US3420778A - Voltage stabilized liquid dielectrics - Google Patents

Description

United States Patent VOLTAGE STABILIZED LIQUID DIELECTRICS Lawrence J. Heidt, Arlington, Mass., assignor to Simplex Wire and Cable Company, Cambridge, Mass., a corporation of Massachusetts No Drawing. Filed July 21, 1966, Ser. No. 566,766 U.S. Cl. 25263.7 13 Claims Int. Cl. H01b 3/20 ABSTRACT OF THE DISCLOSURE A method of increasing the allowable voltage stress rating, resistance to aging and electric discharge of oils including the naturally occurring or synthetic paraffinic, naphthenic or moderately aromatic hydrocarbon oils is disclosed. This is accomplished by adding to the oil an additive in the amount of from .1 percent up to the limit of solubility of the additive in the oil. Suitable additives include o-nitrotoluene, o-nitroanisole, Z-nitrodiphenylamine, o-nitrobiphenyl, 2,4-dinitrotoluene, diphenylamine, benzoguanamine, triphenyl formazan, anthranilonitrile and mixtures thereof. The liquid dielectric compositions can be used for electrical insulation in combination with plastic or paper films or alone in cables, transformers, potheads, capacitors and otherwise.

The present invention relates to dielectric compositions and in particular to liquid dielectric compositions containing additives which enhance their voltage stability.

Liquid dielectric compositions are frequently used for electrical insulation in combination with plastic or paper films or alone in cables, transformers, potheads, capacitors and otherwise. Principal among these dielectric compositions are oils such as mineral oils of the paraffinic or naphthenic types, and synthetic hydrocarbon oils such as polybutene and polyisobutene oils, but these compositions are subject to certain limitations of dielectric strength and deterioration under electric discharge. Improvement of these properties would enable the use of smaller quantitles of the compositions thereby reducing the overall size of the electrical component in which they are used.

It is the object of this invention to provide an oil additive for increasing the allowable stress rating and the resistance to aging and electric discharge of the more common liquid dielectric materials.

It is a further object of this invention to provide voltage stabilized dielectric compositions which permit reduction in the seize and cost of electrical components in which they are utilized.

The additives of the present invention which enhance the dielectric strength of liquid compositions such as hydrocarbon oils are certain substituted aromatic hydrocarbon compounds characterized in part by having an electron acceptor group and an electron donor group potentially hydrogen bonded together by a reversibly transferable proton. Some of these additive are among those disclosed in Heidt copending application Ser. No. 372,301 filed June 3, 1964, as voltage stabilizing additives for solid olefin polymers such as polyethylene. Dielectric breakdown in solid polyolefin dielectrics was not thought to be related, and indeed may not be related to dielectric breakdown in liquid dielectrics. Thus in a solid polyolefin dielectric breakdown occurs under high voltage stress by the movement of electrons through the dielectric which produces ionization at points where the dielectric is not homogeneous, for example, at the location of voids or solid particles which are inevitably found in solid polyolefin dielectrics because of the need of mechanical shaping, for example, by extrusion. Breakdown occurs when such ionization is suflicient to weaken the dielectric such 'ice that an avalanche of electrons escapes through it at a point of ionization producing a treeing burn out in the dielectric. The additives of the above Heidt application are thought to capture high energy electrons escaping through the solid dielectric and to dissipate the energy of the electron. The initial cause of dielectric breakdown in liquids obviously differs, as the hydrocarbon oils employed for this purpose are highly refined and do not require mechanical working to shape them. Accordingly, it was therefore not anticipated that the substituted aromatic hydrocarbons which function as voltage stabilizers in solid polyolefin dielectric might similarly serve to stabilize liquid hydrocarbon dielectrics.

The preferred voltage stabilizer additives are, of course, those materials which enhance voltage stability up to the very highest applied voltages. Materials which are both effective stabilizers and also readily available in commerce are most desirable. In accordance with the present invention it is postulated that effective additives have in common the following:

(1) An electron acceptor group, especially a strongly unsaturated radical, e.g. one containing a 1r bond, such as -NO CO, CN, phenyl and polycyclic aromatics.

(2) An electron donor group, especially one containing a transferable proton such as amino and lower, i.e. up to eight carbon atoms, alkyl radicals, e.g. -NH and --CH (3) Potential hydrogen bonding between the acceptor and donor group by a transferable proton, such as when the acceptor and donor groups are ortho with respect to one another, e.g. on a benzene ring.

