Electrical Insulation A Field for Chemical FR W K
M.
CLARK,
Exploitation
Technical Consultant on Insulation, General Electric Co., l^ittsiield, Mass.
traditionally considered t n e p r o b l e m ol Ihe electrical e n g i n e e r a n d t h e p h y s i c i s t , t h e field of e l e c t r i c a l i n s u l a t i o n o f f e r s c o m plex a n d varied o p p o r t u n i t i e s t o t h e c h e m i s t * . . P r o b l e m s to b e s o l v e d b y t h e c h e m i s t i n c l u d e t h e s y n t h e s i s of i n s u l a t i n g c o m pounds molccularly designed to meet the required need HiiJECTRiCAL· insulation has been traditionally considered as the problem of the electrical engineer and the physicist. The continuing demand for insulations capable of operation at higher voltages and at higher temperatures, however, has brought the realization that progress can be assured only by a careful chemical analysis of the problems presented and the synthesis of insulating compounds molecularly designed to fill the required need. The chemical opportunities presented are complex and varied. Depending upon the type of apparatus, the insulation required may be solid or liquid. As a solid it must in some applications be hard and mechanically strong; in others, soft and pliable. As a liquid, certain types of electrical apparatus demand a low dielectric constant and a low viscosity ; in others, the demand requires a highly viscous liquid having a high dielectric constant. Through all the applications, however, runs the demand for a high degree of chemical stability even in the possible presence of air and moisture and under extremes of temperature which may be as low as —50° C. and as high as 100° C. or higher. The insulation must be inert to the possible contamination and catalytic effect of materials of construction which include the commonly used metals such as iron, copper, lead, and aluminum, and their alloys as well as a variety of varnishes, resins, gums, and fillers. The problem is not simple but because of its complexity does offer a fertile field for chemical ingenuity and exploitation.
development of this type. Their remarkable thermal stability and chemical inertness promise a continued extension of their usefulness. But in practically all highvoltage electrical equipment and in many types of low-voltage apparatus, the principal insulations are paper and mineral oil, materials whose use can be traced back to the early days of electrical manufacture. The industrial user of such products is inevitably confronted with the problems of chemical stability which arc made more difficult by the lack of suitable laboratory tests for gaging their probable behavior in commercial use. An example is the need for an accredited laboratory test for evaluating the sludgeforming tendency of mineral oil in transformer use. Industrial operation of electrical machines most frequently is limited by the temperature and voltage stability requirements of its component insulation. Dielectric instability during the normal commercial use of properly designed and properly manufactured electrical apparatus is a manifestation of chemical change. Chemically stable materials are characterized by an equal stability in dielectric properties in the absence of conducting contaminants.
Adoption- of Plastics The efforts of the chemist have been most widely concerned in the past with the development of improved insulating varnishes and resins. Toward this objective, considerable progress has been made. Most of the older insulating varnishes have been replaced by their synthetic counterparts. The use of naturally occurring resins and gums is giving way to the adoption of Bakélite, glyptal, and other synthetic plastics. The silicone resins and varnishes represent the most recent 2976
CHEMICAL
In electric transformers, capacitors, a.nd cables, although trie chemical and dielectric properties o f insulating papers have been improved a s the result of research and development, no substantial progress has been made i n the synthesis of a sheet material which, can foe substituted for paper in high-volt*age apparatus in order to remove the, tempérât un; limitations now considered necessary for successful operation. At present such equipment is limited to operation a t temperatures lower than about 85° Ο Synthetic chemistry ha*, however, produced a series of dielectric liquids which have largely replaced the mineral oil formerly used as an imprégnant in capacitors. As a dielectric and cooling medium in transformers, these synthetic liquids are finding increasing favor in place of initierai oil in installations where it. is desired to avoid the danger of fire and explosion. Such applications include a, variety o f indoor transformer installations, railway locomotives, and mining equipment. These synthetic and nonflammable liquids have been designated as Askarels. Their development and commercial application are milestones in insulation progress. He moved Danger
