WO1997030498A1

WO1997030498A1 - Internal transformer chimney

Info

Publication number: WO1997030498A1
Application number: PCT/US1997/002633
Authority: WO
Inventors: Stewart W. Durian; Charles M. Martin; Jack C. Moore; Donald Stachewicz; Dennis O. Conway
Original assignee: Cooper Industries, Inc.
Priority date: 1996-02-20
Filing date: 1997-02-20
Publication date: 1997-08-21
Also published as: AU2131997A

Abstract

A chimney (20) is disclosed for use in fluid-cooled equipment to create a non-forced convective flow that efficiently transfers heat from the heat source to the outer surface of the equipment. The chimney (20) maximizes the heat transfer rate out of the equipment, and therefore, reduces the average temperature inside the equipment by forming inner and outer fluid cooling columns that heat and cool the fluid, decreasing heat transfer between the heating and cooling columns and increasing the fluid velocity and turbulence of the fluid columns. The increase in cooling efficiency allows reduction of the fluid used in a given transformer, thereby reducing the weight and cost of the transformer.

Description

INTERNAL TRANSFORMER CHIMNEY TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to the field of electrical distribution and power equipment, including transformers. More particularly, the present invention relates to a cooling system for electrical power distribution equipment, and still more particularly, to a chimney adapted to maximize the rate of heat transfer away from a particular piece of heat-generating electrical equipment. The present invention also relates to modifications of the equipment itself, such as transformer tanks, that can further maximize the advantages of provided by the increased rate of heat transfer. BACKGROUND OF THE INVENTION

Many types of conventional electrical equipment contain a dielectric fluid for dissipating the heat that is generated by energized components, and for insulating those components from the equipment enclosure and from other internal parts and devices. Examples of such equipment include transformers, capacitors, switches, regulators, circuit breakers and reclosers. A transformer is a device that transfers electric power from one circuit to another by electrical magnetic means. Transformers are used extensively in the transmission of electrical power, both at the generating end and the user's end of the power distribution system. A distribution transformer is one that receives electrical power at a first voltage and delivers it at a second, lower voltage.

A distribution transformer consists generally of a core and conductors that are wound about the core so as to form at least two windings. The windings (also referred to as coils) are insulated from each other, and are wound on a common core of magnetically suitable material, such as iron or steel. The primary winding or coil receives energy from an alternating current (AC) source. The secondary winding receives energy by mutual inductance from the primary winding and delivers that energy to a load that is connected to the secondary winding. The core provides a circuit or path for the magnetic lines of force (magnetic flux) which are created by the alternating current flow in the primary winding and which induce the current flow in the secondary winding. The core and windings are typically retained in an enclosure for safety and to protect the core and coil assembly from damage caused by the elements or vandalism.

The transformer windings or coils themselves are typically made of copper or aluminum. The cross section of the conductors forming the coil must be large enough to conduct the intended current without overheating. Transformers may be insulated with shellac, varnish, enamel, polymer or paper. Small transformers, those rated less than 50 kVA and lø, a varnished primary wire and nylon tape insulated secondary bar conductor may be used. For 75 to 500 kVA lø transformers and 75 to 1500 kVA 3ø transformers, a varnished primary wire and paper insulated secondary bar conductor may be used. For larger units, such as 3ø transformers rated 1000 to 10,000 kVA, both the primary and secondary conductors are typically insulated with paper. The insulation must provide not only for normal operating voltages and temporary overvoltages, but also must provide the required insulative levels during transient overvoltages as may result from lightning strikes or switching operations.

Distribution transformers used by the electric utilities in the United States operate at a frequency of 60 Hz (cycles per second). In Europe, the operating frequency is typically 50 Hz. Where the size and weight of the transformer are critical, such as in aircraft, transformers are typically designed to operate at a frequency of from 400 to 4,000 cycles per second. These high frequency applications allow the transformer to be made smaller and lighter than the 50 Hz and 60 Hz transformers designed for power distribution by the electric utilities.

The capacity of a transformer to transmit power from one circuit to another is expressed as a rating and is limited by the permissible temperature rise during operation. The rating of a transformer is generally expressed as a product of the voltage and current of one of the windings and is expressed in volt-amperes, or for practical purposes, kVA (kilovolt-amperes). Thus, the kVA rating of a transformer indicates the maximum power for which the transformer is designed to operate with a permissible temperature rise and under normal operating conditions.

