US2703390A - Explosion-safe transformer - Google Patents

Description

March 1, 1955 w. MARKS ExPLosIoN-SAFE TRANSFORMER Fild Oct. l1

. 1.41. ve 1i.

Invent-.OTH Louis VV. Marks,

His 'Attorney 2,703,390' EXPLOSION-SAFE TRANSFORMER Louis W. Marks, Pittsfield, Mass., assignor tov General Electric Company, a corporation of New York Application october 11, 1951, seriarNu. 250,859-

1 claim. (ci. ass-.96)

This invention relates to stationary electrical induction apparatus and more particularly to improvementsin porcelain-clad liquid-filled instrument transformers'. By the term porcelain I wishto include all brittle ceramic and vitreous dielectric materials.

This invention relates to instrument' transformers of the tylpe disclosed in Patent 2,529.1,135 to Guglielmo Gamilli, assigned to the same assignee asl the present invention.

One of the diicultes which has-been encounteredy in the operation of porcelain-clad instrument transformers of the high voltage type is that electricalI failure ofr the transformer causes the evolutionfof a considerable quantity of gas at high pressure which may cause shattering of the porcelain shell with consequent danger to persons and property in the vicinity of the instrument transformer. j

When such a conventional porcelain=clad liquid-filled transformer fails internally, the resultant electric arc de"- composes the liquid andl solid insulation and there is a rapid increase in the amount of gas which is a product of such decomposition, This causes the production of very high pressures, and, infact, pressure waveshave been observed having a peak value in excess' of 1,000 pounds per square inch and the maximum value' ofthese pressure waves is often reached in $6000 of a second or less. This very steep front of the pressure wave almost invariably cracks the porcelain, and the energy in theex` panding gas may accelerate the porcelain fragments to a high velocity with resulting danger' to nearby personnel and possible damage to adjacent equipment.

Accordingly, it is an object of this invention to pro'- vide al new and improvedv highvoltage instrument trans former in which danger of shattering of' the' cylindrical porcelain insulator is reduced-to aminimum.

It is a further object of this' inventionvto provide a new and improved high voltage instrument transformer in which means are provided to reduce the magnitude and velocity of the pressure waves resulting fromdecompo sition of the liquid and solid insulation due to electric arcs within the transformer apparatus.

l't is a further object' of this invention to provide a new and improved construction forhigh voltage' instrument transformers in which means are provided for relieving the 'gas pressure within the transformer before lit reaches the porcelain-clad' portion of Ithe transformer asfseinlbily, while at the same time reducing the magnitude and velocity of the pressure waves which approach the porcelain-clad portion of the transformer.

In accordance with these objectives, this inventionprovides a high voltage instrument transformer in which the core and coils are contained within ar metal tanks lled with an inert filler material such as sand or gravel' which acts as a restriction to the* passage of hig'li-pressurey gas into the porcelain bushing. One or more pressureV relief devices are provided in the sides of the metalv containing tank, and the high voltage leads are conducted away from the transformer winding through a porcelain bushing mounted on the metal enclosure, a barrier insulating cylinder being interposed between the' high voltage leads and the internal surface of the porcelain bushing which provides a duct between the metal tank and an expan'-` sion chamber at the upper portion of theporcelain bushing. The barrier cylinder projects into the metal tank in which the core and coils are contained to provide a rc-entrant orifice thereby retarding the passage' of explosive gases into the interior of the porcelain bushing.

g United States PatentO novel are set forth with particularity in the appended clair'ns. My invention itself, however, both as to its organlzation and use, together with further objects andy advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 is an eleva tion view, partially in section, of a high voltage instrument transformer incorporating the features of my invention; Fig. 2 is a graph showing the pressure relationships existing in thle bushing structure of Frilg. 1 upon the occurrence of a fault; and Fig. 3 is a graph showing the relation of pressure to time required for opening of `the relief diaphragm used in the structure of Fig. 1.

