transformers too, the mixture of the invention allows the same insulating properties to be achieved as when using SF6 at the mentioned pressure levels.

[0026] In particular, the present invention allows for a lower GWP potential transformer to be achieved compared to a transformer of the same construction but using an SF6 containing medium.

[0027] In the context of the present invention, it has been found that by using the insulating medium defined in claim 1, a higher partial pressure of heptafluoroisobutyronitrile can be achieved than the partial pressure of this compound that can be achieved in a gas mixture containing carbon dioxide at the same total pressure to reach the same dew point or minimum operating temperature. Without wishing to be bound by theory, the use of nitrogen in a fluoronitrile-containing gas mixture allows for the Poynting effect to be achieved, which partially compensates for the increase in dew point caused by the use of heptafluoroisobutyronitrile, which has a relatively high boiling point. Combined with the relatively high dielectric strength inherent to nitrogen, the insulating medium according to the present invention allows for dielectric insulation performance similar to that of SF6.

[0028] In addition, heptafluoroisobutyronitrile has been found to exhibit high compatibility with other materials contained in the device. In particular, the insulating medium according to the present invention has been found to have a relatively low permeability with respect to sealing components typically used in electrical devices using SF6. Therefore, the dielectric insulating properties present in the insulating space can be maintained over time, which also contributes to the high safety of the device re-established by the present invention.

[0029] Finally, as mentioned above, replacement of SF6-based potential transformers by those with improved environmental friendliness can be achieved without requiring changes to the overall design of the transformer and the selection of components and materials used. The concept of the present invention is therefore clearly distinguished from the concept disclosed in EP-A-3118955, whereby the design of the equipment is modified to allow for a future switchover from SF6 to a more environmentally efficient insulating gas.

[0030] The technical effect achieved by the present invention is particularly pronounced when the amount of heptafluoroisobutyronitrile in the gaseous mixture is between 3.5 and 4.5 mol %, preferably about 4 mol %.

[0031] According to a particularly preferred embodiment, the amount of oxygen (O2) in the gaseous mixture is between 4 and 6 mol %, preferably about 5 mol %.

[0032] According to a further preferred embodiment, the amount of nitrogen (N2) in the gaseous mixture is between 89.5 and 92.5 mol %, preferably about 91 mol %.

[0033] All percentages refer to the total molar content of the gas mixture. In practice, the preparation of gaseous mixtures is always subject to tolerances. Whenever a range for a gaseous mixture is given, the range also encompasses the tolerances. When no range is given, the value refers to the nominal value.

[0034] As mentioned above, the present invention allows potential transformers to operate at low temperatures without encountering the problem of condensation of the medium. Specifically, the rated minimum operating temperature of potential transformers is -5°C or lower, which generally applies to indoor applications of potential transformers. In the alternative case of outdoor applications, the rated minimum operating temperature of potential transformers is preferably -25°C or lower, more preferably -30°C or lower. Most preferably, the rated minimum operating temperature of outdoor applications of potential transformers is -30°C, but it can also be -40°C, -50°C, or -60°C.

[0035] Particularly high insulating performances can be achieved in the case of potential transformers in which the dielectric insulating medium is present in the insulating space at a pressure in the range from 3 bar absolute to 12 bar absolute, preferably from 3 bar absolute to 11 bar absolute, more preferably from 8 bar absolute to 11 bar absolute, the pressures referring to a reference temperature of 20° C. Surprisingly, it has been found that even in these high pressure ranges no condensation of the insulating medium occurs, or occurs only to a negligible extent.

[0036] As mentioned above, the dielectric insulating medium of the present invention is also advantageous in view of its high compatibility with other materials contained in the potential transformer in which it is used, particularly seals, solid insulators, etc. In particular, the dielectric insulating medium is compatible with seals selected from the group consisting of EPDM rubber and nitrile rubber, as well as seals consisting of butyl rubber, which are commonly used in electrical equipment designed to use SF6. Even with these seals, the insulating medium of the present invention has been found to have a relatively low permeability. Specifically, a leakage rate of only 0.1% / y has been measured for EPDM sealants.

[0037] The sealing parts that seal the insulating space are usually in the form of O-rings. The sealing material used for the sealing parts is preferably EPDM rubber (ethylene propylene diene monomer rubber), but may alternatively be nitrile rubber and butyl rubber, including unmodified butyl rubber and modified butyl rubber, in particular chlorobutyl rubber (CIIR) or bromobutyl rubber (BIIR).

[0038] This is particularly true when the dielectric insulating medium contains only small amounts of carbon dioxide or is free of carbon dioxide. Thus, according to a preferred embodiment, the dielectric insulating medium contains less than 5 mol % carbon dioxide, preferably less than 2 mol % carbon dioxide, and most preferably is at least essentially free of carbon dioxide.

[0039] Considering the high material compatibility achievable by using the dielectric insulating medium of the present invention, it is preferred that the insulating space be sealed by a sealing component comprising a sealing material selected from the group consisting of EPDM rubber, nitrile rubber, and butyl rubber.