of Fire
The terril. Askarel has been applied to that class o f insulating liquid which is not only non flammable in itself but which when decomposed by the electric arc evolves onlv nonflammable and nonexplosive gases. The use of Askarels in transformers h a s removed the danger of fire and chemical explosion inevitably associated with t b e use of mineral oil. In addition, as a result, of their resistance to oxidation, the use of Askarels has eliminated the problem of sludge formation. In capacitors, their higher dielectric constant has enabled (he electrical engineer to produce a capacitor which i s smaller by about 40% than a mineral oil-treated capacitor of the same electrical capacitance. When mineral oil is subjected to the electric arc it is decomposed to its constituent atoms, carbon and hydrogen. The basic chemical problem in the development of trie Askarels was the elimination of the hydrogen, the presence of which, together with the products of its reaction with carbon to give gaseous hydrocarbons, was responsible for the explosive hazard presented b y the use of mineral oil. The dielectric a.nd stability requirements imAND
ENGINEERING
NEWS
posed on the liquid for use in transform ers and capacitors eliminated most of the substituants which were chemically pos sible in the basic hydrocarbon molecule in order to remove the arc-formed hydrogen. The halogens, however, presented many possibilities. Chlorinated hydrocarbons could be synthesized with excellent dielec tric properties. Sufficiently chlorinated, they were nonflammable. Given a chlo rine content which was chemically equiva lent to the hydroge^n present in the mole cule, hydrogen chloride was substantially the sole gaseous product obtained when the chlorinated liquid was decomposed by the electric arc even in the presence of air. The dielectric constant obtained was de termined by the type of molecule and the degree of its chlorination. Tt was possible to synthesize chlorinated compounds, however, which exhibited values of dielec tric constant as high as 5, values twice that of mineral oil. Such liquids presented many advantages for use in capacitors and transformers. Problem One of Synthesis With the establishment of the basic principles by means of which a nonsludging, nonflammable, and explosionproof di electric liquid could be obtained, the chem ical problem was one of synthesis. The chlorination of paraffinic hydrocarbons yielded compositions possessing many of the required dielectric and physical prop erties for successful application. Their chemical instability, however, eliminated this type of molecule from serious consid eration. Operation at temperatures up to 100° C. under the electric stresses of nor mal transformer and capacitor use resulted in the formation of free chlorides, with attendant chemical corrosion and dielectric deterioration. The aromatic hydrocar bons, howTever, yielded chlorinated prod ucts possessing excellent chemical and di electric stability under the most difficult conditions envisaged for commercial prac tice. Their development and application represent the chemical solution of a dif ficult insulation problem and indicate the mutually profitable results which can be expected to accrue from the cooperative effort of the chemist and the electrical en gineer. There are many chemical possibilities in the formulation of dielectric liquids of the Askarel class. The essential chemical requirement is the demand that there shall be at least a chemical equivalency of chlorine and hydrogen in the molecule in order to eliminate the fire and explosive characteristic of the liquid and the gases evolved when the liquid is decomposed by the electric arc. Among the compounds which have been found of practical value either alone or in blend with each other are trichlorobenzene, p e η t a c h l o r o d i p h e n y l , pentachlorodiphenyl oxide, pentachlorophenylbenzoate, hexachlorodiphenylmethane, pentachlorodiphenyl ketone, and pentachloroethylbenzene. The Askarels V O L U M E
2 5,
NO.
4 1 » .
