Abstract:
A system ( 27 ) for reducing the temperature of cooling oil for a power transformer ( 12 ) includes a heat exchanger ( 44 ) interposed in the cooling oil system. The heat exchanger ( 44 ) relies upon a liquid-to-liquid exchange of heat from the heated oil to a coolant flowing through the heat exchanger. In one embodiment, the coolant provided to the heat exchanger is obtained from an absorption chiller ( 65 ). Heat energy is provided to the chiller ( 65 ) from a heat storage device ( 80 ). In a specific embodiment, the heat storage source ( 80 ) can be a phase change material device. In a preferred cooling system, a programmable controller ( 55 ) determines the activation and operation of the system. The controller ( 55 ) can sense transformer or cooling oil temperature to trigger activation. In a preferred embodiment, the controller ( 55 ) compares a current temperature history against a temperature profile to anticipate increased cooling requirements. In certain embodiment, excess, off-peak or waste heat from the transformer ( 12 ) itself is provided to the heat storage device ( 80 ) or to the phase change heat exchanger.

Description:
This application claims the benefit of Provisional application Ser. No. 60/166,065 filed Nov. 17, 1999. 

   BACKGROUND OF THE INVENTION 
   The present invention pertains to transformers, and, in particular, to an apparatus and method for cooling power transformers during use. 
   Power transformers are employed within power supply systems in order to transform, transmit and distribute electricity for end-user consumption. Transformers are designated by high and low side operating voltages, and sized as to capacity of the volts and amperes being carried. For example, large-size transformers are utilized as transmission transformers, which step up the voltage along the power supply chain, as well as distribution transformers, which step down the voltages for distribution. 
   One shortcoming of existing transformers is their susceptibility to operational problems associated with high temperatures of operation, both internal and external to the transformers. Typically, in order to maintain rated capability and preserve useful life of the transformer and all of its constituent parts, maximum temperature within the transformer should be maintained below the lesser of 95° C. (203° F.) and a temperature that is 65° C. above ambient temperature. The failure to keep the transformer temperature so regulated can result in failure of the transformer or perhaps the significant reduction of its useful life, each of which results in high cost to the industry due to the need to replace the destroyed transformer units. 
   Moreover, due to the directly proportional relationship between temperature and electrical resistance, when the temperature of the copper windings in the transformer core increases, the efficiency of the transformer decreases, thereby resulting in a loss of power output (watts) proportional to transformer core heating. Moreover, during use, the temperature inside the transformer tends to increase due to the electrical current flowing through the conducting windings and the micro-current flowing in the magnetic steel core. 
   Some prior attempts at controlling transformer temperature have been relatively crude. For instance, one common approach has been to simply drench the transformer with a water spray when ambient conditions suggest the risk of excessive transformer temperature, or when a high temperature condition is sensed. 
   In another approach, oil baths have been provided for the inner workings of the transformer. In different prior art applications, such oil baths were designed to operate on several levels. First, a “self cooled” level essentially relies on convention currents within the transformers insulating and cooling oil to draw heat away from the core. A second level uses a forced circulation of the insulating oil through heat exchangers/radiators integral with or separate from the transformer which utilize ambient air around the heat exchangers to absorb the heat energy of the cooling oil. A third level uses the forced oil circulation of the second level but adds electric fans, powered by energy supplied from the transformer itself or other sources of power in the substation, to force air circulation over the external radiators thus increasing heat removal from the oil and therefore the transformer windings, and thereby increasing transformer efficiency. These fans, which are selectively operated when transformer temperature rises are sufficiently large, are controlled by a controller connected to temperature sensors located in and on the transformers. 
   A prior art system that uses fans is diagrammatically shown in FIG.  1 . The transformer, generally designated  10 , is of conventional design and includes a casing or housing in which is disposed a soft iron core  12  with copper windings  14  there-around. The core and windings are immersed in a bath of cooling oil  15 . A nitrogen gas blanket  16  at the top of the internal volume of the transformer housing maintains the quality of the oil within the housing. 