Modern transformers are highly efficient, and typically operate with efficiencies in the range of 97-99%. The losses in the transformation process arise from several sources, but all losses manifest themselves as heat. As an example of the heat that is generated by even relatively small, fluid-filled distribution transformers, it is not uncommon for a 15 kVA mineral oil-filled transformer to operate with temperatures inside the transformer enclosure exceeding approximately 90°C continuously.

A first category of losses in a transformer comprises losses resulting from the electrical resistance in the conductors that constitute the primary and secondary windings. These losses can be quantified by multiplying the electrical resistance in each winding by the square of the current conducted through the winding (typically referred to as I²R losses). Similarly, the alternating magnetic flux (or lines of force) generates current flow in the core material as the flux cuts through the core. These currents are referred to "eddy currents" and also create heat and thus contribute to the losses in a transformer. Eddy currents are minimized in a transformer by constructing the core of thin laminations and by insulating adjacent laminations with insulative coatings. The laminations and coatings tend to present a high resistance path to eddy currents so as to reduce the current magnitudes, thereby reducing the I²R losses.

Heat is also generated in a transformer through an action known as "hysteresis" which is the friction between the magnetic molecular particles in the core material as they reverse their orientation within the core steel which occurs when the AC magnetic field reverses its direction. Hysteresis losses are minimized by using a special grade of heat-treated, grain-orientated silicon steel for the core laminations to afford its molecules the greatest ease in reversing their position as the AC magnetic field reverses direction.

Although conventional transformers operate efficiently at relatively high temperatures, excessive heat is detrimental to transformer life. This is because, as discussed above, transformers contain electrical insulation that is used to prevent energized components or conductors from contacting or arcing over to other components, conductors, structural members or other internal circuitry. Heat degrades the insulation, causing it to loose its ability to perform its intended insulative function. Further, the higher the temperatures experienced by the insulation, the faster the insulation material degrades and the shorter the life of the insulation becomes. The insulating materials in contact with the conductors in the winding are of prime concern, since these are in the vicinity of the highest temperatures in the transformer. Thus, maintaining the temperature of the insulation materials in the windings below that which will induce excess loss of transformer life is a primary concern. When insulation fails, an internal fault or short circuit may occur. Such occurrences could cause the equipment to fail. Such failures, in turn, typically lead to system outages. On occasion, equipment can fail catastrophically and endanger personnel who may be in the vicinity. Accordingly, it is of utmost importance to maintain temperatures within the transformer to acceptably low levels.

To prevent excessive temperature rise and premature transformer failure, distribution transformers are generally provided with a liquid coolant to dissipate the relatively large quantities of heat generated during normal transformer operation. The coolant also functions to electrically insulate the transformer components and is often therefore referred to as a dielectric coolant. A dielectric coolant must be able to effectively and reliably perform its cooling and insulating functions for the service life of the transformer which, for example, may be up to 20 years or more. The ability of the fluid and the transformer to dissipate heat must be such as to maintain an average temperature rise below a predetermined maximum at the transformer's rated kVA. The cooling system must also prevent hot spots or excessive temperature rises in any portions of the transformer. Generally, this is accomplished by submerging the core and coil assembly in the dielectric fluid and allowing free circulation of the fluid. The dielectric fluid covers and surrounds the core and coil assembly completely and fills all small voids in the insulation and elsewhere within the enclosure where air or contaminants could otherwise collect and eventually cause failure of the transformer.

As the core and coil assembly generates heat, the heat is transferred to the surrounding dielectric fluid. The heated fluid transfers the heat to the tank walls and ultimately to the surrounding air. Most, but not all, conventional distribution transformers include a headspace of air or inert gas, such as nitrogen, above the fluid in the tank. The headspace allows for some expansion of the dielectric fluid that will occur with an increase in temperature. Unfortunately, the headspace is also a thermal insulator and reduces the efficiency of heat transfer from the fluid to the tank's cover. In such designs, because the cover or the top of the transformer tank is not in physical contact with the coolant, it has a lower temperature that the tank walls and therefore cannot dissipate as much heat per unit surface area. The cooling must be sustained by the other surfaces of the enclosure that are in contact with the fluid.

