Abstract:
An electrostatic shield for controlling the electrostatic field between a high voltage conductor and a low voltage conductor in an instrument transformer is provided. The instrument transformer has a current transformer and a voltage transformer. The current transformer has a split core which includes a first core segment and a second core segment. When the first core segment adjoins the second core segment, a current transformer is formed, having a core formed from the first and second core segments. The high voltage conductor runs between the first and second core segments of the current transformer. The first core segment is encapsulated in a polymer resin and when encapsulated, forms a first encasement. The second core segment has a low voltage winding mounted thereon. The electrostatic shield is disposed between the low voltage winding and the high voltage conductor. A second encasement is formed by encapsulating the electrostatic shield, low voltage winding and second core segment in a polymer resin.

Description:
FIELD OF INVENTION 
     The present application is directed to an electrostatic shield for controlling electrostatic field stress in a split core instrument transformer. 
     BACKGROUND 
     This invention relates to instrument transformers and more particularly to an electrostatic shield for controlling the electrostatic field in a split core instrument transformer. 
     Instrument transformers include current transformers and voltage transformers and are used to measure the properties of electricity flowing through conductors. Current and voltage transformers are used in measurement and protective applications, together with equipment, such as meters and relays. Such transformers “step down” the current and/or voltage of a system to a standardized value that can be handled by associated equipment. For example, a current transformer may step down current in a range of 10 to 2,500 amps to a current in a range of 1 to 5 amps, while a voltage transformer may step down voltage in a range of 12,000 to 40,000 volts to a voltage in a range of 100 to 120 volts. Current and voltage transformers may be used to measure current and voltage, respectively, in an elongated high voltage conductor, such as an overhead power line. 
     A conventional current transformer for measuring current in a high voltage conductor typically has a unitary body with an opening through which the conductor extends. Such a conventional current transformer has a unitary core, which is circular or toroidal in shape and has a central opening that coincides, at least partially, with the opening in the body. With such a construction, the current transformer is mounted to the conductor by cutting and then splicing the conductor. As can be appreciated such cutting and splicing is undesirable. Accordingly, current transformers having two-piece or split cores have been proposed. Examples of current transformers having split cores are shown in U.S. Pat. No. 4,048,605 to McCollum, U.S. Pat. No. 4,709,339 to Fernandes and US20060279910 to Gunn et al. 
     The control of electrostatic field stress is an issue in a split core current transformer having a high voltage conductor disposed between the split core segments, one of which core segments has a low voltage conductor wound thereon. Uncontrolled electrostatic field stress between the high and low voltage conductors can cause partial discharges that will eventually erode the insulating material between the high and low voltage conductors and the split core segments. While electrostatic shields are available to reduce the electrostatic field stress experienced between high and low voltage conductors, there is room for improvement in electrostatic shields. 
     Accordingly, the present invention is directed to an electrostatic shield for controlling the electrostatic field in a current transformer. 
     SUMMARY 
     An instrument transformer for measuring the properties of electricity flowing in an elongated conductor comprises a first core segment and a second core segment, each having at least one end surface. A first encasement formed of a polymer resin encapsulates the first core segment except for the at least one end surface. The second core segment has a low voltage winding wound thereon. An electrostatic shield is provided for connection to the elongated conductor. A second encasement formed of a polymer resin encapsulates the electrostatic shield, the low voltage winding, and the second core segment except for the at least one end surface. The electrostatic shield is embedded in the polymer resin of the second encasement and disposed slightly beneath an outer planar surface of the second encasement. 
     A method of making an instrument transformer comprises providing a first core segment and encapsulating the first core segment in a polymer resin to form a first encasement. The method of making an instrument transformer further comprises providing a second core segment, mounting a low voltage winding to the second core segment, providing an electrostatic shield between a high voltage conductor and the low voltage winding, and positioning the electrostatic shield above and out of contact with the low voltage winding. A second encasement is formed by encapsulating the second core segment, low voltage winding and electrostatic shield in a polymer resin. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, structural embodiments are illustrated that, together with the detailed description provided below, describe exemplary embodiments of an electrostatic shield for a transformer. One of ordinary skill in the art will appreciate that a component may be designed as multiple components or that multiple components may be designed as a single component. 
