Abstract:
A transformer is provided along with a combination of bridging tap-changers to provide a wide range of selectable output voltages in discrete, relatively small voltage steps where the highest voltage is more than double the lowest output voltage. Relatively inexpensive, off-the-shelf, bridging tap-changers are utilized in conjunction with transformer winding schemes to provide a low winding loss ratio.

Description:
The present invention claims the benefit of the filing date of provisional application, U.S. Serial No. 60/175,647, filed on Jan. 12, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrical power distribution equipment and more particularly to electrical transformers and switching means therefor. 
     2. Description of the Prior Art 
     In some electrical power distribution systems it is desirable to provide a plurality of selectable, often incrementally different, voltage outputs from system transformers. A range of such selectable voltage outputs from a single transformer may be achieved through the use of transformers having a number of isolated multiple tap primary and/or secondary windings interconnected to appropriate switching mechanisms. Commonly used in such applications are bridging tap changers and series-parallel-series (S-P-S) switches. 
     Bridging tap changers may take the form of several stationary electrical contacts arranged in an arcuate array with a movable contact mounted on an insulating rotor. Rotation of the rotor brings the movable contact into bridging contact with any selected pair of adjacent stationary contacts. Bridging tap changers may be connected to provide selectable voltage outputs by interconnecting the ends and/or taps of transformer windings so as to bypass any or all selected portions (turn groups) of windings. 
     Two-position series-parallel (S-P) switches and multiple position S-P-S switches are used to connect multiple transformer windings, some of which may be tapped, into various series-parallel-series configurations as well as full series or full parallel configurations. S-P and S-P-S switches characteristically are ganged switch pairs where each switch of each pair has a common terminal. 
     Bridging tap changers having six stationary contacts (five positions) or eight stationary contacts (seven positions) are the most commonly used in the industry. For this reason, five and seven-position bridging tap changers are easily obtainable as “off-the-shelf” items and are relatively inexpensive. Bridging tap changers with a greater number of positions are usually made to order and therefore are more expensive and have longer delivery times. 
     It is at times expedient to interconnect bridging tap changers in a manner to provide a common terminal. Such a configuration is achieved in the prior art by “jumper wiring” every other stationary contact of a bridging tap changer in common. Thus, the rotary contact becomes, in effect, the common terminal since in every position of the changer it is in contact with one of the “jumped” stationary contacts. When so wired, a five-position bridging tap changer becomes, in effect, a three-position device and a seven-position bridging tap changer becomes a four-position device, each with a common terminal. 
     In constructing tap changing selectable output transformers it is desirable to provide a wide range of output voltages available in discrete, relatively small voltage steps. Among the design factors to be considered are cost and ready availability of material or parts, such as switches. Also to be considered, are the winding losses and in particular the winding losses of the highest loss configuration relative to the losses of the lowest loss configuration. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the present invention to provide a method and apparatus that will provide output voltages in small increments between the highest and lowest output voltage in multiple-tap power transformers. 
     It is another object of the present invention to provide a method and apparatus that will provide a wide range of output voltages in multiple tap power transformers where the highest voltage output is greater than twice the lowest voltage output and the winding loss factor is low. 
     The foregoing objects are achieved as is now described. In the illustrated embodiments of the present invention, there are provided a series of switched, multiple tap power transformers offering a wide range of selectable output voltages in discrete, relatively small voltage steps wherein the highest voltage is more than double the lowest output voltage. Further, the highest to lowest loss ratio is less than most comparable prior art systems. The present invention uses “off-the-shelf,” relatively inexpensive, and readily available switches in conjunction with less elaborate and thus less expensive transformer winding schemes than comparable prior art systems. 
     All objects, features, and advantages of the present invention will become apparent in the following detailed written description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
         FIGS. 1 ,  2  and  3  depict schematic diagrams of prior art multi-tap transformer switching configurations; 
         FIG. 4  is a schematic diagram of a transformer switch in which a preferred embodiment of the present invention may be implemented; 
         FIG. 5  depicts a second variant of a transformer switch in accordance with a preferred embodiment of the present invention; 
         FIG. 6  illustrates a third variant of a transformer switch in accordance with a preferred embodiment of the present invention; 
         FIG. 7  depicts a fourth variant of a transformer switch in accordance with a preferred embodiment of the present invention; 
         FIG. 8  illustrates a fifth variant of a transformer switch in accordance with a preferred embodiment of the present invention; 
         FIG. 9  depicts a sixth variant of a transformer switch in accordance with a preferred embodiment of the present invention; and 
         FIG. 10  illustrates a high-level flow diagram of a method for providing output voltages in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , there is shown in schematic diagram form a single phase power transformer having a single untapped primary winding P (primary), a pair of isolated individual untapped secondary windings (secondary), S 1  and S 2 , and a pair of isolated individual tapped secondary windings, S 3  and S 4 . Windings S 1  and S 2  are interconnected by a two-position series parallel switch (non-bridging type) SW 1 , and windings S 3  and S 4  are interconnected by a seven-position bridging tap changer switch SW 2 . The S 1 , S 2  and SW 1  system is serially connected to the S 3 , S 4  and SW 2  system as shown. 
