Patent Publication Number: US-2023147165-A1

Title: Wireless transmissions through a transformer tank

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 U.S.C. § 371 national stage application of PCT International Application No. PCT/EP2021/054345 filed on Feb. 22, 2021, which in turn claims foreign priority to European Patent Application No. 20167922.2, filed on Apr. 3, 2020, the disclosures and content of which are incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to communication of sensor readings of a liquid-filled transformer. 
     BACKGROUND 
     Continuous measurements of various parameters such as temperature, humidity, pressure, etc. from an active transformer are used for monitoring of its operating conditions and consequently for its reliability. When the transformer is enclosed and embedded in oil, it becomes particularly difficult to both power sensors and transmit power and acquired data inside a transformer, as well as to transmit the sensor data to the outside of the transformer tank. Especially the latter problem has yet to be resolved. The transfer of data/information to the outside of a tightly closed transformer tank, which is impermeable to liquid and air, is a challenge. Any hole made in the tank, for passing a cable there through, will be problematic because it may with time (e.g. due to aging of the insulation, for instance comprising an O-ring) eventually start leaking. Also, it may be problematic since such a cable will pass from high electrical potential within the transformer to a low electrical potential outside of the transformer. 
     US2002/107657 discloses an apparatus for measuring contact pressure exerted by a winding compression element on a winding in a power transformer in a tank. A sensor and a sensor antenna are arranged in the region of an upper compression element. An electronic checking device is provided outside the tank. A checking antenna in the tank is connected to the checking device via a radio frequency bushing, which passes through the wall of the tank. 
     DE2427830 relates to an electric heat sensor which is soldered to a conductor of a winding or inserted between parallel conductors of the winding. A transmitted signal is scanned by a receiving antenna attached to an inner wall of a transformer tank and passed through a bushing insulator to the outer side of the tank. 
     US2019/286146 shows a submersible ROV for inspecting a liquid-filled housing, such as for an electrical transformer. The ROV wirelessly communicates, via hole(s) in the housing, with a base station outside the housing. 
     SUMMARY 
     It is an objective of the present disclosure to provide improved communication of sensor data from the inside of a liquid-filled transformer to the outside of the transformer tank. 
     According to an aspect of the present disclosure, there is provided a transformer system comprising a power transformer comprising a metal tank filled with an electrically insulating liquid, and a wireless sensor arrangement submerged in the insulating liquid within the tank. The sensor arrangement comprises a radio transmitter for wirelessly transmitting sensor readings to the outside of the transformer through an opening in the tank, said opening being provided with a liquid-tight seal comprising a solid insulator for preventing leakage from the tank of the insulating liquid, and wherein the radio transmitter is configured for transmitting the sensor readings using a carrier frequency within the range of from 100 kHz to 1 MHz. 
     According to another aspect of the present disclosure, there is provided a method of transmitting sensor readings in an embodiment of the transformer system of the present disclosure, the method comprising the sensor arrangement obtaining sensor readings on the power transformer, and the radio transmitter transmitting the sensor readings using a carrier frequency within the range of from 100 kHz to 1 MHz. 
     It has now been realised that radio waves may be able to pass through an opening of a transformer tank having a liquid-tight (i.e., liquid impermeable) seal comprising a solid insulator, e.g., of a dielectric material. The data/information about the sensor readings can thus be sent wirelessly from the inside of the power transformer, typically at a HV potential, to the outside of the power transformer, typically at low electrical potential. The radio waves will pass through the solid insulation from the inside of the transformer tank (which is typically of metal and thus generally shielding electromagnetic, i.e. radio, waves). There is thus no need for a cable passing through the tank, nor passing from high to low electrical potential. In some embodiments, the seal is also gas-tight, for preventing air from leaking into the transformer tank and polluting the insulation liquid, e.g., mineral oil and/or an ester liquid. 