       Further, in the accompanying drawings and description that follow, like parts are indicated throughout the drawings and written description with the same reference numerals, respectively. The figures are not drawn to scale and the proportions of certain parts have been exaggerated for convenience of illustration. 
         FIG. 1  is a front view of an instrument transformer embodied in accordance with the present invention; 
         FIG. 2  is a schematic sectional view of the instrument transformer taken along line A-A in  FIG. 1 ; 
         FIG. 3 a    is a top view of an electrostatic shield embodied in accordance with the present invention; 
         FIG. 3 b    is an isometric view of the electrostatic shield; 
         FIG. 3 c    is a front view of the electrostatic shield; 
         FIG. 3 d    is a right side view of the electrostatic shield; 
         FIG. 4  is a sectional top view of a current transformer embodied in accordance with the present invention; and 
         FIG. 5  is a sectional side view of the current transformer having an alternative low voltage winding configuration. 
     
    
    
     DETAILED DESCRIPTION 
     It should be noted that in the detailed description that follows, identical components have the same reference numerals, regardless of whether they are shown in different embodiments of the present invention. It should also be noted that in order to clearly and concisely disclose the present invention, the drawings may not necessarily be to scale and certain features of the invention may be shown in somewhat schematic form. 
     As used herein, the abbreviation “CT” shall mean “current transformer”. 
     Referring now to  FIGS. 1 and 2 , there are shown views of an instrument transformer  10  embodied in accordance with the present invention. The instrument transformer  10  includes a current transformer  12  and a voltage transformer  14 . One of ordinary skill in the art will recognize that the instrument transformer  10  may be embodied as a current transformer  12  alone. The current transformer  12  and the voltage transformer  14  are arranged in a cover section  18  and a base section  20  that are releasably secured together. The voltage transformer  14  is fully disposed in the base section  20 , while the current transformer  12  is partially disposed in the cover section  18  and partially disposed in the base section  20 . The current transformer  12  is operable to measure the current in a high voltage conductor (such as high voltage conductor  38 ), while the voltage transformer  14  is operable to measure the voltage in the high voltage conductor  38 . The voltage transformer  14  also supplies power to the electronics for the instrument transformer  10 . 
     The cover section  18  includes a top or first core segment  24  encapsulated in a top or first encasement  26  formed from one or more polymer resins in a cover casting process. The first core segment  24  is generally U-shaped and is comprised of ferromagnetic metal, such as grain-oriented silicon steel or amorphous steel. The first core segment  24  may be formed from layers of metal strips or a stack of metal plates. An electrostatic shield  28  is disposed over and covers the first core segment  24 , except for the ends thereof. The electrostatic shield  28  may be formed from one or more layers of semi-conductive tape that are wound over a layer of closed cell foam padding that encompasses the first core segment  24 . The first encasement  26  fully covers the first core segment  24  except for the ends thereof, which are exposed at a bottom surface of the first encasement  26 . At least a portion of the bottom surface of the first encasement  26  is substantially flat (planar) so as to permit the bottom surface to be disposed flush with a top surface of a second encasement  46  of the base section  20 . 
     An electrostatic shield  55  embodied in accordance with the present invention is depicted in  FIGS. 3 a -3 d    and is disposed between the high voltage conductor  38  and a low voltage winding  54 . The electrostatic shield  55  is embedded within a polymer resin of the second encasement  46  and located slightly beneath a substantially planar surface of the second encasement  46 . For example, the electrostatic shield  55  may be located at a depth of about 3.175 mm to about 19.05 mm from the substantially planar surface of the second encasement  46 . Additionally, the electrostatic shield  55  may be located at a distance of about 12.7 mm to about 25.4 mm from the low voltage winding  54  or ground components. 
     The electrostatic  55  shield is generally oval in shape and extends laterally through the second encasement, shielding the low voltage winding  54  from the high voltage conductor  38 . The electrostatic shield  55  may be embodied as a solid, perforated or mesh sheet formed from a semi-conductive or conductive material such as aluminum, brass, copper, cellulose impregnated with a conductive or semi-conductive material, or any material having similar properties. In one embodiment, the perforated or mesh sheet allows a polymer resin to permeate through the openings in the electrostatic shield  55  during a casting process, the casting process to be described in further detail below. 
