Abstract:
A method. The method includes determining that a failure has occurred in a power cell of a multi-cell power supply. The method also includes moving a part of a first contact which is connected to first and second output terminals of the power cell from a first position to a second position, moving a part of a second contact which is connected to a first input terminal of the power cell from a third position to a fourth position, and moving a part of a third contact which is connected to a second input terminal of the power cell from a fifth position to a sixth position.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of U.S. Provisional Patent Application No. 60/848,153, filed on Sep. 28, 2006. 
    
    
     BACKGROUND 
     This application discloses an invention that is related, generally and in various embodiments, to a method for bypassing a power cell in a multi-cell power supply. 
     In certain applications, multi-cell power supplies utilize modular power cells to process power between a source and a load. Such modular power cells can be applied to a given power supply with various degrees of redundancy to improve the availability of the power supply. For example,  FIG. 1  illustrates various embodiments of a power supply (e.g., an AC motor drive) having nine such power cells. The power cells in  FIG. 1  are represented by a block having input terminals A, B, and C; and output terminals T 1  and T 2 . In  FIG. 1 , a transformer or other multi-winding device  110  receives three-phase, medium-voltage power at its primary winding  112 , and delivers power to a  load  130  such as a three-phase AC motor via an array of single-phase inverters (also referred to as power cells). Each phase of the power supply output is fed by a group of series-connected power cells, called herein a “phase-group”. 
     The transformer  110  includes primary windings  112  that excite a number of secondary windings  114 - 122 . Although primary winding  112  is illustrated as having a star configuration, a mesh configuration is also possible. Further, although secondary windings  114 - 122  are illustrated as having a delta or an extended-delta configuration, other configurations of windings may be used as described in U.S. Pat. No. 5,625,545 to Hammond, the disclosure of which is incorporated herein by reference in its entirety. In the example of  FIG. 1  there is a separate secondary winding for each power cell. However, the number of power cells and/or secondary windings illustrated in  FIG. 1  is merely exemplary, and other numbers are possible. Additional details about such a power supply are disclosed in U.S. Pat. No. 5,625,545. 
     Any number of ranks of power cells are connected between the transformer  110  and the load  130 . A “rank” in the context of  FIG. 1  is considered to be a three-phase set, or a group of three power cells established across each of the three phases of the power delivery system. Referring to  FIG. 1 , rank  150  includes power cells  151 - 153 . rank  160  includes power cells  161 - 163 , and rank  170  includes power cells  171 - 173 . A master control system  195  sends command signals to local controls in each cell over fiber optics or another wired or wireless communications medium  190 . It should be noted that the number of cells per phase depicted in  FIG. 1  is exemplary, and more than or less than three ranks may be possible in various embodiments.  
       FIG. 2  illustrates various embodiments of a power cell  210  which is representative of various embodiments of the power cells of  FIG. 1 . The power cell  210  includes a three-phase diode-bridge rectifier  212 , one or more direct current (DC) capacitors  214 , and an H-bridge inverter  216 . The rectifier  212  converts the alternating current (AC) voltage received at cell input  218  (i.e., at input terminals A, B and C) to a substantially constant DC voltage that is supported by each capacitor  214  that is connected across the output of the rectifier  212 . The output stage of the power cell  210  includes an H-bridge inverter  216  which includes two poles, a left pole and a right pole, each with two switching devices. The inverter  216  transforms the DC voltage across the DC capacitors  214  to an AC output at the cell output  220  (i.e., across output terminals T 1  and T 2 ) using pulse-width modulation (PWM) of the semiconductor devices in the H-bridge inverter  16 . 
     As shown in  FIG. 2 , the power cell  210  may also include fuses  222  connected between the cell input  218  and the rectifier  212 . The fuses  222  may operate to help protect the power cell  210  in the event of a short-circuit failure. According to other embodiments, the power cell  210  is identical to or similar to those described in U.S. Pat. No. 5,986,909 and its derivative U.S. Pat. No. 6,222,284 to Hammond and Aiello, the disclosures of which are incorporated herein by reference in their entirety. 
       FIG. 3  illustrates various embodiments of a bypass device  230  connected to output terminals T 1  and T 2  of the power cell  210  of  FIG. 2 . In general, when a given power cell of a multi-cell power supply fails in an open-circuit mode, the current through all the power cells in that phase-group will go to zero, and further operation is not possible. A power cell failure may be detected by comparing a cell output voltage to the  commanded output, by checking or verifying cell components, through the use of diagnostics routines, etc. In the event that a given power cell should fail, it is possible to bypass the failed power cell and continue to operate the multi-cell power supply at reduced capacity. 
