Polyol Ester for Transformers

Abstract

A dielectric insulating fluid composition for a transformer including a trimethylol propane ester corresponding to general formula I:

Description

DETAILED DESCRIPTION OF THE INVENTION

It has been found that trimethylol propane esters with saturated fatty acids as acid components satisfy the stated requirements in excellent fashion.

Accordingly, the present invention relates to compositions containing trimethylol propane esters (TMP esters) corresponding to general formula I:

where R¹, R² and R³ may be the same or different and represent a linear or branched alkyl group containing 5 to 11 carbon atoms, the ester having a viscosity of <23 mm²/s at 40° C. and a fire point of >300° C. These esters are eminently suitable as dielectric insulating fluids for transformers.

The low viscosity which, in a preferred embodiment, is 20 mm²/s at 40° C. or lower leads to an improved heat-dissipating effect and, hence, to a longer useful life of the transformers. The low viscosity can be achieved by selected acid components for the esterification. The substituents R¹, R² and R³ in formula (I) consist of linear or branched C_5-11 alkyl groups. It is preferred to use substituents with linear or branched C_7-9 alkyl groups. The substituents have to be saturated to achieve the required oxidation stability. All the substituents in a polyol ester may be the same, only two may be the same or all may be different. Preference is attributed to a distribution of C_7-9 substituents which are formed in the esterification of trimethylol propane with an acid mixture of C_8-10 fatty acids, the fire point having to be above 300° C. and the viscosity reaching the described preferred ranges of <23 mm²/s at 40° C. The larger the number of C atoms, the higher the fire point, but the higher also the viscosity. Since these values run counter to one another, there is an optimum C chain distribution of the substituents R¹, R² and R³ for each pair of values.

This class of trimethylol propane esters comply with IEC 61099 and are classified as non-hazardous to water by the Deutsches Bundesamt für Umwelt (UBA, Berlin). Accordingly, their biodegradability is well above the standard of Water Hazard Class 1 and reaches a level of >60% BOD/COD or CO₂ evolution or >70% DOC removal after 28 days. This corresponds to the limits for ready biodegradability (as determined, for example, by OECD Method 301).

The compositions according to the invention have good thermal properties and excellent dielectric properties. In order further to improve the properties of the insulating fluid, antioxidants and/or metal deactivators may be, and preferably are, used. In another embodiment, the composition according to the invention additionally contains—based on the ester—0.005 to 1.0% by weight of an antioxidant and/or 0.01 to 2.0% by weight of a metal deactivator. The preferred quantity of antioxidant is between 0.01 and 0.5% by weight and, more particularly, is 0.1% by weight, based on the ester. The preferred quantity of metal deactivator is between 0.1 and 1.0% by weight and, more particularly, is 0.1% by weight, based on the ester.

The antioxidant is preferably selected from the group consisting of bis-hydroxytoluene, hydroquinone, 4-tert.butyl catechol, naphthol, phenyl naphthylamines, diphenylamines, phenylic thioethers, tocopherols and mixtures of the substances mentioned.

The metal deactivator is preferably an organic hetero compound selected from the group consisting of triazoles, tolyltriazoles, dimercaptothiadiazolines and mixtures of the substances mentioned.

The present invention also relates to the use of the composition according to the invention containing trimethylol propane esters (TMP esters) corresponding to general formula I:

where R¹, R² and R³ may be the same or different and represent a linear or branched alkyl group containing 5 to 11 carbon atoms, the ester having a viscosity of <23 mm²/s at 40° C. and a fire point of >300° C., as dielectric insulating fluids for transformers.

The transformers in question are power transformers, distribution transformers, mast transformers, load stepping switches or change-over switches.

EXAMPLES

Production Example Ester T1

250 g trimethylol propane are mixed under nitrogen with 850 g fatty acid. After addition of 0.05 g tin(II) oxalate, the mixture is heated with vigorous stirring to 230° C. The reaction begins at 160° C. with intensive elimination of water. After reaching the reaction temperature of 230° C., the reaction is continued to an acid value of 20. Vacuum is then applied and the reaction completed at the same temperature. The reaction mixture is then cooled and the catalayst is neutralized by addition of 50% NaOH solution. The reaction mixture is dried and filtered through Celatom.

In a second step, 0.1% antioxidant and 0.1% metal deactivator are added to ester T1, an oxidation test is carried out to IEC 61099 and the characteristics are determined. The characteristics of a fully additive-charged ester, including the dielectric properties to IEC 61099, are set out by way of Example below:

	Physical properties	Unit	IEC 61099	Ester T1
	Color	HU	<200	122
	Appearance		Clear	Clear
	Density at 20° C.	g/cm3	<1	0.9451
	Kinematic viscosity
	at 40° C.	mm2/s	<35	19.3
	at −20° C.	mm2/s	<3000	468
	Flash point	° C.	>250	273
	Fire point	° C.	>300	313
	Crystal formation		None	None
	Pour point	° C.	<−45° C.	−49

	Chemical Properties	Unit	IEC 61099	Ester T1
	Water content	mg/kg	<200	100
	Neutralization value	mg KOH/g	<0.03	0.06
	Peroxide value			0.2
	Oxidation stability
	Total acid content	mg KOH/g	<0.3	0.01
	Total sludge content	% by wt.	<0.01	—

Electrical properties	Unit	IEG 61099	Ester T1
Breakdown voltage	kV	>45	75
Dielectric loss factor		<0.03	0.025
at 90° C./50 Hz
Direct-current resistance at 90° C.	Gohmm	>2	8.3

Claims

1. A dielectric insulating fluid composition for a transformer, comprising a trimethylol propane ester corresponding to general formula I:

2. The composition according to claim 1, wherein R1, R2, and R3 are the same or different and each represents a linear or branched alkyl group containing 7 to 9 carbon atoms.

3. The composition according to claim 1, wherein the ester has a viscosity of 20 mm2/s at 40° C. or lower.

4. The composition according to claim 1, further comprising, based on the ester, 0.005% to 1.0% by weight of an antioxidant and/or 0.01% to 2.0% by weight of a metal deactivator.

5. The composition according to claim 4, wherein the antioxidant is selected from the group consisting of bis-hydroxytoluene, hydroquinone, 4-tert-butyl catechol, naphthol, phenyl naphthylamines, diphenylamines, phenylic thioethers, tocopherols, and mixtures thereof.

6. The composition according to claim 4, wherein the metal deactivator is an organic hetero compound selected from the group consisting of triazoles, tolyl triazoles, dimercaptothiadiazoles, and mixtures thereof.

7. The composition according to claim 1, incorporated into a transformer as a dielectric insulating fluid.

8. The composition according to claim 7, wherein the transformer is selected from the group consisting of power transformers, distribution transformers, mast transformers, load stepping switches, and change-over switches.