Abstract:
A filter apparatus suitable for electrical substation high-voltage transformer tap changers and similar oil-filled equipment removes moisture and other contaminants from the insulating oil bath in which the tap contacts are immersed. The apparatus can be configured and subsequently reconfigured for use with any of a variety of filter media using a common pump and control mechanism and an assortment of adapters.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to filtration of oil-filled switching apparatus for electrical substations and other high-voltage, high-power applications. More particularly, the invention relates to filtering apparatus and methods for maintaining cleanliness and satisfactory electrical properties in the coolant oil that fills tap changers. 
     BACKGROUND OF THE INVENTION 
     It is known in the manufacturing of power distribution apparatus to include automatic transformer tap changing that can adjust the power fed to factories, subdivisions, apartment houses, and other large loads as often as several times a day or more as the load varies. This load variation changes the voltage drop across substantially fixed resistances such as distribution wiring, requiring compensating adjustments in transformer tap connections. 
     The contact portion of a tap changer is in some embodiments fully immersed in insulating oil for cooling and reduction of arcing. The immersed switching events can nonetheless exhibit some arcing, which tends to break down the oil, leaving contaminating particles. In addition, the tap changer is normally not hermetically sealed, so that there is some opportunity in many systems for water vapor to enter the system, which vapor can be absorbed by the oil, can be entrained as a corrosion promoter, or can otherwise become an active contaminant. 
     It is further known in the power distribution industry to provide a surplus volume of oil in a reservoir in each unit of the apparatus in order to increase thermal mass and to promote thermal regulation without continuous recourse to pumping. Installation of filters and pumps permits periodic filtering to capture undesirable materials for subsequent removal. Industry has developed a variety of solutions to the power distribution and switching apparatus oil contamination problem. This has lead to diversity in the capabilities of the apparatus that controls such parameters as filtering system run time and run interval, as well as of styles, of filters. In some examples, the filter or the pump may be located inside the apparatus, so that it is necessary to deenergize, drain, and disassemble the apparatus simply to gain access to the pump and/or filter to troubleshoot it; in other examples, filtering resources are limited in capability with respect to the volume of oil in a device requiring filtering. 
     Accordingly, there is a need in the art for an apparatus and method for adapting pumped filter systems to accept more than one type of filter, as well as further adapting pumped filter systems to operate either in a heavily automated mode or with very detailed user command control. 
     SUMMARY OF THE INVENTION 
     The above needs have been met to at least some degree by a novel filter adapter and method as herein described. 
     In accordance with one embodiment of the present invention, a filter system for filtering oil comprises a pump to draw oil from a reservoir external to the filter system and introduce it into the filter system under pressure; a first fitting directing flow from the pump, where the first fitting is able be disconnected without permitting significant oil leakage under a condition of applied pump pressure; a filter to remove contaminants from the oil urged through it from the first fitting; a second fitting directing flow from the filter, to allow the filter to be disconnected without permitting significant oil leakage under a condition of applied pump pressure; and a return line to return filtered oil to the reservoir from the filter system. 
     In accordance with another embodiment of the present invention, a filter system for filtering oil comprises means for drawing oil from a reservoir; means for pressurizing oil while allowing it to flow from means for pressurizing; means for directing flow of oil from means for pressurizing; means for capturing contaminants entrained in a flow of oil; and means for directing flow of oil back to a reservoir from which the oil was previously drawn. 
     In accordance with yet another embodiment of the present invention, a process for filtering a fluid comprises the steps of drawing fluid from a reservoir; pressurizing the drawn fluid using an apparatus for pressurizing; directing the drawn and pressurized fluid to a filtering medium capable of removing contaminants therefrom; passing the drawn and pressurized fluid through the filtering medium so that contaminants are removed; and directing the reduced-contaminant fluid back to the reservoir from which it was initially removed. 
     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 front view of a retrofit filter unit ready to be mounted to a tap changer side wall. 
         FIG. 2  is a right side view of the retrofit filter unit of  FIG. 1 . 
         FIG. 3  is a ladder-type schematic diagram of an implementation of the timing and control system within a filter unit. 
     
    
    
     DETAILED DESCRIPTION 
     In a preferred embodiment of the present invention, an oil filtering apparatus and method is provided that allows removal and replacement of a filter without deenergizing, disassembly, or spillage and that permits the non-spilling feature to be retained with other filter styles by using a novel filter adapter. This adapter assembly may be fitted to the filter ports on the pumping system in place of the original style of filter. Like the original style of filter, the adapter assembly actuates moderate-pressure connectors that allow a filter assembly to be removed while its transformer is in use with negligible leakage—typically no more than one or two drops per connecting or disconnecting event. Since the pump can be configured to stop passing oil through the filter for a number of hours per day, there can be ample opportunity to perform periodic maintenance following removal, even without securing pump power. Further, an overpressure bypass can route the oil past the filter in the event that the pump begins to run while the filter assembly is removed. 
     Once a filter adapter has been removed from the pumping subsystem, it can be disassembled and the filter cartridge removed, typically in a controlled, workshop environment. After any needed cleaning, it can receive a replacement filter cartridge and any desired testing can be performed. Finally, the filter adapter can be reinstalled by mating the two dripless fittings on the pump housing with the corresponding, fittings on the filter adapter. Adding of any needed makeup oil is a separate activity. 
     An alternate approach employs the maintenance worker&#39;s carrying of a spare cartridge assembly per tap changer, permitting a complete round of filter exchanges to be performed in the field, followed by refurbishing and restocking the first set of filter cartridges without needing to make a second trip to each site. 
