US2274781A - Method and apparatus for insulating and cooling stationary induction devices - Google Patents

Description

March 3, 1942- H. ENSMINGER 2,274,781

METHOD AND APPARATUS FOR INSULATING AND COOLING STATIONARY INDUCTION DEVICES Filed Jan. 29, 1940 3 Sheets-Sheet 1 r22 7 3mm Iwylum'zyer MWM H. ENSMINGE R 2,274,781 METHOD AND APPARATUS FOR INSULATING AND COOLING Mal-c113, 1942.

STATIONARY INDUCTION DEVICES Filed Jan 29, 1940 3 Sheets-Sheet 2 grwwwm March 3, 1942. H, s m 2,274,781

METHOD AND APPARATUS FOR INSULATING AND COOLING STATIONARY INDUCTION DEVICES' Filed Jan. 29, 1940 3 Sheets-Shegt 3 sesses'several disadvantages.

Patented Mar. 3, 1942 METHOD AND APPARATUS FOR INSULATING AND COOLING STATIONARY INDUCTION DEVICES Harry Ensminger, Chicago, IlL, mignor to Cardox Corporation, Chicago, 111., a corporation of Illinois Applicationlanuary 29, 1040, Serial lilo. 316,282

25 Claims.

This invention relates to new and useful improvements in methods and apparatus for insulating and cooling stationary induction devices and'deals more particularly with large outdoor power transformers.

It is the present accepted practice to employ special dry transil oils as an insulating medium for large outdoor power transformers. Such oils are, of, course, inflammable and present a serious fire hazard. The maintaining of an atmosphere ofv an inert gas above the oil insulation in transformer casings has been adopted to a limited extent to lessen this fire hazard. Such a practice, however, is entirely ineflective ifan explosion occurs which ruptures the transformer casing. When a transformer casing is ruptured, the inert gas is lost to the surrounding atmosphere and the insulating oil may ignite and burn freely. Because of this serious fire hazard problem, the research laboratories of many large electrical equipment manufacturers are at the present time seeking a satisfactory non-inflammable insulating medium for these large outdoor power transformers.

The copper windings now used have a very large cross-sectional area so as to avoid burnt copper and so that the large surfaces will transcause shorting with consequent-electrical failure.

The special transil oils now being usedhave a high dielectric value but are believed to introduce certain power inefficiencies. For example, it'is considered that the oil supports the corona conditions about the transformer windings, and corona conditions coupled with internal arcing are .partly responsible for the formations of colloidal practical figure of 132,000 volts;

mit heat rapidly to the oil. This use of such use of fins is enhanced with quite a number of.

installations by circulating air past the casings by means of blower fans. Attempts have been made to control the temperatures of the insulating oil by placing a coil in the transformer casing and circulating water through the coil. This practice of using water as a cooling medium pos- 'As hydrant or city water is usually used, its cooling effect is considerably lower in the summer than it is in the winter, yet it is during the summertime when the cooling of theoil isfmost needed. Also, the use of water in cooling coils placed intransformer casings is not considered to be good practice be-v cause of the likelihood of the coils becoming loose and leaking. Water in a transformer casing will holding the Cores of transformers have constant heat losses which are known as hysteresis losses. This type of loss is brought about by the friction of the molecules in the core iron and the constant change of magnetic field. There also is a conlarge outdoor power transformers and the,mainv taining of the carbon dioxide as a liquid having a reasonably low vapor pressure, by constantly temperature of a liquid at a low value.

In replacing the present special transil oils with liquid carbon dioxide, the fire hazard resulting from the use of the highly inflammabl oils is, of course, entirely eliminated. Carbon dioxide is an excellent fire extinguishing medium.

Although not as high as transil oils, liquid carbon dioxide has good dielectric properties, it can be kept free from the entrainment of any water by the use of a small amount of drier, and it is harmlessto the insulation employed about the internal terminal blocks and leads that are con 'ventional with power transformers.

By maintaining the liquid carbon dioxide at a low temperature value, which may be as low as 69.9' F., if desired, a constant operating temperature condition will be provided. This constant low temperature operating condition will permitexisting transformer units to be operated safely at almost any load as the present day difilculties regarding overloading will. be entirely overcome. With transformers particularly designed for using liquid carbon dioxide as an insulating medium, it will be possible to reduce the amount of copper used in the windings by as much as 40% or 50% if present day load capacities are maintained. By reducing the required 1 amount of copper to this extent, considerably seasons of a year will provide numerous additional operating advantages.