   Positioned proximate the top of the transformer housing is an outlet connected via a top isolating valve  18  to a conduit  20  that leads to a radiator or heat exchanger, generally designated  22 . In this prior system, radiator  22  includes finned cooling tubes  24  through which the cooling oil is circulated. The tubes are oriented in a series of spaced apart rows and columns to allow the passage of ambient air there-around for cooling purposes. A plurality of motor-driven fans  26  are designed to draw air over and around the finned cooling tubes  24  in order to provide forced-air ambient cooling. The outlet of radiator  22  is plumbed to a sealed, motor-driven pump  28  that pumps the cooling oil through conduit  30 , bottom isolating valve  32 , and back into the internal volume of the transformer housing. 
   During operation, pump  28  forces cooling oil into the base of the transformer as indicated by arrow  33 . As the oil travels upward, as indicated at  35 , over and through the various openings provided within the internal workings of the transformer (such as the core  12  and windings  14 ), the temperature of the cooling oil increases as it draws off heat, and thereby cools the transformer parts which have increased in temperature due to their operation. The now heated oil passes through the oil outlet at  37  into conduit  20  and is routed through radiator  22 . Ambient air, which is ducked into the area in which the radiator is installed, is drawn over the cooling tubes  24  by fans  26  to cool the oil passing through tubes  24 . The ambient air which has been heated as the energy is drawn off the cooling oil is discharged to the atmosphere, and the cooled fluid is returned to pump  28  for recirculation through the transformer. 
   While the prior art cooling system shown in  FIG. 1  does provide some benefit, its cooling limitations result in some transformers being operated in conditions which are undesirable. Specifically, limits imposed by ambient conditions, most specifically temperature humidity, can result in the cooling oil passing entirely through the heat exchanger without a sufficient removal of heat energy, such that over time the temperature of the cooling continues to build, and the cooling capabilities of such oil then declines. In the end, the cooling oil may become too hot to prevent the transformer from exceeding recommended temperatures. 
   Also, transformer utilization and consequently load current loading typically occurs during the highest ambient temperature conditions. For example, the temperature of the oil bath may be elevated on days when the ambient conditions are extremely warm and humid, and consequently the oil is not adequately cooled and the temperature continues to build in the transformer until damage can be done to the transformer workings. 
   Thus, it would be desirable to provide a cooling system for power transformers which overcomes these and other shortcomings of the prior art. 
   SUMMARY OF THE INVENTION 
   The present invention provides an apparatus and method for improved cooling of the inner workings of a power transformer. The apparatus modulates transformer core heat while at the same time providing a high efficiency source of cooling which is less susceptible to having heat build up in the cooling oil due to ambient condition variation and core heat due to transformer loading. The apparatus selectively routes cooling oil for the transformer into a heat exchanger provided with a source of chillate more effective than ambient air. The present invention may utilize off-peak energy provided by the transformer to power the system via the use of heat exchangers that store energy for subsequent use. In one embodiment, the heat exchangers can utilize phase change material for heat storage. 
   One advantage of the present invention is that it provides a system for effectively cooling the inner workings of a transformer for all atmospheric conditions in order for the transformer to not be subjected to heat which damages its structural integrity and/or its efficiency. 
   Still another advantage of the present invention is that the cooling system can use a heat exchanger operated with chillate provided by the use of off-peak energy, thereby decreasing the cost of operations. Yet another advantage is that the heat energy of the transformer can be utilized in providing the energy required to operate the heat exchanger that cools the cooling oil routed in the transformer. 
   A further advantage is accomplished by features of the present invention that effect properly cooling of the transformer cooling oil for any ambient conditions, thereby maximizing the electrical capacity of the transformer. 
   One object of the inventive system is to improve the overall efficiency of the transformer by removing heat generated by current flow through the transformer. Another object is to increase the utilization and extend the useful life of the transformer serviced by the inventive cooling system. 
   The above mentioned and other advantages and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic front view, in partial cross-section, of a prior art cooling system for a power transformer. 
