High temperature transformers

Abstract

 A transformer whose windings are insulated with cellulose-based materials and are immersed in a carboxylic ester liquid, the liquid and the load capacity rating of the transformer being such that the normal winding hot-spot temperature is in the range 110°C to 130°C, preferably in the region of 130°C, and thus substatially hotter than 98°C.

Description

[0001] This invention relates to liquid-immersed power distribution transformers, in particular those which use cellulose-based winding insulation materials.

[0002] Liquid-immersed power distribution transformers are employed to change the voltage at which electrical power supplied from a high voltage supply system reaches the user. Typically the supply voltage is 11,000 volts, and the user voltage is 440 volts. The transformer power rating is typically in the range 50 to 10,000 kVA.

[0003] Such 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 will be fitted with radiators or alternatively constructed with corrugations. Provision will be made for expansion of the liquid as it becomes heated.

[0004] The windings will be formed from wire, strip or foil, usually of copper. Wire will usually be enamelled and strip will usually be wrapped with an electrically insulating cellulose-based paper. Layers of similar paper will be placed between layers of foil and also between adjacent turns of some of the windings.

[0005] The insulating and cooling liquid will usually circulate 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 will be a mineral oil. Otherwise an ester or silicone liquid will be employed. Various esters for this application are described in U.K. Patent No. 1,602,092.

[0006] 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 cellulose-based insulation materials used there. As the temperature rises the rate of deterioration of the cellulose increases, more or less doubling in the range 80-140°C for every 6°C rise. The "normal" rate of deterioration (i.e. that which gives a normal life expectancy of several tens of years) is assumed to occur at a temperature of 98°C. In the case of naturally circulating transformers, it is the usual practice to design for a "top liquid temperature rise" of 55°C. This is calculated to give a winding hot spot temperature of 98°C when the ambient temperature is 20°C. Similarly the design for a transformer with pumped circulation would be based on a top liquid temperature rise of 40°C.

[0007] In a few special cases, notably concerning rail transportation, where the need to reduce transformer weight has been the principal consideration, polyamide interturn and conductor insulations have been substituted wholly or in part or those based on cellulose. The use of this type of material in combination with either the ester or silicone liquids mentioned earlier has enabled transformers to be designed with a top liquid temperature rise of 120°C without a reduction in life expectancy. The result has been transformers significantly lighter than cellulose-insulated mineral oil filled designs of similar load-carrying capacity.

[0008] Outside of these special cases, liquid-immersed high temperature power distribution transformers with polyamide insulations have not found a more general application, due to their relatively high cost, to which the polyamide has contributed significantly.

[0009] The use of certain aliphatic esters as insulating liquids in electrical apparatus in general is disclosed in U.K. Patent No. 1,602,092, which is hereby incorporated by reference. Specifically this patent disclosed the esters of trimethylolpropane triheptanoate, trimethylolpropane tricaprylate and pentaerythritol tetrapelargonate and listed their viscosities, boiling points, electrical loss tangents, resistivities, permittivities, electrical breakdown strengths, fire points and autoignition temperatures.

[0010] The object of this invention is to provide liquid-immersed transformers with improved load-carrying capacity manufactured with standard cellulose-based insulation materials. It is a further object that the life expectancy of these transformers should be similar to that of existing types.

[0011] The invention provides, according to a first aspect, a transformer whose windings are insulated with cellulose-based materials and are immersed in a carboxylic ester liquid, the liquid and the load capacity rating of the transformer being such that the normal winding hot-spot temperature is in the range 110°C to 130°C, preferably in the region of 130°C, and thus substantially hotter than 98°C.

[0012] In the case of naturally-circulating liquid the transformer would be designed typically for a top liquid temperature rise of 85°C.

[0013] According to a further aspect, the invention provides a transformer whose windings are insulated with cellulose-based materials and are immersed in a carboxylic ester liquid which includes as a major constituent 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

-CH₂-O-

-R′
or (in the case that the ester is polymeric)

[0014] The liquid may comprise a single such ester or a mixture of such esters.

[0015] Preferably in an ester employed as an insulating and cooling liquid according to the invention each R is

-CH₂-O-

-R′
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. 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.

[0016] The esters suitable for use in accordance with the invention will preferably contain a conventional antioxidant such as a substituted phenol. An example of a typical ester is given below with details of its properties.

EXAMPLE 1

[0017] Pentaerythritol heptanoate/octanoate, produced by reacting pentaerythritol (C(CH₂OH)₄) with a mixture of heptanoic (C₆H₁₃CO₂H) and octanoic (C₇H₁₅CO₂H) acids.

PROPERTIES :-
viscosity at 20°C	95 centistokes
firepoint	310°C
pourpoint	-48°C
permittivity at 20°C	3.2
breakdown strength	50kV
tan δ at 20°C	0.0001
resistivity at 20°C	100 x 10 ¹²ohm-cm

[0018] Cellulose based insulation materials suitable for use in accordance with the invention are generally as specified in IEC 641. They may be in the form of paper or board manufactured from sulphate woodpulp, cotton, jute hemp or various mixtures of these. The properties of a typical paper are given in the following example.

EXAMPLE 2

[0019] Soft porous paper of high purity and high oil-­absorption to IEC 641 class P.4-1, manufactured from 100% woodpulp. Thickness 0.25mm.

PROPERTIES :-
density	1.0 - 1.3 gm/cm³

tensile strength
lengthwise	77 N/mm² min
crosswise	38 N/mm² min

elongation at break
lengthwise	2.0 N/mm² min
crosswise	8.0 N/mm² min
moisture content	8% max
ash content	1% max
conductivity of aqueens extract	8 mS/m max
pH of aqueous extract	6-9
oil absorption	10% min

electric strength
in air	10.5 kV/mm min
in oil	55 kV/mm min

Claims

1. A liquid-immersed power distribution transformer having windings insulated with a cellulose-based material or materials wherein the windings are immersed in a carboxylic ester liquid, the liquid and the load capacity rating of the transformer being such that the normal winding hot-spot temperature is in the range 110° to 130°C.

2. A liquid-immersed power distribution transformer having windings insulated with a cellulose based material or materials, wherein the windings are immersed in a carboxylic ester liquid which includes as a major constituent 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

-CH₂-O-

-R′

3. A transformer according to Claims 1 or 2 wherein the carboxylic ester liquid is polymeric and includes as a major constituent 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 and R is

4. A transformer according to cla8im 2 or 3 wherein the liquid comprises a mixture of said esters.

5. A transformer according to Claim 2 incorporating a carboxylic ester in which each R is of the form

-CH₂-O-

-R′

6. A transformer according to Claim 2 incorporating a carboxylic ester liquid in which one R is ethyl and the other R is -CH₂-O-C-R′.

7. A transformer according to Claim 2 incorporating a carboxylic ester liquid in which R′ is hexyl (C₆), heptyl (C₇) or octyl (C₈).

8. A transformer according to Claim 7 wherein R′ is a straight chain or a branched chain alkyl group.

9. A transformer according to Claim 1 wherein the carboxylic ester liquid includes an ester of pentaerythritol, and heptanoic or octanoic acid.

10. A transformer according to Claim 9 wherein the carboxylic ester liquid includes a tetra-ester of pentaerythritol.

11. A transfer according to Claim 9 wherein the carboxylic ester liquid includes n-heptanoic and/or n-octanoic acid.

12. A transformer according to Claim 1 wherein the carboxylic ester liquid includes at least one ester of trimethylol propane, di-trimethylol propane and di-pentaerythritol.