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(11) | EP 1 764 807 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Liquid immersed electrical transformer |
| (57) The present invention relates to compact transformer comprising one or more solid
materials for insulating at least some of the conductive parts and a liquid for both
insulating at least some of the conductive parts and for heat transfer and cooling,
wherein the solid material comprises a high temperature insulation material, and the
liquid comprises a biodegradable, fire safe, high temperature coolant which can be
used for voltage levels above 36kV. |
BACKGROUND OF THE INVENTION
[0001] High voltage small power transformers are widely used for the transmission of electricity to increase the voltage of generated power or to decrease the transmission voltage down to low voltage levels for household or industrial applications. Typically the transmission voltage for this range of transformers is 10,000 to 20,000 volts, though higher voltages up to 36 kV may be used, providing more efficient transmission, other factors permitting.
[0002] Such transformers are either dry-type, air or gas cooled, or liquid filled. Liquid filled transformers are comprised principally of a core and windings immersed in a cooling and insulating liquid contained within a tank. The tank may be sealed or open to the atmosphere and may be either completely or only partially filled with liquid. To dissipate the heat produced primarily in the windings, but also in the core by the passage of electric current, the tank can be fitted with radiators or alternatively constructed with corrugations. Provision is often made for expansion of the liquid as it becomes heated.
[0003] The windings are usually formed from wire or foil, copper or aluminium. Wire is typically enamelled or wrapped with an electrically insulating cellulose-based paper. Sheets of similar paper are placed between turns of foil and also between adjacent layers of the windings. The insulating and cooling liquid usually circulates round the windings and core by natural convection, but it may sometimes be pumped. In the majority of cases, where the fire-risk is acceptable, the liquid is a mineral oil.
[0004] The load carrying capacity of the transformers described above is limited by the effect of increasing current on the temperature in the windings, which in turn affects the life of the mineral oil impregnated cellulose-based insulation materials used.
[0005] The size available to transformers is often limited, for example, when installed within the tower of a wind turbine, or in city substations. The surface area available for cooling and ultimately the transforming capacity are consequently restricted. The increasing demand for electricity generated from renewable sources has lead to larger wind turbines with higher power outputs, requiring transformers that are still compact and can manage additional heat and higher voltages. Similarly, increasing power consumption in cities necessitates higher capacity distribution transformers that are space efficient. A transformer operating at elevated temperatures requires more regular inspections and replacements. In offshore environments, the economic impact of frequent servicing is considerable owing to the difficulty and expense of access; a long, maintenance-free lifespan is desirable.
[0006] Furthermore, components mounted at sea suffer increased risk of corrosion. Leaks of coolant through corroding casing causes pollution and incurs financial penalties in some jurisdictions.
[0007] There is clearly a need for a transformer that is small, with an increased voltage capacity and heat tolerance and that is environmentally cleaner than conventional transformers.
SUMMARY OF SOME EMBODIMENTS OF THE INVENTION
[0008] One embodiment of the present invention is a compact transformer for operation at temperatures above 100 deg C, comprising one or more solid materials for insulating at least some of the conductive parts and a liquid for both insulating at least some of the conductive parts and for heat transfer and cooling, wherein the solid material comprises a high temperature insulation material, and the liquid comprises a biodegradable, fire safe and high temperature coolant allowing use with voltage levels above 36kV.
[0009] Another embodiment of the present invention is a compact transformer as described above, wherein its thermal profile is optimised to the thermal capabilities of the components and accessories present in the transformer.
[0010] Another embodiment of the present invention is a compact transformer as described above, wherein said liquid is any of biodegradable synthetic, natural esters, agricultural esters.
[0011] Another embodiment of the present invention is a compact transformer as described above, wherein said liquid has suitable insulating properties, biodegradability and chemical stability at temperatures above 100 deg C.
[0012] Another embodiment of the present invention is a compact transformer as described above, wherein said solid material has insulating properties, and chemical stability in liquid at temperatures above 120 deg C.
[0013] Another embodiment of the present invention is a compact transformer as described above, wherein said solid material is aramid.
[0014] Another embodiment of the present invention is a compact transformer as described above, wherein said thermal profile includes full, partial or split segment cooling ducts.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The object of this invention is to provide highly reliable, compact liquid-immersed transformers with improved load-carrying capacity. It is a further object that the life expectancy of these transformers should be improved over that of existing types. It is further object to provide low-environmental impact transformers with low fire hazard. It is further object to surpass the typical medium transmission voltages above 36 kV.