       FIG. 2  is a diagrammatic front view, in partial cross-section, of portions of a cooling system for a power transformer of the present invention. 
       FIG. 3  is a diagram of transformer useful hours as a function of transformer oil temperature, comparing the performance of prior art cooling systems with the performance of a cooling system constructed according to the teachings of the present invention. 
       FIG. 4  is diagrammatic view illustrating one embodiment of the components used to produce chillate for the heat exchanger of FIG.  2 . 
       FIG. 4A  is cross-sectional view, taken along line  4 A— 4 A of  FIG. 4 , of the phase change material heat exchanger, and further showing in dashed lines the electrical connection between the heat exchanger heating elements and a source of power. 
       FIG. 5  is a diagrammatic plan view of a power transformer cooling system of the present invention suitable for use with a 100 MVA power transformer. 
       FIG. 6  is a diagrammatic plan view of another power transformer cooling system of the present invention. 
       FIG. 7  is a diagrammatic representation of a power substation having a number of transformers, each including a cooling system in accordance with one embodiment of the present invention. 
   

   Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the invention, the drawings are not necessarily to scale and certain features may be exaggerated or omitted in order to better illustrate and explain the present invention. 
   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. The invention includes any alterations and further modifications in the illustrated devices and described methods and further applications of the principles of the invention which would normally occur to one skilled in the art to which the invention relates. For example, while the illustrated embodiment is a retrofit to an existing system, the inventive cooling system can be incorporated into the design specifications for a newly constructed power transformer array. 
   Referring now to  FIG. 2 , there is diagrammatically shown selected portions of one embodiment of a transformer cooling system  27  of the present invention. In this embodiment, the transformer and cooling system of the prior art shown in  FIG. 1  has been retrofit with a cooling system of the present invention to provide improved transformer cooling capabilities during many operating conditions possibly experienced by the transformer. Thus, the inventive cooling system is being used to augment the cooling capabilities of a conventional cooling system. This embodiment is illustrative and not intended to be limiting as the inventive cooling system alternatively may be used as the sole cooling system for the cooling oil of a transformer. 
   With reference numerals in  FIG. 2  the same as in  FIG. 1  for corresponding parts, the inventive cooling system introduces a three-way valve, diagrammatically shown at  40 , into conduit  20 . Valve  40  is plumbed to a conduit  42  that connects to a supplemental heat exchanger, abstractly shown at  44 , which is used to cool the cooling oil flowing around the transformer. Although shown for purposes of illustration in  FIG. 2  as being above or proximate conduit  20 , three-way valve  40  is positioned in conduit  20  in order to selectively redirect cooling oil such that the oil bypasses the radiator  22  and instead flows into conduit  42  and then through the chilled working fluid heat exchanger  44 . This bypassing is programmed (using controller  55  described herein) to occur when fans  26  and radiator  22  are not going to be capable of properly cooling the cooling oil. 
   In one feature of the invention, a return line  46  from heat exchanger  44  is connected to circulating pump  28 . Return line  46  introduces oil cooled during its passage through heat exchanger  44  to pump  28  for recirculation through transformer  10 . The temperature to which the oil is cooled depends on the transformer&#39;s loading, ambient temperature conditions, and the cooling system, which temperatures are preferably low enough not to degrade the life or efficiency of the transformer. Preferably, a check valve or a two-way valve  47  is interposed in return line  46  to control the recirculation flow provided to the transformer. In one embodiment, this valve  47  can be controlled in conjunction with valve  40 . 
   Heat exchanger  44  utilizes a chilled working fluid or chillate to sensibly reduce the temperature of the cooling oil from the transformer routed through the heat exchanger  44 . The chillate is introduced into heat exchanger  44  through conduit  48  connected to a conventional source of chillate (not shown). The chillate that has been heated when being used to reduce the temperature of the cooling oil is discharged from heat exchanger  44  through conduit  50 , which returns the chillate to the chillate source for reuse. Heat exchanger  44  can be any one of a variety of differently configured heat exchangers known in the art, such as a shell and tube heat exchanger. However, in the most preferred embodiment of the invention, the heat exchanger relies upon a fluid-to-fluid transfer of heat energy between flows of working fluids (i.e, cooling oil and chillate). 