It has been determined that the corona condition about transformer windings will be materially improved and, of course, the formation of colloidal carbon will beentirely eliminated.

The consensus of opinion of electrical engineers responsible for the maintenance of high power transformers is that the hysteresis loss is less when transformers are operated under present day winter operating conditions. Modern transformers are required to have an efficiency rating of at least 90%. It is calculated that the use of low temperature liquid carbon dioxide lowers the hysteresis losses and the loss through the copper windings to such an extent that the best efficiency ratings obtainable under any operating conditions with modern transformers will be improved by from 3% to 4%. A 3% to 4% improvement in the operating efficiency of a modern transformer must be considered as a real accomplishment when the margin left for improvement is only 10%. i

It is the primary object of this invention to provide an improved form of insulating medium for large outdoor power transformers.

A further important object of the invention is the provision of an insulatingmedium for. transformer which will eliminate many of the present day operating difficulties resulting from the use of special transil oils.

A still further primary object of the invention is to provide a constant temperature operating condition for large outdoor transformers during all seasons of the year.

Other objects and advantages of the invention will be apparent during the course of the follow ing description.

in the accompanying drawings forming a part of this specification and in which like numerals are employed to designate like parts throughout the same,

Figure 1 is a diagrammatic view of a bank of transformers to which has been applied a system for maintaining the transformer casings filled with liquid carbon dioxide which is maintained at a preselected'low temperature, and its corresponding low vapor pressure, a.

Figure 2 is an elevational view of a single transformer with the liquid carbon dioxide system of Fig. 1 applied thereto, Figure 3 is an elevatlonal view of a single large transformer with refrigeratingmeans for maintaining the carbon dioxide insulating medium in a liquid state and at a desired low temperature without necessitating the withdrawal of the liquid carbon dioxide from the transformer casing,

Figure 4 is a partly elevational view and partly sectional view of a transformer casing filled with liquid carbon dioxide which is maintained at a low temperature,

Figure 5 is a view similar to; Fig. 1 but illustrates a modified form of circulating system for the liquid carbon dioxide lnsula'tingmedium, and

illustration are shown the preferred embodiments of this invention, and particularly referring to Figs. 2 and 4, the reference character III designates the casing of a transformer having a suitable cover portion H. Positioned within the casing I0 is a conventional core and winding assembly l2. No attempt has been made to disclose the details of this assembly, the manner in which it is mounted in the transformer casing 10, or the specific. lead wire connections and the way in which they extend outwardly of the casing.

Fig. 2 discloses this transformer casing H], with its cover H, positioned within a completely en-' closing housing I3 which is of suitable construction to insulate the transformer casing l0, and its contents, against the absorption of heat from the surrounding atmosphere. No attempt has been made to disclose the details of this insulating housing as such details form no part of this invention. Merely for the sake of aiding in designating the character of the construction, a high tension bushing I4 and a low tension bushing ii are illustrated as projecting from the top of the housing l3. As has been explained above, it is the purpose of this invention to employ liquid carbon dioxide as the insulating medium for the core and winding assembly 12 arranged within the transformer casing I0. Fig. 4 discloses the transformer casing l0 as being filled with liquid carbon dioxide l6. To maintain this transformer casing filled, an auxiliary reservoir or chamber H is illustrated in Fig. 2 as being arranged ata higher level than the level of the transformer casing. This reservoir or chamber H has pipingf l8 interconnecting its lower portion with the upper portion of the transformer casing [0. Figs. 2 and 4 disclose the piping II as being connected to the cover ll of the transformer. A suitable shut-off valve I9 is arranged in the piping 18 which is normally left open but is closed when it is necessary to servicelthe transformer; A filling valve and connection 20 is suitably coupled to the piping ll. This valve and connection cooperate with the pressure equalizing valve andconnectlon II which is suitably coupled to the upper portion of the reservoir or chamber H. A drain valve and connection 22 is joined by the pipe 23 withthe lower portion of the transformer casing I.