(4) Reversibility of the proton transfer between the acceptor and donor groups, such as in the keto-enol isomerization.

(5) The structure and bonds between the acceptor and donor groups which favor transfer of charge and energy, such as a planar or near planar structure of a conjugated system of alternating single and double bonds, e.g. an aromatic ring structure.

(6) Adequate size and complexity of the conjugated system to provide for electron capture and subsequent energy dissipation without producing irreversible bond rupture.

(7) Adequate dispersibility of the additive in the liquid dielectric to provide a sufficient number of distributed centers for the capture of electrons moving in the dielectric under an electric field.

Generally speaking the relative electron afiinities of the atomic residues are in the order of Cl being greater than 0. O is in turn greater than N which in turn is greater than C. The aceptor groups have electron affinities in the order of NO being greater than CN which is greater than C=O in turn greater than phenyl. The relative electron donor strengths of groups such as -N(CH are greater than NH which is greater than CH in turn greater than phenyl. The Hammett sigma and sigma plus values are a rough measure of the relative electron acceptor and donor properties of the atoms and groups. These sigma values have been determined and tabulated by many people, especially P. R. Wells, Chemical Reviews, pp. 171-219, April 1963; see the tables on page 181 and 189 of this article.

Effective voltage stabilizers in accordance with the present invention include such aromatic hydrocarbons as O- nitrotoluene; O-nitroanisole; Z-nitrodiphenylamine; O- nitrobiphenyl; 2,4-dinitrotolnene diphenylamine, benzoguanamine, triphenyl formazan anthranilo nitrile. The above compositions may be used alone or admixed with each other, or for ease in handling they also may be admixed with hydrocarbon oils preferably aromatic oils as the aromatic structure itself provides a voltage stabilizing effect. The admixture of such oils and additives to form voltage stabilized compounds of solid olefin polymers such as polyethylene is disclosed in copending Hunt application Ser. No. 367,718 filed May 15, 1964.

It is important to the realization of improved voltage stability that the additive of the present invention be used in soluble amounts since crystallization during use may adversely affect the performance of the oils whether used as a cable insulation impregnant, a capacitor insulation impregnant or as a filler for transformers or potheads. Generally, at least 0.1 part by weight of additive per 100 parts by weight of oil is necessary to impart significant voltage stabiilty and, therefore, it is a requirement of the additives of this invention that they be soluble in the oils to which they are added at least to the extent of 0.1%. The amount of additives used may vary between 0.1 part by weight per 100 parts by weight of oil up to the limit of solubility of the additive in the oil and is preferably between .25 part and 4.0 parts by weight of additive per 100 parts by weight of oil. The effect of increasing the amount of additive is most pronounced at additive concentrations between 1.0 part and 2.0 parts by weight per 100 parts by weight of oil. At concentrations above 3.0 parts by weight of additive per 100 parts by weight of oil the effect of increasing the amount of additive decreases noticeably so that only negligible improvement in voltage stability occurs at concentrations above 4.0 or 4.5 parts by weight of additive per 100 parts by weight of oil. The solubilities of the additives vary according to the particular oil used, the more aromatic oils having greater solvent power than the paraffin oils, and this variation should be oils, cable oils such as Sun XX, a polybutene' oil having a low viscosity and a low temperature viscosity coefficient which is useful with paper or plastic tape cables or as a pothead filler and a G & W 235, a medium viscosity polybutene oil suitable for use in high voltage cables especial- 1y cables using plastic films instead of paper tapes. The voltage stabilizing effect is not limited to those oils specifically designed as dielectrics, but has been found to occur in other hydrocarbon oils. Primol 355, a pure paraffinic white oil used for pharmaceutical purposes exhibits enhanced voltage stabiilty when solube amounts of voltage stabiilzers of this invention are incorporated in it. Thus this invention can lead to the use of particular hydrocarbon oils as dielectrics which were not heretofore suitable for the purpose.

To demonstrate the voltage stabilizing characteristics of the additives of this invention, 15 minute stepwise tests were run on samples each having six layers of paper impregnated with the dielectric liquid compound under test. In each instance a test was run on one six layer sample impregnated with the oil only and other samples containing the oil plus at least 1% of an additive according to the present invention. The samples were subjected stepwise to increasingly higher voltage stress using crossed cylindrical electrodes having three layers of paper on each electrode. The electrode assembly was immersed in the compound being tested. Results are reported as the average of 5 or more tests on each in Table 1 below in which the composition of the various compounds tested is set forth in the column identified by the letters A-I, inclusive, in parts by weight of the additives in the same row at the left hand column of the table.