In order to improve the rate of heat transfer from the core and coil assembly, transformers may include a means for providing increased cooling, such as fins on the tank that are provided to increase the surface area available for heat transfer, or radiators or tubes attached to the tank that are provided so that the hot fluid that rises to the top of the tank may cool as it circulates through the tubes and returns at the bottom of the tank. These tubes, fins or radiators provide additional cooling surfaces beyond those provided by the tank walls alone. Fans may also be provided to force a current of air to blow across the heated transformer enclosure, or across radiators or tubes to better transfer the heat from the hot fluid and heated tank to the surrounding air. Alternatively, some transformers include an internal oil circulation system. Such a cooling system may include a pump to circulate the dielectric coolant, or may rely on natural convection to circulate the coolant. However, it is undesirable for various reasons to employ mechanical pumps or other complicate auxiliary apparatus to effect liquid circulation. Similarly, thermally induced convection currents in the liquid coolant are sometimes not sufficient to provide desirable or necessary cooling characteristics.

To adequately transfer heat away from the transformer core and coil assembly so as to maintain an acceptably low operating temperature, conventional transformers require relatively large volumes of dielectric fluid. For example, a standard 15 kVA pole mounted single phase distribution transformer housed in a cylindrical container and having a head space of air above the fluid may contain approximately ten gallons of fluid. Every gallon of fluid increases the weight of the transformer by approximately eight pounds. Thus, for the example given above, the fluid alone adds over eighty pounds to the transformer. The weight of the dielectric fluid also may require that a transformer enclosure be made of heavier gage steel than would be required for a smaller transformer, or may require that special or stronger hangers or supports be provided. Such additions also increase the weight and cost of the transformer.

Unfortunately, increasing the size of the transformer has undesirable consequences even beyond the size and weight considerations discussed above. Transformers, particularly the common pole mounted distribution transformers, are frequently mounted in areas congested by other electrical distribution equipment, including other transformers, conductors, fuses, and surge arrester, as well as by telephone and cable TV lines and cables. Important minimum clearances must be maintained between the energized transformer terminals and all other nearby equipment and lines and all grounded structures, including the transformer's own grounded tank. Accordingly, because of the height of conventional transformers, a dimension that, in great part, is dictated by the fluid volume, maintaining the appropriate clearance is ever-increasingly becoming a problem when trying to locate and mount the transformer. Thus, there are cost advantages and weight savings that can be obtained from a transformer design that will effectively dissipate heat using less-than-conventional volumes of dielectric coolant.

Other significant drawbacks are directly associated with the size and weight of conventional transformers. Providing a transformer design that is smaller and lighter than conventional, similarly-rated transformers would save costs associated with shipping and storing larger and heavier equipment, and may ease installation difficulties and lessen installation costs given that a smaller transformer may not require the same equipment or personnel to install as a larger, heavier unit.

Accordingly, despite the advances made in transformer and dielectric fluid technology, there remains a need in the art for a transformer that is smaller, lighter weight and that contains less dielectric coolant than conventional transformers. Preferably, a reduction in size is effected by maximizing the heat transfer rate from the core and coil to the coolant and from the coolant to the air, so that less coolant is required. It is also desirable to reduces the temperature-induced degradation of the insulation, by reducing the maximum temperatures to which the insulation is exposed.

These and other objects and advantages of the invention will appear and be understood from the following description. SUMMARY OF THE INVENTION

The present invention relates to improvements in a chimney used in fluid-cooled, heat- generating equipment to create a non-forced convective flow that efficiently transfers heat from the heat source to the outer surface of the equipment. The present chimney maximizes the heat transfer rate out of the equipment, and therefore reduces the average temperature inside the equipment, by forming inner and outer fluid cooling columns that heat and cool the fluid, respectively, decreasing heat transfer between the heating and cooling columns, and increasing the fluid velocity and turbulence of the fluid columns. BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of a preferred embodiment of the invention reference will now be made to the accompanying drawings wherein:

Figure 1 is a cross-sectional view of a transformer showing a chimney according to the prior art;

Figure 2 is a cross-sectional view of a transformer showing an improved chimney according to the present invention;

