Abstract:
A current transformer for a ground fault circuit breaker used on a circuit having at least one line conductor and a neutral conductor includes a toroidal core having a circular opening defining a center point and a multi-turn winding wound on the core. A first guide member is disposed on one side of the core, and a second guide member is disposed on another side of the core. The first and second guide members each have a hole for receiving the line conductor and a hole for receiving the neutral conductor formed therein. The guide members thus position the conductors with respect to the core. Also included is a method of correcting asymmetries in the current transformer. The method includes measuring the magnitude and orientation of any asymmetries, and then altering the current transformer based on the measured magnitude and orientation of the asymmetries so as to eliminate the asymmetries.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to current transformers and more particularly to current transformers used in ground fault circuit breakers. 
     Ground fault circuit breakers for alternating current distribution circuits are commonly used to protect people against dangerous shocks due to line-to-ground current flow through someone&#39;s body. Ground fault circuit breakers must be able to detect current flow between line conductors and ground at current levels as little as 5 milliamperes, which is much below the overload current levels required to trip conventional circuit breakers. Upon detection of such a ground fault current, the contacts of the circuit breaker are opened to deenergize the circuit. 
     Current transformers are an integral part of ground fault circuit breakers in that such circuit breakers typically include two of the transformers. A first current transformer, referred to as the ground fault or sense transformer, is used to sense ground fault currents. The sense transformer has as its primary windings the conductors of the distribution circuit being protected, which are encircled by the core, and a multi-turn winding wound on the core. (In the case of a one pole breaker, the line and neutral conductors both go through the sense transformer core, and in the case of a two pole breaker, the two line conductors and the neutral conductor all go through this core. For the sake of example, the following discussion relates to a one pole breaker.) During normal conditions, the current flowing in one direction through the line conductor will return in the opposite direction through the neutral conductor. This produces a net current flow of zero through the transformer, and the multi-turn winding provides no output. However, if a fault (that is, a leakage path) is established between the line conductor and ground, return current will bypass the transformer and flow through the ground back to the grounded side of the source supplying the circuit. Thus, more current will be flowing in one direction through the transformer than in the other, producing a current imbalance. Such a current imbalance produces uncancelled flux in the sense transformer&#39;s core, resulting in an output from the multi-turn winding that trips the circuit breaker mechanism. 
     A second current transformer, referred to as the ground neutral transformer, is commonly used to detect neutral-to-ground faults. A neutral-to-ground fault is an inadvertent short between the neutral conductor and ground that may occur due to a fault such as a wiring error by the electrician installing the circuit breaker. Such a leakage path on the load side of the sense transformer does not in itself produce a shock hazard; however, the occurrence of a grounded neutral at the same time as a ground fault on a line conductor will cause the ground fault circuit breaker to be less sensitive in detecting ground fault currents, thereby creating a hazardous situation. A neutral-to-ground fault reduces the sensitivity of the sense transformer as a ground fault sensing device because such a fault tends to provide a return current path via the neutral conductor for a large portion of the line-to-ground leakage current. To the extent that line-to-ground leakage current returns to the source via the neutral conductor, it escapes detection by the sense transformer. Consequently, the sense transformer may not respond to a hazardous ground fault. 
     In one known application, the ground neutral transformer comprises a core that encircles the neutral conductor (the ground neutral core can, but need not, encircle the line conductor too) and has a multi-turn winding wound thereon. When a neutral-to-ground fault occurs, an inductively coupled path between the sense transformer and the ground neutral transformer is closed. The resultant coupling produces an output in the ground fault sense transformer that trips the circuit breaker mechanism. 
     Such circuit breakers provide generally satisfactory operation. However, because of a current transformer&#39;s finite permeability, a dipolar asymmetry in the magnetic properties of the transformer&#39;s core and/or multi-turn winding will exist if the conductors are not symmetrically located in the opening of the transformer. The sense transformer of a ground fault circuit breaker must be able to detect a current imbalance as little as 5 milliamperes in the presence of hundreds of amperes of current. Thus, even a small dipolar asymmetry can produce an unacceptable error that will degrade the sense transformer&#39;s ability to detect ground fault currents. 
     Conventional current transformers often address this problem with magnetic shielding around the core, but magnetic shielding adds considerable cost to the current transformer. Magnetic shielding also increases the volume of the transformer. This can be a problem in ground fault circuit breakers because it can be difficult to package two transformers, the large #12 or #14 conductors, and a printed circuit board (which contains standard circuit breaker circuitry), into the small allotted volume provided in existing circuit breaker housings. This is particularly the case in residential applications for which compact, half-inch circuit breakers are now available. 
     It is also known to use high saturation core materials, such as those available under the trademark Permalloy, to minimize the dipolar asymmetry. However, such materials are typically more expensive than other common core materials such as ferrite. 
