US2986593A

US2986593A - Gas bubble prevention system for liquid insulated electrical apparatus

Info

Publication number: US2986593A
Application number: US735478A
Authority: US
Inventors: Herbert B Detrick; Kurt H Weber
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1958-05-15
Filing date: 1958-05-15
Publication date: 1961-05-30
Anticipated expiration: 1978-05-30

Description

May 30, 1961 GAS BUBBLE PREVENTION SYSTEM FOR LIQUID INSULATED ELECTRICAL APPARATUS Filed May 15, 1958 DETRICK ET AL 2,986,593

frzflnffirs, Herberf B. Defr/c/c, KW? H. Weber, /5 5M 732,1,

United States Patent GAS BUBBLE PREVENTION SYSTEM FOR LIQUID INSULATED ELECTRICAL APPARATUS Herbert B. Detrick, Pittsfield, Mass., and Kurt H. Weber, Shaker Heights, Ohio, assignors to General Electric Company, a corporation of New York Filed May 15, 1958, Ser. No. 735,478

6 Claims. (Cl. 174-12) This invention relates to electrical apparatus, and more particularly to an improved means for preventing the formation of gas bubbles in the dielectric liquid of stationary electrical induction apparatus of the type having an electric device immersed in a dielectric liquid which is blanketed by a gas in a sealed container.

Stationary electrical induction apparauts, such as transformers or the like, are frequently immersed in a dielectric liquid, such as oil. In order to prevent contamination of the dielectric liquid by contact with air, the apparatus tanks are frequently sealed, and in order to allow for thermal expansion of the liquid in the tank, an inner gas space is provided within the tank contacting the liquid. As an example, a gas frequently employed in such capacity in electrical apparatus is nitrogen.

Where a gas and liquid are in such contact, some of the gas dissolves into the liquid, depending upon the kind of gas and liquid employed and the temperature and pressure of the fluids. The relationship between the amount of gas that dissolves in the liquid and the pressure of a gas may in some cases be a simple linear relationship. As an example, oil at 25 C. exposed to a nitrogen pressure of one atmosphere will dissolve 8.9% by volume of nitrogen, and when the nitrogen pressure is two atmospheres, the oil will dissolve 17.8% by volume of nitrogen.

If a closed system of transformer oil blanketed by nitrogen is given time to reach equilibrium at two atmospheres of pressure, so that the amount of nitrogen dissolved in the oil is 17.8% of the oil volume, and if the pressure is then released, for example, as a result of decrease in oil temperature in response to decrease in load, loss of excitation, or decrease in ambient temperature, the nitrogen Will remain in the oil if the oil is undisturbed. The oil is then said to be in a supersaturated state, since the quantity of nitrogen exceeds the equilibrium quantity of 8.9% corresponding to its new environment at one atmosphere. If a suflicient time is permitted, the excessive nitrogen will diffuse out of the oil and eventually the system will restore itself to equilibrium corresponding to one atmosphere and 25 C., that is, 8.9% nitrogen by volume. This return to equilibrium may require an extended period of time, depending upon the surface to volume ratio of the oil and the amount of motion in the oil due to convection or other minor disturbances.

If the oil remains undisturbed, the gas will diffuse out of the oil in an orderly manner. There are several operating conditions, however, that can cause supersaturated oil to evolve gas in the form of gas bubbles throughout the oil. For example, such evolution of gas bubbles may arise from the agitation of oil by oil circulation pumps, the electrical shock of a transient voltage, or the mechanical shock of a short circuit. While the extent of supersaturation required to produce bubbles is dependent upon variables other than the extent of saturation, it has been found that, with nitrogen in transformer oil, bubble formation can be expected if the oil 2,986,593 Patented May 30, 1961 is over saturated by 12% to 20%, i.e., when the pressure differential between gas in the oil and gas above the oil is between 2 and 3 pounds per square inch.

Due to the difference in dielectric constant and dielectric strength of nitrogen and oil, the formation of nitrogen bubbles in the oil has been found to reduce the dielectric strength of the oil by as much as two-thirds of its electrical strength without gas bubbles. Needless to say, this reduction of dielectric strength is highly undesirable, and may result in severe damage to or failure of the transformer.

