Patent Document

FIELD OF THE INVENTION 
     The present invention relates generally to oil-filled switching apparatus for electrical substations and other high-voltage, high-power applications. More particularly, the invention relates to apparatus and methods for maintaining an environment free of excessive pressure and explosive vapors in head space above the oil that fills load tap changers. 
     BACKGROUND OF THE INVENTION 
     It is known in the manufacturing of power distribution, apparatus to include, with power transformers, automatically controlled load tap changers that can adjust the voltage at which power is fed to factories, subdivisions, apartment houses, and other large loads, typically several times per day but as often as hundreds of times per day, in response to variations in the applied load. These variations in the applied load change the voltage drops across such substantially fixed resistances as distribution wiring; the changes in the voltage drops in turn demand compensating adjustments in transformer winding connections to minimize errors in the available voltage, with the intent of maintaining at each distributed load as close to a constant voltage as practicable. 
     Transformer winding switching is performed by devices known to the art as load tap changers, so called because they are engineered to switch from one tap to another on a transformer while carrying kiloamp-level current loads. The contact portion of a load tap changer (LTC) is in some embodiments fully immersed in one of several blends of mineral oil, where the term oil may refer to one of a variety of petroleum distillates which are in the liquid state at room temperature, for insulation, cooling, and reduction of arcing. Numerous petroleum distillates may be suited to particular applications, as determined by operating temperature range, viscosity requirements, water absorption, electrical properties such as dielectric coefficient, conductivity, and change in electrical properties with moisture concentration, temperature, and the like. 
     The non-oil-filled gas volume at the top of the open chamber in a tap changer, transformer, or other device is termed ullage. The pressure in the ullage in an LTC tends to change slowly with outside temperature, as the oil volume typically can provide a significant thermal reservoir. 
     Despite the presence of insulating oil, the immersed tap switching events can produce arcing, which tends to break down the oil, leaving contaminating particles as well as liquid and gas hydrocarbon molecules of various molecular weights. A portion of the contaminating particles can be deposited on the sliding contacts of the LTC, building up a resistive layer and increasing contact heating, with the waste heat ultimately coupled to the oil. Removal of these deposits is promoted by abrasion between the sliding contacts during each tap change. Another portion of the contaminating particles can remain in suspension in the oil until mechanically removed by passing the oil through a filter. Still another portion of the contaminating particles may sink to the bottom of the oil volume, while others float to the surface or form foams. 
     An LTC can be vented rather than being hermetically sealed, so that there is some opportunity in many systems for water vapor and other airborne contaminants to enter the system; the contaminants can be absorbed by the oil, can be entrained as corrosion promoters, and can be shown to directly lower the dielectric constant of the oil. A variety of known technologies can serve for suppression of entrainment of water vapor, such as the use of a desiccant within the ullage of the LTC. 
     Another phenomenon evident in some LTCs, in the presence of dissolved oxygen and water in mineral oil subjected to arcing events, is formation of organic acids and other reactive chemical compounds, some of which can be destructive of some components of the system. 
     Accordingly, there is a need in the art for an apparatus and method capable of providing to some extent a continuously refreshed nonreactive gas atmosphere in an LTC and associated subsystems, balancing requirements for fresh supplies of gas against assured minimization of combustibles, oxidizers, and other corrosives in all accessible regions of the LTC, both continuously during operation and at a rapidly restoring rate after servicing, while avoiding to at least some extent the requirement for periodic maintenance and its associated expenses. 
     SUMMARY OF THE INVENTION 
     The above needs have been met to at least some degree by a novel nonreactive atmosphere control apparatus and method, as herein described. 
     In accordance with one embodiment of the present invention, a gas remover system that provides capability for expelling gases from a load tap changer (LTC) comprises a nitrogen generator to extract nitrogen from the atmosphere; a feed line to introduce the nitrogen extracted by the nitrogen generator into an ullage in the LTC; and an orifice to establish an outflow rate of nitrogen along with entrained vapor phase contaminants, if present, from the LTC ullage to the atmosphere. 