Figure 3 is a cross-sectional view of a transformer showing a first alternative embodiment of an improved chimney according to the present invention;

Figure 4 is a cross-sectional view of a transformer showing a second alternative embodiment of an improved chimney according to the present invention;

Figure 5 is a top view of the inside of a transformer showing a third alternative embodiment of an improved chimney according to the present invention; Figure 6 is a top view of the inside of a transformer showing a fourth alternative embodiment of an improved chimney according to the present invention;

Figure 7 is a cross-sectional view of a transformer showing a fifth alternative embodiment of an improved chimney according to the present invention;

Figure 8 is a cross-sectional view of a transformer showing a sixth alternative embodiment of an improved chimney according to the present invention;

Figure 9 is a cross-sectional view of a transformer showing a seventh alternative embodiment of an improved chimney according to the present invention;

Figure 10 is a cross-sectional view of a transformer showing a eighth alternative embodiment of an improved chimney according to the present invention;

Figures 11A and 1 IB are top and cross-sectional views, respectively, of a transformer showing a ninth alternative embodiment of an improved chimney according to the present invention;

Figures 12A and 12B are top and cross-sectional views, respectively, of a transformer showing a tenth alternative embodiment of an improved chimney according to the present invention; and

Figures 13A and 13B are top and cross-sectional views, respectively, of a transformer showing a eleventh alternative embodiment of an improved chimney according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

The present invention will be discussed hereinafter in the context of transformers for the purposes of simplicity, but it will be understood that the concepts disclosed and described herein will be applicable in other fluid-cooled, heat generating electrical equipment. Referring initially to Figure 1, as known in the art, a transformer 10, comprising a core/coil assembly 12 housed in a tank 14 and submerged in dielectric coolant 16, may include a basic chimney 20. Chimney 20 is a barrier installed between the outer surface of core/coil assembly 12 and the transformer tank wall 14 such that it directs the flow of coolant 14, in the transformer tank. To this end, the basic chimney 20 is spaced away from both the tank wall 14 and the outer surface of the core/coil assembly such that it forms a fluid heating column 22 and a fluid cooling column 24. A lower passage 26 is provided adjacent its lower end to allow coolant to flow from fluid cooling column 24 to fluid heating column 22. The top edge of chimney 20 is located some distance below the upper coolant surface forming an upper passage 27 to allow coolant to flow from fluid heating column 22 to fluid cooling column 24. The chimney directs the coolant flow so that the coolant that flows from the top of the core/coil assembly 12 driven by natural convection forces, must rise to within a short distance of the coolant surface before entering the top of fluid cooling column 24.

This invention consists of modifications that improve the performance of a transformer chimney. First, referring to Figure 2, it has been found that reducing the exit (top) flow area of a right cylindrical chimney through the use of a restrictor plate 29 affixed to the top of the chimney improves flow properties. The restrictor is a circular plate of material having an outer diameter such that it can be sealed to the chimney and a central opening with a diameter smaller than the chimney diameter. By adding a flow restriction of a particular size and configuration, the fluid flow rate can be controlled so as to provide a minimum temperature gradient between the winding(s) and the fluid. For example, in a transformer designed for a 65°C temperature rise, it is preferred that the temperature gradient be less than about 15°C and more preferably less than about 8°C. Using an improved chimney according to the present invention, the temperature gradient has been reduced from approximately 12-lt C to approximately 8-11°C. Also, the controlled fluid exit will reduce the amount and likelihood of flow recirculation in fluid heating column 22. Fluid recirculation within either fluid column 22 or 24 is undesirable, as it reduces the rate of heat transfer.

According to the present invention, passage 26 is configured such that: a) the fluid is forced to remain near or in contact with the tank wall 14 to very near the bottom of the tank, and b) proper restriction is added to control flow rate. By forcing the fluid to flow to the bottom of the tank, more of the tank wall surface and the tank bottom are brought into play for heat dissipation. Also, the cooling leg of the thermo-siphon flow is lengthened to its fullest extent, thus enhancing flow. In addition, by providing an engineered and controlled entrance configuration, the flow of coolant can be directed to particular portions of the core/coil assembly 12. For example, since core losses are reduced as the core temperature increases, it may be desirable to direct the flow of coolant from passage 26 through and around the coils while avoiding the core to the extent possible. It may also be desirable to configure the outlet of passage 26 such that the fluid is split in a desired proportion between flow through the coils and flow around the outside of the coils. An optimal fluid split is one that achieves the lowest coil to oil gradient.