[0016] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art. The articles "a" and "an" are used herein to refer to one or to more than one, i.e. to at least one of the grammatical object of the article. By way of example, "a winding" means one winding or more than one winding.
[0017] Throughout this application, the term "about" is used to indicate that a value includes the standard deviation of error for the device or method being employed to determine the value.
[0018] Throughout this application, the term "transformer" is used to indicate distribution and small power transformers, used as either step-up or step-down, in generator applications in general and wind turbine generators in particular, for public electricity distribution and supply of electric energy to industrial facilities.
[0019] The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1, 2, 3, 4 when referring to, for example, a number of windings, and can also include 1.5, 2, 2.75 and 3.80, when referring to, for example, temperatures).
[0020] The present invention relates to a compact electricity transformer comprising one or more solid materials for insulating at least some of the conductive parts and a liquid for both insulating at least some of the conductive parts and for heat transfer and cooling, wherein the solid material comprises a high temperature insulation material, and the liquid comprises a fire-safe, high temperature and biodegradable coolant.
[0021] In a preferred aspect, the present invention relates to a compact electricity transformer comprising a tank provided with a magnetic core, a plurality of windings, an insulating system made of solid material and a liquid for both insulating at least some of the windings and for heat transfer and cooling.
[0022] Conductive parts of a transformer includes the wire, windings, and internal and external connecting elements which conduct electricity. Windings are well known in the art and refer to the coils of wire disposed around a magnetic core and/or around other windings. The wire present in a winding may be at least partly insulated using a solid material of the present invention. For example, such solid material of the present invention may be used to insulate the wire throughout a winding. Alternatively, such solid material may be disposed at temperature hot spots, and conventional insulation provided elsewhere. Conventional insulation materials include wire enamel insulation such as polyvinyl acetal, , or polyesterimide with polyamide-imide overcoat.
[0023] The solid material of the present invention may be used as a winding-to-winding insulator. Where a transformer comprises a pair of concentrically arranged windings, the inner winding may be insulated from the outer winding by means of one or a plurality of layers of solid material. Alternatively, or in addition radial insulators may be disposed between adjacent windings arranged around a cylindrical core. Alternatively, or in addition screens and barriers of the solid material may be used as winding-to-earth insulator, the earth potential being represented by the core, tank and mechanical construction of the active part (i.e. the core coil assembly). Alternatively, or in addition the solid material may be used to create cooling ducts optimised to the maximal thermal performance, the optimisation including full and partial ducts as well as split cooling duct segments. The person skilled in the art of transformer construction would employ high temperature insulating solid material described herein as appropriate, for example at the temperature hot spots.
Liquid
[0024] The liquid in which the windings and components are immersed has suitable insulating properties, biodegradability and chemical stability at temperatures above 100, 105, 110, 120, 130, 140, 150, 160, 180, 200, 250, 300 or 320 deg C, preferably at temperatures above 100 deg C. Suitable liquids include biodegradable synthetic, natural esters, and agricultural esters. The liquids listed in Table 1 provide examples of preferred liquids.
TABLE 1: Examples of liquids suitable for use in the present invention
| Name | Liquid |
| A | Pentaerythritol fatty acid ester |
| B | Synthetic ester |
| C | Agricultural ester |
[0025] A liquid according to the present invention may be a synthetic ester liquid that includes a carboxylic ester as a major component. A liquid according to the present invention may be a synthetic ester liquid that includes as a major component one or more monomeric or polymeric esters of the general formula :
where R' are the same or different alkyl groups having five to ten carbon atoms, R are independently H or Me or Et or
or (in the case that the ester is polymeric)
[0027] Preferably in a synthetic ester each R is
or one R is ethyl and the other R is -CH2-O-C-R'. R' is preferably hexyl (C6), heptyl (C7) or octyl (C8). R' may be a straight chain or a branched chain alkyl group. Preferred esters for use in accordance with the invention are esters (particularly tetra-esters) of pentaerythritol, i.e. C(CH2OH)4, and heptanoic or octanoic (particularly n-heptanoic and n-octanoic) acid.
[0028] Esters of trimethylol propane (particularly 1,1,1 trimethylol propane), di-trimethylol propane and di-pentaerythritol, i.e. (CH2OH)3 C-CH2-O-CH2-C (CH2OH)3, are also suitable, particularly when the acid is heptanoic or octanoic acid.