     In the prior art arrangement shown in  FIG. 1 , the secondary winding turns ratio between the taps of S 3  and S 4  are equal to one unit each, the turns ratio between the winding ends of S 3  and S 4  are equal to six units each, and the turns ratio between the winding ends of S 1  and S 2  are equal to seven units. The system of  FIG. 1  can provide fourteen different voltage outputs from thirteen units to twenty-six units of voltage in steps of one voltage unit each. 
     A design such as that of  FIG. 1  has the advantage of using lower cost, off-the-shelf switches but this design exhibits relatively high losses at the lower voltage outputs and relatively large voltage increments. 
     The secondary winding losses asserted herein are calculated using the following assumptions: constant power frequency, constant applied sinusoidal voltage, constant kVA load, equal resistance in each turn and equal impedance in each turn. Core loss, primary winding loss, lead loss, stray loss and eddy current loss are ignored. The relative value of the various tapping schemes are thus compared on a consistent basis. 
     The prior art shown schematically in  FIG. 2  is similar to that of FIG.  1 . The system of  FIG. 2  can provide eighteen different voltage outputs from seventeen to thirty-four voltage units in steps of one voltage unit each. The system of  FIG. 2  requires turns ratios as follows: one unit between adjacent taps T 1 ′ through T 4 ′ and between T 4 ′ and winding end E 2 ′; one unit between adjacent taps T 5 ′ through T 8 ′ and between tap T 5 ′ and winding end E 3 ′; eight units between winding ends E 1 ′ and E 2 ′ and between E 3 ′ and E 4 ′; and nine units between E 5 ′ and E 6 ′ and nine units between E 7 ′ and E 8 ′. The  FIG. 2  system uses a more expensive switch SW 2 ′ as well as other materials of comparable cost to those in the system of FIG.  1 . 
     Prior art systems similar to that shown in  FIG. 3  have allowed a relatively large number of voltage steps, twenty-five for the system shown. But such a system requires special (more expensive) switches, such as the five-position series-parallel-series switch shown, and are limited to a reduced voltage range with the highest voltage available being no more than twice the lowest voltage. Alternatively, special ganged, nine position, bridging tap changers “jumper wired” for five positions may be substituted. 
     The present invention combines three, five, and/or seven-position tap changers, which are off the shelf, relatively inexpensive switches and are readily available with unique but inexpensive tapped winding transformers. 
     In the first embodiment of the present invention, shown in  FIG. 4  of the drawings, the schematically illustrated transformer  10  has a primary winding  11 , two isolated center tapped secondary windings  12  and  13  and two additional isolated secondary windings  14  and  15 . Electrical access leads  16  through  24  provide electrical contact to the winding ends and taps as shown. The turns ratios of the secondary windings are 6:6:1:1 for windings  12 ,  13 ,  14 , and  15 , respectively. 
     Leads  16  through  21  are interconnected through a pair of ganged five-position bridging tap changers  25  and  26  and wired as a three-position series-parallel-series switch. Leads  21  through  24  are interconnected through a three-position bridging tap changer  27 . Both three-position and five-position bridging tap changers are relatively inexpensive, off-the-shelf switches. 
     According to the invention, the system of  FIG. 4  provides a selection of any of nine distinct voltage outputs at transformer secondary output terminals  28  and  29  in one voltage unit steps from six voltage units to fourteen voltage units. The ratio of maximum to minimum secondary winding losses in the system of  FIG. 4  is 1.296. The prior art systems of FIG.  1  and  FIG. 2  can be adapted to achieve similar voltage range but the loss ratios are significantly higher, 1.496 and 1.510 respectively. The prior art of  FIG. 3  cannot be adapted to this voltage range. 