TAB LE I Components, Parts by Weight Base 011 (Transil 10-0) L. J.

Heidt, 19 July 1966 Base Oil (Primol 355) Aromatic Oil (CD101) 2,4-dinitrot oluene Diphenylamine 2-nitrodiphenylamine O-nitrobiphenyl O-nitrotoluene- O-nitroanisole. Dielectric Breakdown, volts per mil per hr.

Compounds Tested D E F G B C H I In those compounds containing (JD-101, the other additives were first mixed with the CD-lOl and then the mixture was added to the base oil. CD101 is a hydrocarbon oil having an aromatic content in excess of a specific gravity of 1.035, a pour point of 20 F., viscosity SUS of at 100 F. and 36 at 210 F.

A second series of 30-minute step-rise tests were run on samples each having six layers of paper impregnated with liquid dielectric compounds. The tests were conducted in accordance with the procedure described above with reference to Table I. The results are similarly reported in Table II as the average of five or more tests on each kind of compound, the composition of the compounds in Table II being identified in the columns headed by the letters K-U, inclusive, and the explanations of Table I are equally applicable.

TABLE II Compounds, Parts by Weight Base Oil (Sun XX) Base Oil (G & W 235) Aromatic Oil (CD-101) 2,4-d1nitrot0luene Diphenylamine O-nitrotoluene Compounds Tested L M N 0 P Q, R S T U Dielectric Breakdown, volts per mil perhr 540 595 665 705 705 591 688 725 715 715 700 In those compounds containing OD101, the other additives were first mixed with the (DD-101 and then the mixture was added to the base 011.

The negligible benefits to be gained from increased additive concentrations may be observed by comparison of Example K with Examples N-O and P-Q-R in Table II. Thus, in Examples P and Q the additionof 1 and 2 parts by weight of diphenylamine resulted in dielectric breakdown values increased by 10 and 27.5% respectively above the base oil valve of 540; whereas in Example R the relatively large addition of 5 parts by weight of diphenylamine resulted in a mere 7% further increase in the dielectric breakdown value.

The following are examples of typical voltage stabilizing oil additives and oil additive combinations used accordng to the present invention:

EXAMPLE 1 Benezoguanamine.-Used alone or in combination with any one of O-nitrotoluene, O-nitrobiphenyl, 2,4-dinitrotoluene (the latter only to the extent that it is soluble) in amounts of .25 part of 4.5 parts by weight of total additives per 100 parts by weight of oil.

EXAMPLE 2 Triphenyl formazan.Used in amounts of .25 part to 0.5 part by weight per 100 parts by weight of oildepending upon the particular oil usedalone or in combination with .25 part to 4.0 or 4.25 parts by weight of any one of O-nitrotoluene, O-nitrobiphenyl, 2,4-dinitrotoluene (the latter only to the extent that it is soluble).

parts by weight per 100 parts by weight of oil, alone or in combination as defined by Examples 1 through 3.

EXAMPLE 5 O-nitrobiphenyl.-Used in amounts of .25 part to 4.5 parts by weight per 100 parts by weight of oil alone or in combination as defined in Examples 1 through 3 above.

EXAMPLE 6 2,4-dinitrotoluene.Used in amounts of .25 part to 2.0 parts by weight per 100 parts by weight of oil-in greater amounts where solublealone or in combination as defined in Examples 1 through 3.

O-nitroanisole.-Used in amounts of .25 part to 2.0 parts by weight per 100 parts by weight of oil due to its limited solubility in most oils.

EXAMPLES 9-19 Examples of typical voltage stabilized transformer oils in accordance with the present invention are shown in Table III below.

TABLE III Example Numbers Components, Parts byWeight 9 10 11 12 13 14 15 16 l7 18 19 Kazan 2iiilc(g{laI(lgi1D1 O1BC1)). ::III:I: 10g 100 100 100 100 100 100 100 100 100 100 2,4-dim'trotoluene Diphenylamine 2-nitrodiphenylamine O-nitrobiphenyl.

O-nitrotoluene. O -nitroanisole Benzoguanimine Triphenyl formazan In those compounds containing CD-101, the other additives were first mixed with the (JD-101 and then the mixture was added to the base oil.

