Patent Publication Number: US-2023144416-A1

Title: Air insulated switch with very compact gap length

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority from the U.S. Provisional Application No. 63/278,222, filed on Nov. 11, 2021, the disclosure of which is hereby expressly incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     Field 
     This disclosure relates generally to a grounding switch for grounding an energized conductor in an underground switchgear and, in particular, a grounding switch for grounding a high voltage cable connected to underground switchgear. 
     Discussion of the Related Art 
     Power distribution networks include components that control the flow of power throughout the network. For underground power distribution networks it is typical to have electrical disconnect switches, fuses and/or circuit breakers (used to control power flow and to protect and isolate electrical equipment) to be assembled and packaged in an assembly know as switchgear, where the assembly is enclosed in an external housing that is mounted underground or mounted on, for example, a concrete pad. An underground mounted switchgear always has a dead-tank design, i.e., external surfaces of all components are grounded for safety and operational reasons. 
     Electrical cables going into and out of switchgear are connected to terminals of certain electrical devices or equipment, such as switches, within the switchgear. For an underground switchgear high voltage cables are typically connected through T-body connectors with conical interfaces to terminal conductors of the switchgear. The external surface of the T-body is grounded for safety and operation reasons. Whenever utility lineman have to workj on an existing cable, they have to ground it for safety reasons. Typically, they have to connect the cable with the T-body from the switchgear terminal and to “park” it on a separate isolated cone. This operation is cumbersome and hazardous because sometimes an arc can be created in a very limited space. Therefore, grounding switches are often employed in switchgear to connect the cables to ground as a safety measure when such personnel are present for replacing and servicing the equipment and devices in the switchgear. However, these grounding switches typically have long air gaps between the high voltage electrode that is connected to the cable and a ground electrode to prevent breakdown between the electrodes when the switch is open, which adds significant size and cost to the switchgear. 
     SUMMARY 
     The following discussion discloses and describes a grounding switch for grounding a high voltage cable in an underground switchgear. The switch includes a solid insulation housing, an outer grounded semicon layer on its external surface and a fixed electrode extending into one end of the housing and being encapsulated in the solid insulation housing. The switch also includes a ground electrode positioned at an opposite end of the housing from the fixed electrode, an air gap formed in the solid insulation housing between the fixed electrode and the ground electrode, and a movable electrode that is moved within the air gap to electrically connect and disconnect the fixed electrode to and from the ground electrode. A shielding electrode is electrically coupled to the fixed electrode proximate the air gap, and shapes the distribution of the electric field to have a high field region to be mostly contained within the solid insulation. In one embodiment, the shielding electrode is an annular electrode having a cup shape that encircles an end of the fixed electrode, where the shielding electrode includes a cylindrical body portion and a protrusion that extends from the body portion towards the ground electrode. 
     Additional features of the disclosure will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a broken-away, cross-sectional type view of a grounding switch having a reduced air gap in an open position; 
         FIG.  2    is a broken-away, cross-sectional type view of the grounding switch shown in  FIG.  1    in a closed position; and 
         FIG.  3    is an isometric view of a shielding electrode in the grounding switch shown in  FIG.  1   . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the disclosure directed to a grounding switch including a shielding electrode for grounding a high voltage cable is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, in the discussion herein the grounding switch is employed in underground switchgear. However, as will be appreciated by those skilled in the art, the grounding switch may have other uses and applications. 
       FIG.  1    is a broken-away, cross-sectional type view of a grounding switch  30  in an open position and  FIG.  2    is a broken-away, cross-sectional type view of the grounding switch  30  in a closed position that can be used in underground switchgear to connect, for example, a high voltage cable, or other connection, to ground as a safety measure when the switchgear is being serviced and/or parts are being replaced by technicians, where the cable is not otherwise grounded. The switch  30  includes an outer semicon external surface layer  32  that encloses a solid insulation housing  34 . A fixed high voltage electrode  36  extends into the semicon layer  32  through a top end  38  of the semicon layer  32  and is mostly encapsulated within the solid insulation housing  34 , where the electrode  36  is connected, for example, to the high voltage cable (not shown in  FIGS.  1  and  2   ) through a conical joint interface at a top end  46  of the electrode  36 . A bottom end  48  of the electrode  36  is spaced some distance from a ground electrode  50  connected to ground across an air gap  52  formed in the solid insulation housing  34 . The electrode  36  is cylindrically shaped and includes a center bore  54 . A movable cylindrical electrode  56  is connected to an insulating pull rod  58  that is actuated by a suitable actuator (not shown), such as a solenoid, to move the electrode  56  into and out of the bore  54  through an opening  44  in the bottom end  48  and connect the fixed electrode  36  to and disconnect the fixed electrode  36  from the ground electrode  50 , where the movable electrode  56  is shown in the up and ground disconnect position in  FIG.  1    and in the down and ground connect position in  FIG.  2   . 
     Because of high voltages involved, in the known grounding switches, the air gap  52  between the high voltage electrode  36  and the ground electrode  50  was significantly longer, such as 10 inches, to prevent arcing therebetween during normal operation (and during impulse overvoltage on the cable) when the switch is open. However, that increases the overall size of the grounding switch. In order to reduce the length of the air gap  52 , the grounding switch  30  includes an annular shielding electrode  60  electrically coupled to and extending around the bottom end  48  of the fixed electrode  36 , as shown, where the shielding electrode  60  can be metallic or insulating with a conductive surface. The shielding electrode  60  has a cup shape defined by an annular body portion  62 , a top rim  64  and a protrusion  66 . An isometric view of the shielding electrode  60  is shown in  FIG.  3    separated from the grounding switch  30 . This shape of the shielding electrode  60  and its position with respect to the semicon layer  32 , the grounded electrode  50  and the air gap  52  causes the electric field created by high voltage on the fixed electrode  36  to be non-uniform and enhanced at a tip of the protrusion  66 , and, due to non-uniformity, to become significantly lower as it extends in the solid insulation housing  34  towards the grounding electrode  50  and towards the air gap  52 . If a sufficient distance is provided between the tip of the protrusion  66  and the air gap  52 , the high field region is caused to be mainly confined within the solid insulation housing  34  so that the electric field within the air gap  52  is significantly reduced and is approximately uniform. For a specified maximum impulse voltage, this shape of the electric field distribution prevents breakdown from occurring in the air gap  52 , which prevents arcing across the air gap  52  to the ground electrode  50 . 
     In one non-limiting embodiment, for the length of the air gap  52  of 2.5″ between the electrodes  36  and  50  and with the overall OD of the switch  30  of 5.6″, the switch  30  has an impulse withstand voltage of more than 160 kV. All of the electric field concentrations are located in the solid insulation housing  34  where they cannot lead to impulse breakdown at a maximum required impulse withstand voltage. Even though the solid insulation housing  34  can take a very high impulse field without breakdown, the continuous AC field in the solid insulation housing  34  still has to be limited to account for possible electrical aging. The field in the solid insulation housing  34  at a continuous AC voltage has to be lower than a certain empirical maximum continuous electric field that depends on the material of the solid insulation housing  34 . The shape of the shielding electrode  60  can be determined iteratively by any available electromagnetic field calculation software. For a given impulse voltage and a given continuous AC voltage the geometrical parameters (which include the length and OD of the air gap  52 , OD of the solid insulation housing  34 , the length and ID of the protrusion  66  and the rounding radiuses of the protrusion  66 ) are varied in order to provide a maximum field in the air gap  52  to be below 3 kV/mm and a maximum field on the tip of the protrusion  66  to be below the empirical maximum continuous field for the given solid insulation material. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the disclosure as defined in the following claims.