Referring now to Fig. 1 of the drawing, there is shown a metal tank 1 in which is contained an instrument transformer core and coil assembly indicated generally at 2. ln this case, the core and coil assembly 2 comprises a current transformer having a high voltage primary winding3 of relatively few turns which links a low voltage secondary winding 4 which is wound on a generally annular-shaped magnetic core. Both the primary winding 3 and the secondary winding 4 are of generally annular shape, each respective winding being positioned in a plane substantially perpendicular to the plane of the other winding. The leads from the primary Winding 3 are conducted upwardly through an insulating sleevey which is `contained inside of porcelain 'bushing 7. Bushing 7 is mounted in sealed relation with respect to `the upper surface of tank 1. For this purpose, a bottom clamping ring 8 surrounds the outer circumferential surface of the lower end of porcelain bushing 7 and is cemented to the surface of bushing '7 by a suitable bonding means 9. Clamping ring 8 is suitably bolted to the cover member 10 of the tank 1, thereby holding porcelainl bushing 7 rigidly in place with respect to tank ll. Suitable gaskets are provided between the bottom surface of porcelain bushing 7 and the cover member 1t) of metal tank 1.

To aid in protecting the porcelain bushing 7 from the effects of steep front high pressure waves produced by an internal fault in the transformer, a barrier cylinder 11 is interposed between the inner surface of the porcelain bushing 7 and the radially outer surface of insulating sleeve 6. The barrier cylinder 11 extends from the upper portion of the interior of the porcelain bushing 7 where'the high voltage leads 12 and 13 emerge for attachment to their respective high voltage terminals 14 and 15, to below the lower end of the porcelain bushing 7 Cylinder 11 projects into the upper portion of tank l to a position closely adjacent the point where high voltage leads 12 and 13 emerge from primary high voltagel winding 3, thereby forming a reentrant oriice, as will be explained more fully hereinafter. insulating cylinder 11 is supported in position by a disk-shaped plate member 16 which engages the outer circumference of cylinder 11 where cylinder 11 passes into tank 1. Plate 16 is bolted to the under surface of cover member 10 of tank 1, and the outer periphery of insulating cylinder 1I is suitably recessed to provide for engagement with the radially inner periphery of disk plate 16.

lln my preferred construction, insulating cylinder 11 is formed of a strong relatively resilient material, such as a suitable paper which has not been impregnated with any hardening binder. ln a modified embodiment, a paper cylinder which has been resin impregnated and baked may be used. The cylindrical'barrier 11 should have a wall thickness of about one inch. j The cylindrical b'ar` rier 11 helps to protect the porcelain bushing from the effects of steep front high pressure waves produced by internal faults in the transformer, as will be hereinafter explained. `ln addition to its function of protecting the porcelain against steep wave fronts of pressure, the barrier cylinder 11 also serves as an insulating medium.

A metal cap member 17 is positioned on top of the bushing in sealed relation therewith, cap 17 being bolted to a top clamping ring which is cemented to the outer circumferential surface of the upper end of porcelain bushing 7 by any suitable bonding material 9.

TheA metal cap 17 has a convex outer surface which provides an expansion space for evolved gases. The space formed by the convex surface of cap 17 and the space afroaso within the bushing above the level of liquid and filler material has been designated by the number 18.

If desired, as an aid in protecting the transformer from transient overvoltages, a suitable by-pass resistor 19 having an inverse voltage-resistance characteristic may be connected between one of the high voltage primary terminals and the cap member 17 which in turn is connected to the other primary terminal.

A terminal box 26 is mounted on the exterior surface of metal tank 1, and low voltage secondary leads 21 and 22 are brought out through the side of tank 1 through a suitable bushing.