[0029] Liquids suitable for use in accordance with the present invention preferably comprise a conventional antioxidant such as a substituted phenol. The liquid should conform with the technical specifications mentioned in IEC TS 60076-14 (Part 14: Design and application of liquid immersed power transformers using high temperature insulation materials).
Solid material
[0030] The solid material used to insulate at least some of the conductive parts in a transformer has suitable insulating properties, and chemical stability in liquid at temperatures typically above 100, 105, 110, 120, 130, 140, 150, 160, 180, 200, 250, 300 or 320 deg C, preferably at temperatures above 120 deg C. Suitable materials include those indicated in Table 2 below. The solid material should conform with the technical specifications mentioned in IEC TS 60076-14.
TABLE 2: Examples of solid materials suitable for use in the present invention
| Name | Solid material |
| A | Aramid |
| B | Polyesterimide |
| C | Polyester and epoxy glass |
| D | Teflon |
[0031] The solid material may be available in the form of paper, film, sheet, board or other shapes for mechanical applications used within the transformer. As already described elsewhere, the solid material may insulate the windings but may not necessarily be present in every winding. It may be limited to temperature hot spots. A layer tertiary winding, for example, may be composed of conventional materials since it may operate at a lower temperature than the other windings. Typically, the conductor insulation and the radial and axial spacers separating the coils windings may comprise high-temperature solid materials. Conventional cellulose may be used in other areas such as cylinders and angle rings that operate at conventional temperatures.
[0032] The combination of the solid materials as indicated in Table 1 above and the liquids as indicated in Table 2 are optimized in respect of prolonged operation at high temperatures, in respect of biodegradability and in respect of safely and economically transforming voltages greater than 36 000 V. The combination is stable at prolonged elevated temperatures, and retains the insulation characteristics at voltages as high as 200 000 V. While a typical mineral oil filled transformer operates at 100 deg C in the oil and 105 deg C in the windings without undue aging, higher temperatures are not tolerated. Mineral oil filled transformers tend to be bulky and therefore often fail to fit into dimensional limitations. Overheating can lead to fire owing to the low-flash point of mineral oil, and subsequent damage to the transformer and the surroundings. Costly fire protection and special installation provisions are required. Silicone fluids and esters, on the other have a higher flash point than mineral oil, typically above 320 deg C. They can also operate above 100 deg C without aging. Esters have two main advantages compared to silicone fluids: ester is biodegradable and is electrically stable at voltages above 36 000 V. A transformer according to the present invention can operate safely at higher temperatures and higher voltages, guaranteeing fire and environmental safety. Accordingly, their size can be reduced without detriment due to the loss of cooling surface area.
Additional features
[0033] A transformer of the present invention may further comprise additional features such as found in conventional transformers. Such features may include, for example, an overload protection system configured to disconnect the transformer from the generating source in the event of excess load. Other features may include, for example, a liquid preservation system that excludes oxygen, a moisture-removing breather, tap changer, overpressure protection, liquid level sensors, thermal protection.
Preferably, the thermal profile of the transformer is optimised to the thermal capabilities of these components and accessories.
Incorporation into structures
[0034] One embodiment of the present invention is an offshore windturbine, comprising a rotor, nacelle and tower, further comprising a transformer as described above, co-operatively connected to the power output of the turbine. Preferably, the transformer is located in the base of the tower or inside the nacelle.
1. A compact transformer for operation at temperatures above 100 deg C, comprising one
or more solid materials for insulating at least some of the conductive parts and a
liquid for both insulating at least some of the conductive parts and for heat transfer
and cooling, wherein the solid material comprises a high temperature insulation material,
and the liquid comprises a biodegradable, fire safe and high temperature coolant allowing
use with voltage levels above 36kV.
2. A compact transformer according to claim 1, wherein its thermal profile optimised
to the thermal capabilities of the components and accessories present in the transformer.
3. A transformer according to claims 1 or 2 wherein said liquid has suitable insulating
properties, biodegradability and chemical stability at temperatures above 100 deg
C.
4. A transformer according to any of claims 1 to 3 wherein said liquid is any of biodegradable
synthetic, natural esters, agricultural esters.
5. A transformer according to claim 4 wherein said liquid comprises a carboxylic ester
liquid.
6. A transformer according to any of claims 1 to 5, wherein said solid material has insulating
properties, and chemical stability in liquid at temperatures above 120 deg C.
7. A transformer according to claim 6 wherein said solid material is aramid.
8. A transformer according to any of claims 2 to 7 wherein said thermal profile includes
full, partial or split segment cooling ducts.