     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 preferred embodiment of an oil filtration system intended as an add-on or retrofit unit, ready to be affixed to a tap changer. An electrical controls case  10  can be fed with alternating current (AC) power through a bottom-feed conduit port  12 . The case  10  in the preferred embodiment has three indicator lamps  14 ,  16 , and  18  and a single control  20 . Controlled power can be fed out of the electrical controls case  10  via a second conduit port  22  to an electric motor and pump assembly  24 . An output from the motor and pump assembly  24  feeds the pressurized oil into a manifold  26  with a pressure gauge  28 . The manifold  26  is equipped with an automatic control valve  30  for overpressure bypass, to allow the oil to shunt around a blocked or missing filter  38 , and control switches comprising a low pressure sensor  32  to allow the controller to shut down the motor if the oil inlet line  34  pressure drops to an unacceptably low level, such as by blockage, and a high pressure sensor  36  to likewise provide high pressure fault data to the timer circuit to direct shutdown in event of an overpressure not related to a filter blockage. 
     A filter  38  can be mounted in a suitable location, such as one that provides a compact overall configuration, one that promotes ease of access, one that minimizes plumbing lengths, or in accordance with another criterion. In the preferred embodiment, a distinctly large canister filter  38  with feed lines  40  and  42  near the filter&#39;s bottom is installed to the side of the pump  24  opposite the manifold  26 , fitted within a slot  44  that retains the filter  38  in its preferred orientation. The flexible feed lines  40  and  42  from the filter  38  in the preferred embodiment are routed behind the control box  10  to the manifold  26 , where inlet  44  and outlet  46  fittings plug into mating connectors  48  and  50 , respectively, on the manifold  26 . 
       FIG. 1  further shows an oil inlet  52  on the pump  24 . This port is plumbed by the user in add-on systems. Mounting tabs  54  allow the unit to be affixed to any convenient location, preferably directly to a face of the tap changer, as by spot welded studs or another method as suited to the application. 
       FIG. 2  is a side view that shows some features blocked in  FIG. 1 . Return oil flow to the tap changer comes from the manifold  26  by way of a fitting  56 . Routing of the feed lines  40  and  42  from the filter  38  to the manifold  26  is shown to pass through a clearance hole  58  in a filter mounting bracket  60 , the latter also visible in  FIG. 1 . 
     The embodiment shown in  FIGS. 1 and 2  is one of several possible embodiments that permit an end user to choose a preferred filter style. The default filter configuration for a similar tap changer oil filter not incorporating the invention uses a more compact, sealed cartridge not suitable for refurbishment. The filter canister  38  shown in the preferred embodiment uses a paper filter element inside, and can be disassembled to replace the element. Alternate embodiments can use other replacement elements, a motor vehicle-style screw-in cartridge attached to a mounting device with the appropriate input and output fittings provided, or a comparable arrangement. For many such embodiments, a unique or generic mounting or support bracket for the filter assembly may be necessary. 
     The length of the flexible feed lines  40  and  42  between the manifold  26  and the filter  38  is limited by the drop in pressure due to line losses; typically, a well-chosen pump can function with a combination of input and output line lengths between the tap changer and the filter unit, plus the internal flexible line lengths, of roughly 75 feet, without needing to increase line inside diameter beyond the default one-half inch. Under circumstances where system configurations can result in destructive resonances between line structure, fluid dynamic properties of the oil, and the excitative properties of the pump, it may be necessary to select critical line lengths or to use damping devices to permit system operation at all temperatures in unattended systems. A resonance sensor in a fluid line can be added to detect incipient faults under extreme conditions. As noted above, the low-pressure detector can shut down operation in event of a line fracture. 
     The ladder diagram in  FIG. 3  shows the basic functionality of the control system of a tap changer filter apparatus in accordance with a preferred embodiment. 120 volts alternating current (VAC) input is provided as an input from a user installation to a terminal strip or equivalent termination arrangement  62 . A switch element  64  on the control switch  20  applies the 120 VAC to the electronics board  66  at terminal  2 . The Power On indicator light  18  is on whenever the switch  22  is set to the ON position. Terminal  3  on the electronics board  66  is the AC return to neutral, completing the primary circuit. 
     Continuing the description of a representative apparatus in accordance with a preferred embodiment, common terminal  15  on the electronics board  66  feeds sense power to the two external switches labeled LOW FLOW  32  and HI PRESS  36 . For the embodiment shown, the low flow switch contacts  32  used in the circuit are normally closed, such as when sitting in the shipping carton. This normal state is the same state that occurs during low flow conditions. The overpressure contacts  36  used in the embodiment are normally open, and close during overpressure conditions. Thus each of the faults completes a circuit, connecting terminal  15  to terminal  18  and terminal  19 , respectively, which causes power to be applied to the LOW FLOW  18  and HI PRESS  20  indicators respectively. Each of these indicator circuits is bypassed by a comparatively high-value resistor  68  and  70 , respectively, to provide a stable electrical circuit. 
     Concluding the description of a representative apparatus according with a preferred embodiment, terminal T of the electronics board  66  applies power to an external motor  72 , a part of the motor and pump assembly  24 , shown in  FIGS. 1 and 2 . 
     The content of the electronics board  66  is comprised in large part of circuitry that implements a conventional timing circuit, namely a power supply, clock oscillator, and counters that are controlled by so-called dual-in-line package, or DIP, switches. At the completion of a delay interval, the activation time for the motor occurs and a booster circuit on the board  66  activates a small on-board power relay that drives the motor. No auxiliary relay contacts are shown for customer use in the preferred embodiment, although such a feature would represent a straightforward extension of the concept and would allow remote monitoring of the run cycle without risk of damage to the working components of the system. 
     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.