EXAMPLE 3 Dipheny1amine.Used alone or in combination with any one of O-nitrotoluene, O-nitrobiphenyl, 2,4-dinitro- EXAMPLES 20-29 Examples of voltage stabilized capacitor impregnant oils are shown in Table IV below.

O-nitrebiphenyl O-nitrotoluene- O-nitroanisole Benzoguanimine Triphenyl formazan 25 Anthranilo nitrile *In those compounds containing (JD-101, the other additives were first mixed with the (JD-101 and then the mixture was added to the base oil.

toluene (the latter only to the extent that it is soluble) in amount of .25 part to 4.5 parts by weight of total additive per 100 parts by weight of oil.

EXAMPLE 4 Examples 30-40 Examples of typical voltage stabilized oils suitable for use in oil impregnanted cables having paper or plastic film O-nitrotoluene.Used in amounts of .25 part to 4.5 insulation are shown in Table V below.

TABLE V Example Numbers Components, Parts by Weight 31 32 33 34 35 Base Oil (Sun XX) Aromatic Oil (CDl0l)* 2,4-dinitrotoluene Diphenylamine 2-nitrodiphenylamine O-nitrobiphenyl- O-nitrotoluene. O-nitroanis0le Benzoguanimin Triphenyl formazan- Anthranilo nitrile-.

In those compounds containing (JD-101, the other additives were first mixed with the (JD-101 and then the mixture was added to the base 011.

The voltage stabilized oils of Examples 30-40 are useful as impregnants in cables which are insulated with synthetic tapes such as polyphenylene oxide filaments. These tapes, when impregnated with the stabilized oils of this invention are able to carry very high voltages with low power losses.

Examples 41-51 Examples of typical voltage stabilized pothead filling oils are shown in Table VI below.

TABLE VI Example Numbers Components, Parts by Weight Base Oil (Indopol H150) 100 100 100 100 100 Base Oil (Aronite l2E) Aromatic Oil (CD-101). 2, 4-dinitrotoluene Diphenylamine... 2-nitrodiphenylam O-nitrobiphenyl. O-nitrotoluene O-nitro anisole Benzoguanimine Triphenyl formazan... Anthranilo nitrile *In those compounds containing (JD-101, the other additives were first mixed with the OD-lOl and then the mixture was added to the base oil.

The above examples of voltage stabilized oils suitable for use in transformer, pothead capacitor and oil impregnanted cable insulations are intended to be merely typical of the compositions which can be prepared according to the present invention. It will be obvious that variations of the particular oil and additive compositions and the concentrations used can be substantially varied within the scope of this invention. In addition dielectric oils which are stabilized according to the present invention can contain other conventional additives such as antioxidants and the like.

It will be noted that some of the additives consist of or include amines, e.g. diphenylamine. In using these additives in contact with copper components precaution should be taken to avoid dissolution of the copper in the oil by reaction with the amine. The use of, for example, tinned copper is suggested. While the use of such precautionary measures may add to the expense of the additive composition, the presence of amines is frequently helpful since they act as thermal stabilizers for the oils.

I claim:

1. A method of increasing the allowable voltage stress rating, resistance to aging and electric discharge of an oil selected from the group consisting of naturally occurring and synthetic paraffinic, naphthenic and moderately aromatic hydrocarbon oils and mixtures thereof comprising adding to said oil an amount between .1 percent up to the limit of solubility of an additive selected from the group consisting of o-nitrotoluene, o-nitroanisole, 2-nitrodiphenylamine, o-nitrobiphenyl, 2,4-dinitro- 5. The method of claim 1 wherein the additive includes o-nitrotoluene.

6. The method of claim 1 wherein the additive includes o-nitroanisole.

7. The method of claim 1 wherein the additive includes 2-nitrodiphenylamine.

8. The method of claim 1 wherein the additive includes o-nitrobiphenyl.

9. The method of claim 1 wherein the additive includes 2,4-dinitrotoluene.

10. Te method of claim 1 wherein the additive includes diphenylamine.

11. The method of claim 1 wherein the additive includes benzoguanamine.

12. The method of claim 1 wherein the additive includes triphenyl formazan.

13. The method of claim 1 wherein the additive includes anthranilonitrile.

References Cited Chemical Abstracts, vol. 46, col. 10744b, Fifth Decennial Index, pp. 4250 and 1050.

LEON D. ROSDOL, Primary Examiner.

J. D. WELSH, Assistant Examiner.

US. Cl. X.R.