To place this unit in operation, the connections 20 and II are coupled to a source of supply of liquid carbon dioxide which is preferably at a preselected subatmospherlc temperature and its corresponding vapor pressure. The expression subatmospheric temperature" is used throughout the specification and claims and-is intended to mean any desired temperature falling below a normal atmospheric temperature of 70 F. The sub atmospheric temperature which will be used most frequently will fall within the range of from 0 F. to -40 F. The valve and connection 20 will be joined with suitable piping, not shown, to the lower or liquid space of the source-of subatmospheric temperature carbon dioxide, and the valve and connection 2| will be suitably attached to the upper portion or vapor space of the said source-of liquid carbon dioxide. The system, including the reservoir II, the pipe I8, and the transformer casing ID, will befilled with liquid carbon dioxide to a suitable liquid level in the reservoir H from the source of supply by any suitable carbon dioxide liquid transferring mechanism or method. The-connection between the vapor spaces of thereservdlr I1 and the source or supply will permit theiransformer system and the source of supply to be maintained at the same vapor pressure during filling of the transformer system. After the system is filled to the desired liquid level in the reservoir or chamber I1, the valves and .connectlons and 2| will be disconnected from the source of supply by closing these two valves. I

As the liquid carbon dioxide maintained in the sary to cool or refrigerate the carbon dioxide. The preferred way of accomplishing this desired result is to employ a mechanical refrigerating machine, diagrammatically illustrated at 24. This refrigerating apparatus is connected by inlet and discharge pipes with a coil 25 which is ar liquid body or mass in the lower portion of the reservoir l1. Due to the connection provided by the piping l8, any vapors which form in the transformer casing III, as a result of absorbing heat through the insulating housing I3, will be per mittedto percolate or bubble up through the liquid 'column'maintained in the piping l8 and the liquid bath in the reservoir l1 until it reaches the vapor space of the reservoir. As has been The carbon in its branch line 34.is closed. In this way.

, the remaining transformers of the bank may be transformer casing I0 is to be held at a desired subatmospheric temperature, it becomes necesexplained, the vapors accumulating in the vapor space of the reservoir l1 will be condensed by the cooling coil 26 of the refrigerating apparatus 24. As vapors leave the transformer casing l0, liquid carbon dioxide will flow by gravity from the reservoir I1 to refill the transformer casing 10.

Referring now to Fig. 1, there is disclosed a transformer system which differs from the system allel with the liquid carbon dioxide supply reserthe reservoir and the manifold-pipe 32.

illustrated in Fig, 2 solely by including a bank of three transformers which are coupled in parvoir. Each one .ofthe transformers includes an insulating housing 21 which encloses a transformer structure of the type illustrated in Figs. 2 and 4. ,The high and low tension bushings 23 and 23 are illustrated as projecting from each one of the housings 21. A reserve supply reservoir or chamber is located at a higher level than the transformers within the housings 21.'

This reservoir is connected by a pipe line 3| with a manifold pipe 32. The pipe line 3| is connected with the lower portion or the liquid space of the reservoir 30. A shut off valve 33 is located in the line 3| to close communication between Con- , nected with this manifold pipe are the three j branch lines 34, which extend into the housings I 21 for connection with the covers of the transformer casings, not shown, located within the housings. lines 34 for selectively cutting off any one of these lines from the manifold line 32. It is to be under- "stood that the valves 33 and 35 normally are Valves 35 are located in the branch.

open. The valve 33 is closed whenever it is desired to place out.of operation all three of the transformer units. When it is desired to service any one of the. transformerunits, the valve 35 maintained in operation while the disconnected transformer is being serviced. When it is desired to fill this system with liquid carbon dioxide, the filling valve and its connection 36 and the pressure equalizing valv and itsconnection 31 are employed in the manner described for valves and connection 20 and 2| of Fig. 2. An auxiliary filling valve and connection 38 is providedin the manifold line 32.

Whenever it is desired to completely drain the liquid carbon dioxide from any one of the transformer casings located in a housing 21, the drain valve and connection 39 for that unit is employed. Of course, the shut-off valve 35 for the unit to be drained is closed prior to the opening of the valve 39.

'The liquid carbon dioxide in this system is maintained at the desired subatmospheric temperature, and its corresponding low vapor pressure, in the same manner as that described in connection with Fig. 2. The reservoir 30, there- 'fore, has a conventional refrigerating apparatus 40 associated therewith. The inlet and dischargepipe lines 4| extend from the refrigerating apparatus 40 to the coolin coil 42 arrangedln the vapor space of the reservoir 30. This cooling coil functions to condense the carbon dioxide vapors which accumulate in the vapor space of the reservoir 30.

Referring now to Fig. 3, there is disclosed a,

single transformer unit of larger size than that illustrated'in the previously referred to figures. This unit includes an insulating housing 43 which confines a transformer casing 44 therein. This whole unit is mounted on wheels 45 for travel along the rails 46. ,The high and low tension bushings 41 and 48 project from the top of the housing 43.

The transformer casingof this unit isof sufficient capacity to house its own complete supply of liquid carbon dioxide. In other words, an auxiliary reservoir or chamber is not employed.