       FIG. 5  illustrates, schematically a system similar to that of  FIG. 4  in that transformer  10 ′ has a pair of isolated center tapped windings  12 ′ and  13 ′ interconnected by a pair of ganged five-position bridging tap changers  25 ′ and  26 ′. Transformer  10 ′ differs from transformer  10  in having a pair of isolated secondary windings  14 ′ and  15 ′ that are each center tapped and a turns ratio of 10:10:2:2 in the windings  12 ′,  13 ′,  14 ′ and  15 ′. The interconnection of windings  14 ′ and  15 ′ is through a five-position bridging tap changer  27 ′, as shown. The system of  FIG. 5  provides fifteen different voltages available at one unit increments from ten to twenty-four voltage units. The secondary winding ratio, highest to lowest, is 1.333. The prior art systems of FIG.  1  and  FIG. 2  can be adapted to achieve similar voltage range but the loss ratio is significantly higher, 1.495 and 1.511 respectively. The prior art of  FIG. 3  cannot be adapted to this voltage range. 
     In the transformer system schematically depicted in  FIG. 6 , twenty-one different voltage outputs are provided in one unit steps from fourteen to thirty-four voltage units. The ratio of highest to lowest winding loss is 1.349. The loss ratios of similar voltage range using the systems of FIG.  1  and  FIG. 2  are 1.491 and 1.511 respectively. The prior art of  FIG. 3  cannot be adapted to this voltage range. The system of  FIG. 6  comprises a transformer  10 ″ with an isolated pair of center tap secondary windings  12 ″ and  13 ″ interconnected by switches  25 ″ and  26 ″ wired as a three-position series-parallel-series switch and a pair of secondary windings  14 ″ and  15 ″ each having winding taps  30 ″,  31 ″,  32 ″ and  33 ″, respectively, dividing the windings into thirds. Secondary windings  14 ″ and  15 ″ are interconnected through a seven-position bridging tap changer  27 ″, as shown. 
     Referring to  FIG. 7  the transformer  50  comprises a primary winding  51  and four isolated secondary windings, two of which,  52  and  53 , are tapped at thirds and two of which,  54  and  55 , have no taps. Windings  52  and  53  have their end leads  56  and  59  and  60  and  63  and tap leads  57  and  58  and  61  and  62  interconnected, as shown by a pair of ganged seven-position bridging tap changers  68  and  69  connected as four-position series-parallel-series switches. End leads  64 ,  65 ,  66  and  67  of secondary windings  54  and  55 , respectively, are interconnected, as shown, by a three-position bridging tap changer  70 . 
     In the system of  FIG. 7 , windings  54  and  55  each comprise a one unit turns group and windings  52  and  53  each comprise a nine unit turns group (three units per tapped section). The transformer system of  FIG. 7  then provides twelve unique voltage outputs in one unit steps from nine voltage units to twenty voltage units having a highest to lowest ratio of winding losses of 1.250. The winding losses of FIG.  1  and  FIG. 2  adapted for comparable voltage range are 1.491 and 1.503 respectively. The prior art of  FIG. 3  cannot be adapted to this voltage range. 
     In the embodiment schematically illustrated in  FIG. 8 , the secondary windings  54 ′ and  55 ′ are center tapped with their leads  64 ′,  65 ′,  66 ′ and  67 ′ and their tap leads  71 ′ and  72 ′ interconnected by a five-position tap changer  70 ′. Secondary windings  52 ′ and  53 ′ are each tapped at thirds similar to windings  52  and  53  of  FIG. 7  but are differently related to the secondary windings  54 ′ and  55 ′. Specifically, windings  54 ′ and  55 ′ each comprise a two unit turns group and windings  52 ′ and  53 ′ each comprise a fifteen unit turns group (i.e., five units per tapped section). 
     The system of  FIG. 8  thus provides twenty unique voltage outputs in one unit steps from fifteen voltage units to thirty-four voltage units with the highest to lowest winding loss ratio of 1.275. The winding losses of FIG.  1  and  FIG. 2  adapted for a comparable voltage range are 1.494 and 1.507 respectively. The prior art of  FIG. 3  cannot be adapted to this voltage range. 
     In the sixth embodiment of the present invention shown in  FIG. 9 , the secondary windings  52 ″ and  53 ″ are, as in the previous two embodiments, tapped at thirds as are secondary windings  54 ″ and  55 ″. The turns ratio relationship between the four secondary windings of the system of  FIG. 8  is such that windings  54 ″ and  55 ″ each comprise a three unit turns group and windings  52 ″ and  53 ″ each comprise a twenty-one unit turns group. 