     In accordance with another embodiment of the present invention, a gas remover for expelling gases from an LTC comprises means for extracting nitrogen gas from the atmosphere; means for urging the extracted nitrogen gas into an ullage in an LTC; and means for establishing a substantially continuous outflow of nitrogen from the ullage to the atmosphere along with entrained vapor phase contaminants, if present. 
     In accordance with yet another embodiment of the present invention, a process for expelling gases from an LTC is comprised of the steps of extracting nitrogen gas from the atmosphere; urging the extracted nitrogen gas into an ullage in an LTC; and establishing a substantially continuous outflow of nitrogen from the ullage to the atmosphere along with entrained vapor phase contaminants, if present. 
     There have thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
     In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
     As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods, and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a load tap changer configured to include the inventive apparatus. 
         FIG. 2  is a front view without the door of a nitrogen gas generator of the type used to maintain nitrogen gas charge in a transformer and its associated load tap changer and other apparatus. 
         FIG. 3  is a perspective view of a representative transformer that uses a load tap changer and can accept the inventive apparatus. 
         FIG. 4  is a system block diagram showing a transformer, to which are affixed a load tap changer and a nitrogen gas generator. 
     
    
    
     DETAILED DESCRIPTION 
     In a preferred embodiment of the present invention, a nitrogen gas based contaminant gas remover apparatus and method is provided, which allows displacement of gases through a generally continuous bleed of nitrogen introduced from a nitrogen source and released using a vent orifice. The expelled gases may include contaminant, corrosive, explosive, and/or pressurizing gases, for example. With a nonreactive gas overpressure in place, opportunity for the introduction of oxidants from outside the LTC system is minimized, and with a continuous bleed, virtually all water, oxygen, vapor-phase oxidants, combustible vapors, and other contaminants introduced, such as low-mass breakdown products from the oil, can escape into the atmosphere, leaving the LTC largely free of oxidants and other contaminants. 
     The invention will now be described with particular reference to the drawing figures, in which like reference numerals refer to like parts throughout. 
       FIG. 1  shows a representative load tap changer (LTC)  10  with an associated motor box  12 . Sight glasses  14 , one for each phase of the AC power handled by the transformer  12 , permit a technician to look inside the LTC  10  to examine the cleanliness of the mineral oil inside and the condition of the taps between which the LTC  10  switches in order to compensate for load current variations. 
       FIG. 2  shows the interior of a representative nitrogen generator  18  intended to support a power transformer, and including sufficient surplus capacity to support a preferred embodiment of the present invention. An air compressor  20  is shown along with a fan-forced heat exchanger  22  within the nitrogen generator  18 ; for a preferred embodiment, such an air compressor  20  can be designed to operate intermittently, for example for up to several years with minimal maintenance. 
     A pressure regulator panel  24  can establish preferred pressures for some or all of the functions of the nitrogen generator  18 . The controlled pressures can include the air compressor  20  air pressure output, which can include a failure mode shutdown threshold as well as a regulated level with a feedback control function; control over the air pressure level fed into the filter membrane  26 ; regulation of the filter membrane  26  nitrogen output pressure, whether by the use of feedback control to the input, by the use of output bleed, or both; nitrogen pressure fed into a makeup nitrogen reservoir bottle or bottles  28 ; minimum/maximum controlled nitrogen pressure into the ullage  22  of the LTC  10 , and a makeup nitrogen output pressure control. 
     Regulator valves are particularly well suited to the task of pressurizing multiple devices. A multiplicity of regulator valves can, for example, be required with high-power transformers. In high-power transformers, the transformer itself may need a clean and isolated supply, and may not generate significant amounts of contaminants. An associated LTC  10  sharing the same nitrogen generator  18 , meanwhile, may produce contaminants on a daily basis, and require continuous purging flow. Using a separate flow regulator for each function can assure satisfactory performance without undue complexity. In some embodiments, multiple flow regulators can use a piping arrangement that is common in part to two or more of the regulators. 