Fig. 1. Reduction in size of capacitors resulting frown u.se of pentachlorodi phenyl in place of mineral oil as a di electric in most common use consist of the chlori nated benzenes and the chlorinated diphenyls. Trichlorobenzene, the simplest and first Askarel liquid developed for electrical use possesses an objeetionally high crystalliz ing temperature (10° C ) . This occurs in the normal range of the operating tempera ture of most electrical machines. The crystalline nature of solid trichlorobenzene is conducive to interstitial ionization at low voltage with resultant cumulatively destructive effects and the ultimate dielec tric failure of the total insulation. Pentaehlorinated diphenyl, however, can be produced in a normally liquid condition. Although it, too, possesses a solidification value within the i.ormal temperature' range of operation, the resinous type of solid which is formed at temperatures be low -f-10° C. has excellent electrical prop erties and has been found to be of profits able application in electrical apparatus where liquidity at low temperatures is not a fundamental requirement. Such an ap plication is the use of pentachlorodiphenyl rn the electric capacitor. Pentachlorodiphenyl is chemically sta ble, nonoxidizing, nonflammable, and evolves hydrogen chloride as substantially the only gaseous product when decom posed by the electric arc. But, what is more important t o the capacitor designer and user, it possesses a dielectric constant
of 5. This is more than twice as great as that of mineral oil. An electric capacitor is used commercially to promote the more efficient transmission and utilization of electric power. It con sists essentially of thin aluminum foil electrodes separated from each other b y a multiplicity of thin paper sheets, the num ber and thickness depending upon the voltage for which the capacitor is to be rated. The dielectric and electrodes are wound into rolls and assembled in packs of various types and sizes to meet the speci fied electrical requirements. Less Sgyace Required The electrical capacitor supplies cdeetrical capacitance or microfarad^. Since the electrical capacitance cf a capacitor is a function of the dielectric constant of its constituent insulation, any change in the molecular configuration which affects the dielectric constant of one or both of the components of the insulation, liquid or solid, will be reflected in the physical size of the completed capacitor. Tin; re placement of mineral oil (dielectric con stant 2.2) by pentachlorodiphenyl (di electric constant 5) not only provided the capacitor engineer with a stable, reproduc ible, and nonflammable liquid but resulted in a marked reduction in capacitor .«ize. As illustrated in Fig. 1, the capacitor im pregnated with pentachlorodiphenyl has only about 6 0 % of the physical bulk of an oil-treated capacitor with the same total dielectric thickness and equal electrical eapacitance. The reduction in size al lows the use of capacitors in circuits from which they had previously been barred because of space limitations. But what, is more important to the chemist and to the electrical industry, the adoption of pen ta chlorodi phenyl in capacitor manufacture represented for the first time the replace ment on a large scale of a cheap, naturally occurring liquidlike mineral oil, by a more expensive product of synthetic chemistry. Its use constitutes a signpost toward future progress.
Fig. 2. Ttvo thousantl-kv-a. load-center unit substation for secondary selective service, one of 18 units with their transformers installed on the factory floor. This practice is permissible tvhen an Asharel liquid is used
OCTOBER
13,
1947
2977
Fig. 3. Corrosion by hydrogen chloride formed when is arced is eliminated by ilissolving tin tetraphenyl B u t pentachlorodiphenyl, despite its excellence as a capacitor imprégnant, was unsatisfactory as a dielectric and cooling liquid in transformers. A transformer in operation represents a carefully factored engineering balance between heat generation and h e a t dissipation. Sludge formation a n d deposition in mineral oil m a y upset this thermal balance. In like manner, the solidification of pentachlorodiphenyl a t temperatures below 10° C. and its high viscosity a t the higher t e m p e r a t u r e s of Qormal operation a r e objectionable. These objections are eliminated by the use of liquid blends. A solution containing equal parts of pentachlorodiphenyl and trichlorobenzene fully meets the physical, chemical, and dielectric requirements of transformer use. T h e properties of a typical transformer Askarel a r e : Burn point Arc-formed gases Pour point Specific gravity (15.5/15.5° C.) Color (ΑΡΗ) Viscosity (37.8° C.) Dielectric strength (25° C.) Oxidation characteristics Dielectric constant (25° C.)