In some instances a supply of gas, for example, from a pressurized cylinder of gas, is provided in order to maintain the pressure of the gas cushion within predetermined limits, since it has been found that substantial gas bubble evolution may occur if the range of pressures within the transformer tank is above about 2.5 p.s.i.g. In this type of system, which is known as the automatic gas seal type of oil preservation system, when the gas pressure reaches a maximum desired level the gas is vented to the atmosphere and when the pressure reaches a minimum level more gas is admitted to tank from the gas supply. While such a system may satisfactorily prevent evolution of gas bubbles if the range between the maximum and minimum pressures within the transformer tank is sufliciently narrow, the maintenance of such necessarily narrow limits may be highly undesirable due to the large quantities of gas that are normally expended in this type of oil preservation system as a result of periodic temperature cycling during operation of the transformer.

It is therefore an object of this invention to provide an improved system for the prevention of gas bubble evolution in electrical apparatus of the type having an electric device immersed in a dielectric liquid.

It is a further object of the invention to provide a gas bubble prevention system for maintaining a fluid pressure in electric apparatus of the type having electric device immersed in a dielectric liquid, the system maintaining the pressure in such apparatus within a sufliciently narrow range that evolution of gas bubbles in a liquid does not occur, and also not requiring the expenditure of excessive amounts of gas from a gas supply connected to a gas cushion in the apparatus.

Still another object of this invention is to provide a gas bubble prevention system for electrical apparatus of the type having an electrical device immersed in a dielectric fluid in a sealed tank and having a gas cushion contacting the liquid, the gas for said gas cushion being supplied from a pressurized gas supply, said system being characterized in that a narrow range of gas pressure is permitted in said gas cushion without the excessive expenditure of gas from the gas supply.

In accordance with one aspect of the invention, we provide means for preventing the evolution of gas bubbles in electrical apparatus of the type comprising a sealed enclosure containing a dielectric liquid in contact with a gas, and having an electric device immersed in the liquid. The means comprises storage tank means, means for withdrawing the gas from the enclosure to the storage tank means at a predetermined maximum enclosure pressure, and means for releasing gas from the storage tank to the enclosure, when the pressure in the enclosure is below a predetermined pressure which is less than the maximum desired pressure.

In the preferred arrangement of the invention, a supply of pressurized gas is provided to supply gas to the tank at a minimum pressure, and the gas is released from the storage tank to the enclosure at a pressure between the maximum pressure at which gas is withdrawn from the enclosure to the tank and the minimum pressure at which gas is supplied by the gas supply.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which we regard as our invention, it is believed that theinvention will be better understood from the following description taken in connection withthe accompanying drawing.

The single figure in the drawing illustrates a cross sectional view of a transformer and schematically shows the pressure regulation system of the invention as it may be connected to the transformer.

Referring now to the drawing, therein is illustrated an electrical apparatus such as a transformer comprising a sealed tank 11 partially filled with a dielectric liquid 12 such as oil. A gas space 13 is provided in the upper portion of the tank 11, to act as a cushion for volumetric changes in the oil 12. The transformer core 14 and coils 15 are immersed in the dielectric liquid 12, an external connection to the windings 15 is provided by way of bushings 16 mounted on and extending through the exterior walls or top of the tank 11.

A supply of gas, such as pressurized gas cylinder 20, is connected, by way of regulating valve 21 and conduit 22, to supply gas at a predetermined minimum pressure to the gas space 13 in tank 11. A gas storage tank 25 is also connected, by way of a regulating valve 26 and conduit 22, to the gas space 13 of the apparatus tank 11, and the storage tank 25 is also connected to the gas space 13, by a separate path including a pump 27 and a check valve 28. The pump 27 is arranged to withdraw gas from the gas space 13 through a conduit 29, and the check valve 28 is arranged to prevent gas from flowing back through the pump 27 and conduit 29 to the gas space 13. The regulating valves 21 and 26 are arranged to regulate the flow of gas respectively from cylinder and storage tank to the tank 11. A relief valve 30 may be provided in the conduit 22, for example, in order to relieve excessive pressures in the apparatus tank 11.

A pressure responsive device, such as a bellows 31 is provided within the gas space 13. The bellows 31 is mechanically coupled to suitable switching means 32 which is electrically connected to effect the energization of the pump 27 in response to the pressure in the gas space 13.