     The bypass device  230  is a single pole single throw (SPST) contactor, and includes a contact  232  and a coil  234 . As used herein, the term “contact” generally refers to a set of contacts having stationary portions and a movable portion. Accordingly, the contact  232  includes stationary portions and a movable portion which is controlled by the coil  234 . The bypass device  230  may be installed as an integral part of a converter subassembly in a drive unit. In other applications the bypass device  230  may be separately mounted. When the movable portion of the contact  232  is in a bypass position, a shunt path is created between the respective output lines connected to output terminals T 1  and T 2  of the power cell  210 . Stated differently, when the movable portion of the contact  232  is in a bypass position, the output of the failed power cell is shorted. Thus when power cell  210  experiences a failure, current from other power cells in the phase group can be carried through the bypass device  230  connected to the failed power cell  210  instead of through the failed power cell  210  itself. 
       FIG. 4  illustrates various embodiments of a different bypass device  240  connected output terminals T 1  and T 2  of the power cell  210 . The bypass device  240  is a single pole double throw (SPDT) contactor, and includes a contact  242  and a coil  244 . The contact  242  includes stationary portions and a movable portion which is controlled by the coil  244 . When the movable portion of the contact  242  is in a bypass position, one of the output lines of the power cell  210  is disconnected (e.g., the output line connected  to output terminal T 2  in  FIG. 4 ) and a shunt path is created between the output line connected to output terminal T 1  of the power cell  210  and a downstream portion of the output line connected to output terminal  172  of the power cell  210 . The shunt path carries current from other power cells in the phase group which would otherwise pass through the power cell  210 . Thus, when power cell  210  experiences a failure, the output of the failed power cell is not shorted as is the case with the bypass configuration of  FIG. 3 . 
     The bypass devices shown in  FIGS. 3 and 4  do not operate to disconnect power to any of the input terminals A, B or C in the event of a power cell failure. Thus, in certain situations, if the failure of a given power cell is not severe enough to cause the fuses  222  (see  FIG. 2 ) to disconnect power to any two of input terminals A, B or C, the failure can continue to cause damage to the given power cell. 
     SUMMARY 
     In one general respect, this application discloses a method for bypassing a power cell in a multi-cell power supply. According to various embodiments, the method includes determining that a failure has occurred in a power cell of a multi-cell power supply. The method also includes moving a part of a first contact which is connected to first and second output terminals of the power cell from a first position to a second position moving a part of a second contact which is connected to a first input terminal of the power cell from a third position to a fourth position, and moving a part of a third contact which is connected to a second input terminal of the power cell from a fifth position to a sixth position.  
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the invention are described herein by way of example in conjunction with the following figures. 
         FIG. 1  illustrates various embodiments of a power supply, 
         FIG. 2  illustrates various embodiments of a power cell of the power supply of  FIG. 1 ; 
         FIG. 3  illustrates various embodiments of a bypass device connected to an output of the power cell of  FIG. 2 ; 
         FIG. 4  illustrates various embodiments of a bypass device connected to an output of the power cell of  FIG. 2 ; 
         FIG. 5  illustrates various embodiments of a system for bypassing a power cell of a power supply; 
         FIG. 6  illustrates various embodiments of a system for bypassing a power cell of a power supply; 
         FIGS. 7-9  illustrate various embodiments of a bypass device; 
         FIG. 10  illustrates various embodiments of a system for bypassing a power cell of a power supply; 
         FIG. 11  illustrates various embodiments of a system for bypassing a power cell of a power supply; 
         FIG. 12  illustrates various embodiments of a system for bypassing a power cell of a power supply;  
         FIG. 13  illustrates various embodiments of a circuit for controlling a bypass device; 
         FIGS. 14 and 15  show the per-unit DC current and power available for various per-unit DC voltages at V 1  of the circuit of  FIG. 13 ; 
         FIG. 16  illustrates various embodiments of a circuit for controlling a bypass device; 
         FIG. 17  illustrates various embodiments of a circuit for controlling a bypass device; 
         FIG. 18  illustrates various embodiments of a circuit for controlling a bypass device; and 
         FIG. 19  illustrates various embodiments of a circuit for controlling a plurality of bypass devices. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that at least some of the figures and descriptions of the invention have been simplified to focus on elements that are relevant for a clear understanding of the invention, while eliminating, for purposes of clarity, other elements that those of ordinary skill in the art will appreciate may also comprise a portion of the invention. However, because such elements are well known in the art, and because they do not necessarily facilitate a better understanding of the invention, a description of such elements is not provided herein. 
