Patent Application
20050095177
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.
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.
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.
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.
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
Returning to
The LTC 10 shown in
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.