, The transformer casing 44 is supplied with a suitable amount of liquid carbon dioxide and a vapor space is left in the casing above the level of the liquid. To fill the transformer casing 44 to the desired level, a combined filling and draining valve and connection 49 and a pressure equalizing valve and connection .50 are employed to accomplish this end. In this system, the vapors are condensed right in the vapor space of the transformer casing 44 by means of the cooling coil 5|. This coil is connected by pipes 52 to th refrigerating apparatus 53. In Fig. 5 there is disclosed a bank'of transformer units which are connected to an auxiliary reservoir. This system differs from the system of Fig. l by having separate piping for the return of vapors from the upper portions of the transformer casings to the upper portion of the auxiliary reservoir instead of compelling the vapors to percolate or'bubble up through the liquid -column or columns connecting the transformer casings with the reservoir and by having the lower portion of the reservoir connected by piping to the lowerportions of the transformer casings. This modified system will be described in detail as follows: I

Each transformer casing is enclosed in .a suitable insulating housing 54. The high tension and low tension bushings 55 and 56 project from the top of the housing 54. An auxiliary reservoir 51 is provided and is'located at a higher level than the transformer casings. The lower portion of this reservoir is connected by a pipe line 58 to a manifold pipe line 59. This manifold pipe line has the branches 60 which are connected to the lower portions or liquid spaces of the transformer casings positioned within the insulating housings 54. A shut off valve 6| is located in the pipe line 58. shut off valves 62 are located in the several branch lines 50. By closing the valve the supply of liquid to the several transformers from the reservoir 51 is stopped. By closing any one of the valves 62 in the branch lines 50, the associated transformer is disconnected from the source of supply of liquid carbon dioxide maintained in the reservoir. By means of these several valves, either all of the, transformer units or any selected one of theunits may be conditioned for servicing. Prior to the servicing operation, however, the liquid carbon dioxide should be drained from the transformer or transformers by means of the drain valves and connections 53.

Extending from the vapor space of the reservoir 51 is a pipe line 54 which connects with a manifold pipe 65. This manifold pipe has branch lines 85 which extend to and are connected with the upper portions of the transformer casings located in the insulating housings 54. A threeway valve 51 is located in the pipe line 54. Shutof! valves 58 are located in the respective branch lines 66. By positioning the valve 51 so that communication between the vapor space of the reservoir 51 and the manifold pipe 65 is cut off, the flow of carbon dioxide vapors from the several transformer casings will be prevented. The various valves 88 may be closed when an one or more of the transformer units are to be serviced.

To fill the system with the desired amount of liquid carbon dioxide at the preselected subatmospheric temperature, the filling valve and connection 59 is attached to the liquid space of a source of supply of liquid carbon dioxide. The branch connection I0, extending from the threeway valve 57, is connected with the vapor space of the source of liquid carbon dioxide supp y. The system is then filled in the manner describedgin connection with the forms of the invention disclosed in Figs. 1 and 2. An auxiliary filling connection and valve ii is provided in the manifold pipe 59 for aiding in filling the system with liqold.

It will be appreciated that as vapors are formed in the transformer casings located within the insulating housings 54, these vapors will fiow into the vapor space of the auxiliary reservoir 51 through the branch lines 55, the manifold line 55, and the main pipe line Bl. The iransformer casings will be refilled with liquid carbon dioxide to take the place of the exhausted vapors through the'pipe line 58, the manifold line 55, and the branch'lines 60.

To maintain the liquid in the entire system at a desired subatmospheric temperature, and its corresponding low vapor pressure, a mechanical refrigerating apparatus 12 is associated with the reservoir 51. Pipe lines 13 connect this refrigerating apparatus to the coil 14 located in the vapor space of the reservoir 51. This cooling coil I will condense the vaporsin this space as they carbon dioxide liquid'and the vapors will be provided. The reservoir for this modification may either be located at a higher level than the transformer casing to allow for gravitational fiow of the liquid from the reservoir into the transformer casing or the reservoir may be located at any desired level on a plane with or lower than the transformer casing and a fluid pump em ployed for providing a forced circulation of the 80 also is located in the pipe line ll. From the lower-portion orliquid space of the reservoir 46, a pipe lineal extends to a pump casing 82. The connection of the pipe 8! to the pump casing is at the inlet port of the liquid pump. The outlet of this pump is connected by a pipe line I! with the lower portion of th transformer casing located within the insulating housing-15. A filling valve and connection 84 is provided in the pipe line 8|. A shut-off valve 55 also is provided in this line. A second shut-off valve 56 is provided in the pipe line 83. When it is desired to fill this system with the desired amount of liquid carbon dioxide at a preselected subatmospheric temperature and its corresponding low vapor pressure. the filling valve and connection 84 is coupled with the liquid space of the source of supply of carbon dioxide. The branch pipe 19 is connected to the vapor space of the source of liquid supply. The valves 84 and II then are opened and the filling of the system may be carried out in the same manne as described in connection with Figs. 1 and 2. When the system is properly filled, the valve I8 is arranged so that the branch line ll will connectwith the vapor space of the reservoir 15. The branch" line 19 will be disconnected from the reservoir and the pipe line TI. The filling valve 84, also, will be closed. When it is desired to drain the transformer casing, th drain valve and connection 81 will be employed for that purpose.

