Patent Document

TECHNICAL FIELD  
         [0001]    This invention relates to current sensors used in electrical switchgear.  
         BACKGROUND  
         [0002]    Current sensors are used in the electric power industry to measure current flowing in electrical systems. In particular, current sensors may be used in electrical switchgear such as circuit breakers, reclosers, and switches to determine when a fault has occurred in the electrical system.  
         SUMMARY  
         [0003]    In one general aspect, an electrical switchgear device includes a conductor, a base, and a current sensor positioned to detect current in the conductor and attached to the base using a support element. The device also includes an apparatus mounted to the base to interrupt current through the conductor when a signal from the current sensor indicates a predetermined condition. A housing positioned on the base encapsulates the current sensor, the support element, the current interrupting apparatus, and the conductor.  
           [0004]    Embodiments may include one or more of the following features. The housing may include a solid insulating material. The support element may include a rigid tube. The support element may be bent at an end coupled to the current sensor. The bent end of the support element may include a support strip shaped to match a curvature of the current sensor.  
           [0005]    The current sensor may include a sensor conductor that produces a signal. The support element may be hollow—in this case, the sensor conductor is drawn through the support element to control circuitry. The sensor conductor and the support element may be hermetically sealed. The support element may be hermetically sealed to the base.  
           [0006]    The support element may be metallic or non-metallic. In either case, the support element may be coated with a semi-conductive paint.  
           [0007]    The housing may encapsulate the current sensor, the support element, the current interrupting apparatus, and the conductor such that there is no dielectric interface between the current sensor and the conductor.  
           [0008]    In another general aspect, a method of producing an electrical switchgear device includes securing a support element to a current sensor. The current sensor is mounted relative to a main conductor by securing the support element to a surface of a mold that houses a current interrupter and a portion of the conductor. A prepared material is injected into the mold to encapsulate the support element, the current sensor, the conductor, and the current interrupter. The injected material is permitted to solidify to form a housing.  
           [0009]    Embodiments may include one or more of the following features. The support element may be secured to the current sensor by drawing sensor conductors from the current sensor through a hollow passage of the support element. The support element may be secured to the current sensor by bending a first end of the support element and attaching to the first end a support strip shaped to match a curvature of the current sensor. The support element may be secured to the current sensor by securing the support strip to the current sensor.  
           [0010]    The support element may be secured to the surface of the mold by connecting a second end of the support element to a post positioned at the surface of the mold. The second end of the support element may be connected to the post by hermetically sealing the second end to the post. The second end of the support element may be connected to the post by drawing sensor conductors from the current sensor through a hollow passage of the post. The method may include removing the mold from the housing and securing the housing to a tank that houses additional components.  
           [0011]    The electrical switchgear exhibits improved overall dielectric performance because all of the components are encased into a single housing with no dielectric interfaces. Moreover, the electrical switchgear exhibits a longer life because of reduced failure associated with dielectric breakdown at interfaces. Manufacturing of the electrical switchgear is more economical due to simplification of the current sensor design.  
           [0012]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description, the drawings, and the claims.  
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0013]    [0013]FIG. 1 is a cross section of an electrical switchgear with an exemplary mounting device for a current sensor.  
         [0014]    [0014]FIG. 2 is a side view of a three-phase electrical switchgear of FIG. 1.  
         [0015]    [0015]FIG. 3 is a front view of the three-phase electrical switchgear of FIG. 2.  
         [0016]    [0016]FIG. 4 is a flowchart of a procedure for forming a housing of the electrical switchgear of FIGS.  1 - 3 .  
         [0017]    [0017]FIG. 5 is a cross section of an electrical switchgear that includes an improved current sensor mounting system.  
         [0018]    [0018]FIG. 6 is a perspective view of a mold used in forming the electrical switchgear of FIG. 8.  
         [0019]    FIGS.  7 - 9  are perspective views of alternative mounting devices for current sensors used with electrical switchgear.  
         [0020]    [0020]FIGS. 10 and 11 are perspective views of current sensors used in the electrical switchgear of FIGS. 5 and 6.  
         [0021]    [0021]FIG. 12 is a perspective view of a three-phase electrical switchgear that incorporates the electrical switchgear of FIGS. 5 and 6.  
         [0022]    [0022]FIG. 13 is a flowchart of a procedure for forming a housing of the electrical switchgear of FIGS. 5 and 6.  
         [0023]    Like reference symbols in the various drawings indicate like elements. 
     
    
     DETAILED DESCRIPTION  
       [0024]    The invention provides improved techniques for supporting a current sensor in electrical switchgear. For ease of explaining the improved technique, electrical switchgear constructed according to a current technique are discussed relative to FIGS.  1 - 4 , prior current sensor mounting systems are discussed relative to FIGS.  7 - 9 , and electrical switchgear constructed according to the improved technique is discussed relative to FIGS. 5, 6, and  10 - 13 .  