       FIG. 5  illustrates various embodiments of a system  250  for bypassing a power cell (e.g., power cell  210 ) of a power supply. As shown in  FIG. 5 , the system  250   includes bypass device  252  connected to the output terminals T 1  and  12 , a bypass device  254  connected to input terminal A, and a bypass device  256  connected to input terminal C. Although the system  250  is shown in  FIG. 5  as having respective bypass devices connected to input terminals A and C, it will be appreciated that, according to other embodiments, the respective bypass devices may be connected to any two of the input terminals A, B and C. 
     The bypass devices  252 ,  254 ,  256  may be mechanically-driven, fluid-driven, electrically-driven, or solid state, as is described in the &#39;909 and &#39;284 patents. For purposes of simplicity, each bypass device will be described hereinafter in the context of a bypass device which includes one or more electrically-driven contactors which are connected to the output of a power cell. As described hereinafter, a given bypass device may be embodied as a single-pole single-throw (SPST) contactor, a single-pole double-throw (SPDT) contactor, or a multi-pole contactor. 
     Bypass device  252  is a single pole double throw (SPDT) contactor, and includes a contact  258  and a coil  260 , The contact  258  includes stationary portions and a movable portion which is controlled by the coil  260 . The bypass device  252  operates in a manner similar to that described hereinabove with respect to bypass device  240  of  FIG. 4 . The bypass device  254  is a single pole single throw (SPST) contactor, and includes a contact  262  and a coil  264 . The contact  262  includes stationary portions and a movable portion which is controlled by the coil  264 . Else bypass device  256  is a single pole single throw (SPST) contactor, and includes a contact  266  and a coil  268 . The contact  266  includes stationary portions and a movable portion which is controlled by the coil  268 . In general, in the event of a failure, bypass devices  254 ,  256  disconnect the  cell input power at substantially the same time that bypass device  252  creates a shunt path for the current that formerly passed through the failed power cell. 
     The condition associated with the creation of the described shunt path and the disconnection of cell input power from at least two of the cell input terminals may be referred to as “fill-bypass”. When the full bypass condition is present, no further power can flow into the failed cell. As described with respect to  FIG. 2 , the fuses  222  of the power cell may operate to help protect the power cell in the event of a short-circuit failure. However, in certain situations (e.g., when fault current is low), the fuses  222  may not clear quickly enough to prevent further damage to the failed power cell. According to various embodiments, the bypass devices  254 ,  256  are configured to act quicker than the fuses  222 , and the quicker action generally results in less damage to the failed power cell. 
       FIG. 6  illustrates various embodiments of a system  270  for bypassing a power cell (e.g., power cell  210 ) of a power supply. The system  270  includes a single bypass device  272  which achieves the combined functionality of the bypass devices  252 ,  254 ,  256  of  FIG. 5 . The bypass device  272  is a multi-pole contactor which includes a first contact  274  connected to the output terminals T 1  and T 2  of the power cell, a second contact  276  connected to the input terminal A, and a third contact  278  connected to the input terminal C. Each of the contacts  274 ,  276 ,  278  include stationary portions and a movable portion. Although the second and third contacts  276 ,  278  are shown in  FIG. 6  as being connected to input terminals A and C, it will be appreciated that, according to other embodiments, the second and third contacts  276 ,  278  may be connected to any two  of the input terminals A, B and C. The bypass device  272  also includes a single coil  280  which controls the movable portions of the contacts  274 ,  276 ,  278 . 
       FIGS. 7-9  illustrate various embodiments of a bypass device  300 . The bypass device is a multi-pole contactor, and may be identical to or similar to the bypass device  272  of  FIG. 6 . The bypass device  300  includes a first contact which includes stationary portions  302 ,  304  and movable portion  306 , a second contact which includes stationary portions  308 ,  310  and a movable portion  312 , and a third contact which includes stationary portions  314 ,  316 ,  318 ,  320  and a movable portion  322 . The bypass device  300  also includes a coil  324  which controls the movable portions  306 ,  312 ,  322  of the first, second and third contacts. The stationary portions  304 ,  310  of the first and second contacts may be connected to any two of the input terminals A, B and C of a power cell. The stationary portions  314 ,  318  of the third contact may be respectively connected to the output terminals T 1  and T 2  of a power cell. The movable portions  306 ,  312 ,  322  of the first, second and third contacts are shown in the normal or non-bypass position in  FIGS. 7 and 8 , and are shown in the bypass position in  FIG. 9 . 