During normal operation of the apparatus 10, the temperature of the dielectric liquid 12 will vary depending, for example, upon the electrical load supplied by the apparatus and the atmospheric temperature. These temperature variations result in volumetric variation of the liquid, and, hence, variations in the pressure in the gas space 13. The pressure responsive means 31 is adjusted to eifect the energization of the pump 27, by means of the switch means 32, at a predetermined maximum pressure within the gas space 13. When the pump 27 is energized, gas is withdrawn by way of the conduit 29, pump 27 and check valve 28 to the storage tank 25. The withdrawal of gas from the gas space 13 continues until the energization of the pump 27 is stopped in response to detection of a second predetermined pressure within the gas space 13 by the pressure responsive means 31. The second predetermined pressure at which the operation of the pump ceases is of course lower than the maximum predetermined pressure at which the operation of the pump commences. The check valve 28 serves to prevent passage of the gas from the storage tank 25 back through the pump 27 and conduit 29 to the gas space 13.

When the pressure in the gas space 13 drops below a third predetermined pressure (below the pressures at which the pump 27 is energized and de-energized) the regulating valve 26 opens to permit release of gas from the storage tank 25 to the gas space 13 by way of conduit 22. The diiferential pressure between the maximum predetermined pressure and the pressure at which valve 26 opens is sufiiciently narrow that gas bubble evolution will not occur in the apparatus.

If the pressure within the gas space 13 drops below a predetermined minimum value (less than the pressure of opening of the regulating valve 26) such as may occur if insufiicient gas is present in the storage tank 25, the regulating valve 21 opens to permit release of gas from the pressurized gas cylinder 20 to the gas space 13 by way of the conduit 22. The relief valve 30 may be set to relieve pressure in the gas space 13 at a predetermined pressure above the maximum desired operating pressure within the gas space.

As an example of typical settings of the valve and pressure responsive device to prevent evolution of gas in the liquid 12, the pressure responsive means 31 may be set to energize the pump 27 at a pressure of 1.5 p.s.i.g. and de-energize the pump at a pressure of 1.0 p.s.i.g. The regulating valve 26 may be set to open when the pressure in the gas space 13 falls below 0275 p.s.i.g., and the regulating valve 21 may be set to open when the gas pressure in the gas space 13 falls below 0.5 p.s.i.g. With these settings, the pressure in the gas space 13 is maintained between the pressures 0.5 p.s.i.g. and 1.5 p.s.i.g., a diiferential of one pound per square inch in pressure, which is sufiicient to prevent the evolution of bubbles in the liquid 12. The relief valve 30 may be set to relieve pressure in the gas space 13 at 2.5 p.s.i.g. in the event of sudden buildup of pressure in the tank 11. It will be understood, of course, that the above example is purely illustrative, and other pressure settings of the valve and pressure responsive device may be employed to attain a desired pressure range within the gas space.

Since variations in the pressure in gas space 13 are compensated for by withdrawing gas to the tank 25 and releasing gas from the tank 25, the present invention provides a fluid pressure regulating system having the advantages of the previously employed automatic gas seal type of oil preservation system without having the disadvantage of the large expenditure of gas resulting from pressure variations within the gas space 13, and the expenditure of gas is not dependent upon the narrowness of range of pressures within the gas space.

It will be understood, of course, that while the form of the invention herein shown and described constitutes the preferred embodiment of the invention, it is not intended herein to illustrate all of the possible equivalent forms or ramifications thereof. It will also be understood that the words employed are words of description rather than of limitation, and that various changes may be made without departing from the spirit and scope of the invention herein disclosed, and it is aimed in the appended claims to cover all such changes as fall within the true spirit and scope of the invention.

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

1. Means for preventing the evolution of gas bubbles in electrical apparatus of the type comprising a sealed enclosure containing a dielectric liquid in contact with a gas and an electric device immersed in said liquid, said means comprising storage tank means, pump means operatively connected between said tank means and enclosure to withdraw gas from said enclosure to said tank means when the pressure in said enclosure is between a predetermined maximum value and a second value, a check valve operatively connected between said tank means and pump means to prevent gas compressed in said tank means from flowing back through said pump to said sealed enclosure, and valve means operatively connected between said tank means and enclosure to release gas from said tank means to said enclosure when the pressure in said enclosure is below a predetermined value less than said predetermined maximum value.

