Abstract:
The invention relates to an on-load tap changer having an energy storage mechanism for changing over, without any interruptions, between different winding taps of a tap-changing transformer on load. According to the invention, the energy storage mechanism has one or more compression springs, which are tensioned at the beginning of a changeover operation. In comparison with the tensile springs used to date with such energy storage mechanisms, the compression springs have markedly greater long-term strength.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US national stage of PCT application PCT/EP2006/010580, filed 4 Nov. 2006, published 30 Aug. 2007 as WO 2007/095978, and claiming the priority of German patent application 102006008338.5 itself filed 23 Feb. 2006, whose entire disclosures are herewith incorporated by reference. 
     FIELD OF THE INVENTION 
     The invention relates to an on-load tap changer having a force-storing unit for the uninterrupted switching between different winding taps of a tapped transformer under load. 
     BACKGROUND OF THE INVENTION 
     An on-load tap changer of this type along with an associated force-storing unit is known from publication of the applicant “On-load tap Changer VACUTAP® VV Operating Instructions,” imprint BA164/03de. This on-load tap changer combines the function of a selector for the no-load preselection of the tap to which the changeover is to be made, as well as a load transfer switch for the actual switching. 
     The force-storing unit of this known on-load tap changer has a flexible sheet-metal plate on which several tension springs are attached at one end, the springs being pivoted at their other ends on bearings. The free end of the flexible sheet-metal plate in turn is rotatably mounted on a drive crank. The drive crank that is mounted on an operating shaft is actuated through a one-way clutch means by a drive element that is coaxial with and rotatable on and independently of this shaft. At its bottom end the operating shaft supports a Geneva crank of a Geneva drive. Finally, the Geneva wheel of this Geneva drive is connected to a rotatable switching column supported coaxially inside an insulating tube, this column supporting the switching elements on a radially projecting arm. 
     Fixed tap contacts are provided in the wall of the insulating tube at vertically spaced horizontal planes, these contacts being selectably connectable in each plane to one of the switching elements on the switching column. 
     The functional principle of this known force-storing unit is as follows: 
     At the start of a load-switching operation, i.e. switching from one winding tap to an adjacent different winding tap, the drive element, actuated generally by a drive motor, begins to turn slowly and continuously. The drive element entrains the drive crank via an abutment, and with it also the end of the flexible sheet-metal plate attached thereto, thereby moving and tensioning the tension springs. After an initial rotation angle of 180° when the tension springs have reached their maximum displacement, the drive element disengages from the drive crank due to the one-way clutch. After passing dead center, the next rotation of the drive crank occurs by snap action since the tension springs are still tensioned at this point in time. 
     This rapid rotational motion of the drive crank is transferred to the Geneva crank of the Geneva drive, and thus to the switching column; the switching elements attached thereto switch by snap action to each of the adjacent fixed tapping contacts. The tap changing operation has concluded. 
     This force-storing unit has proven successful over many years for the widest variety of load tap changers. Nevertheless, the tension springs used do not have unlimited mechanical fatigue strength but must be replaced after a predetermined operating life or permissible number of operations for reason of safety. This has not been a problem up to now since generally inspections are performed on the on-load tap changer in any case after this permissible number of operations. Due to the high reliability of the vacuum-type switching cells now available and used, and the overall advantages of vacuum switching technology, it is now possible, however, to increase the maintenance intervals for on-load tap changers such that the limited mechanical fatigue strength of the employed tension springs has increasingly proven to be a disadvantage for the known force-storing units. 
     OBJECT OF THE INVENTION 
     The object of the invention is therefore to provide an on-load tap changer of the generic kind with a force-storing unit that has energy storage springs as the force-storing unit, this mechanism having a higher mechanical fatigue strength. 
     SUMMARY OF THE INVENTION 
     The problem is solved by an on-load tap changer with a force-storing unit in which the force-storing unit is a compression spring supported on one end against a fixed spring abutment, a pull rod ( 10 ) is mounted on the drive crank, and at least one movable spring abutment is provided on the pull rod, another end of the compression spring being supported on the fixed spring abutment. 