     Referring now to  FIGS. 3 a , 3 b   , and  4 , the electrostatic shield  55  has a gap  59  that prevents a continuous conductive path around the first and second core segments  24 ,  44 . The electrostatic shield  55  has a generally arcuate recess  66  that runs from a first side of the electrostatic shield  55  to an opposing, second side of the electrostatic shield  55 . The high voltage conductor  38  is disposed slightly above the recess  66 . The high voltage conductor  38  does not touch the electrostatic shield  55 . The electrostatic shield has one or more cut-outs  43  through which the second core segment  44  slightly extends. The electrostatic shield has one or more openings  49  for threaded bolts  34 . 
     The electrostatic shield  55  is electrically connected to the high voltage conductor  38  through lead wires  35  that run from the electrostatic shield  55  to metallic inserts  37  The metallic inserts  37  are embedded in the polymer resin and are further attached to clamps  42  in direct connection with the high voltage conductor  38 . The electrostatic shield  55  is at about the same potential as the high voltage conductor  38 . 
     Referring now to  FIGS. 1 and 2 , a plurality of bore inserts  30  extend through the first encasement  26  from the top to the bottom thereof. The bore inserts  30  are arranged around the first core segment  24  and are adapted to receive threaded bolts  34  for securing the cover section  18  to the base section  20 , as will be further described below. A main passage  36  extends laterally through the first encasement  26  and is adapted to accommodate a high voltage conductor  38 , such as an overhead power line. The high voltage conductor  38  may carry electricity at a voltage from about 1 kV to about 52 kV. When the instrument transformer is installed and the high voltage conductor  38  extends through the main passage  36 , a connector  40  electrically connects the un-insulated high voltage conductor  38  to the first core segment  24  and the second core segment so that the first core segment  24 , second core segment  44 , connector  40 , and threaded bolts  34 , are at about the same potential as the high voltage conductor  38 . The connector  40  may be connected to a terminal  41  mounted on the outside of the first encasement  26  and the terminal  41  may then be electrically connected to the first core segment  24  by an internal conductor. The connector  40  may be connected to the high voltage conductor  38  by a clamp  42 . 
     The base section  20  includes a bottom or second core segment  44  encapsulated in a bottom or second encasement  46  formed from one or more polymer resins in a base casting process. The second encasement  46  has a plurality of circumferentially-extending sheds  47 . The second core segment  44  is also generally U-shaped and has the same construction as the first core segment  24 . In one embodiment, the first and second core segments  24 ,  44  are produced by constructing a single core and then cutting the core in half. The second encasement  46  fully covers the second core segment  44  except for the ends thereof, which are exposed at a top surface of the second encasement  46 . At least a portion of the top surface of the second encasement  46  is substantially flat (planar) so as to permit the top surface to be disposed flush with the bottom surface of the first encasement  26  of the cover section  12 . When the cover section  12  is secured to the base section  20 , the exposed ends of the first and second core sections  24 ,  44  abut each other, thereby forming (or re-forming) a core of the current transformer  12 . 
     The second core segment  44  is supported on a cradle  48  having a C-shaped middle section and opposing peripheral flanges. The cradle  48  is formed from an epoxy resin or any material having similar properties. Mounts  50  are secured to the flanges and have threaded interiors for threadably receiving ends of the bolts  34  extending through the bore inserts  30 . A layer of closed cell foam padding, an insulation tube  52  and a low voltage winding  54  are disposed over the second core segment  44  and the middle section of the cradle  48 , with the closed cell foam padding being disposed over the second core segment  44  and the insulation tube  52  being disposed between the layer of closed cell foam padding and the low voltage winding  54 . The insulation tube  52  is composed of a dielectric material and electrically insulates the low voltage winding  54  from the second core segment  44 . The insulation tube  52  may be comprised of a dielectric resin (such as an epoxy resin), layers of an insulating tape or a phenolic kraft paper tube (i.e., a kraft paper tube impregnated with a phenolic resin). The low voltage winding  54  is wound around the insulation tube  52  and is comprised of a plurality of turns of a conductor composed of a metal, such as copper. An electrostatic shield  56  is disposed over and covers the low voltage winding  54 . The electrostatic shield  56  may be formed from one or more layers of semi-conductive tape that are wound over the low voltage winding  54 . The cradle  48 , the insulation tube  52  and the low voltage winding  54  are all encapsulated in the second encasement  46 . 
