Electrical Bushing Gas Sampling Apparatus and Method

High voltage bushings used within power transformers generally contain a quantity of dielectric oil for cooling and insulating the bushing electrodes. Over long periods of operation, the gas space above the surface of the oil dielectric can become contaminated by the gaseous byproducts of the decomposition of the insulation material used within the bushing. A sampling of the gaseous material in the space above the oil dielectric can therefore give an indication as to the condition of the oil dielectric and the paper dielectric materials within the bushing for incipient fault detection. Current means for sampling the gaseous materials existing above the oil dielectric generally involve surrounding the top portion of the bushing oil fill hole plug with an airtight enclosure and evacuating, then introducing the gaseous material to within the enclosure. There is often the question as to whether the atmosphere within the enclosure dilutes or otherwise affects the measured pressure and composition of the gaseous materials being tested.

The purpose of this invention is to describe methods and apparatus for rapidly and reliably obtaining samples of the gaseous materials existing above the oil dielectric in electrical bushings without the requirement for auxiliary housings or similar airtight enclosures.

The invention comprises an adapter for connecting with a high voltage electrical bushing to allow a gas sample to be withdrawn from the expansion space above the oil dielectric within the bushing. The adapter includes a sealable membrane connected to the bushing by means of a passage plug member. A hypodermic syringe is inserted through the passage and penetrates the sealable membrane without introducing any external gaseous or atmospheric materials to the bushing. A further embodiment includes a gauge member which can be removably inserted through the membrane for direct reading of the gas pressure within the bushing.

FIG. 1 is a front perspective view, in partial section, of an electrical bushing used with the method and apparatus of the instant invention;

FIG. 2 is a side view of the bushing of FIG. 1 including the gas-sampling apparatus according to the invention; and

FIG. 3 is a side perspective view of the assembled apparatus depicted in FIG. 1.

FIG. 1 shows an electrical bushing 10 of the type including a porcelain skirt section 11 and a top and bottom electrode, 12, 13, which are internally electrically connected. In order to insulate and cool the bushing a quantity of oil dielectric 14 is included within the major interior portion of the bushing. To determine the level of oil dielectric 14 a sight glass 15 is provided within the metal section 16 which supports top electrode 12. To provide for the filling and draining of oil dielectric 14 a passage 17 is included within metal section 16 and includes internal thread 18 for insertion of a pipe plug.

FIG. 3 shows a portion of bushing 10 including a sealable membrane 23 which functions to provide a gas-tight seal between passage 17 and the interior of bushing 10. Membrane 23 replaces the usual O ring member that is employed with the pipe plug described earlier. Membrane 23 is formed from a material such as silicon, rubber or plastic which can "heal" itself after being penetrated by a sharp object such as a hypodermic needle. A first plug 19 having a plurality of external threads 20 for engaging with internal thread 18 on bushing 10 and having formed therein internal threads 21 is employed for holding membrane 23 in place. A passage 22 in the form of a concentric hole is provided within plug 19 to provide access to membrane 23. In one embodiment of the instant invention, bushing 10 is provided with membrane 23 and plug 19 is threadedly inserted within passage 17 to provide an airtight closure. When samples are required to be taken of the gaseous materials above the oil dielectric 14, shown in FIG. 1, a hypodermic syringe 29 is inserted within passage 22 by means of a hollow needle 30 having a side port 5 which prevents needle clogging and membrane tearing to penetrate membrane 23. Plunger 31 is then slowly retracted to withdraw a quantity of the gaseous material within hypodermic syringe 29. The gaseous sample can then be brought to a gas analyzer, such as a gas chromatograph, for analysis. To facilitate fastening plug 19 within passage 17, a portion of passage 22 can comprise a hex-like configuration to accommodate an Allen wrench. A second membrane 24 can be inserted within groove 9 provided within plug 19 in order to further seal passage 22 from the atmosphere. Membrane 24 is held in place by a second plug 25 containing external threads 26 for engaging with internal threads 31 within groove 9. Second plug 25 includes a passage 8 and a tapered guide hole 27 for facilitating and guiding the passage of needle 30. Passage 8 being slightly larger than syringe needle 30 prevents unguided penetrations and tatterings of membranes 23, 24 and thereby promotes longer membrane life. With second plug 25 and membrane 24 in place within plug 19 and plug 19 in place within slot 17, needle 30 is inserted within passage 8 through membrane 24, through passage 22 and through membrane 23 to obtain the gas sample. Membrane 24 is made of a material similar to that for membrane 23 and has similar sealable properties. One of the main reasons for the use of membrane 24 is to determine whether membrane 23 remains gastight over many applications. Needle 30 can be inserted through membrane 24 without penetrating membrane 23 to sample the atmosphere existing in the close vicinity of membrane 23. If gaseous contaminants are found external to membrane 23 and internal to the area intermediate membrane 24 and membrane 23, then membrane 23 should be replaced or a new membrane 24 will then be relied upon for sealing. The provision of tapered slot 27 within second plug 25 is to facilitate the entrance of needle 30 within passage 8 without damaging needle 30. In the event that second plug 25 and membrane 24 are not employed, passage 22 can have a tapered slot similar to slot 27 to facilitate the entrance of needle 30 directly to within passage 22. In order to determine the pressure of the gaseous contaminants or the nitrogen blanket, which is oftentimes employed, a gauge 32 can be employed in the following manner. Gauge 32 is first evacuated by connecting needle 33 to a vacuum system and opening valve 34 to cause indicator 7 to come to a reference zero. Valve 34 is then closed and needle 33 is removed from the vacuum system. Needle 33 is then inserted in a manner similar to that described earlier for operation of hypodermic syringe 29, and valve 34 is opened in order to obtain a pressure reading on 32 of the gaseous material existing within bushing 10. The movement of indicator 7 then provides a measure of the gas pressure within bushing 10 and, if readings are taken over extended periods of time, can provide an indication as to whether the nitrogen blanket within bushing 10 is leaking to the atmosphere or whether the dielectric materials are deteriorating. Besides using hypodermic syringe 29 to remove a small sample of the gaseous material within bushing 10, it can also be used to supply nitrogen gas to bushing 10 in the following manner. Hypodermic syringe needle 30 and 30A assembly are connected with a nitrogen supply hose with pipe fittings. The nitrogen gas can then be introduced into bushing 10 by the insertion of needle 30 in the manner described earlier.

The interconnection between plugs 25, 19 and membranes 24, 23 is shown at 6 in FIG. 3. The complete unit 6 can be installed at the time of manufacture of bushing 10 or in the field as a replacement for the oil fill plug described earlier or offered as an option for bushings made prior to the invention.