         [0025]    Referring to FIGS. 1 and 2, electrical switchgear  100  includes a current interrupter  105 , an insulated operating rod  110 , and a conductor  115  encapsulated in a solid polymer that makes up a housing  120 . The housing  120  is mounted on a tank or base  130  that houses additional components. For example, in electrical switchgear  100 , the tank  130  typically houses an electromagnetic actuator mechanism, a latching mechanism, and a motion control circuit.  
         [0026]    The housing  120  is manufactured of a solid polymer such as an epoxy or other solid insulating material. Solid dielectric insulation eliminates the need for insulating gas or liquid, thereby greatly reducing switch life-cycle maintenance costs. The solid dielectric insulation may be made of a cycloaliphatic epoxy component and an anhydride hardener, mixed with silica flour filler.  
         [0027]    A current sensor  135  is mounted externally to the housing  120  and is partially supported by a coupler  140  attached to the tank  130 . The current sensor  135  measures direction and magnitude of current flowing though the conductor  115  based on the principle of induction. The current sensor  135  is typically formed from a conductor wound around a magnetic core. In this way, alternating current through the conductor  115  induces a current through the conductor in the current sensor  135 . Wires from the current sensor  135  are directed through the coupler  140  and into the tank  130  to the appropriate control or relay circuitry. Before mounting, the current sensor  135  is also encased in a housing  145  using a solid polymer.  
         [0028]    Referring also to FIG. 3, the electrical switchgear  100  may be implemented in a threephase electrical switchgear power system  300 . In this case, electrical switchgear  100  is used for each phase of the power system. The three electrical switchgear  100  are mounted on a tank  305  that is designed like tank  130  to hold the additional components.  
         [0029]    Referring also to FIG. 4, the housing  120  may be formed using a procedure  400  for casting. In one implementation, the procedure  400  is an automatic pressure gelation (APG) procedure. Initially, cycloaliphatic epoxy material is prepared, for example, by preheating and degassing in special equipment provided with vacuum (step  405 ). The mold houses the current interrupter  105  and conductor  115 , as shown in FIG. 1. Then, the preheated and degassed material is pumped under pressure into the mold at a higher temperature, which provides the necessary energy to disrupt the equilibrium of the system to start gelation and crosslinking processes in the material(step  410 ). When the desired crosslinking and gelation of the material is completed, an encapsulation or housing  120  is formed (step  415 ) and then removed from the mold (step  420 ). The gelation and crosslinking processes provide a housing  120  with a desired glass transition temperature, which enhances its dielectric and mechanical properties and enhances its ultraviolet protection and weather resistance. Alternatively, the housing  120  may be molded by other procedures, for example, vacuum casting.  
         [0030]    After the housing is removed from the mold (step  420 ), the current sensor housing  145  (which contains the current sensor  135 ) is mounted to the conductor  115  portion that extends from the housing  120  and the coupler  140  is mounted to the tank  130  (step  425 ). The current sensor housing  145  may be formed using a procedure similar to procedure  400 . The current sensor  135  is then connected to appropriate control or relay circuitry associated with the electrical switchgear (step  430 ).  
         [0031]    Referring to FIGS. 5 and 6, electrical switchgear  500  is similar in design and operation to electrical switchgear  100  in many respects. The switchgear differ primarily with respect to the positioning, design, and manufacture of current sensor  505 . In electrical switchgear  500 , the current sensor  505  is mounted relative to conductor  115  prior to molding of the current sensor  505  or the conductor  115 .  
         [0032]    Prior electrical switchgear designs that employ a system of mounting the current sensor to the conductor prior to molding are shown as mounting systems  700 ,  800 ,  900  in FIGS.  7 - 9 . However, these other mounting systems  700 ,  800 , and  900  cause dielectric problems between the surface of the current sensor and the conductor. Often, the dielectric failure rate of mounting systems  700 ,  800 , and  900  may be high.  
         [0033]    Referring to FIG. 7, in mounting system  700 , the current sensor  135  is pre-cast into a molding  705  and is supported directly on the conductor  115  through an opening  710 . However, this mounting system  700  may cause dielectric failures subsequent to molding along an interface between the pre-cast sensor and the epoxy material that forms the electrical switchgear housing.  
         [0034]    As shown in FIG. 8, in mounting system  800 , the current sensor  135  is supported on the conductor  115  using elastic bands  805  such as rubber bands or O-rings. Although mounting system  800  is fast and inexpensive, dielectric failures may occur following casting of the current sensor  135  because the epoxy material shrinks as it cures and leaves small cracks or deformations along the elastic bands  805 . One way to address this problem is to ensure that the thermal coefficient of expansion of the elastic bands is close to or matches that of the epoxy.  
         [0035]    Referring also to FIG. 9, in mounting system  900 , the current sensor  135  is mounted on a stand  905  that is positioned on an inner surface of the current sensor mold. The stand  905  is encapsulated along with the current sensor  135  during molding. When using this approach, care must be taken to ensure that the stand  905  does not move out of place during the molding process, which could cause damage or marring of the mold surface. The material used in the stand  905  must be one capable of withstanding molding temperatures. Again, the presence of a dielectric interface may cause problems.  