The carbon dioxide in this system is maintained at the desired subatmospheric temperature by means of a mechanical refrigerating device I! which is associated-with the reservoir ll. Inlet and discharge pipe lines I! connect the refrigerating apparatus "with a cooling coil ll located in the vapor space of the reservoir I5.

'This cooling coil functions in the manner pre- ,stood that a fluid pmnpmay be located in the pipe line it of the system disclosed in Pig. 5 if it is desired to provide a forced circulation of carbon diozddethrough'this system. r

The above description covers one mode of operation of a system of the WI illustrated in Pig.

6. However. the efficiency of this type of system the transformer coils may be I be taken as preferred examples of the same, and that various changes in the shape, size, and ar-' can be materiallyimproved by subcooling the liquid carbon dioxide before it enters the transformer casing positioned within the insulating.

housing 15. In explanation, it will be appointed out that under certain operating conditions of this system with no subcooling of the liquid before it enters the transformer there will be experienced vaporization of the liquid at the surfaces of the transformer coils, conductors, and the like as a result of passage of heat from these transformer parts into the liquid. The vapor film thus formed and interposed between the parts of the transformer and the actual body of carbon dioxide liquid will prove to be less efilcient for heat transfer than direct liquid contact would be. To overcome this surface vaporization, the temperature of the liquid in the transformer casrangement of parts may be resorted to without departing from the spirit of theinvention or the scope of the subjoined claims.

Having thus described the invention, I claim:

, 1. A method of insulating and cooling power transformers comprising insulating the transformer casing against the absorption of heat from the surrounding atmosphere, insulating the transformer windings against electrical losses by maintaining the transformer casing filled with liquid carbon dioxide to a level which will cover ing should be maintained several degrees lower an additional refrigerating device, not shown, by

means'of the pipe lines 94 and 95. Any suitable refrigerant may be circulated through the heat exchange. chamber 93 in this manner and the liquid carbon dioxide passing through the chamber in the pipe line 83 will be subcooled the desirednumber of degrees.

. This subcooling of the liquid carbon dioxide without materially affecting the pressure in the pipe line 83 is possible because in a circulating system which includes a pump that is capable of creating pressure, the pressure on the discharge side of the pump is a function of the pump capacity and the resistance to flow of the combined system on the discharge side of the pump. This pressure can be maintained constant as long as there is no variation in the condition of the fluid being pumped between one measurement of pressure and another. Therefore, the liquid carbon dioxide flowing at all times from pump 82 to the transformer casing through the pipe line 83 will be at a substantially constant pressure, and it may be subcooled without materially aifecting this constantpressure.

Th use of this subcooling mechanismwill permit the maintenance of a given level of liquid in the transformer casing under given requirementsv of heat absorption and the heat exchange capacity of the chamber 93 may be employed to control the level of liquid in the transformer casin idea of assuring direct liquid contact with ered by proportioning the size of the opening or the flow capacity of the valve 80 in the transformer discharge line 11 in order to maintain the pressure in the transformer casing, by means of the pump 82. above the normal vapor pressure which would correspond with the temperature of the liquid carbon dioxide in the transformer casing.

From the above explanation of the system disclosed in Fig. 6, it will be understood that this It is to be understood that the forms of this the windings and the core, and refrigerating the carbon dioxide to maintain the liquid in the easing at a preselected subatmospheric temperature and its corresponding low vapor pressure.