[0040] Also, as noted above, in combination with nitrogen, fluoronitrile-containing insulating media have been found to have dew points lower than that of fluoronitrile itself. More specifically, dew point measurements of an alternative insulating medium containing heptafluoroisobutyronitrile mixed with a carrier gas containing nitrogen and oxygen have revealed a dew point of −36° C., which is lower than the dew point of isolated heptafluoroisobutyronitrile at the same partial pressure used in the mixture (which is about −29° C.) and substantially lower than the dew point of a ternary mixture similar to that defined above but using carbon dioxide instead of nitrogen (which is about −27° C.).

[0041] Due to the low dew point of the dielectric insulating medium, charging of the potential transformer can be carried out on-site by introducing the gas mixture directly from the respective storage and transport device. Therefore, on-site commissioning is very simple and does not require sophisticated mixing procedures to prepare a composition containing the correct amounts of components.

[0042] The present invention is further illustrated by the following examples in conjunction with the drawings. [Brief explanation of the drawings]

[0043] [Figure 1] 1 is a side view of a potential transformer according to the invention in the form of a current transformer; FIG. [Figure 2] 2 shows the potential transformer of FIG. 1 in longitudinal section through a first section plane; [Figure 3] 2 shows the potential transformer of FIG. 1 in longitudinal section through a second section plane; DETAILED DESCRIPTION OF THE INVENTION

[0044] Example Potential transformers are provided that are designed to use SF6 as the dielectric insulating medium.

[0045] An alternative insulating medium containing 91 mole percent nitrogen, 4 mole percent heptafluoroisobutyronitrile, and 5 mole percent oxygen is then filled into the insulating space by supply pipes connected to respective filling valves in a housing surrounding the insulating space.

[0046] The potential transformers filled in this way could pass the dielectric tests by IEC for reduced impulse withstand voltage, switching impulse withstand voltage, power frequency withstand voltage, and partial discharge measurements.

[0047] The dew point of the mixture was determined by slowly and continuously cooling the fluids and monitoring their respective pressures, the drop in pressure indicating the point at which condensation begins. The dew point of the mixture was determined to be −36°C, a lower dew point than that of isolated heptafluoroisobutyronitrile at the same partial pressure, which was −29°C.

[0048] The alternative gas mixture has further been found to be compatible with most materials used in potential transformers designed to use SF6 as the dielectric insulating medium, therefore no design modifications or significant material changes are necessary.

[0049] Regarding gas tightness, EPDM O-rings used as standard SF6 sealing components in the equipment demonstrated acceptable permeability to the alternative gas mixtures used. Specifically, the permeability of nitrogen through the EPDM O-rings was found to be reduced by a factor of seven compared to the permeability of carbon dioxide.

[0050] In the particular embodiment shown in Figure 1, the isolation transformer (1) is provided with a filling valve (2) at its bottom, through which an insulating medium is introduced into an insulating space (4) surrounded by a housing (6) so as to surround the electrically active parts arranged in the insulating space, in particular the coil (8) and a part of the main conductor (10) arranged in the insulating space, as shown in Figures 2 and 3. The insulating space is sealed by several sealing parts, the main sealing part (12) of which is shown in Figure 3.

Claims

1. science fiction ６ 1. A potential transformer of a type designed to use an insulating medium containing 3 to 5 mole percent heptafluoroisobutyronitrile, 4 to 11 mole percent oxygen (O ２ ), and 84-93 mole % nitrogen (N ２ ) .... A potential transformer, wherein the potential transformer is of the type described above designed for use with SF 6 at pressures above 5 bar absolute.

2. science fiction ６ 1. A potential transformer of a type designed to use an insulating medium containing 3 to 5 mole percent heptafluoroisobutyronitrile, 4 to 11 mole percent oxygen (O ２ ), and 84-93 mole % nitrogen (N ２ ) ....

1. A potential transformer, wherein the dielectric insulating medium contains less than 5 mole percent carbon dioxide.

3. The oxygen (O ２ 3. The potential transformer according to claim 1, wherein the amount of said cation exchange resin is 4 to 6 mol %.

4. 3. The potential transformer according to claim 1, wherein the amount of heptafluoroisobutyronitrile in the gaseous mixture is 3.5 to 4.5 mol %.

5. The nitrogen (N ２ 3. The potential transformer according to claim 1, wherein the amount of α- and β-membered carboxylic acid is 89.5 to 92.5 mol %.

6. 3. The potential transformer according to claim 1, wherein the rated minimum operating temperature is −5° C. or lower and −60° C. or higher.

7. 3. The potential transformer according to claim 1, wherein the rated minimum operating temperature is −30° C.

8. 3. A potential transformer according to claim 1, wherein the dielectric insulating medium is present in the insulating space at a pressure in the range from 3 bar absolute to 12 bar absolute.

9. 3. The potential transformer according to claim 1, wherein the insulating space is sealed by a sealing part containing a sealing material selected from the group consisting of EPDM rubber, nitrile rubber, and butyl rubber.

10. 3. The potential transformer of claim 1, wherein the potential transformer is one of a high voltage potential transformer, a current transformer, and a substation transformer.

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