The velocity of the fluid flow as it circulates through the transformer is a function of the cross-sectional area through which the fluid flows. It is preferred to locate the smallest cross- sectional area at the top of the chimney.

Referring now to Figure 3, another modification comprises constructing the chimney walls out of a thermally insulating material so as to provide a thermal barrier 30 between heating and cooling columns 22, 24. By providing thermal barrier 30 between the flow zones, the temperature differential between the flow zones can be maximized. Because the convective flow rate depends on the relative densities of the fluid in heating and cooling columns 22, 24, increasing the temperature difference between them increases flow rates.

Referring now to Figure 4, an alternative embodiment of the present invention comprises a chimney having thick, curved walls 40 that are sculpted or shaped to maximize convective flow. This shape includes parabolic walls that funnel the fluid in each direction. By eliminating sharp corners from the fluid flow path, circulatory flow is maximized, resulting in maximal heat transfer away from the core/coil assembly. It will be understood that the functional aspects of each of the three embodiments disclosed in Figures 2-4 can be combined advantageously without departing from the scope of the present invention.

Additional modifications to the chimney design shown in Figure 4 entail constructing the shaped chimney 50 from a material that is sufficiently rigid that it can support its own weight and maintain its required shape without additional supports or standoffs from either the tank wall 14 or the core/coil assembly 12, such as polymeric material, wood, pressboard or rigid cardboard construction. The material from which chimney 50 is constructed is also preferably impermeable or virtually impermeable to the flow of coolant therethrough.

Still referring to Figure 4, the chimney may be constructed such that the chimney itself has a relatively large volume and displaces a significant amount of coolant, thereby reducing the total amount of coolant required in the tank. It has been found that as much as 90% of the fluid volume that is used in a conventional transformer can be displaced in the present transformer without affecting thermal performance.

Shaping the chimney also allows the cross-sectional flow area to be altered continuously along the circulation path. By continuously altering the cross-sectional area of the flow channel to offset the changes in fluid density corresponding to temperature it is possible to provide a constant fluid velocity, and uneven flow losses can be reduced. The equation that relates velocity, mass flow, density, and flow area is:

p A

Where: V = velocity m = mass flow rate p = density (varies with temperature)

A = flow area

In order to better control the cross-sectional flow area, a additional solid piece 53 may be included around the lower edge of the tank wall 14, where wall 14 meets the tank floor. Comer piece 53 defines a more efficient version of passage 26 between cooling column 24 and heating column 22 and eliminates dead space in which non-circulating fluid could reside.

Still referring to Figure 4, if the chimney 20 is to displace a significant amount of coolant, it is preferred that the chimney be constructed out of a material having a relatively low density, so that the weight of the transformer can be reduced. If the chimney weighs less than the fluid displaced, the overall weight of the transformer decreases. It is known that the fluid film thickness in cooling column 24 can be as small as 0.25 inches with no loss in the transfer of heat to the tank wall. Likewise, the film thickness in heating column 22 can be as small as 0.18 inches with no loss in the transfer of heat to the fluid. As the thickness of the fluid layer decreases, flow in the layer becomes more turbulent. In general, turbulent flow is a better transferor of heat than laminar flow. Fluid film thickness in heating column 22 and cooling column 24 should be small enough to ensure that the available natural convection driving forces cause all flow each column to be uni-directional. If the channel is too wide, fluid in the column tends to recirculate. Recirculation can cause premature cooling of the fluid and viscous flow losses.

Referring now to Figure 5, in applications where it is not feasible to construct a chimney completely around a core/coil assembly 12 due to electrical connections 28 or other construction requirements, such as in padmounted transformers where the connections are on a frontplate, it has been found that a chimney modified to accommodate these connections still facilitates heat transfer. Testing on a 3ø padmount transformer has demonstrated that this variation reduces the winding temperature rises similarly to an unmodified chimney.