     A nitrogen source feeding a manifold that has several regulator valves can provide the variety of pressure feeds required by the components of a transformer system. Such a manifold can include a second regulator valve to charge the LTC  10  at a high rate, such as by employing ten times the normal overpressure, in order to purge the LTC  10  after it has been opened or otherwise allowed to receive a large contamination influx, as well as during climate-induced sudden pressure drops. 
     The exemplary embodiment shown in  FIGS. 1 and 2  is representative of several possible embodiments that can permit development of a broad range of system configurations suited to particular applications. A comparatively small number of nitrogen generator system sizes spread over a wide range of output flow rates, for example, can be used to provide the nitrogen needed for a broad range of sizes of transformers and their associated LTCs. 
     Returning to  FIG. 1 , a nitrogen feed line  32  from an output port  30  of the nitrogen generator  18  carries low pressure nitrogen to the LTC  10  and applies a nitrogen overpressure to the ullage  34  above the oil volume  36  in the LTC  10 . The outflow orifice  38  shown in phantom in  FIG. 1  is located inside the LTC  10  within the ullage  34  volume above the oil  36 . 
       FIG. 3  shows a representative prior art transformer  40  with an affixed load tap changer  10 . Provision of a nitrogen generator  18  to pressurize a power transformer  40  is known in the art to assure maintenance of a nitrogen overpressure in the transformer ullage  42  above the windings of the transformer  40 . The oil-filled interior  44  of the transformer  40  represents a stable and substantially inert environment, provided any gas leakage is restored with nitrogen. The size of the transformer  40 —comparable in some cases to the size of an over-the-road truck cab—and the criticality of its maintaining a stable amount of nitrogen can dictate the use of a nitrogen generator  18  with enough surplus capacity to support an inert-gas-charged LTC  10  without adding additional equipment other than manifolds and check valves, and without increasing the size and capacity of the nitrogen generator  18 . 
       FIG. 4  shows the exemplary inventive system in block diagram form. Here, the compressor  20  provides high-pressure air to the nitrogen extractor  26 , which can furnish nitrogen substantially free of contaminants to a multiplicity of regulators. The primary regulator can be seen as the low-pressure regulator  46 , which, through an LTC backflow preventer  48 , feeds the ullage  34  within the LTC  10 . An orifice  38  establishes a controlled and substantially constant flow rate of nitrogen into the atmosphere by way of an orifice check valve  50 . A high-pressure bypass regulator  52  can provide an alternate flow path to reload the LTC  10  when a pressure sensor  54  detects that the pressure has dropped below a critical level, driving a control valve  56  that allows the bypass regulator  52  to flow nitrogen into the ullage  34 . An alternative method using a manual control valve on the high-pressure regulator  52  is potentially feasible since the principal need for makeup gas may come from servicing, for which an operator can be available who can activate and deactivate such a manual valve. Nitrogen from the nitrogen extractor  26  can also feed a storage system comprising a tank regulator  58  and one or more storage tanks  28 ; the stored nitrogen can provide a substantially constant supply, which can be particularly useful to perform the rapid replenishment activity described above. As in a transformer  40  without the inventive apparatus, another regulator, here termed a transformer regulator  60 , can establish and regulate the nitrogen charge within the transformer  40 , using a transformer backflow preventer  62  to prevent contaminated gases from feeding back into the nitrogen generator system and a pressure release  64  to vent to the atmosphere in event of sudden pressure rises within the transformer  40 . 
     The LTC  10  shown in  FIG. 1  includes a preferred embodiment of the inventive apparatus. The tap changing mechanisms inside are fully submerged in oil  24  in normal operation, with the oil  24  normally receiving a low nitrogen overpressure, which can in some embodiments be on the order of one-half PSI, roughly 3% above the external atmosphere. The level of pressure differential established for a particular embodiment can be maintained by the low-pressure regulator  46 , a component of the regulator panel  24  dedicated to this function. The orifice  38  establishes a flow rate suitable for the nitrogen generator  18  of the embodiment. A nitrogen flow rate suitable for a representative LTC  10  may be on the order of two standard cubic feet of nitrogen per day. 