     With the leads of windings  52 ″ and  53 ″ connected by series-parallel-series switch, ganged switches  68 ″ and  69 ″, as in the previous embodiments, and the leads of windings  54 ″ and  55 ″ are interconnected through a seven-position bridging switch  70 ″, as shown, the system of  FIG. 9  provides twenty-eight different voltage outputs available in one voltage unit steps from twenty-one to forty-eight voltage units. The highest to lowest winding loss ratio of the system of  FIG. 9  is 1.286. The highest to lowest loss ratios of FIG.  1  and  FIG. 2  adapted to this voltage range are 1.495 and 1.508 respectively. The prior art of  FIG. 3  cannot be adapted to this voltage range. 
     It will be apparent to those familiar with the art that the fixed connection between terminal  63 ″ of winding  53 ″ and terminal  70 ″ of winding  54 ″ can be a internal coil connection as well as an external connection. An internal connection essentially makes winding  53 ″ and  54 ″ one continuous winding. This alternative construction may be adapted to any of the embodiments depicted in  FIGS. 4 ,  5 ,  6 ,  7 ,  8  and  9 . 
     Referring now to  FIG. 10 , a flow diagram is depicted of a method for providing selectable voltage outputs in accordance with the present invention. The process begins with step  1002 , which depicts providing a transformer (single phase, two phase, or three phase) with multiple taps on a secondary winding (secondary). The process continues with step  1004 , which illustrates providing interconnecting bridging tap-changers: a two stage, ganged bridging tap-changer in a series-parallel-series configuration and a single stage bridging tap-changer to the secondary of the transformer for providing incremental output voltages. 
     The process then proceeds to step  1006 , which depicts connecting selected winding points on the secondary winding with selected contacts on the two stage, ganged bridging tap-changers. The process next passes to step  1008 , which depicts connecting windings via various switch positions of the two stage bridging tap-changer and single stage bridging tap-changer combination. If the switch is moved to a first position, the process proceeds to step  1010 , which illustrates corresponding windings being connected in parallel. The process continues to step  1016 . If the switch is moved to a second switch position, the process passes to step  1012 , which depicts connecting corresponding windings being connected in series. The process then continues to step  1016 . If the switch is moved to any other position, the process instead passes to step  1014 , which illustrates connecting a portion of the corresponding windings in parallel and in addition, a portion of the windings in series. 
     Corresponding windings include those winding turns from one selected winding tap to another selected winding tap, and all the windings in between, physically connected to a particular switch, including a ganged switch. With reference to the figures, a corresponding winding includes a pair of taps from a winding with one end having a polarity opposite that of the other end and including all the taps between the pair. 
     The process continues from step  1014 , step  1012  or step  1010  to step  1016 , which depicts a single stage bridging tap-changer interconnected to the first switch. If the second switch is moved into a first switch position, the process passes to step  1018 , which illustrates corresponding windings being fully bypassed. If the tap-changer is in a second switch position, the process moves to step  1020 , which depicts the corresponding windings being connected in series. If the tap changer is in any other position, the process passes to step  1022 , which illustrates a portion of the corresponding windings being series connected. 
     The present invention achieves voltage steps of 1/48th of a fully series connected winding utilizing off the shelf seven position bridging tap-changers. The prior art of  FIG. 3  can achieve voltage steps of 1/48th of a fully series connected winding, but only with the use of a specially designed switches, but cannot achieve the voltage range of the present invention. The present invention achieves a voltage range from a lowest voltage up to 2.286 times the lowest voltage. Though the prior art  FIGS. 1 and 2  can achieve the wider voltage range of the present invention a significantly higher loss factor is incurred. The present invention has a winding loss ratio, highest to lowest, of 1.286. 
     While the designations “primary” or “input” and “secondary” or “output” windings have been used herein, arbitrarily, to designate various windings of the embodiments disclosed, it is well recognized by those skilled in the art that the switches and tapped winding arrangements may be used in the primary or input windings and the other windings used as secondary or output windings. It will be recognized as well that any of the systems of the present invention may be used in multiples on polyphase electrical systems. 
     Thus, there has been disclosed a new transformer topology that may inspire others to make changes and modifications still within the spirit and scope of the invention. The above detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.