2. Means for preventing the evolution of gas bubbles in electrical apparatus of the type comprising a sealed enclosure containing a dielectric liquid in contact with a gas and an electric device immersed in said liquid, said means comprising storage tank means, pump means connected to P p as from said enclosure to said tank means, means for detecting the fluid pressure within said enclosure, said pressure detecting means being connected to energize said pump means to withdraw gas from said enclosure when the pressure therein is between a predetermined maximum value and a second value, a check valve operatively connected between said tank means and pump means to prevent gas compressed in said tank means from flowing back through said pump to said sealed enclosure, and valve means operatively connected between said tank means and said enclosure to release gas to said enclosure when the pressure therein is below a predetermined value less than said predetermined maximum value, the difierential between predetermined pressures being sufiiciently narrow to prevent the evolution of gas bubbles in said liquid.

3. Means for preventing the evolution of gas bubbles in electrical apparatus of the type comprising a sealed enclosure containing a dielectric liquid in contact with a gas and an electric device immersed in said liquid, said means comprising storage tank means, pump means connected to pump gas from said enclosure to said tank means, means for detecting the fluid pressure within said enclosure, said detecting means being operatively connected to eflect the energization of said pump means when the pressure within said enclosure reaches a first predetermined value and to continue the energization of said pump means until the pressure within said enclosure drops to a second predetermined value, means for preventing return flow of gas from said storage tank means to said enclosure through said pump means, and valve means connected between said tank means and enclosure to release gas from said tank to said enclosure when the pressure in said enclosure is below a third predetermined pressure less than said second predetermined pressure, the differential between said first and third predetermined pressures being sufficiently narrow to prevent the evolution of gas bubbles in said liquid.

4. Means for preventing evolution of gas bubbles in the dielectric liquid of electrical apparatus of the type having an electric device immersed in said liquid in a sealed enclosure, and in which said enclosure also contains a gas in contact with said liquid, said means comprising storage tank means, compressor means responsive to fluid pressure within said enclosure to pump gas from said enclosure to said tank to maintain the fluid pressure within said enclosure between a predetermined maximum value and a second value, means for preventing return flow of gas from said storage tank means to said sealed enclosure through said compressor means, and valve means responsive to fluid within said enclosure to release gas from said storage tank means to said enclosure to maintain the fluid pressure within said enclosure above a predetermined minimum value, the differential between said minimum and maximum pressures being sufiiciently narrow to prevent the evolution of gas bubbles in said liquid.

5. An electrical transformer comprising a sealed enclosure containing a transformer core and winding coil, a dielectric liquid in contact with said core and winding partially filling said enclosure, a gas that is soluble in said liquid filling the remainder of said enclosure, means for preventing the evolution of gas bubbles in said liquid comprising a supply of pressurized gas connected to said enclosure, a gas storage tank, a compressor pump connected to said enclosure and storage tank to pump gas from said enclosure to said storage tank, means for detecting the fluid pressure within said enclosure, the pressure detecting means being connected to energize said compressor pump to withdraw gas from said enclosure when the pressure therein attains a pre-determined maximum value, a check valve operatively connected between said storage tank and compressor pump to prevent gas compressed in said storage tank from flowing back through said pump to said enclosure, a valve operatively connected between said supply of pressurized gas and said enclosure to release gas to said enclosure when the pressure therein is below a pre-determined minimum value less than said predetermined maximum value, and other valve operatively connected between said storage tank and enclosure to release gas from said storage tank to said enclosure when the pressure therein below a pre-determined value intermediate said maximum and minimum pre-determined values, the diiferential between the maximum and intermediate pressures being sufficiently narrow to prevent the evolution of gas bubbles in said liquid.

6. A transformer as recited in claim 5 in which said means for detecting the fluid pressure within said enclosure comprises a bellows located entirely within said enclosure and mechanically coupled to switching means which is electrically connected to effect the energization of said compressor pump.

References Cited in the file of this patent UNITED STATES PATENTS 1,671,492 Skinner May 29, 1928 1,689,352 Maxon Oct. 30, 1928 1,979,407 Pike v- Nov. 6, 1934 1,982,045 Clithero Nov. 27, 1934 2,194,604 Malsbary Mar. 26, 1940 2,200,610 Weichsel May 14, 1940 2,464,026 Clark Mar. 8, 1949 2,575,726 Peck Nov. 20, 1951 FOREIGN PATENTS 629,224 France July 19, 1927 273,797 Switzerland May 16, 1951 OTHER REFERENCES Publication, General Electric Review, May 1953, pp. 58-59.