     Accordingly, there is a need for a current transformer that provides accurate output without using magnetic shielding or expensive materials. 
     SUMMARY OF THE INVENTION 
     The above-mentioned need is met by exemplary embodiments of the present invention which provide a current transformer for a ground fault circuit breaker used on a circuit having one or more line conductors and a neutral conductor. The current transformer includes a toroidal core having a circular opening defining a center point and a multi-turn winding wound on the core. A first guide member is disposed on one side of the core, and a second guide member is disposed on another side of the core. The first and second guide members each have a hole for receiving the line conductor and a hole for receiving the neutral conductor formed therein. The guide members thus position the conductors with respect to the core. In addition, a method of correcting asymmetries in the current transformer is provided. The method includes measuring the magnitude and orientation of any asymmetries, and then altering the current transformer based on the measured magnitude and orientation of the asymmetries so as to eliminate the asymmetries. 
     The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
     FIG. 1 is a schematic, cross-sectional view of an exemplary embodiment of the current transformer of the present invention. 
     FIG. 2 is a plan view of a guide disk from the current transformer of FIG.  1 . 
     FIG. 3 is a schematic representation of a first approach to correcting asymmetries in a transformer. 
     FIG. 4 is a schematic representation of a second approach to correcting asymmetries in a transformer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 schematically shows a current transformer  10  in cross-section. In a preferred embodiment of the present invention, the current transformer  10  is used in a ground fault circuit breaker that is connected in a two-way alternating current circuit line that delivers electrical energy from a power source (not shown) to a load (not shown). The circuit line has a line conductor  12  and a neutral conductor  14  grounded at the power source as is known in the art. While a transformer in a ground fault circuit breaker is being used as an example to facilitate disclosure of the present invention, it should be recognized that the current transformer of the present invention is not limited to use in ground fault circuit breakers and can be used in many transformer applications. 
     The current transformer  10  includes a toroidal core  16  having a circular opening that defines a center point. The core  16  encircles both the line conductor  12  and the neutral conductor  14 , so that the conductors  12  and  14  function as the single turn winding of the transformer  10 . The core  16  is fabricated using a magnetic material, preferably a relatively inexpensive core material such as iron or ferrite. The transformer  10  also includes a multiturn winding  18  that is uniformly wound on the core  16 . In a ground fault circuit breaker, the multi-turn winding  18  is electrically connected to conventional circuitry, which, in response to a multi-turn winding output, triggers a trip device that opens the breaker contacts, thereby deenergizing the conductors  12  and  14 . 
     The transformer  10  includes a pair of guide members  20  disposed on opposite sides of the core  16 . Each guide member  20  has a flat disk portion  22  and a cylindrical extension  24  extending perpendicularly from the disk portion  22 . The cylindrical extension  24  is centered with respect to the disk portion  22  and has a radius that is smaller than the radius of the disk portion  22 , but greater than the inside radius of the core  16  with the multi-turn winding  18 . Thus, the cylindrical extension  24  fits snugly within the circular opening of the toroidal core  16 , thereby centering the disk portion  22  with respect to the core  16 . The guide members  20  are made of a non-conducting material such as plastic or fiberglass. 
     Each guide member  20  has two holes  26  formed therein through which the line and neutral conductors  12  and  14 , respectively, are inserted. As best seen in FIG. 2, which shows a single guide member  20 , the holes  26  of each guide member  20  are both located very close to the center of the disk portion  22  and are arranged symmetrically with respect to the center of the disk portion  22 . By virtue of the cylindrical extension  24  centering the disk portion  22  with respect to the core  16 , the holes  26  of each guide member  20  are also located symmetrically with respect to the core  16 . Thus, the guide members  20  assure that the line and neutral conductors  12  and  14  are symmetrically located in the opening of the core  16 , thereby reducing and controlling the dipolar magnetic field from the single turn winding (i.e., the conductors  12  and  14 ) of the transformer  10 , and thereby reducing dipolar asymmetry without using magnetic shielding or expensive core materials. By locating the holes  26  of each guide member  20  as close as possible to the center point of the corresponding disk portion  22 , the effect of quadripole and higher moments will be minimized. 
     The holes  26  are all sized such that the line conductor  12  and the neutral conductor  14  will fit tightly within its corresponding holes  26 . Thus, the guide members  20  will be held in place against the top and bottom of the core  16  by a friction fit between the conductors  12  and  14  and the guide members  20 . Optionally, the guide members  20  could be bonded to the core  16  with a suitable adhesive. 
     Although exemplary embodiments of the present invention have been described in terms of a one pole circuit breaker having one line conductor and one neutral conductor, and thus two holes  26  in each guide member  20 , the present invention is also applicable to other breakers such as two pole breakers. In this case, each guide conductor would have three holes for the two line conductors and the neutral conductor. The three holes would be arranged symmetrically with respect to the center of the guide member. 