     As shown in  FIG. 7 , the bypass device  300  also includes electrical terminals  326  connected to the coil  324 , a steel frame  328  which surrounds the coil  324 , a first insulating plate  330  between the steel frame  328  and the stationary portions  304 ,  308 ,  310 ,  312  of the first and second contacts a second insulating plate  332  between the steel frame  328  and the stationary portions  314 ,  316  of the third contact, and first and second support brackets  334 ,  336 . The bypass device  300  further includes a non-magnetic shaft  338  which passes through the coil  324 , through openings in the steel  frame  328 , through respective openings in first and second insulating plates  330 ,  332 , and through respective openings of the first and second support brackets  334 ,  336 . 
     Additionally, the bypass device  300  also includes a first biasing member  340  between the first support bracket  334  and a first end of the non-magnetic shaft  338 , a second biasing member  342  between the second support bracket  336  and a second end of the non-magnetic shaft, and a position sensing device  344  which is configured to provide an indication of the position (bypass or non-bypass) of the movable portions  306 ,  312 ,  322  of the first, second and third contacts. 
     Although not shown for purposes of simplicity in  FIGS. 7-9 , one skilled in the art will appreciate that the bypass device  300  may further include a plunger (e.g., a cylindrical steel plunger) which can travel axially through an opening which extends from the first end of the coil  324  to the second end of the coil  324 , permanent magnets capable of holding the movable portions of the contacts in either the bypass or the non-bypass position without current being applied to the coil  324 , a first insulating bracket which carries the moving portions  306 ,  312  of the first and second contacts, a second insulating bracket which carries the moving portion  322  of the third contact, etc. 
     In operation, permanent magnets (not shown) hold the plunger in either a first or a second position, which in turn holds the movable portions  306 ,  312 ,  322  of the contacts in either the non-bypass position or the bypass position. When the electrical terminals  326  receive pulses of current, the pulses of current are applied to the coil  324 , thereby generating a magnetic field. Depending on the polarity of the applied pulse and the position of the plunger, the applied pulse may or may not cause the plunger to change its position. For example, according to various embodiments, if the plunger is in  the first position and the movable portions  306 ,  312 ,  322  of the contacts are in the non-bypass position, a positive current pulse will change the plunger from the first position to the second position, which in turn changes the movable portions  306 ,  312 ,  322  of the contacts from the non-bypass position to the bypass position. In contrast, if a negative current pulse is applied, the plunger will stay in the first position and the movable portions  306 ,  312 ,  322  of the contacts will stay in the non-bypass position. 
     Similarly, according to various embodiments, if the plunger is in the second position and the movable portions  306 ,  312 ,  322  of the contacts are in the bypass position, a negative current pulse will change the plunger from the second position to the first position, which in turn changes the movable portions  306 ,  312 ,  322  of the contacts from the bypass position to the non-bypass position. In contrast, if a positive current pulse is applied, the plunger will stay in the second position and the movable portions  306 ,  312 ,  322  of the contacts will stay in the bypass position. 
       FIG. 10  illustrates various embodiments of a system  350  for bypassing a power cell (e.g., power cell  210 ) of a power supply. The system  350  is similar to the system  250  of  FIG. 5 . The system  350  includes a first contact  352  connected to the output terminals T 1  and T 2  of the power cell, a second contact  354  connected to the input terminal A of the power cell, and a third contact  356  connected to the input terminal C of the power supply. Each of the contacts  352 ,  354 ,  356  include stationary portions and a movable portion. Although the second and third contacts  354 ,  356  are shown in  FIG. 10  as being connected to input terminals A and C, it will be appreciated that, according to other embodiments, the second and third contacts  354 ,  356  may be connected to any two of the input terminals A, B and C.  