     In addition to the intended almost unlimited mechanical fatigue strength of the compression springs employed, the breakage of which is practically impossible, the advantage of the invention lies in the fact that the amount of force of the force-storing unit can be continuously adjusted due to the use of a threaded adjustable pull rod. As a result, it easily becomes possible to adjust the force-storing unit in advance to a wide variety of types of load tap changers and to adjust the force-storing unit in terms of the force released after triggering. The force-storing unit according to the invention is usable with a wide variety of on-load tap changers; it can be used both with equipment switching under oil using mechanical tap contacts and also with those having vacuum switching cells. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The following discussion explains the invention in more detail based on the drawings. In the drawings: 
         FIG. 1  is a perspective view illustrating the top portion of an on-load tap changer according to the invention with a force-storing unit; and 
         FIG. 2  is a schematic sectional side view of such an on-load tap changer with a force-storing unit. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  will be described in detail first. Shown is a base plate  1  on which the entire force-storing unit and drive arrangement for actuating the on-load tap changer is mounted and which in turn is mounted normally at an upper end of a dielectric cylinder  29  ( FIG. 2  only) carrying a multiplicity of contacts  30  of which only one is shown. The base plate  1  has support brackets  2 . A gear wheel  3  is shown that is connected to an unillustrated drive-motor shaft. The gear wheel  3  drives a drive element  4  by means of its gear teeth  5 . The drive element  4  in turn has two symmetrical catches  6  and  7  that coact with a drive crank  8 . This will be explained in more detail later. A head  9  of a pull rod  10  is pivoted on the drive crank  8 . The pull rod  10  is connected to additional parts of the force-storing unit according to the invention that will be explained in detail further below. The drive crank  8  is attached to an operating shaft  11  that passes vertically downward completely through the base plate, the lower end of the shaft having a Geneva crank shown in  FIG. 2  at reference  12 . This Geneva crank  12  extends into a Geneva wheel  13  that in turn is connected to a schematically illustrated switching column  31  carrying an arm  32  having an outer end forming a contact  33  engageable with one the contacts  30 . 
     A spring tube  14  is provided according to the invention surrounding the described pull rod  10 . One end of the spring tube  14  is linked to a bearing block  15 ; it is horizontally pivotal by means of a vertical pivot pin  16 . An inner compression spring  17  is provided between the pull rod  10  and the spring tube  14 ; an outer compression spring  18  is mounted concentrically outside the spring tube  14 . In the embodiment illustrated here, an arrangement comprising the inner compression spring  17  and the outer compression spring  18  was selected to generate strong forces. Within the scope of the invention, however, it is also possible to provide only a single compression spring, optionally either inside or outside the spring tube  14 . It is also possible to provide more than two compression springs. In the embodiment of the invention illustrated in  FIG. 1 , the inner compression spring  17  and the outer compression spring  18  are each braced at one end against a fixed spring abutment  19  that is connected to the spring tube  14 . At the other end, the inner compression spring  17  is braced against a movable inner spring abutment  20 , shown only in  FIG. 2 , while the outer compression spring  18  is braced against a movable outer spring abutment  21  forming with the inner abutment  20  a common spring abutment. The inner and outer spring abutments  20  and  21  are connected to the free end of the pull rod  10 . The inner spring abutment  20  here is attached directly to the pull rod  10 . To attach the outer spring abutment  21  surrounding the spring tube  14  to the pull rod  10 , longitudinal slots  22  are provided on both sides in the spring tube  14 . A support pin or mounting bolt  23  projects outward through these longitudinal slots  22  and attaches the pull rod  10  and the outer spring abutment  21  to each other. Also illustrated in  FIG. 1  is a roller  24  that cooperates with another Geneva wheel  25  that in the known manner actuates a position indicator  26 , and also in the known manner actuates a preselector. 
     The arrangement according to the invention as illustrated in  FIG. 1  functions as follows: 
     In order to initiate a switchover operation, i.e., each actuation of the on-load tap changer, a drive motor rotates the gear wheel  3  by the unillustrated drive-motor shaft. This rotation is transmitted through the gear teeth  5  to the drive element  4 . Depending on the direction of rotation, which depends on whether the next load transfer is to occur in the direction “higher” or “lower,” one of the two catches  6  or  7  of the drive element  4  positively engages the drive crank  8  and simultaneously rotates this crank  8 . At the same time, the pull rod  9  attached to the drive crank  8  is moved simultaneously; the pull rod  10  is displaced and the compression springs  17  and  18  are compressed. After the drive crank  8  rotates 180°, the pull rod  10  has reached its new end position; the compression springs  17  and  18  are under maximum compression. After passing through dead center, the previously positively engaged catch  6  or  7  is disengaged and rotation of drive crank  8 , and thus of operating shaft  11 , is quickly terminated since the compression springs  17  and  18  are released by snap action. This rapid rotational motion is transmitted through the Geneva crank  12  and the Geneva wheel  13  to the switching column, not shown, of the on-load tap changer. Fast switching is effected between adjacent fixed tapping contacts within the dielectric cylinder. 
       FIG. 2  illustrates a sectional side view of the described arrangement according to the invention. For the sake of completeness, those other components known from the art are not described in detail here. 
     What is evident here is that, in an especially advantageous development of the invention, one end of the pull rod  10  is secured by a screwthread  27  into the inner abutment  20 . Due to this thread  27 , especially advantageously in the form of a fine-pitch thread, what results is an adjustable pull rod  10  in which the effective length between fixed spring abutment  19  and movable spring abutments  20  and  21  can be easily modified. As a result, the spring lengths of the compression springs  17  and  18 , and thus the effective spring loads, can be continuously and easily adjusted. In this embodiment, the force-storing unit can be readily adapted to the specific requirements in terms of the energy of the released energy storage springs, which energy can vary as a function of the switch type and switching sequence. 
     It would also be possible within the scope of the invention to implement the adjustment capabilities of pull rod  10  by means of an additional thread  28  on the pull rod head  9 .