   Three-way valve  40  is connected to a temperature sensing and control mechanism or controller, shown abstractly at  55 , that controls the operation of valve  40 . Controller  55  can also be programmed and operatively connected in a conventional fashion so as to control the remainder of the inventive cooling system, such as the two-way valve  47  or the flow of chillate from the chillate source, in a manner that will be understood by one of skill in the art in view of the explanation below. Preferably, the controller  55  can be a conventional programmable controller that generates control signals as a function of various input signals. In one specific embodiment, the controller  55  can be programmed to control valve  40  as a function of the temperature of the cooling oil. In this specific embodiment, controller  56  can include temperature sensors installed internally and/or externally to the transformer housing or tank, such as suspended in the cooling oil in the top third of the housing in particular. 
   In order to optimize transformer performance, controller  55  can be configured so that as the sensed temperature of the transformer insulating and cooling oil begins to rise, digital signals from the sensors can be provided to a solid state integrated thin film device which extrapolates the rise curve to determine if the maximum cooling oil temperature will exceed an acceptable level during a predetermined ensuing time period. Of course, this same determination can be made at a software level with an appropriately configured controller  55 . Typically, maximum temperatures within the transformer must be maintained below 95° C. (203° F.) or limited to a 65° C. rise above ambient temperatures in order to maintain rated capability of the transformer and preserve useful life. If these parameters are anticipated to be exceeded, then controller  55  automatically opens valve  40 , as well as other necessary valves, in proper sequence to start the chilling process. For example, when the chillate source has produced chillate at a proper temperature, valves are opened in sequence to allow the chillate to flow through the “cooling” heat exchanger  44 . The controller  55  can substantially simultaneously open valve  40  to route the high temperature cooling oil for temperature reduction. 
   Due to this “look ahead” feature of the system control architecture, the internal components of transformer  10  are never allowed to approach temperatures which could reduce the useful life or efficiency of the transformer. Furthermore, the controller  56  can be programmed to operate the inventive cooling system within any range of ambient and cooling oil temperatures to maximize power transfer of the transformer. In an alternative approach, the controller  55  can include stored information indicative of a temperature history of a specific transformer. For instance, transformers typically exhibit a generally uniform temperature response to operating loads and ambient conditions. Each transformer responds differently and each has a different threshold temperature response before a deleterious condition arises. The controller  55  can maintain a temperature history or profile for every transformer connected to the cooling system. Actual temperature data from the cooling oil temperature sensors for each transformer can be compared against this profile to determine if a problematic temperature condition is anticipated. 
   It is within the scope of the present invention for chillate to be supplied to heat exchanger  44  in any fashion which is known to the skilled artisan. For example, transformer station electricity can be used to produce chillate by powering vapor compression refrigeration equipment, or an absorption chiller and/or natural sources of groundwater, lakes, etc. (e.g., operating a pump that brings groundwater to the heat exchanger). Such vapor compression refrigeration equipment may be operated on-line, in other words during the time period such chillate is actually needed, or off-line, or both on-line and off-line. When operated off-line, chillate produced by the vapor compression refrigeration equipment can be stored adequately as a fluid or ice which can later be used for removing core heat. Still further, the power transformer itself is a robust source of heat that can be used to power an absorption refrigeration or chiller as described more fully below. When the transformer heat is the source of energy for its own cooling, a cooling system even more energy efficient than achieved by the use of fans or vapor compression refrigeration may be realized. 
   The graph in  FIG. 3  helps illustrate the benefits obtained by the cooling system depicted in FIG.  2 . The graph relates transformer life, in useful hours, to the temperature rise in the transformer cooling oil. The curve C depicts the limits of performance for the traditional forced oil and air cooling systems of the prior art. These prior systems are typically unable to predictably maintain the transformer performance to the left of the curve C, which means that the transformer may operate in a dangerous range. However, with the present invention, the transformer performance is always maintained in the “safe” region, as represented by the operating range R in FIG.  3 . 