' Does not burn Noncombustible - S 5 ° C. 1.54 100 56 seconds Sayboht Universal 30 kv. ' Does not oxidize nor sludge 4.8
T h e use of Askarels in transformers has resulted in t h e elimination or reduction of m a n y of t h e restrictions heretofore applied to transformer operation. T h e replace ment of mineral oil with t h e consequent elimination of t h e possibility of fire a n d chemical explosion p e r m i t s t h e installation of t h e transformer on t h e factory floor or other easily accessible location, t h u s re moving t h e necessity for transformer vaults or other expensive installation. These a d v a n t a g e s of t h e Askarels have been recognized b y a modification of t h e National Electric Codes which govern transformer installation a n d operation. A typical industrial Askarel transformer installation is illustrated i n Fig. 2. Subsequent
2978
liquid Askarel
plication, t h e immediate objectives formu lated by the insulation engineer were fully a t t a i n e d . B u t as occurs often in industrial manufacture, the solution of one problem begets another. I n this instance, the modi fication of t h e basic molecule to produce the nonexplosive hydrogen chloride in stead of explosive hydrogen when the transformer liquid was arced gave birth t o problems associated with t h e presence of hydrogen chloride. Transformers do not often fail in com mercial use. T h e y last for y e a r s with only a m i n i m u m of attention. B u t no t y p e or manufacture is perfect a n d occasionally through one fault or another, due t o de fects in manufacture o r in operation, fail ures will occur. With t h e use of an Askarel liquid, such failures n o longer present the danger of fire a n d chemical explosion, and the hydrogen chloride formed can be vented to t h e outside or eliminated b y
(Quotoons REQ. U. S. RATENT OFFICE) Judging from the number of per sons who sit and look at it, work must be the most fascinating thing in the world.
Challenge Electrical Research is expensive, but nobody has invented a cheaper way to stay in business.
In the course of my travels, I have found that the world is filled with people who are willing to give you directions; the only trouble is that most of them don't know how to get there either.
Problem
With t h e development of suitable As karels for capacitor and transformer ap
an Mskarel in the
chemical absorbers. In passing from the point of dielectric failure (arcing) to the outside, however, the hydrogen chloride necessarily passes through a n d is in part dissolved by t h e Askarel liquid. T h i s dis solved hydrogen chloride nia-y caxise de terioration of the cellulose Lusula/tion of the transformer. Unless p r o m p t l y and properly handled, therefore, whaat was originally a minor electrical fsuilure subject t o easy correction may entail an extensive repair. T h e chemical solution was to provide the liquid with a "chafer" or ""getter" which would be capable o f combining quickly and efficiently with t h e dissolved hydrogen chloride, thereby rendering it incapable of promoting further trans former deterioration. T3he obvious chemi cal solution, however, i s complicated b y the dielectric requirement th_at tfcte "get ter" m u s t in itself possess good dielectric properties. I t must be chemically stable and without deleterious effect on the ex cellent chemical and dielectric stability of the Askarel liquid. Furthermore, it must be capable of use in low concerntration such that it does n o t detract from "the rxonflanamability and nonexplosive chiarae^Êeristics which were fundamentally budlt in. the Askarel molecule. T i n tetraphenyl has been found "fco meet these exacting requirements, It is a white solid, melting a t about 226° C - , is nonvolatile, and possesses excellent dielectric properties. T h e m a r k e d ability of t i n tetraphenyl t o combine with hydrogen chloride allows its use i n low coricentrations. T h e present synthetic nonflammable a n d nonexplosive transformer Askarel, therefore, is compounded w i t h tin tetraphenyl. Fig. 3 illustrates the protection which is provided by i t s presence. Each assembly was arced extensively and to t h e same degree. Each -was t h e n a l lowed to stand under conditions s t least equal to the most severe possibilities of commercial practice. T h e assembly filled with the Askarel containing tick tetraphenyl was substantially unaffected! by t h e arcing treatment of the l i q u i d . T i i e insulation in the assembly which contained no t i n tetraphenyl w a s chemically a t tacked, blackened, a n d mechanically weakened.
—O. (ALL
A.
BATTISTA
R I G H T · RESERVED)
of
the Industry
There are m a n y problems -awaifcing t h e chemist who i s willing t o sturdy tfcte needs of t h e electrical industry. N o t all ^ r e concerned with insulation. Marty are; metallurgical and analytical- Some concern the development of resbns antd adJhesives, improved gasket materials, a n d t-he like. But none offer a grea-ter p r o m i s e or a greater challenge t o t h e chemist and his profession t h a n the development o f chemical compounds, molecularly constructed and chemically synthesized to -meet a properly recognized electrical insulation requirement.
C H EMI CAL A N D
ENGINEEKINC
NE^/S