2. A method of insulating and cooling power transformers comprising insulating the transformer casing against the absorption of heat from the surrounding atmosphere, insulating the transformer windings against electrical losses by filling the transformer casing with liquid carbon dioxide, maintaining at a higher level than the level of the transformer casing a reserve supply of carbon dioxide, establishing liquid and vapor flow paths between the transformer casing and the reserve supply of carbon dioxide whereby vapors formed in the transformer casing will be delivered to the reserve supply and liquid will be delivered to the transformer casing from the reserve supply to take the place of the withdrawn vapors, and maintaining the liquid in the entire system at a preselected low temperature by refrigerating the carbon dioxide at the said reserve supply. I

3. A method of insulating and cooling power transformers comprising insulating the transformer casing against the absorption of heat from the surrounding atmosphere, insulating the transformer windings against electrical losses by filling the transformer casing with liquid carbon dioxide, maintaining at a higher level than the level of the transformer a reserve supply of care la'bon dioxide, establishing liquid and vapor iiow system may be operated either-with or without the subcooling step.

paths between the transformer casing and the reserve supply of carbon dioxide whereby vapors formed in the,transformer casing will be delivered to the reserve supply and liquid will be delivered to the transformer casing from the reserve supply to take-the place of the withdrawn vapors, and maintaining the liquid in the entire 'system at a preselected low temperature, and its having a-vapor space above the liquid level maintained therein, establishing a liquid flow path for the gravitational flow of liquid carbon dioxide the reserve supply chamber, and maintaining the carbon dioxide in the entire system at a preseinvention herewith shown and described" are to lected low temperature value bycondensing the vapors in the vapor space of the reserve supply chamber.

5. A method of insulating and cooling power transformers comprising insulating the transformer casing against the absorption of heat from the surrounding atmosphere, insulating the transformer windings against electrical losses by filling the transformer casing with liquid carbon dioxide, providing a chamber for the storage of a reserve supply of carbon dioxide, said chamber having a vapor space above the liquid level maintained therein, establishing a flow path between the liquid space of said chamber and the bottom portion of the transformer casing, establishing a flow path between the upper portion of the transformer casing and the vapor space of the reserve supply chamber, maintaining a forced circulation of carbon dioxide through the transformer casing, the reserve supply chamber, anddioxide liquid to submerge the core and windings while leaving a vapor space above the liquid level, and maintaining the carbon dioxide confined in the transformer casing at a preselected subatmospheric temperature, and its corresponding low vapor pressure, by condensing the vapors in the vapor space of the transformer casing and allowing the liquid drops of condensation to fall back into the liquid bath in the casing.,

I. Stationary induction apparatus of the type described, comprising a transformer casing, heat insulating means entirely enclosing the casing, a core and winding assembly positioned in the casing, a body of liquid carbon dioxide in the casing submerging the core and winding assembly, and means for refrigerating the carbon dioxide to maintain it as a liquid at a preselected subatmospheric temperature and its corresponding low vapor pressure.

3. Stationary induction apparatus of the type described, comprising-a transformer casing, heat insulating means entirely enclosing the casing, a core and winding assembly positioned in the casing, an auxiliary reservoir, piping for conmeeting the reservoir and the transformer casing, a body of carbon dioxide filling the transformer casing, the piping, and the reservoir, and means operatively associated with the reservoir for maintaining the carbon dioxide at a preselected low temperature and its corresponding low vapor pressure.

9. Stationary induction apparatus of the type described, comprising a transformer casing, heat insulating means entirely enclosing the casing, a core and winding assembly positioned in the casing, anauxiliary reservoir, piping connecting the reservoir and the transformer casing, a body of liquid carbon dioxide filling the transformer casing, the piping, and the reservoir with the exception of a vapor space left in the upper portion of the reservoir, a mechanical refrigeratconnected to the refrigerating apparatus and arranged in the vapor space of the reservoir for selected subatmospheric temperature and its corresponding low vapor pressure.

l0. Stationary induction apparatus of the type described, comprising a transformer casing, heat insulating means entirely enclosing the casing, a core and winding assembly positioned in the casing, an auxiliary reservoir, piping connecting the upper portion and the lower portions, respectively, of the reservoir and the transformer casing, a body of liquid in the transformer casing, the piping, and the reservoir with the carbon dioxide vapors being permitted to collect in the upper portion of the reservoir, said piping allowing the transfer of liquid carbon dioxide from the reservoir to the transformer casing and transfer of vapors from the transformer casing to the reservoir, and means for refrigerating the carbon dioxide operatively associated with the reservoir.