   Referring now to FIG.  4  and  FIG. 4A , one configuration of equipment suitable for providing chillate to the supplemental heat exchanger  44  of  FIG. 2  is shown. In  FIG. 4 , the transformer  20  and the related components shown in  FIG. 2 , and other than heat exchanger  44 , are indicated abstractly at  60 . In this embodiment, an absorption chiller or refrigeration generator, generally designated  65 , is used as the source of chillate for heat exchanger  44 . 
   Conduit  48  is operably connected to absorption chiller  65  to deliver a cooled working fluid or chillate from the absorption chiller  65  to the shell and tube heat exchanger  44 . The chillate can be water preferably at a temperature between about 42° F. and 60° F. The chiller  65  is controlled by the controller  55  to provide a modulated temperature fluid. Conduit  50  is operably connected to absorption chiller  65  to return the warmed chillate for recooling. Absorption chiller  65  is connected, via pipes  72  and  74 , to a cooling tower  70  of traditional design which allows the excess absorbed heat from the absorption chilling process to be rejected to the atmosphere. Pipe  72  conveys high temperature water, such as at temperatures of between about 90° F. and 130° F., to cooling tower  70  for cooling. Pipe  74  returns lower temperature water, such as at temperatures of between about 700° F. and 100° F., to absorption chiller  65  for cooling uses. 
   The heat energy used to power absorption chiller  65  is supplied by high temperature water, such as at temperatures of between about 200° F. and 240° F., delivered by pipe  78  from a phase change material (PCM) heat exchanger  80 . Steam may also be the source of heat energy. Pipe  82  returns lower temperature water from absorption chiller  65  to PCM heat exchanger  80  for reheating. IN a most preferred embodiment, PCM heat exchanger  80  can be of a type described in co-pending U.S. patent application Ser. No. 09/607,853, entitled “Phase Change Material Heat Exchanger With Heat Energy Transfer Elements Extending Through the Phase Material,” the teachings of which are incorporated herein by reference in their entirety. 
   As abstractly shown in  FIG. 4A , the output of the transformer station  85  is electrically connected to the heat energy transfer elements  87  embedded in the phase change material  89 . Electricity from the transformer station  85  is dispatched to heat the elements  87  and melt the phase change material  89 . This process is preferably initiated by a controller, such as controller  55 , at a time of day when the dispatch load is less than the transformer rating, including possibly off-line times such as at night when the inventive cooling system is either less needed. PCM heat exchanger  80  is designed such that the heat energy given up as the phase change material solidifies from a molten state is transferred to the water passed through the annulus of the heat exchanger and routed to the absorption chiller  65 , such that chiller  65  can produced chillate used to condition the transformer oil passing through heat exchanger  44 . 
   Further description of the internal operations of the absorption chiller  65  is not provided herein, as the general operation is known in the art. For example, the configuration and workings of a chiller  65  are further described in U.S. Pat. No. 4,936,109, the teachings of which are incorporated herein by reference in their entirety. 
   Referring now to the diagrammatic top view of  FIG. 5 , the cooling system of  FIG. 4  is more further shown retrofit or adapted to cool an existing power transformer  10  with a 100 MVA capacity rating, for example. The existing transformer  10  is shown already equipped with four forced oil air ambient cooling radiators  22  and their associated pumps  28  connected in a conventional fashion, and as described above with reference to  FIG. 2 , to the internal volume of the transformer. As described above with reference to  FIG. 2 , each inlet to the radiator  22  is provided with a bypass valve, such as valve  40 , connected to a common high temperature oil line  42  plumbed to a heat exchanger, such as heat exchanger  44 . The transformer oil cooled by passage through heat exchanger  44  is returned by the multi-branched pipe  46  for reintroduction into the transformer at the four inlets proximate the four radiators  22 . In one specific embodiment, the heat exchanger is provided with a heat transfer capacity of about seventy-five therms, and although shown as a single unit, may be comprised of multiple smaller units that together provide the required cooling capacity. 