     The solid insulator may e.g., be within a conventional bushing used for passing an electrical conductor, e.g., a phase, through the transformer tank. Additionally or alternatively, the solid insulator may be a sealing element between an outside of the (metal) transformer tank and a metal cap covering a hole in the transformer tank, e.g., a hole for accessing the inside of the tank during installation or maintenance. Depending on the physical characteristics of the opening, the insulating liquid and the solid insulator, the frequency of the carrier wave used for wirelessly transmitting the sensor readings by radio may be especially adapted to pass through the solid insulation to the outside of the tank. 
     It is to be noted that any feature of any of the aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of any of the aspects may apply to any of the other aspects. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings. 
     Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the element, apparatus, component, means, step, etc.” are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. The use of “first”, “second” etc. for different features/components of the present disclosure are only intended to distinguish the features/components from other similar features/components and not to impart any order or hierarchy to the features/components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG.  1    is a schematic sectional side view of a transformer system in accordance with some embodiments of the present invention. 
         FIG.  2    is a schematic view in longitudinal section of a bushing, in in accordance with some embodiments of the present invention. 
         FIG.  3   a    is a schematic sectional detail of a transformer tank having an opening covered by a metal cap, in accordance with some embodiments of the present invention. 
         FIG.  3   b    is a schematic top view of an embodiment of the metal cap of  FIG.  3     a.    
         FIG.  4    is a schematic flow chart of a method in accordance with some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown. However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description. 
       FIG.  1    illustrates a transformer system  10  comprising a power transformer  1 , e.g., a HV transformer, and a sensor arrangement  11 . 
     The transformer  1  comprises an inductive arrangement  8 , comprising transformer windings, submerged in an electrically insulating liquid  3  contained in a tank  2 , typically completely filling the tank. The liquid may be any suitable transformer liquid, e.g., an oil such as a mineral oil, and/or an ester liquid. The tank is typically of metal, e.g., steel, which is radio frequency shielding. The tank  2  has at least one opening  4  through a wall  9  of the tank. For instance, the transformer may comprise at least one, but typically a plurality of, bushing(s)  5  for passing an electrical conductor  6 , e.g., for an electrical phase, through an opening  4   a  in a wall  9  of the tank  2 . Also, the transformer may comprise a service opening  4   b  in a wall  9  of the tank  2 , which is covered by a metal cap  7 , typically of the same material as the tank  2 . Thus, the opening  4   a  is provided with a liquid-tight seal comprising the bushing  5 , while the opening  4   b  is provided with a liquid-tight seal comprising the cap  7 . 
     The sensor arrangement  11  is submerged in the insulating liquid  3  and comprises a sensor  13  configured for measuring a property of the transformer, e.g., of the inductive arrangement  8 . The sensor  13  is connected with a radio transmitter  12  of the sensor arrangement ii, for wirelessly transmitting sensor readings to the outside of the transformer  1  through an opening  4   a  and/or  4   b  in the tank  2 . The radio transmitter  12  may be configured for transmitting the sensor readings using a predetermined carrier frequency within a frequency range which is empirically selected in view of the physical characteristics of the opening  4 , solid insulator and insulating liquid  3  for enabling the radio transmission to pass through the opening  4 . It has been determined that a carrier frequency within either of the ranges 100 kHz to 1 MHz and 300 MHz to 10 GHz may be especially useful for passing through a sealed opening  4 . 
       FIG.  1    also illustrates a few possible different paths (a), (b) and (c) for the wireless transmission of the sensor readings from the radio transmitter  12 . Path (a) is through the opening  4   a , which is sealed by means of the bushing  5 , through solid insulation within the bushing. Path (b) is also through the opening  4   a , which is sealed by means of the bushing  5 , but through solid insulation between the bushing  5  and the tank  2 , e.g., solid insulation (possibly comprising an O-ring or the like) arranged between a flange  25  (see  FIG.  2   ) of the bushing  5  and an outside of the wall  9  of the tank  2 . Path (c) is through the opening  4   b , which is covered by the cap  7 , through solid insulation between the tank  2  and the cap  7 , e.g., solid insulation (possibly comprising an O-ring  31  or the like, cf.  FIG.  3   a   ) arranged between the cap  7  and the outside of the wall  9  of the tank  2 . 