     The system  350  also includes a first coil  358  which controls the movable portions of the first contact  352 , a second coil  360  which controls the movable portion of the second contact  354  and a third coil  362  which controls the movable portion of the third contact  356 . According to various embodiments, the coils  358 ,  360 ,  362  are embodied as contactor coils. According to other embodiments, the coils  358 ,  360 ,  362  are embodied as magnetic latching coils which do not need to have continuous power applied to the coils in order to hold the plunger in its first or second position and/or to hold the moving portions of the contacts  352 ,  354 ,  356  in the non-bypass or bypass position. The first contact  352  and the first coil  358  may collectively comprise a first contactor, the second contact  354  and the second coil  360  may collectively comprise a second contactor, and the third contact  356  and the third coil  362  may collectively comprise a third contactor. 
     The system  350  further includes a first local printed circuit board  364  in communication with the first coil  358 , a second local printed circuit board  366  in communication with the second coil  360 , and a third local printed circuit board  368  in communication with the third coil  362 . Each of local printed circuit boards  364 ,  366 ,  368  are configured to control the respective movable portions of the contacts  352 ,  354 ,  356  via the respective coils  358 ,  360 ,  362 . In general, each of the local printed circuit boards  364 ,  366 ,  368  are configured to receive commands from, and report status to, a master control device (e.g., master control system  195  of  FIG. 1 ) that is held near ground potential. Each of the local printed circuit boards  364 ,  366 ,  368  are also configured to deliver pulses of energy to the respective coils  358 ,  360 ,  362  as needed to change the position of the movable portions of the respective contacts  352 ,  354 ,  356 , and to  recognize the position of the movable portions of the respective contacts  352 ,  354 ,  356 . Each of the local printed circuit boards  364 ,  366 ,  368  may obtain control power from the input lines which are connected to input terminals A, B, C of the power cell. As shown in  FIG. 10 , one or more position sensing devices (labeled PSD in  FIG. 10 ) may be utilized to provide the local printed circuit boards  364 ,  366 ,  368  with the respective positions of the movable portions of the contacts  352 ,  354 ,  356 . According to various embodiments, the position sensing devices may be embodied as switching devices, hall effect sensors, optical sensors, etc. 
     For embodiments where the coils  358 ,  360 ,  362  are latching coils, the local printed circuit boards  364 ,  366 ,  368  may each include a DC capacitor which can store enough energy to switch the plunger and/or the movable portions of the respective contacts  352 ,  354 ,  356  between positions. Each of the local printed circuit boards  364 ,  366 ,  368  may also include a power supply which restores the stored energy after a switching event, using AC power from the input lines connected to the input terminals A, B, C of the power cell. 
       FIG. 11  illustrates various embodiments of a system  370  for bypassing a power cell (e.g., power cell  210 ) of a power supply. The system  370  is similar to the system  350  of  FIG. 10 . The system  370  includes a first contact  372  connected to the output terminals T 1  and T 2  of the power cell, a second contact  374  connected to the input terminal A of the power cell, and a third contact  376  connected to the input terminal C of the power supply. Each of the contacts  372 ,  374 ,  376  include stationary portions and a movable portion. Although the second and third contacts  374 ,  376  are shown in  FIG. 11  as being connected to input terminals A and C, it will be appreciated  that, according to other embodiments, the second and third contacts  374 ,  376  may be connected to any two of the input terminals A, B and C. 
     The system  370  also includes a first coil  378  which controls the movable portions of the first contact  372 , a second coil  380  which controls the movable portion of the second contact  374 , and a third coil  382  which controls the movable portion of the third contact  376 . According to various embodiments, the coils  378 ,  380 ,  372  are embodied as contactor coils. According to other embodiments, the coils  378 ,  380 ,  382  are embodied as magnetic latching coils which do not need to have continuous power applied to the coils in order to hold the plunger in its first or second position and/or to hold the moving portions of the contacts  372 ,  374 ,  376  in the non-bypass or bypass position. 
     According to various embodiments, the first contact  372  and the first coil  378  are portions of a first bypass device, the second contact  374  and the second coil  380  are portions of a second bypass device, and the third contact  376  and the third coil  382  are portions of a third bypass device. For such embodiments, the system  370  includes a plurality of bypass devices. 
     In contrast to the system  350  of  FIG. 10 , the system  370  includes a single local printed circuit board  384  which is in communication with the first coil  378 , the second coil  380 , and the third coil  382 . The local printed circuit board  384  is configured to control the respective movable portions of the contacts  372 ,  374 ,  376  via the respective coils  378 ,  380 ,  382 . Thus, the local printed circuit board  384  is similar to the local printed circuit boards described with respect to  FIG. 10 , but is different in that the local printed circuit board  384  is configured to drive three coils and recognize the  respective positions of the movable portions of three contacts. In general, the local printed circuit board  384  is configured to receive commands from, and report status to, a master control device (e.g., master control system  195  of  FIG. 1 ) that is held near ground potential. 