         [0036]    Referring again to FIGS. 5 and 6, the electrical switchgear  500  includes a current sensor  505  mounted directly to tank  130  by a support element  507 , with this mounting being done prior to molding. An expanded mold  600  (FIG. 6) is shaped to include the current interrupter  105 , the conductor  115 , and the current sensor  5 O 5 . After molding, a housing  510  encapsulates the current interrupter  105 , the conductor  115 , the current sensor  505 , and the support element  507 . As discussed below, this current sensor mounting system eliminates or significantly reduces dielectric interfaces that may cause subsequent failures.  
         [0037]    [0037]FIGS. 10 and 11 show the current sensor  505  and the support element  507  separate from the housing  510 . The support element  507  may include a passage through which conductors  1000  from the current sensor  505  are drawn and connected to appropriate circuitry in the switchgear  500 . The current sensor  505  may be painted with a semi-conductive paint or covered with semi-conductive tape to guarantee an intimate ground contact to the epoxy surface surrounding current sensor  505 .  
         [0038]    In one implementation, the support element  507  may be made of a non-metallic rigid tube. In this case, the tube may be painted with a semi-conductive paint to shield any air that may be within the tube. In another implementation, the support element  507  may be made of a metallic rigid tube, which may be coated with a semi-conductive paint to provide shielding if the epoxy tends to pull away from the tube during subsequent curing or temperature cycling extremes.  
         [0039]    To facilitate attachment of the support element  507  to the current sensor  505 , a first end of the support element  507  may be bent. A support strip  1005  may be secured to the first end of the support element  507  and formed to match the curvature of the current sensor  505 . The support strip  1005  may be metallic or coated, as needed. The support strip  1005  may be secured to the current sensor  505  using any suitable device, such as semi-conductive tape  1010 , that shields air that may be trapped between the support strip  1005  and the current sensor  505 .  
         [0040]    Referring again to FIGS. 5 and 6, the other end of the support element  507  connects with a short post  520  at the bottom of the mold. The short post  520  is hollow, to permit passage of the conductors  1000  from the support element  507  to the switchgear circuitry. The short post  520  and the support element  507  may be sealed where they meet using any suitable material, such as, silicone rubber tubing. Additionally, the conductors  1000  and the support element  507  may be sealed where they meet using, for example, an appropriately sized silicone rubber washer and a coating of room temperature vulcanizing rubber. Epoxy or other materials may be used to seal the support element  507  to short post  520  or the conductors  1000  to the support element  507 . In any case, these sealing materials are selected to withstand preheat and molding temperatures that typically reach around 155° C. and to prevent unwanted air flow.  
         [0041]    Referring to FIG. 12, electrical switchgear  500  may be implemented in a three-phase electrical switchgear system  1200 . In this case, electrical switchgear  500  is positioned on each phase of the power system. Electrical switchgear  500  are mounted on a tank  1205  that houses additional components.  
         [0042]    Referring also to FIGS. 5 and 13, the housing  510  may be molded using a procedure  1300  for encapsulating the current interrupter  105 , conductor  115 , current sensor  505 , and support element  507 . In one implementation, the procedure  1300  is an automatic pressure gelation (APG) procedure. Initially, the current sensor  505  is assembled in relation to the conductor  115  by securing the support element  507  to the mold  900  (step  1305 ). In this way, the mold  600  houses the current interrupter  105 , conductor  115 , current sensor  505 , and support element  507 . The epoxy material is prepared, for example, by preheating and degassing in special equipment provided with vacuum (step  1310 ). Then, the prepared material is pumped under pressure into the expanded mold  600  at a higher temperature (step  1315 ). The higher temperature provides the necessary energy to disrupt the equilibrium of the system to start gelation and crosslinking processes in the material. When the processes are complete, the housing  510  is formed (step  1320 ) and the formed housing  510  is removed from the expanded mold  600  (step  1325 ). Alternatively, the housing  510  may be cast by other procedures, for example, vacuum casting.  
         [0043]    In any case, the design and mounting of the current sensor  505  and the procedure  1300  for forming the housing  510  reduce or eliminate the dielectric problems between the surface of the current sensor and the conductor. In particular, the current sensor  505  design and mounting eliminates a dielectric interface between the current sensor  505  and the conductor  115 . Dielectric failure rates within the housing  510  may be significantly reduced. Moreover, dielectric failure rates approaching  0 % are possible with additional modifications to a shielding of the current sensor  505 .  
         [0044]    The current sensor  505  may be connected to appropriate control or relay circuitry associated with the electrical switchgear at any appropriate time before, during, or after procedure  1300 .  
         [0045]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims. For example, the current sensor support structure of FIGS. 5, 6, and  10 - 13  may be implemented in any electrical switchgear such as fault interrupters, reclosers, breakers, or switches.

Technology Category: 5