       FIG.  2    illustrates a bushing  5  which may be arranged through an opening  4   a  in the tank  2 . The bushing may be substantially rotational symmetric and arranged for allowing the conductor  6  to run through a central longitudinal through hole through the bushing. The bushing  5  comprises a condenser core  26  which is arranged around the conductor  6 , insulating the conductor from the surroundings, especially from the tank  2 . The condenser core  26  comprises a solid insulator  21  and electrically conductive field-grading layers  22 , e.g., in the form of aluminium foils. The field-grading layers  22  may form substantially concentric tubes around and in parallel with the conductor  6 . The field-grading layers may e.g., be interleaved between layers of wound material of the solid insulator, e.g., of cellulose based paper, when producing the condenser core  26 . Typically, outer field-grading layers  22  have a reduced longitudinal extension L 1  (corresponding to height of the concentric tubes) compared with inner field-grading layers  22  such that the longitudinal extension is gradually reduced from the innermost field-grading layer to the outermost field-grading layer. 
     Each of the field-grading layers may have a thickness of less than 100 μm, e.g., within the range of 10 to 40 μm, and the radial distance L 2  between the field-grading layer and a neighbouring inner our outer field-grading layer may be within the range of 0.1 mm and 10 mm, e.g., about 1 mm, corresponding to a plurality of turns of a web of the solid insulator  21  material. The longitudinal extension (height) L 1  of the field-grading layer may be within the range of 1-10 m. Typically, the bushing is filled with the same insulating liquid  3  as the tank  2 . However, the bushing is liquid-tight may be gas-tight according to some embodiments, providing a liquid-tight seal of the opening  4   a  in the tank when the bushing is placed there through. The solid insulator  21  may be impregnated with the insulating liquid, e.g., oil-impregnated paper. 
     It has been found that, a radio transmissions (path (a) of  FIG.  1   ) using a carrier frequency within the range of from 100 kHz to 1 MHz, may pass through the solid insulator  21  in the gap  23  between two neighbouring field-grading layers  22  (i.e., two field-grading layers which do not have any field-grading layer between them, but typically only solid insulator  21 , possibly impregnated with insulating liquid  3 ). 
     The bushing  5  may conventionally also comprise an outer, weather shed insulator  24  and/or a flange  25  for fastening the bushing to the outside of the wall  9  of the tank  2 . 
       FIG.  3   a    illustrates an opening  4   b  in a wall  9  of the tank  2 , e.g., a service hole for accessing the interior of the tank during installation and/or service of the transformer  1 . The opening  4   b  is covered by a metal cap  7  (e.g., substantially flat) which is typically fastened to the wall  9  by means of metal fastening means  32  such as screws and/or bolts, herein exemplified with screws  32 . The cap  7  may have any suitable shape, as viewed from atop, e.g., a rectangular or circular shape. A solid insulator  31 , e.g., comprising an O-ring of an elastic material, is arranged between the metal cap  7  and an outside of the tank wall  9  continuously around the opening  4   b  for providing a liquid-tight, and preferably also gas-tight, seal of the opening  4   b.    
     When the cap  7  is fastened to the tank wall  9  by means of the metal fastening means  32 , at least one slit  33  is formed between the outer surface of the wall  9  and the inner surface (i.e., the surface facing the opening  4   b ) of the metal cap  7 . Each of the at least one slit  33  has a length l 1  defined by the distance (typically the straight distance) between two neighbouring metal fastening means  32  (i.e., between two metal fastening means which do not have a further metal fastening means between them, e.g., along an outer edge of the cap  7 ). In such a slit  33 , delimited by the metal cap, wall and fastening means, it has been found that radio waves (of path (c) of figure i) may pass through the solid insulator  31  without being shielded by said metal cap, wall and fastening means. Similarly, radio waves of path (b) of  FIG.  1    may pass through a solid insulator  31 , with the cap  7  exchanged for the flange  25  of a bushing  5 , i.e., in a slit  33  delimited by the outer surface of the wall  9  and the inner surface (i.e., the surface facing the opening  4   a ) of the flange  25 , and by two neighbouring fastening means, e.g., bolts or screws,  32  fastening the flange  25  to the wall  9  around the opening  4   a.    
