Abstract:
A high voltage, high current telescopic disconnect switch suitable for use in isolated phase bus duct has two fixed conductors and a movable telescopic conductor coaxially disposed within the first fixed conductor and movable to a closed position bridging the first and second fixed conductors. A plurality of contact fingers are mounted to the fixed conductors and have contact surfaces that extend into engagement with the telescopic conductor to make electrical contact between the fixed conductors through the telescopic conductor when in a closed position. The contact surface of each of the fingers comprises a first tip portion that extends beyond the fixed conductor and has a first radius that is slightly greater than the radius of the telescopic conductor so that the first tip portion presents an arcuate width that overlies in wiping electrical contact the telescopic conductor when in the closed position.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to electrical switches and, more particularly, to high voltage, high current telescopic disconnect switches suitable for use in isolated phase bus duct. 
   BACKGROUND OF THE INVENTION 
   Utility company power plants have large generators typically generating at medium voltages of, for example, 13,800 volts to 34,000 volts and current ratings of 5,000 amps to 30,000 amps. Typically this voltage is stepped up by transformers to much higher voltages in order to transmit the energy over long distances. The connection between the generator and the step-up power transformer is usually made by isolated phase bus duct. The bus duct usually comprises three phase conductors each comprising an inner conductor and a outer conductive housing surrounding, and electrically insulated from, the inner conductor. 
   A circuit breaker is typically provided in the isolated bus duct to protect the generator by isolating the generator in the event of a short circuit condition or a fault in the step-up transformer. To service the generator, the circuit breaker is opened and then the disconnect switch is opened to isolate the generator. 
   The disconnect switch utilized to isolate the generator may comprise a telescopic switch. The telescopic disconnect switch also finds application in a pump storage plant as a phase reversal switch. The telescopic switch comprises two fixed or stationary conductors and a hollow moveable cylindrical conductor which resides in a first one of the fixed conductors and telescopes between first and second positions to engage and disengage, respectively, contact fingers on a second one of the fixed conductors. Contact between both of the fixed conductors and the moving cylindrical conductor is made by contact fingers mounted on the circumference of both fixed conductors. The moving conductor has a smaller diameter than the two fixed conductors and moves along the same axis as the fixed conductors to slide inside one of the fixed conductors until it reaches the fully opened position. The contact fingers have two flat contact surface portions. The first flat contact surface portion contacts one of the fixed cylindrical conductors along a point of contact engagement and the flat second contact portion extends beyond the fixed conductor for sliding wiping engagement and contact along another point of contact engagement with the movable cylindrical conductor. The contact points of engagement provide an effective electrical engagement however the points also limit the surface making electrical contact between the telescopic conductor and the fixed conductor. 
   These existing telescopic disconnect switches with the contact fingers having the flat contact surface portion are able to operate between open and closed positions for cycles of about 500 operations before major maintenance of the telescopic switch is required. There is now a market driven requirement that these telescopic switches operate for more than 500 operations before requiring maintenance servicing. Accordingly, any improvements in the telescopic switch that enhances the number of operating cycles would be advantageous. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention relates to a high voltage, high current telescopic switch suitable for use in isolated phase bus duct. The telescopic switch has two stationary or fixed conductors and a movable telescopic conductor coaxially disposed with the first fixed conductor and movable to a closed position bridging the first and second fixed conductors. A plurality of electrically conductive contact fingers are mounted to the fixed conductors and have contact surfaces that extend into engagement with the telescopic conductor to make electrical contact between the fixed conductors through the telescopic conductor when in a closed position. The contact surface of each of the fingers comprises a first tip portion that extends beyond the fixed conductor and has a first radius that is slightly greater than the radius of the telescopic conductor so that the first tip portion presents an arcuate width that overlies in wiping electrical contact the telescopic conductor when in the closed position. 