     Even with the conductors  12  and  14  located symmetrically in the opening of the core  16 , dipolar asymmetries can arise due to asymmetries in the core material and geometry and/or asymmetries in the multi-turn winding  18 . In order to avoid using magnetic shielding, a method of manufacturing the current transformer  10  is provided herein whereby inexpensive materials and manufacturing methods are used to produce a transformer, and then additional steps are taken to correct asymmetries arising in the core  16  and/or the multi-turn winding  18 . 
     One such approach includes measuring the magnitude and orientation of the asymmetries of the core  16  prior to winding. As shown schematically in FIG. 3, the unwound core  16  is excited by a cylindrical excitation conductor  28  located exactly at the core&#39;s center of symmetry, and a pick-up coil  30  is placed next to the core  16 , oriented in a direction to pick up only the radial component of the resulting magnetic field. The conductor  28  is connected to an excitation source  32 , and the output of the pick-up coil  30  is monitored. Since the field from the conductor  28  is precisely tangential, there will not be any direct coupling between the conductor  28  and the pick-up coil  30 . Furthermore, if the core  16  is precisely symmetrical, the paramagnetically induced field will also have no radial component. But if the core  16  is not perfectly circularly symmetrical, the induced field will be unbalanced, and a radial component will result. The magnitude of the radial component will be detected by the pick-up coil  30 . 
     The orientation of this radial component can be determined by rotating the core  16  about its axis of symmetry and noting the sinusoidal variation from the pick-up coil  30  with the angle of rotation. A conventional computer would analyze these variations and calculate the amount and location of core material that needs to be removed or added to eliminate the built-in core asymmetry. If core material is needed to be removed this could be accomplished with a grinder. If core material is needed to be added, this could be accomplished by using a paint applicator to apply a magnetic pigment, such as ferrite or powdered iron, to the appropriate location of the core  16 . 
     As an alternative to rotating the core  16  to determine the orientation of the induced field, two pick-up coils can be provided at right angles to each other. These coils will pick up the sine and cosine components of the field, and from these, the magnitude and angle of the induced field can be determined. 
     A second approach includes measuring the magnitude and orientation of the asymmetries of the transformer  10  after the multi-turn winding  18  has been wound on the core  16 . Referring to FIG. 4, the core  16  is shown with the multi-turn winding  18  wound thereon and the multi-turn winding leads  34  extending therefrom. A pick-up coil  36  is located in the opening of the core  16 , at the center of symmetry. The multi-turn winding leads  34  are connected to an excitation source  38  so that the multi-turn winding  18  is excited, and the output of the pick-up coil  36  is monitored. The pick-up coil  36  functions as a transformer winding in that if the multi-turn winding  18  is excited and there is zero pick-up in the pick-up coil  36 , then there will also be zero pick-up in the multi-turn winding  18  when the pick-up coil is excited due to the reciprocity of transformers. Since the pick-up coil generates a dipole field, a zero pick-up condition will occur when there is no dipole component to the transformer leakage field. But when there is a non-zero pick-up in the pick-up coil  36 , this is an indication of a dipolar asymmetry in the core  16  and/or multi-turn winding  18 . 
     The orientation of the induced field can be determined by rotating the core  16  about its axis of symmetry and noting the sinusoidal variation from the pickup coil  36  with the angle of rotation. A conventional computer would analyze these variations and calculate the amount and location of the asymmetry. In this second approach, it would is not practical to make adjustments to the core  16  since it is covered with the multi-turn winding  18 . Thus, corrections to the transformer  10  can be made by spraying magnetically loaded paint on an appropriate location of the wound core, or by adding an arcuate strip of magnetic material adjacent to the outer radius of the wound core. Another technique would be to add an additional winding that has the opposite coupling as the induced field to the core  16 . Typically, such an additional winding will have only a few turns that are generally all wound in a small, selected region. 
     Again, as an alternative to rotating the core  16  to determine the orientation of the induced field, two pick-up coils can be provided at right angles to each other. These coils will pick up the sine and cosine components of the field, and from these, the magnitude and angle of the induced field can be determined. 
     An alternative to modifying the properties of the core and/or the winding, which may be sufficient in some applications, is to orient the guide holes with respect to the core such that the dipole field induced by the two wires is orthogonal to the dipole field induced by the asymmetries of the core or winding. Under these conditions, the dipole field induced by the load current and the neutral return current will not induce any pick-up in the multi-turn winding. Although this will work in single pole applications, it does not work in two pole breakers where three conductors pass through the core and the orientation of the dipole cannot be determined. 
     The foregoing has described a current transformer that minimizes dipolar asymmetries without using magnetic shielding or expensive core materials. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention as defined in the appended claims.