     The low voltage winding  54  may have a single CT ratio or multiple CT ratios. In this regard, it should be noted that a CT ratio is the ratio of the rated primary current (in the high voltage conductor  38 ) to the rated secondary current (in the low voltage winding  54 ). If the low voltage winding  54  has a multi-ratio construction, different combinations of taps may provide a range of CT ratios, such as from 50:5 to 600:5 or from 500:5 to 4000:5. The taps are connected at different points along the travel of the conductor of the low voltage winding  54 . For example, if there are five taps, two of the taps may be connected at opposing ends of the low voltage winding  54  and the other three taps may be connected to the low voltage winding  54  in between the two end taps in a spaced apart manner. Thus, the number of turns of the low voltage winding  54  between different pairs of taps is different, thereby creating different CT ratios. The taps on the low voltage winding  54  are connected by conductors to terminals  57  enclosed in a junction box  58  secured to the base section  20 . 
     The voltage transformer  14  includes a winding structure  60  mounted to a core  62  comprised of ferromagnetic metal, such as grain-oriented silicon steel or amorphous steel. As shown, the core  62  may be comprised of two, abutting rings, each of which is formed from layers of metal strips or a stack of metal plates. The winding structure  60  is mounted to abutting legs of the rings. An insulation tube  64  is mounted to the core  62 , between the core  62  and the winding structure  60 . The insulation tube  64  may be comprised of a dielectric resin (such as an epoxy resin), layers of an insulating tape or a phenolic kraft paper tube. 
     The winding structure  60  comprises a low voltage winding concentrically disposed inside a high voltage winding. The low voltage winding and the high voltage winding are each comprised of a plurality of turns of a conductor composed of a metal, such as copper. Of course, the number of turns in the two windings is different. As with the current transformer  12 , the core  62  and the winding structure  60  of the voltage transformer  14  are each covered with an electrostatic shield, which may have the same construction/composition as the electrostatic shields  28 ,  56 . The high voltage winding of the winding structure  60  is electrically connected to the high voltage conductor  38 . The connection may be through the terminal  41  and the first core segment  24 . The voltage transformer  14  is operable to step down the voltage supplied to the high voltage winding (e.g., about 1-35 kV) to a lower voltage at the output of the low voltage winding. This lower voltage may be about 110-120 volts, or even lower, down to a voltage of about 10 volts. The output of the low voltage winding is connected to the terminals  57  in the junction box  58 . The terminals  57  include terminals for the current measurement output(s) from the current transformer  12  and terminals for the voltage measurement output from the low voltage winding of the voltage transformer  14 . The lower voltage power from the voltage transformer  14  is also used to power the electronics in a control box  100  mounted separately from the instrument transformer  10 . 
     The cover section  18  is secured to the base section  20  by inserting the bolts  34  through the bore inserts  30  of the cover section  18  and threadably securing the ends of the bolts  34  in the mounts  50  of the base section  20 . The bore inserts  30  in the cover section  18  and the mounts of the base section  20  are positioned so as to properly align the first core segment  24  with the second core segment  44  to form a contiguous core for the current transformer  12  when the cover section  18  and the base section  20  are secured together with the bolts  34 . The first encasement  26  and the second encasement  46  may also be formed with corresponding structural features (such as ridges and grooves and holes and posts) that help properly align the cover section  18  and the base section  20 . 
     The cover section  18  may be removed from the base section  20  to permit the instrument transformer  10  to be installed to or uninstalled from the high voltage conductor  38 , i.e., to pass the high voltage conductor  38  through the current transformer  12  or remove the high voltage conductor  38  from the current transformer  12 . The cover section  18  is removed simply by unthreading the bolts  34  from the mounts  50  and separating the cover section  18  from the base section  20 . 
     The first and second encasements  26 ,  46  are formed separately in the cover casting process and the base casting process, respectively. Each of the first and second encasements  26 ,  46  may be formed from a single insulating resin, which is an epoxy resin. In one embodiment, the resin is a cycloaliphatic epoxy resin, still more particularly a hydrophobic cycloaliphatic epoxy resin composition. Such an epoxy resin composition may comprise a cycloaliphatic epoxy resin, a curing agent, an accelerator and filler, such as silanised quartz powder, fused silica powder, or silanised fused silica powder. In one embodiment, the epoxy resin composition comprises from about 50-70% filler. The curing agent may be an anhydride, such as a linear aliphatic polymeric anhydride, or a cyclic carboxylic anhydride. The accelerator may be an amine, an acidic catalyst (such as stannous octoate), an imidazole, or a quaternary ammonium hydroxide or halide. 