   The temperature of the cooling oil outlet from the heat exchanger is a function of the ambient conditions and the transformer loading. When ambient temperatures are high, the heat exchanger works to outlet a cooling oil at a temperature that, in conjunction with the throughput of the cooling oil as controlled by the controller  55 , keeps the transformer from heating up beyond its recommended level. For example, for the embodiment of  FIG. 5  more fully described herein, under peak weather conditions in the summer in the Midwest (ambient air of about 100° F.) during which electricity usage is at its peak so that the transformer  10  is fully loaded, the cooling oil needs to keep the transformer from overheating. In a specific instance, and by way of example only, the temperature of the oil inlet to heat exchanger  44  can be about 221° F. with the temperature of the cooling oil outlet from the heat exchanger  44  at a lower set point, such as about 176° F. If an even a lower temperature of the outlet cooling oil is desired for such conditions, a larger chiller, as well as more heat exchangers than shown in  FIG. 5  for more energy storage, may be required. Of course, other types of heat sinks can be substituted for the described heat exchangers to accomplish the requisite energy storage capacity. Still further, in situations where ambient temperatures are lower, the output of the heat exchangers can be at a lower temperature and can be used to cool the transformer even further (i.e., below 221° F.) to improve transformer efficiency. 
   In the specific embodiment depicted in  FIG. 5 , a 415 ton absorption chiller  65  housed in an all weather building abstractly shown at  66  provides heat exchanger  44  with a flow of a chillate, such as chilled water at about 45° F., through conduit  48 . Conduit  50  returns the warmed chillate from heat exchanger  44  to chiller  65 . Absorption chiller  65  is connected to a cooling tower  70  via pipes  72  and  74  in a conventional fashion. 
   Again by way of example, the absorption chiller  65  can be powered by high temperature water at about 240° F. delivered by pipe  78  connected in parallel to a plurality of PCM heat exchangers  80  of the type described with reference to FIG.  4 . The heat exchangers  80  can comprise twenty units, each constructed from a forty-two foot long, twenty-four inch diameter tube. Each heat exchanger can be filled with about eight tons of phase change material or salt. Pipe  82  is connected in parallel to the heat exchangers  80  to return lower temperature water from absorption chiller  65  for reheating. Heat exchangers  80  can be selectively supplied with off-peak station electric in order to store several million BTUs (such as 15,000,000 BTUs which can be enough for two hours of operations during maximum transformer loading and temperature) within electrically melted phase change material for eventual heating of water for use by chiller  65  or other cooling apparatus. As the stored energy in exchangers  80  is used up over time, the controller which regulates the operation of the cooling system causes additional electrical energy to be pulled from the station electric when the station is not involved with on-peak dispatch to replenish the energy storage of the phase change material. In other words, the electrical power to melt the phase change materials is obtained from the transformer substation during times of lower electrical requirements such as night time and hours during the day when the full electric capacity of the station is not required to serve demand. While these principles of the invention have been explained with reference to a PCM-type heat exchanger, other forms of heat exchanger can be adapted for use with this inventive system. 
   Referring now to  FIG. 6 , there is shown still another embodiment of the transformer cooling system of the present invention. The cooling system of  FIG. 6  is similar to the cooling system of  FIG. 5  except that it further includes a secondary heat exchanger  100  that is employed to allow the absorption chiller to be powered by the heat generated by the transformer. In some instances, during start up of the absorption chiller additional cooling capacity may be required. Moreover, additional heat energy may be needed by the chiller itself to facilitate its start up. 