   In another embodiment, the contact fingers each have a fixed contact surface tip portion that engages a groove in the fixed conductor. The fixed contact surface tip portion has a second radius slightly larger than the radius of the groove of fixed conductor so that an arcuate width of the tip portion extends into the groove in electrical contact therewith. 
   The contact surface made by each of the tip portions of each of the fingers with the telescopic conductor and one of the fixed conductors is increased by the present invention. This increase in contact surface from a point of engagement to a line of engagement reduces contact resistance and improves current capacity while maintaining acceptable temperature rise at the contact. An increase in contact finger conductivity and current capacity is achieved by the present invention while improving wear effect on the contact surface tip portions resulting in more cycles of operation of the telescopic switch between maintenance servicing. 
   In one embodiment there is provided a telescopic switch for use in isolated phase bus duct. The switch comprises first and second spaced apart and axially aligned fixed conductors. The first and second fixed conductors have adjacent end portions each supporting a plurality of individual contact fingers circumferentially positioned thereabout. The switch further comprises a telescopic conductor coaxially disposed with the first fixed conductor and axially moveable between an open position where the telescopic conductor is spaced apart from the second fixed conductor and a closed position where the telescopic conductor bridges the first and second fixed conductors. The telescopic conductor has first outside contact wall surface portions of a first predetermined radius. Each of the individual contact fingers comprises first and second finger contact surface portions. The second contact finger surface portion is held in engagement with a corresponding one of the adjacent end portions of the first and second fixed conductors. The first finger contact surface portion comprises a first contact surface tip portion extending beyond the corresponding one of the adjacent end portions of the first and second conductors, the first contact surface tip portion is of first concave arc shaped width of first radius slightly greater than the first predetermined radius. The first contact surface tip portion overlies in wiping electrical contact one of the first outside contact wall surface portions of the telescopic conductor when in the closed position. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the nature and objects of the present invention reference may be had by way of example to the accompanying diagrammatic drawings in which: 
       FIG. 1  is a perspective view of a telescopic switch embodying the present invention; 
       FIG. 2  is a perspective view showing one fixed conductor, the telescopic conductor and the fingers of the present invention; 
       FIG. 3  is an enlarged perspective view of a portion of the telescopic switch of  FIG. 2 ; 
       FIG. 4  is a partial side sectional view of the telescopic switch shown in the closed position; 
       FIG. 5  is a partial side sectional view of the telescopic switch shown in the open position; 
       FIG. 6  is an end view of one of the contact fingers shown in  FIG. 4 ; and, 
       FIG. 7  is a bottom perspective view of a contact finger. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1  there is shown an embodiment of a telescopic switch  10  that is exemplary of the switch embodying the present invention. Switch  10  is suitable for use in one phase of an isolated phase bus duct. Although not illustrated in  FIG. 1 , it should be understood that three switches each similar to switch  10  may be adapted to be inserted in respective phases of a three-phase isolated phase bus bar distribution system at a point where it is desired to have circuit opening and closing capabilities. Similarly, appropriate connectors (not shown) are used to join the switch  10  to the corresponding conductors of the distribution system. It should further be understood that the telescopic switch of the present invention may be used for any type of system employing a housed or enclosed bus. 
   Referring to  FIG. 1 , the telescopic switch  10  comprises an outer enclosure  12  shown partially broken away. The outer enclosure  12  is generally cylindrical in shape, is hollow, and comprises a conductive material. The enclosure  12  has hanger brackets  18  which may be used for mounting the telescopic switch  10 . The telescopic switch  10  further comprises first and second stationary or fixed conductors  14  and  16 , respectively. Conductors  14  and  16  are located within the hollow enclosure  12  and are each supported relative to the hollow enclosure  12  by three insulated support legs  20  spaced apart 120 degrees around the conductors  14 , 16 . 
   The first and second fixed conductors  14 ,  16  are spaced apart from each other and axially aligned along axis  21 . The first and second fixed conductors  14 ,  16  are generally cylindrical in shape and are hollow. 