     The cover casting process and the base casting process may each be an automatic pressure gelation (APG) process. In such an APG process, the resin composition (in liquid form) is degassed and preheated to a temperature above 40° C., while under vacuum. The internal components of the section being cast (such as the first core segment  24  and the bore inserts  30  in the cover section  18 ) are placed in a cavity of a mold heated to an elevated curing temperature of the resin. The degassed and preheated resin composition is then introduced under slight pressure into the cavity containing the internal components. Inside the cavity, the resin composition quickly starts to gel. The resin composition in the cavity, however, remains in contact with pressurized resin being introduced from outside the cavity. In this manner, the shrinkage of the gelled resin composition in the cavity is compensated for by subsequent further addition of degassed and preheated resin composition entering the cavity under pressure. After the resin composition cures to a solid, the encasement with the internal components molded therein is removed from the mold cavity. The encasement is then allowed to fully cure. 
     It should be appreciated that in lieu of being formed pursuant to an APG process, the first and second encasements  26 ,  46  may be formed using an open casting process or a vacuum casting process. In an open casting process, the resin composition is simply poured into an open mold containing the internal components and then heated to the elevated curing temperature of the resin. In vacuum casting, the internal components are disposed in a mold enclosed in a vacuum chamber or casing. The resin composition is mixed under vacuum and introduced into the mold in the vacuum chamber, which is also under vacuum. The mold is heated to the elevated curing temperature of the resin. After the resin composition is dispensed into the mold, the pressure in the vacuum chamber is raised to atmospheric pressure for curing the proto-encasement in the mold. Post curing can be performed after demolding the proto-encasement. 
     In another embodiment of the present invention, each of the first and second encasements  26 ,  46  has two layers formed from two different insulating resins, respectively, and is constructed in accordance with PCT Application No. WO2008127575, which is hereby incorporated by reference. In this embodiment, the encasement comprises an inner layer or shell and an outer layer or shell. The outer shell is disposed over the inner shell and is coextensive therewith. The inner shell is more flexible (softer) than the outer shell, with the inner shell being comprised of a flexible first resin composition, while the outer shell being comprised of a rigid second resin composition. The first resin composition (when fully cured) is flexible, having a tensile elongation at break (as measured by ASTM D638) of greater than 5%, more particularly, greater than 10%, still more particularly, greater than 20%, even still more particularly, in a range from about 20% to about 100%. The second resin composition (when fully cured) is rigid, having a tensile elongation at break (as measured by ASTM D638) of less than 5%, more particularly, in a range from about 1% to about 5%. The first resin composition of the inner shell may be a flexible epoxy composition, a flexible aromatic polyurethane composition, butyl rubber, or a thermoplastic rubber. The second resin composition of the outer shell is a cycloaliphatic epoxy composition, such as that described above. The encasement is formed over the internal components using first and second casting processes. In the first casting process, the inner shell is formed from the first resin composition in a first mold. In the second casting process, the intermediate product comprising the internal components inside the inner shell is placed in a second mold and then the second resin composition is introduced into the second mold. After the second resin composition (the outer shell) cures for a period of time to form a solid, the encasement with the internal components disposed therein is removed from the second mold. The outer shell is then allowed to fully cure. 
     Referring now to  FIG. 5 , a current transformer  80  is depicted and has the same construction as the instrument transformer  10 , except as described below. The voltage transformer  14  included in the instrument transformer  10  is not part of the current transformer  80 . Additionally, the current transformer  80  has two low voltage windings  77  that are arranged in a different configuration than the single low voltage winding  54  of the instrument transformer  10 . Each of the low voltage windings  77  in the current transformer  80  are mounted to an associated one of opposing ends of the second core segment. The low voltage windings  77  may be connected together in series and further connected to a terminal (not shown). 
     It is to be understood that the description of the foregoing exemplary embodiment(s) is (are) intended to be only illustrative, rather than exhaustive, of the present invention. Those of ordinary skill will be able to make certain additions, deletions, and/or modifications to the embodiment(s) of the disclosed subject matter without departing from the spirit of the invention or its scope, as defined by the appended claims.