In order to relieve pressure developed within the tank 1 on the occurrence of an electric arc, I provide a relief means which may consist of a diaphragm member 23 positioned over an aperture in a side of tank 1. Diaphragm member 23 may be provided with grooved reliefs 24 which permit rupture of the diaphragm at a predetermined pressure. Alternatively, the side of the tank 1 may be provided with grooves which will permit the portion of the tank side enclosed by the grooves to yield at a predetermined pressure. While l have shown only one diaphragm member, obviously more than one diaphragm member may be used.

ln accordance with my invention, l fill the interior of tank 1 with an inert ller material 25 such as ne gravel or sand, and also fill the duct between the barrier insulating cylinder 11 and the insulating sleeve 6 with the same filler material which fills tank 1.

The space between the particles of the ller material is lled with a dielectric liquid such as oil or a suitable synthetic liquid.

The diameter of the particles of the filler material should preferably be in the range between 1/32 inch and Mr inch. The filler material used must be substantially free of conducting particles and be a good dielectric to prevent electrical discharge therethrough.

The barrier cylinder 11 should preferably extend at least 2 inches below the top of tank 1 in order to form a reentrant orifice, and the end of the barrier cylinder which projects into tank 1 should preferably be tapered at an angle in the range l5D with respect to the axis of the cylindrical barrier 11. The duct 5 between the high voltage insulating sleeve 6 and the cylinder 11 should have a small radial width, preferably in the range 1/s inch to 1/2 inch, so as to restrict the passage of gas upwardly through the duct 5. Duct 5 will thus accommodate the coarse filler and will provide a labyrinthlike channel for the normal movement of insulating uid between tank 1 and expansion chamber 1S which occurs as a result of normal temperature changes.

Upon the occurrence of an electric arc which causes the decomposition of liquid and solid insulations contained within the tank 1, the coarse filler material filling tank 1 and the duct 5 between barrier cylinder 11 and insulating sleeve 6 will retard the passage of the evolved gases upwardly into the interior of duct 5. The gas in passing through the coarse filler is cooled due to the throttling effect of the restricted passage through the filler material and also due to the restricted passage at the tapered entrance to the barrier cylinder. The coarse iller becomes jammed in this restricted duct due to the pressure change and serves to increase the impedance to On rapid increases in gas pressure in the tank 1, the grooved diaphragm 23 will open and relieve the pressure before it can build up to a dangerously high level within the porcelain bushing 7. The throttling actions of the inert material and of the restricted re-entrant orifice at the mouth of the barrier cylinder combine to delay the passage of high pressure gas to the bushing 7 and so give the diaphragm member 23 in the metal tank 1 time to rupture before a dangerous pressure condition has developed within the bushing 7.

The re-entrant orifice caused by the projection of barrier cylinder 11 into tank 1 increases the restriction to the flow of a uid into duct 5. Therefore, the expanding gases in attempting to force the insulating liquid and coarse filler up duct 5 meet a high impedance to fluid ow. The steep front pressure wave cannot, therefore, be transmitted rapidly up duct 5, and the pressure front moving up the duct is severely attenuated.

The pressure relationships prevailing in the different parts of the instrument transformer assembly upon the occurrence of a fault are graphically illustrated in Fig. 2 in which pressure is plotted against time in milli-seconds.

Curves (a), (b) and (c) respectively represent the pressures in metal tank 1, midway up duct 5, and in chamlease pressure of the diaphragm 23 is well below the ber 18 upon the occurrence of a fault. As shown by curve (a) the pressure in metal tank 1 rapidly rises to a very high value, reaching a maximum at point P when the diaphragm 23 opens. The diaphragm 23 is so calibrated that it breaks before a dangerous pressure level is reached in duct 5. Thereafter, the pressure in tank 1 declines rapidly because the diaphragm (or diaphragms) 24 is made sufficiently large to relieve pressures which would normally be observed under severe arcing. Curve (b) shows that the pressure midway up the duct 5 is of much smaller magnitude and reaches a maximum value slightly later than the pressure in tank 1. Similarly, as shown by curve c), the pressure in chamber 18 at the top of the instrument transformer is even smaller in magnitude and reaches a maximum value slightly later than the pressure at the midpoint of the duct 5. The pressure in the top chamber is held to a very low and relatively safe value because the relatively small amount of liquid and'gas which passes into chamber 18 is not sufcient'to raise its pressure appreciably. This favorable result is aided by proportioning the volume per inch of the liquid in the duct 5 to be a fraction of the expansion volumeof the chamber 18.