   Shown in  FIG. 1  bridging the first and second fixed conductors  14  and  16  is a telescopic conductor  22 . The telescopic conductor  22  is a hollow cylindrical shaped conductor that also extends along the axis  21  and is co-axial with the first fixed conductor  14 . The outer radius of the telescopic conductor  22  is chosen to be less than the radius the inside surface of the first fixed conductor  14  which allows for the conductor  22  to be moved axially along axis  21  for retraction into the first fixed conductor  14 . When retracted the telescopic conductor  22  is spaced from the second fixed conductor  16  to effectively open the circuit of switch  10 . Movement of the telescopic conductor  22  is controlled by a transmission or gearing mechanism (not shown). The telescopic conductor  22  is shown in  FIGS. 1 and 4  in a closed position closing the circuit, and bridging the space, between the first fixed conductor  14  and the second fixed conductor  16 . The telescopic conductor  22  is shown in its open position in  FIG. 5 . 
   Each of the fixed conductors  14  and  16  has adjacent end portions  24  that support a plurality of individual electrically conductive contact fingers  26  which are circumferentially positioned about adjacent end portions  24  of the first and second fixed conductors  14 ,  16 . As better seen in  FIGS. 2 through 5 , the contact fingers  26  are secured either to the fixed conductor  14  or the fixed conductor  16  by means of bolts  28  passing through apertures  27  ( FIG. 7 ) in the fingers  26  and through aperture  29  in the fixed conductors  14 ,  16 . The bolts  28  each have a head portion (not shown) countersunk into the inner surface of the fixed conductors  14 ,  16  so that the bolt heads do not to touch the telescopic conductor  22 . A compression spring  30  is placed over a threaded end portion of the bolt  28  and a nut  31  is tightened on the threaded end portion of the bolt  28  to control the compression force of the spring  30  on the finger  26 . The mounting of the individual contact fingers  26  to either the first or second fixed conductor  14 ,  16 , present a jaw like configuration  32  ( FIG. 2 ) which makes electrical contact with the telescopic conductor  22 . 
   Referring to  FIGS. 4 to 7 , each of the individual contact fingers  26  has first and second finger contact surface portions  32  and  34  respectively. The contact surface portions  32 ,  34  are spaced apart by intermediate link arms  60  and  61 . Link arm  60  is bent to provide finger end portions  62  that converge towards the telescopic conductor  22 . Collectively, the finger end portions  62  of all the fingers  26  mounted to each of the adjacent end portions  24  present the jaw-like configurations  32 . 
   The first contact surface portion  32  of each finger  26  comprises a contact tip portion  38  that extends beyond conductor  14  or  16 . The contact tip portion  38  makes contact with a corresponding raised contact surface outside wall portion  42  ( FIG. 4 ) of the telescopic conductor  22 . The contact surface tip portion  38  (as best seen in  FIG. 7 ) has a first concave arc shaped line of width W 1 . The contact arc associated with width W 1  has a radius R 2  shown in  FIG. 6  which is slightly greater than, the radius R 1  of the first contact surface wall portion  42  of telescopic conductor  22 . This permits the first contact surface tip portion  38  to overlie in wiping electrical contact the first outside wall contact surface portion  42  of the telescopic conductor  22  when in the closed position. The arcuate line of contact of contact surface tip portion  38  is best shown in  FIGS. 6 and 7 . The wiping effect of this line of contact with the outside wall portion  42  of the telescopic conductor  22  is shown, for illustrative purposes only, as area  75  in  FIG. 3 . 
   The second contact finger surface portion  34  for each finger  26  comprises a second contact tip portion  40 . The second contact finger surface portion  34  is shown in side view in  FIG. 7  to be rounded. It should be understood that this rounded portion may be any shape, such as for example, triangular, so long as the shape provides an arcuate tip portion  40  described in more detail hereafter. The second contact finger surface portion  34  is shown mounted in or extending into a groove  35 . Groove  35  is formed in each of the adjacent end portions  24  of the fixed conductors  14  and  16 . The side curvature  77  of the second contact finger surface portion  34  permits the contact surface portion  34  to be wedged into groove  35 . Consequently, as the telescopic conductor  22  is moved into the closed position shown in  FIG. 4 , the conductor  22  forces the finger  26  against the spring  30 . This increases the contact mating force between contact surface tip portion  40  in the groove  35  at the end portions  24  of the conductors  14 ,  15 . This mating arrangement between contact surface portions  34  and groove  35  also prevents any rotation of finger  26  about the axis of bolt  28  due to unbalanced loading of the first contact surface portion  32  during the closing operation of the switch  10 . 