     In some embodiments, the length l 1  of the slit  33  is within the range of from 0.03 m (corresponding to a frequency of 10 GHz) or 0.1 m (corresponding to a frequency of 3 GHz) to 1 m (corresponding to a frequency of 300 MHz). Then, a carrier wave having a carrier frequency within the range of from 300 MHz to 10 GHz, or to 3 GHz, has been found to pass through the solid insulator  31  to the outside of the tank  2 . The height  12  of the slit  33  should be large enough to prevent direct contact between the wall  9  and the cap  7 , e.g., at least 10 μm such as within the range of 10 μm to 1 cm or 1 mm. The radial length l 3  of the slit  33 , i.e., the distance of overlap between the outer surface of the wall  9  and the cap  7  around the opening  4   b , is preferably less than 10 cm, e.g., within the range of from 0.5 cm to 4 cm. 
       FIG.  3   b    illustrates an embodiment of the cap  7  viewed from the top and having numerous fastening means  32 , e.g., screws and/or bolts, arranged along an outer edge of the cap. In the embodiment of  FIG.  3   b   , the cap is substantially circular, which may be provided in some embodiments. As discussed in relation to  FIG.  3   a   , the length l 1  of the slit  33  may be defined by the straight distance between any two neighbouring fastening means  32 . 
       FIG.  4    is a flow chart of an embodiment of a method of the present disclosure. The method is for transmitting sensor readings in an embodiment of the transformer system  10  discussed herein. The sensor arrangement  11  obtain S 1  sensor readings on the power transformer  1 , e.g., measurements of temperature, pressure, flow rate of the insulating liquid, or the like within the tank  2 , e.g., in or at the inductive arrangement  8 . Then, the radio transmitter  12  transmits S 2  the sensor readings wirelessly using a carrier frequency within the range of from 100 kHz to 1 MHz, e.g., through solid insulation  21  in a bushing  5 , or within the range of from 300 MHz to 10 GHz, e.g. through solid insulation  31  between a cap  7  and a wall  9  of the tank. 
     In some embodiments of the present invention, the solid insulator  21  is comprised in a bushing  5  arranged in the tank opening  4   a . In some embodiments, the solid insulator  21  comprises cellulose-based paper which is impregnated with the insulating liquid  3 . In some embodiments, the solid insulator  21  is arranged between longitudinal electrically conductive field-grading layers  22  in a condenser core  26  of the bushing  5 . In some embodiments, the field-grading layers  22  are arranged at a radial distance L 2  from each other within the range of from 0.5 to 2 mm. In some embodiments, each of the field-grading layers  22  has a longitudinal extension L 1  of at least 1 m, e.g., within the range of from 1 to 10 m. In some embodiments, the radio transmitter  12  transmits, or is configured for transmitting, the sensor readings using a carrier frequency within the range of from 100 kHz to 1 MHz. 
     In some embodiments of the present disclosure, the solid insulator  31  is comprised in a slit  33  formed between a wall  9  of the tank  2  and a metal cap  7  (or possibly flange  25 ) covering the tank opening  4   b  (or  4   a ). In some embodiments, the solid insulator  31  comprises an O-ring of an elastic material arranged between the metal cap  7  or flange  25  and an outside of the tank wall  9 . In some embodiments, the slit  33  has a length li within the range of from 0.03 to 1 m. In some embodiments, the radio transmitter  12  is configured for transmitting the sensor readings using a carrier frequency within the range of from 300 MHz to 10 GHz. 
     The present disclosure has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the present disclosure, as defined by the appended claims.