11. Stationary induction apparatus of the type described, comprising a transformer casing, heat insulating means entirely enclosing the casing, a core and winding assembly positioned in the casing, an auxiliary reservoir, piping connecting the upper portions and the lower portions, respectively, of the reservoir and the transformer casing. a body of carbon dioxide in the transformer casing, the piping and the reservoir with the carbon dioxide vapors being permitted to collect in the upper portion of the reservoir, piping allowing for transfer of liquid carbon dixoide from the reservoir to the transformer casing and transfer of vapors from the transformer casing to the reservoir, a liquid pump coupled in the piping connecting the bottom portions of the reservoir and the transformer casing for providing a forced circulationrof the carbon dioxide in the system, and means for refrigerating the carbon dioxide operatively associated with the reservoir.

12. Stationary induction apparatus of the type described, comprising a bank of transformer casings, a heat insulating housing entirely enclosing each one of the casings, a core and winding assembly positioned in each casing, an auxiliary reservoir, piping connecting the reservoir and all of the transformer casings, a body of carbon dioxide filling the transformer casings, the piping, and the reservoir, and means operatively associated with the reservoir for maintaining the carbon dioxide at a preselected subatmospheric temperature and its corresponding low vapor pressure.

l3. Stationary induction apparatus of the type described, comprising a bank of transformer casings, a separate heat insulating housing entirely enclosing each transformer casing, a core and winding assembly positioned in each casing, an auxiliary reservoir, piping connecting the upper portion of the reservoir with the upper portions of all of the transformer casings, piping connecting the lower portion of the reservoir with ing apparatus, and a cooling coil operatively tain the liquid in the entire at a pre- II the lower portions of all of the transformer casings, a bodyof carbon dioxide in all of the transformer casings, the piping connecting the lower portion of the reservoir with the lower portions of all of the transformer casings, and a p rtion of the reservoir, the upper portion of the reservoir acting as a vapor space for the collection of vapors, said piping allowing for transfer of liquid carbon dioxide from the reservoir to the lower portions of all of the transformer casings and transfer of vapors from the upper portions of all of the transformer casings to the upper portion of the reservoir, and means for refrigerating the carbon dioxide maintained in the entire system operatively associated with the vapor space of the reservoir.

- 14. A method of insulating and cooling power transformers comprising insulating the transformer casing against the absorption of heat from the surrounding atmosphere, insulating the transformer windings. against electrical losses by maintaining the transformer casing filled with liquid carbon dioxide to a level which will cover the windings and the core, and refrigerating the carbon dioxide in the transformer casing to maintain the liquid in the casing at a preselected subatmospherictemperature and its corresponding low vapor pressure.

15. A method of insulating and cooling power transformers comprising insulating the transformer casing against the absorption of heat 18. A method of insulating and cooling power transformers comprising insulating .the transformer casing against the absorption of heat from the surrounding atmosphere, insulating the transformer windings against electrical losses by filling the transformer casing with liquid care bon dioxide, providing a chamber for the storage of a reserve supply of carbon dioxide, said chamber having a vapor space above the liquid level maintained therein, establishing a fiow path between the ,liquid space of said chamber and the bottom portion of the transformer casing, establishing a flow path between the upper portion of the transformer casing and the vapor space of the reserve supply chamber, maintaining a forced circulation of the liquid carbon dioxide at a constant pressure through the first mentioned flow from the surrounding atmosphere, insulating the 16. A method of insulating and cooling power transformers comprising insulating the transformer casing against the absorption of heat from the surrounding atmosphere, insulating the transformer windings against electrical losses by filling the transformer casing with liquid carbon dioxide, maintaining a reserve supply of carbonpath, refrigerating the carbon dioxide in the reserve supply chamber by condensing the carbon dioxide vapors in said chamber to provide a preselected low operating temperature, and corre sponding vapor pressure, for the system, and subcooling'the aforesaidliquid carbon dioxide maintained under forced circulation so that liquid carbon dioxide at a lower temperature than the aforesaid preselected low operating temperature will be fed to the transformer casing while the pressure of said liquid will be maintained at the vapor pressure of the system.

19. A method of insulating and cooling power transformers comprising insulating the transformer casing against the absorption of heat from the surrounding atmosphere, insulating thetransformer windings against electrical losses by filling the transformer casing with liquid carbon dioxide, providing a chamber for the storage of a reserve supply of carbon dioxide, said chamber dioxide, establishing liquid and vapor flow paths between the transformer casing and the reserve supply of carbondioxide, whereby vapors formed in the transformer casing will ,be delivered to the reserve supply and liquid will be delivered to the transformer casing from the reserve supply to take the place of the withdrawn vapors, and maintaining the liquid in the system at a preselected low temperature, and its corresponding low vapor pressure, by condensing the carbon dioxide vapors in the reserve supply.