Referring now to Figure 6, an alternative of the transformer shown in Figure 5 has a chimney 20 comprising a straight baffle 20 extending across the entire width of the tank. The baffle is positioned in the tank so that it defines two areas of vertical flow, namely, a heating column 32 that surrounds the core/coil assembly and a cooling column 34 that does not.

Figure 7 shows an alternative embodiment comprising a frusto-conical exit flow restrictor 36. The frusto-conical exit restrictor 36 provides an annular flow exit 38 having a controlled area. The shape of exit restrictor 36 also provides a smoother flow transition through the exit 38 than does the restrictor plate 29 shown in Figure 2. The smooth transition reduces flow losses and the potential for flow stagnation under the exit resistor as compared to the embodiment shown in Figure 2. The four dimensions d„ α^, x„ and x₂ indicated in Figure 7 affect fluid velocity and are the most important dimensions in configuring the transformer for maximum performance.

Figure 8 is comparable to the embodiment shown in Figure 2, but has an annular chimney exit opening 40 as opposed to the central opening shown in Figure 2. As with the embodiment shown in Figure 7, the annular exit is better than a central exit because of the smaller horizontal flow distances required. The embodiment shown in Figure 8 includes a substantially flat flow restrictor 42 that is simpler to manufacture and takes up less volume than frusto-conical restrictor 36.

Figure 9 embodies a fluid displacement concept. The truncated cone 44 that forms annular exit 46 extends above the oil free surface or is sealed to the tank top in a liquid solid tank. This displaces the volume of fluid above restrictor 36 in Figure 7 without affecting flow.

The embodiment of Figure 10 uses an annular upper restrictor 48 to define both a central hot flow exit opening 47 and an annular cold flow inlet opening 49. Restriction of the flow at two separate points results in better turning of the flow.

For best results, the chimney 20 should be positioned such that it neither comes into contact with the surface of the winding or with the surface of the tank wall that is being affected by its performance during normal full load or elevated load operation. During periods when the transformer is not loaded, or is operating well below normal temperatures, however, conditions may exist whereby the tank walls are in contact with the chimney. These conditions may occur, for example, due to an internal tank pressure less than one atmospheric. Example I 1 ' 2 *^•

Referring now to Figures 11A and 11B, an alternative chimney 52 in accordance with the present invention comprises a length of barrier material 54 across which a plurality of rigid ribs 56 are affixed. Barrier material 54 preferably comprises kraft paper such as is known in the art. Ribs 56 are preferably approximately 0.25 inches square and preferably comprise rigid, electrically insulating material, such as pressboard or wood. Ribs 56 are affixed to barrier material 54 such that one end of each strip is approximately even with one edge of the barrier material, while the second end of each strip extends a short distance, such as 0.375 inches, beyond the second edge of the barrier material. This combination of kraft paper with rigid ribs affixed thereto is hereinafter referred to as rib duct material. The length of rib duct material 54 is approximately equal to or slightly greater than the circumference of core/coil assembly 12. Strip 54 is wrapped around the core/coil assembly 12 with ribs 56 on the inside, and the wrapped assembly is placed inside the transformer tank. Ribs 56 bear on core/coil assembly 12, forming a plurality of substantially vertical channels 58 between it and paper 54, and the extending ends of ribs 56 rest on the tank floor, elevating the lower edge of the barrier so as to provide access to channels 58. The coolant is heated and rises up through channels 58 and is recirculated down around the outside of chimney 52. The width of rib duct material, including the extending strip ends is greater than the height of core/coil assembly 12 such that chimney 52 extends above the top of core/coil assembly 12 to within approximately one inch of the fluid surface or the tank top. This configuration has resulted in up to a 30% reduction in the average coil to average oil temperature gradient over predicted values for comparable transformers without a chimney.

Transformers of various sizes equipped with chimneys as shown in Figures 11A and 11B were tested. For each transformer size, the primary and secondary winding gradients are given in Table 1 below. Predicted values of the same gradients in transformers not equipped with chimneys were calculated using a modeling algorithm with an accuracy of ± 2°C, and are also given below for comparison.