   In this embodiment, high temperature cooling oil from the transformer is introduced into heat exchanger  100 , which may be a shell and tube heat exchanger, via branch  42   a  connected to conduit  42 . Preferably, a controllable valve is interposed between the conduit  42  and branch  42   a , which valve may be controlled by a separate programmable controller as a function of the cooling oil temperature, the status of the absorption chiller  65  or other conditions. Reduced temperature cooling oil is discharged from heat exchanger  100  into branch  101  connected to the inlet of the primary heat exchanger  44 . In a specific embodiment, the secondary heat exchanger can reduce the oil temperature from 221° F. to about 190° F. 
   The working fluid, such as water, to be heated by heat exchanger  100  is delivered by pipe  102  that is connected to pipe  82 , which returns lower temperature water from absorption chiller  65  for reheating. The heated working fluid outlet from heat exchanger  100  enters pipe  104  that is connected to pipe  78  that delivers high temperature water, such as between about 200° F. and 240° F., to absorption chiller  65  to power the production of chillate routed to heat exchanger  44 . Appropriate valving can be connected to the controller of the inventive cooling system through pipes  78  and  82  such that fluids may be selectively routed between absorption chiller  65  and either the PCM heat exchangers  80  or heat exchanger  100 . Such valves cause heat energy to be supplied to absorption chiller  65  from the larger heat energy source of PCM heat exchangers  80  during certain periods, such as during start up of the chiller in an effective manner or when the chillate required by heat exchanger  44  is greater than can be provided by absorption chiller  65  when only powered by the heat from heat exchanger  100 . More particularly, the PCM heat exchangers  80  can provide a surge of heat to enable efficient starting of the chiller when the transformer itself has not “banked” enough heat releasable by heat exchanger  100  to do so, or when “banking” sufficient heat in the transformer would be adverse to the life or efficiency of the transformer. In an alternate embodiment, instead of using PCM heat exchangers  80 , the start up of the chiller can be provided by other known devices such as a mechanical refrigeration device, for example an auxiliary boiler. Heat energy is supplied to chiller  65  by heat exchanger  100  when the absorption chiller has been started and is operating in a continuous mode, or at times that lesser chillate production is required. 
   The preferred embodiment of  FIG. 6  that uses PCM heat exchangers with the adjunct of a system employing the transformer heat (heat exchanger  100 ) allows the system operator to optimize the economics of the transformer cooling invention by selecting those times when electric energy is most available and least costly to charge the PCM system so that it would be available to enhance transformer operation when the equipment and system are in a high stress maximum cost of operation mode. A concurrent benefit of the system as it is configured also allows system operators to elect to “bank” heat stored in the PCM heat exchangers during times when energy costs are the lowest, off-peak times, and bring them back by operating the transformer cooling system to reduce transformer winding and core losses which improves the units throughput efficiency and reduces operating costs, even when the unit is able to operate below critical temperatures because of lower ambient temperatures. 
   In one specific embodiment useful for a 300 MVA transformer, the chiller can be a 354 ton device, such as the Trane Model ABSC-03F. The heat exchanger can be a 16-18 ton PCM unit. The illustrated system can store 3,300,000 BTUs off-peak to augment the heat source for the absorption chiller. 
   In a modification to the embodiment of  FIG. 6 , the outlet line  101  can be connected directly to conduit  46 , thereby effectively bypassing the primary heat exchanger  44 . With this configuration, the secondary heat exchanger  100  can provide the primary cooling for the transformer oil, thereby reducing the output requirements for the absorption chiller. 
   The cooling system of the present invention, such as system  27  described above, can form an integral part of a power generation substation. Thus, as depicted in  FIG. 7 , a number of transformers  10  can be provided with a corresponding cooling system  27 . Each cooling system can include a chiller, such as chiller  65 , connected to a common cooling tower  70 . The programmable controller  55  can be housed in a maintenance or control building. A single controller can be provided for each cooling system, or a common controller can receive temperature and performance data from and issue control signals to every cooling system. Preferably, the components of each cooling system  27  are calibrated to circulate cooling oil at about 360 gpm, and to accomplish on-line chilling up to 118 tons/hr. 
   While this invention has been shown and described as having preferred designs, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.