   Referring to  FIG. 6 , the groove  35  in the second outside wall portion  36  has a radius from the axis  21  shown to be R 3 . The second contact finger surface tip portion  40  is an arcuate line or a second concave arc shaped line of width W 2 . The concave arc shaped line associated with the width W 2  has a radius corresponding to R 4  shown in  FIG. 6 . Radius R 4  is chosen to be slightly greater than, the radius R 3 . Consequently, the second contact surface tip portion  40  extends into the groove  35  in electrical contact therewith along the arcuate width of tip portion  40 . It should be understood that the radius R 3  for the first and second fixed conductors  14  and  16  may be different resulting in the radius R 4  for the contact fingers  28  attached to the first fixed conductor  14  being different than the radius of the contact fingers  26  attached to the second fixed conductor  16 . 
   It should be understood that end portions of the telescopic conductor  22  may be raised as a raised flange as shown, or alternatively, as circumferentially spaced apart raised pads to facilitate or alter the radius of the contact surface wall portions  42  of the telescopic conductor  22 . Similarly, the adjacent end portions  24  of the first and second conductors  14  and  16  may comprise a raised collar as shown, or alternatively, circumferentially spaced apart raised collar pads to which the fingers  26  are attached by use of the bolts  28 . 
   The contact fingers  26  in the embodiment shown comprise copper with silver plating. The contact surface tip portions  38 ,  40  of the fingers  26  are machined and silver plated to respectively provide radii R 2  and R 4  prior to assembly to fixed conductors  14 ,  16 . It should be understood that the radius of each of the arcuate widths W 1  and W 2  respectively of the first and second contact surface tip portions  38 ,  40  are chosen respectively to be slightly greater than the width of the corresponding outside surface wall portions  42  of the telescopic conductor and the groove  35  of the fixed conductor because it has been found that during silver plating of the contact fingers  26 , the silver plating is not evenly distributed over the contact tip portions. The silver plating tends to be deposited thicker adjacent the side walls of the contact fingers  26  creating high spots. As a result, if the radius of the contact tip portions  38 ,  40  is chosen to match the radius of the telescopic conductor or the groove, then contact between parts is limited to the high spots. By choosing the radii of the contact surface tip portions  38 ,  40  to be slightly greater, contact is not limited to the high spots but to a greater surface portion of the arcuate line width of the finger contact surface tip portions  38 ,  40 . This line of contact increases with wear of the contact surface tip portion  38 ,  40 . 
   It should be further understood, that while the present invention provides for arcuate width contact surface tip portions  38 ,  40  in the form of an arcuate contact line, during opening and closing of the contacts some wear occurs thickening the line of contact of the tip portions  38 ,  40  and thereby improving contact surface engagement. 
   The use of contact surfaces  32 ,  34  having concave arc curving contact surface tip portions  38 ,  40  with radii R 2 , R 4  utilized in the present invention has been tested and compared to the use of flat surface contact surfaces. A first telescopic switch was built utilizing flat finger contact surface portions in accordance with prior art switches. This first switch built with the fingers having flat contact surface areas was rated for 12000 Amp service. A second switch built with fingers having curved contact surface tip portions  38 ,  40  in accordance with the present invention was also built for testing. Both switches used the same conductor and enclosure sizes. Both switches have undergone heat run and mechanical wear testing, with the following results shown in Table 1 below: 
   
     
       
             
             
             
           
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
               Switch 2 - Present 
             
             
                 
               Switch 1 - Straight 
               Invention - Curved 
             
             
                 
               Fingers 
               Fingers 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               Rating (Amps) 
               12,000 
               13,000 
             
             
               Conductor Temperature 
               49 
               43 
             
             
               Rise [° C.] 
             
             
               Mechanical wear 
               3,000 
               &gt;10,000 
             
             
               capacity - [Maximum 
             
             
               Number of stroke 
             
             
               (condition)] 
             
             
                 
             
           
        
       
     
   
   From the results of the testing, it can be seen that the telescopic switch made in accordance with the present invention has a higher amperage rating, smaller heat rise and improved mechanical wear over the use of flat contact surface fingers. 
   While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the invention disclosed.