The design of the relief diaphragm 24 is closely coordinated with the dimensions of the tank, the volume of the air chamber 18, and the physical dimensions of the barrier cylinder 11 and of the duct 5. Under normal operating conditions, the diaphragm 23 should not fail, leak or open due to the normal forces of expansion and contraction. ln the event of a gradual increase in pressure resulting from a high resistance fault, the static reultimate strength of other parts of the transformer. The time constant of the diaphragm 23 should be such that as the pressure increases, it opens more rapidly. This relationship is shown graphically in Fig. 3 in which pressure necessary to open the pressure relief diaphragm 23 is plotted against time in milliseconds. Thus, the greater the pressure tending to force materials up the duct 5, the more rapidly the pressure is relieved by the opening of diaphragm 23.

In addition to the fact that the combined mass of the filler material and the dielectric liquid, such as oil, which fills the spaces between the particles of the filler material, is greater than that of the dielectric liquid normally used, thereby retarding the movement of the gas up into the porcelain bushing 7, there is the further advantage that the use of the inert material reduces considerably the proportion of dielectric liquid contained in a given tank volume which can disassociate under the heat of an arc.

Thus, it can be seen that I have provided a new and improved construction for high voltage instrument transformers which reduces considerably the danger of shattering-0f the porcelain-clad bushing used with the instrument transformer. l have provided a combination of features in this new construction which cooperate together to provide a relatively explosion-free characteristic: As mentioned hereinbefore, these features include the use of an inert sand or gravel material to restrict the passage of high pressure waves into the porcelain bushing, a relief diaphragm in the side of the tank portion of the structure, a barrier cylinder between the outgoing leads from the transformer windings and the porcelain bushing enclosing those leads, with the end of the barrier cylinder being provided with a special taper and extended to form a re-entrant orifice which act to restrict the passage of gases, liquids and solids through the barrier cylinder.

While there has been shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the invention, and, therefore, it is aimed in the appended claim to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

In a stationary electrical induction apparatus ot' the instrument transformer type, a metal enclosure, said metal enclosure being provided with a wall portion which is rupturable at a predetermined pressure within said enz closure, a magnetic core, inductively related high voltage and low voltage windings magnetically interlinked with said magnetic core, sa1d magnetic core and windings bemetal enclosure, a normally hollow insulating bushing having one end mounted on the upper surface of said metal enclosure, the opposite end of said bushing bein;7 closed by a metallic cap member having an expansion)y chamber, a resilient protective hollow cylindrical insulating liner member positioned Within said bushing member, said liner member extending from closely adjacent said opposite end of said bushing to below said end of said bushing which is mounted on said metal enclosure, said protective liner projecting into said metal enclosure to form a re-entrant orifice therein, the end of said protective liner which projects ing contained within said into said enclosure being tapered radially inwardly away from said end of said protective liner at an angle in the range 15-45 with respect to the axis of said liner, conductor leads from said high voltage winding extending upwardly through said protective liner and being connected to high voltage terminals in the walls of said opposite end of said bushing, said low voltage winding being connected to terminals insulatingly mounted on a side of said metal enclosure, said metal enclosure and said liner member being substantially lled with a filler material subdivided so as to have a particle diameter substantially in the range 1452 inch to 1A inch, the spaces between the particles of said filler material being filled with a dielectric liquid, said re-entrant orifice and said iller material impeding and delaying the transmission of high pressure waves to said expansion chamber and liner member upon the formation of explosive gases in said metal enclosure, said explosive gases being relieved by rupture of said rupturable wall portion.

References Cited in the file of this patent UNITED STATES PATENTS 1,947,085 Hill et al. Feb. 13, 1934 2,529,135 Camilli NOV. 7, 1950 FOREIGN PATENTS 394,825 Great Britain July 6, 1933

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