17. A method of insulating and cooling power transformers comprising insulating the transformer casing against the absorption of heat from the surrounding atmosphere, insulating the transformer windings against electrical losses by filling the transformer casing with liquid'carbon dioxide, providing a chamber for the storage of a reserve supply of carbon dioxide, said chamber having a vapor space above the liquid level maintained therein, establishing a flow path between the liquid space of the chamber and the bottom portion of the transformer casing, establishing a flow path between the upper portion of the transformer casing and the vapor space of the reserve supply chamber, maintaining a forced circulation of the liquid carbon dioxide through the first mentioned fiow path,. refrigerating the carbon dioxide in the reserve supply chamber for proand corresponding lowvapor pressure, for the system, and subcooling the aforesaid liquid carbon dioxide which is maintainedunder forced circulation.

having a vapor space above the liquid level main-' tained therein, establishing a flow path for the liquid carbon dioxide from the reserve supply chamber to the bottom portion of the transformer casing, establishing a vapor fiow path between the upper portion of the transformer casing and the vapor space of the reserve supply chamber,

' and maintaining the carbon dioxide in the entire system at a preselected low temperature value.

I 20. A method of insulating and cooling power transformers comprising insulating the transformer casing against the absorption of heat from the surrounding atmosphere, insulating the transformer windings against electrical lossesby filling the transformer casing with liquid carbon dioxide, providing a chamber for the storage of a reserve supply of carbondioxide, said chamber having a vapor space above the liquid level maintained therein, establishing a flow. path for the liquid carbon dioxide from the reserve supply chamber to the bottom portion of the transformer casing, establishing a vapor flow path between the upper portion of the' transformer casing and the vapor space of the reserve supply viding a preselected low operating temperature.

chamber, and maintaining the carbon dioxide in the entire system at a preselected low temperature value by refrigerating the carbon dioxide in the reserve supply chamber.

21. A method of insulating and cooling power' transformers comprising insulating the transformer casing against the absorptionof heat from the surrounding atmosphere, insulating the transformer winding against electrical losses by 'fllling the transformer casing with liquid carbon dioxide, providing a chamber for the storageof a reserve su ply of carbon dioxide, establishing liquid and vapor flow paths between the transculating liquid carbon dioxide entering the trans former casing.

22. A method of insulating and cooling power transformers comprising insulating the transformer casing against the absorption of heat from the surrounding atmosphere, insulating the transformer winding against electrical losses by filling the transformer casing with liquid carbon dioxide, providing a chamber for the storage of a reserve supply of carbon dioxide, establishing liquid and .vapor flow paths between the transformer casing and the reserve supply chamber to provide for the circulation of the carbon dioxide, cooling the carbon dioxide in the reserve supply chamber at a rate which will prevent absorption of heat from raising the temperature of the carbon dioxide in the system above a predetermined value, and subcooling the circulating liquid carbon dioxide entering the transformer casing so that its temperature will be at a lower value than the aforementioned predetermined value for the system.

23. Stationary induction apparatus of the type described, comprising a transformer casing, heat insulating means entirely enclosing the casing, a core and winding assembly positioned in the casing, an auxiliary reservoir, piping connecting the reservoir and the transformer casing for the circulation of carbon dioxide, a body of carbon dioxide filling the transformer casing, the piping and the reservoir, means operatively associated with the reservoir 'for limiting the temperature of the carbon dioxide in the system to a preselected subatmospheric maximum, and means insulating means entirely enclosing the casing, a

core and winding assembly positioned in the casing, an auxiliary reservoir,'piping connecting the reservoir and the transformer casing for the circulation of the carbon dioxide, a body of 'carbon dioxide filling the transformer casing, the piping, and the reservoir, means operatively associated with the reservoir for limiting the temperature of the carbon dioxide in the system to a preselected subatmospheric maximum, and heat exchange means for subcooling the liquid carbon dioxide entering the transformer casing.

25. Stationary induction apparatus of the type described, comprising a transformer casing, heat insulating means entirely enclosing the casing, a core and winding assembly positioned in the casing, an auxiliary reservoir, piping connecting the reservoir and the transformer casing for the circulation of carbon dioxide, a body of carbon dioxide filling the transformer casing, the piping, and the reservoir, means operatively associated with the reservoir for limiting the temperature of the carbon dioxide in the system to a preselected subatmospheric maximum, means for maintaining a forced circulation of carbon dioxide through the transformer casing, the reservoir, and the piping, and heat exchange means for subcooling the liquid carbon dioxide entering the transformer casing.

HARRY ENSMINGER.

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