Table 1 Transformer Primary Secondary Size Winding Gradient Winding Gradient

With No Chimney With No Chimney Chimney (predicted) Chimney (predicted) lO Kva 11.10 12.14 10.10 12.07

15 Kva 8.65 12.16 8.85 12.04

25 Kva 13.75 13.58 14.25 13.85

37.5 Kva 15.25 15.07 14.35 14.65

50 Kva 9.65 10.35 14.25 12.85

When transformers using the chimney design shown in Figure 11A-B were used, some difficulty was encountered in assembling the transformer. Specifically, because the opening in the tank top through which the core/coil assembly is inserted provides very little clearance, the chimney often sustained damage during insertion of the core/coil assembly.

Example II

Referring now to Figures 12A and 12B, another alternative chimney 60 in accordance with the present invention comprises a length of rib duct material 54 having a length approximately equal to the circumference of tank wall 14. Chimney 60 includes an additional reinforcing layer 65 of pressboard affixed to and coextensive with the paper. According to the embodiment shown in Figures 12A and 12B, the reinforcing layer comprises 0.040 inch thick pressboard. Strip 54 is wrapped into a cylindrical shape, with ribs 56 on the outside, and placed inside the transformer tank prior to placement of the core/coil assembly. Ribs 56 bear on tank wall 14, forming a plurality of substantially vertical channels 62 between the outside of chimney 60 and wall 14. The inside of chimney 60 defines a large heating column 63 surrounding core/coil assembly 12. The coolant is heated in column 63, rises and is recirculated down through channels 62 around the outside of chimney 52. Again, chimney 60 extends above the top of core/coil assembly 12 to within approximately one inch of the fluid surface or the tank top. When tested, this configuration has resulted in an average coil to average oil temperature gradient approximately 1.24°C higher than that of the embodiment shown in Figures 1 IA and

11B.

Example IH

Referring now to Figures 13A and 13B, another alternative chimney 64 in accordance with the present invention comprises a combination of the cylinder 60 shown in Figures 12A and 12B with two additional lengths 66 of reinforced rib duct material spanning the width of cylinder 60 adjacent core/coil assembly 12. Spanning lengths 66 are preferably reinforced with 0.090 inch pressboard. This forms a plurality of channels 62, a heating column 68 and a pair of "D"-shaped dead zones 70. The coolant is heated and rises up through column 68 and is cooled and recirculated down through channels 62 around the outside of chimney 52. Again, chimney 52 extends above the top of core/coil assembly 12 to within approximately one inch of the fluid surface or the tank top. This configuration has resulted in an additional reduction in the average coil to average oil temperature gradient of approximately 3% over that of the embodiment shown in Figures 11 A and 1 IB.

The coolant flow in the tank is driven by the temperature difference between the core/coil assembly 12 and tank wall 14 and the separations of thermal centers of the core/coil assembly 12 and tank wall 14. By forcing increased separation of the thermal centers through the addition of the chimney and enhancements created by this invention, a fixed amount of heat can be transported with a lower temperature differential between core/coil assembly 12 and tank wall 14. This has the effect of allowing the core and coil to operate at a lower temperature without an increase in tank cooling surface area. Thermal center is defined as the vertical location corresponding to the mean surface temperature of either a heating or cooling surface.

In a transformer, the heating surface is the surface of the core/coil assembly and the cooling surface is the tank wall. By forcing the hot coolant exiting the core/coil assembly higher in the tank before it is allowed to come into contact with the cooling surface, the location of the tank wall maximum temperature is moved upward. Also, by forcing the cold coolant adjacent to the lower portions of the tank wall to near the bottom of the tank before it is allowed to flow back into the windings, the location of the tank wall minimum temperature is moved downward and the minimum coolant temperature is also decreased. The vertical separation of the tank wall minimum and maximum temperatures is increased and the thermal center of cooling is moved upward. In a test 15 kVA polemount transformer, the distance between the maximum and minimum tank wall temperatures increased from about 10 inches to about 18 to 20 inches. Because the vertical center of heating within the core/coil assembly remains unchanged, the net result is an increased separation of thermal centers. In a test 15 kVA polemount transformer, the distance between the thermal centers increased from about one inch to about two to three inches.