     Changes in solar irradiance, air temperature, rainfall, and other climatic phenomena, as well as electrical loading, power discharge in the course of switching, and other electrical phenomena, may affect the temperature of the LTC  10 , in turn producing changes in the enclosed volume of the LTC  10 . While the thermal mass of the oil  24  that substantially fills the LTC  10  slows changes to the temperature of the gas comprising the ullage  22 , and hence the volume of the gas, nonetheless the fill pressure from the regulator panel and the pressure reduction through the orifice  26  may not be sufficiently in equilibrium at any given moment to maintain a desirable level of overpressure. 
     In the case of underpressure within the LTC  10 , a second flow path for fill nitrogen may be desirable to shorten the time during which higher outside pressure may force atmospheric gases to enter the ullage  22  through the orifice  26 . This need can also occur after maintenance, when the LTC  10  can have been opened to the atmosphere, in which case water vapor and oxygen can have been introduced while lowering internal pressure within the LTC  10  to atmospheric pressure. A check valve in the orifice  26  vent to the outside atmosphere may help to minimize the effects of this phenomenon by stopping flow in both directions when the overpressure inside the LTC  10  is near zero. A fast feed system that bypasses the low-pressure regulator, or another similar arrangement, may be employed to accelerate pressure restoration. 
     Under some weather conditions, a tendency for contaminants to be urged from the atmosphere into the LTC  10  may be made more severe, for example, by condensed water vapor inside the vent path of a chilled LTC  10 . Such water condensate may form an appreciable and potentially destructive quantity of liquid. Heavy rain, rain driven by strong winds, site flooding, or another climatic phenomenon may represent a source of abundant water that can under some circumstances represent a similar risk to the system. Entry of liquid water into the LTC  10  may be in part resisted by the fitting of an orifice check valve in the form of a float valve into the vent line. A ball with good sphericity may be induced to seal against a seat when floated against the seat by any fluid of higher specific gravity than the ball itself. Other styles of floating devices, such as flappers, may similarly provide a seal against fluids that can lift them. 
     In the case of overpressure inside the LTC  10 , the orifice  26  may continue to vent to the atmosphere, while flow from the nitrogen generator  18  may essentially stop until the pressure within the LTC  10  returns to its preferred overpressure level. A check valve or comparable backflow preventer  48  in the gas feed line from the nitrogen generator  18  to the LTC  10  may serve to substantially prevent higher pressure within the LTC  10  from forcing contaminated fill nitrogen into the low pressure portions of the regulator itself prior to the restoration of the preferred overpressure level through continued venting via the orifice  26 . 
     System faults may occur due to unforeseeable weather extremes, breakdowns of other equipment at a site, premature wearout, and other incidents. Since the nitrogen generator  18  may have logic controls or detectors with logic resources, it can be feasible to connect communication apparatus to the nitrogen generator  18  that can transmit reports of performance degradation before gross failures occur, allowing, for example, focused response by limited numbers of repair crews during major storms. Periodic transmission of system status can provide degradation histories at multiple sites, further enhancing maintenance performance. 
     Reference has been made throughout to nitrogen as a nonreactive gas that can be exceptionally suitable as a fill agent. While the suitability of nitrogen is true for most applications, the attribute of nonreactivity is not unique to nitrogen, and alternate fill gases may be well suited to the task, although alternative fill gases may not as often be readily available. For example, helium has properties that may make it preferable to nitrogen in some regimes, as do the other noble gases, any of which may normally be vented to the atmosphere without harm, as well as some compounds. Helium, moreover, may be available with negligible cost as a petroleum byproduct at an oil refinery. In systems in which a fill gas other than nitrogen is readily available, which gas exhibits comparable or superior properties, that other gas can be used in place of nitrogen by accommodating differences in required pressure, thermal, diffusion, and flow properties, and the like. 
     The use of a nitrogen generator  18  as a nitrogen source has been presented herein as an example of the preferred embodiment. Other embodiments may use other sources, such as liquid nitrogen Dewar storage vessels, sufficient numbers of high-pressure gas storage tanks, or other suitable sources. 
     The many features and advantages of the invention are apparent from the detailed specification; thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to that fall within the scope of the invention.

Technology Category: h