The vertical height of the cliirnney should be as tall as possible while still ensuring that the bottom opening is sufficiently large, and the top opening is below the normal coolant upper level. The bottom openings must be low enough to ensure that the flowing coolant is forced to very near the bottom of the tank, this allows the greatest tank surface area to be involved in dissipating heat. Likewise the top openings should be very near the coolant surface (or tank top for a liquid solid design) to ensure that the coolant is forced to flow as far up as possible. By providing the maximum separation between fluid entrance and exit, the natural convection driving force is maximized.

It will be understood that, while the present invention has been shown and described with respect to a transformer having headspace and a fluid free surface, this invention is also applicable to a tank that is completely filled with coolant. Such completely filled transformers are sometimes referred to as liquid/solid transformers. Likewise, although the present invention is discussed herein in terms of a single core and coil, these same concepts are applicable to rectangular or polygonal or other shape tanks and/or multiple core and coil configurations.

While a preferred embodiment of the invention has been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit of the invention.

Claims

CLAIMS What is claimed is:

1. A chimney for use in a transformer comprising a core/coil assembly housed in a tank and submerged in dielectric coolant, said chimney comprising: a wall portion for guiding the coolant in a cooling loop; and a restrictor affixed to the top of said wall portion for reducing the exit flow area of a channel formed by said wall portion.

2. The chimney according to claim 1 wherein the height of said wall portion is such that said wall portion extends from proximal an upper surface of the dielectric coolant to proximal the floor of the tank.

3. The chimney according to claim 1 wherein said wall portion comprises a thermally insulating material.

4. The chimney according to claim 1 wherein said wall portion has thick, curved walls that are sculpted or shaped to maximize convective flow.

5. The chimney according to claim 1 wherein said wall portion has a parabolic cross- section so as to funnel fluid in each direction.

6. The chimney according to claim 5 wherein said wall portion def s a continuously changing cross-sectional area for the flow channel so as to offset the changes in fluid density corresponding to temperature and thereby provide a constant fluid velocity,

7. The chimney according to claim 1, further including a corner piece positioned around the lower edge of the tank wall where the tank wall meets the tank floor.

8. The chimney according to claim 1 wherein the chimney comprises a straight baffle extending across the entire width of the tank so as to define two areas of vertical flow, namely, a heating column that surrounds the core/coil assembly and a cooling column that does not.

9. The chimney according to claim 1 wherein the chimney includes a frustoconical exit flow restrictor that provides an annular flow exit having a controlled area.

10. The chimney according to claim 1 wherein the chimney has an annular chimney exit opening

11. The chimney according to claim 10 wherein the flow restrictor is substantially flat.

12. The chimney according to claim 1 further including an annular upper restrictor that defines both a central hot flow exit opening and an annular cold flow inlet opening.

13. A method for reducing the amount of dielectric coolant required in a transformer comprising a core/coil assembly housed in a tank and submerged in dielectric coolant, comprising the steps of: providing a chimney in the tank, said dώriney surrounding the core/coil assembly so as to define a heating fluid column and a cooling fluid column; and directed said heating fluid column to particular portions of the core/coil assembly.

14. The method according to claim 13 wherein the flow of coolant from is directed through and around the coils while avoiding the core to the extent possible.

15. The method according to claim 13 wherein the flow of coolant is split in a desired proportion between flow through the coils and flow around the outside of the coils

16. The method according to claim 13 wherein the amount of fluid required is reduced by 90% of the amount required under similar conditions without a chimney.

17. The method according to claim 13 wherein the velocity of fluid circulating in the tank is maintained substantially constant throughout said tank

18. A chimney for use in a transformer comprising a core/coil assembly housed in a tank and submerged in dielectric coolant, said chimney comprising: a wall portion for guiding the coolant in a cooling loop, said wall portion comprising a thermally insulating material and having thick, curved walls that are sculpted or shaped to maximize convective flow;

19. The chimney according to claim 18 wherein said wall portion has a parabolic cross- section that defines a continuously changing cross-sectional area for the fluid flow channel so as to offset the changes in fluid density corresponding to temperature and thereby provide a constant fluid velocity,

20. The chimney according to claim 19, further including a co er piece positioned around the lower edge of the tank wall where the tank wall meets the tank floor.

21. The chimney according to claim 19, further including an exit flow restrictor that provides an annular flow exit having a controlled area.

22. The chimney according to claim 19 further including an annular upper restrictor that defines both a central hot flow exit opening and an annular cold flow inlet opening.