     The local printed circuit hoard  384  is also configured to deliver pulses of energy to the coils  378 ,  380 ,  382  as needed to change the position of the movable portions of the respective contacts  372 ,  374 ,  376 , and to detect the position of the movable portions of the respective contacts  372 ,  374 ,  376 . The local printed circuit board  384  may obtain control power from the input lines which are connected to input terminals A, B, C of the power cell. As shown in  FIG. 11 , one or more position sensing devices (labeled PSD in  FIG. 11 ) may be utilized to provide the local printed circuit board  384  with the respective positions of the movable portions of the contacts  372 ,  374 ,  376 . According to various embodiments, the position sensing devices may be embodied as switching devices, hall effect sensors, optical sensors, etc. 
     For embodiments where the coils  378 ,  380 ,  382  are latching coils, the local printed circuit board  384  may include a DC capacitor which can store enough energy to switch the plunger and/or the movable portions of the contacts  352 ,  354 ,  356  between positions. The local printed circuit board  384  may also include a power supply which restores the stored energy after a switching event, using AC power from the input lines connected to the input terminals A, B, C of the power cell. 
       FIG. 12  illustrates various embodiments of a system  390  for bypassing a power cell (e.g., power cell  210 ) of a power supply. The system  390  is similar to the system  370  of  FIG. 11 . The system  390  includes a bypass device  392  which may be  embodied as a multi-pole contactor. The bypass device  392  may be identical to or similar to the bypass device  300  shown in  FIGS. 7-9 . The bypass device  392  includes a first contact  394  connected to the output terminals T 1  and T 2  of the power cell, a second contact  396  connected to the input terminal A of the power cell, and a third contact  398  connected to the input terminal C of the power supply. Each of the contacts  394 ,  396 ,  398  include stationary portions and a movable portion. Although the second and third contacts  396 ,  398  are shown in  FIG. 12  as being connected to input terminals A and C, it will be appreciated that, according to other embodiments, the second and third contacts  396 ,  398  may be connected to any two of the input terminals A, B and C. 
     In contrast to system  370  of  FIG. 11 , the system  390  includes a single coil  400  which controls the movable portions of the first, second and third contacts  394 ,  396 ,  398 . According to various embodiments, the coil  400  is embodied as a contactor coil. According to other embodiments, the coil  400  is embodied as a magnetic latching coil which does not need to have continuous power applied to the coil in order to hold the plunger in its first or second position and/or to hold the moving portions of the contacts  394 ,  396 ,  398  in the non-bypass or bypass position. 
     The system  390  also includes a single local printed circuit board  402  which is in communication with the coil  400 . The local printed circuit board  402  is configured to control the respective movable portions of the contacts  394 ,  396 ,  398  via the coil  400 . In general, the local printed circuit board  402  is configured to receive commands from, and report status to, a master control device (e.g., master control system  195  of  FIG. 1 ) that is held near ground potential.  
     The local printed circuit board  402  is also configured to deliver pulses of energy to the coil  400  as needed to change the position of the movable portions of the respective contacts  394 ,  396 ,  398 , and to recognize the position of the movable portions of the respective contacts  394 ,  396 ,  398 . The local printed circuit board  402  may obtain control power from the input lines which are connected to input terminals A, B, C of the power cell. As shown in  FIG. 12 , a position sensing device (labeled PSD in  FIG. 12 ) may be utilized to provide the local printed circuit board  402  with the respective positions of the movable portions of the contacts  394 ,  396 ,  398 . According to various embodiments, the position sensing device may be embodied as a switching device, a hall effect sensor, an optical sensor, etc. 
     For embodiments where the coil  400  is a latching coil, the local printed circuit board  402  may also include a DC capacitor which can store enough energy to switch the plunger and/or the movable portions of the contacts  394 ,  396 ,  398  between positions. The local printed circuit board  402  may also include a power supply which restores the stored energy after a switching event, using AC power from the input lines connected to the input terminals A, B, C of the power cell. 
       FIG. 13  illustrates various embodiments of a circuit  410  for controlling a bypass device (e.g., bypass device  392  of  FIG. 12 ). The circuit  410  may be embodied as a printed circuit board having integrated circuits, discrete components, and combinations thereof. The circuit  410  may be utilized, for example, to provide the functionality of one or more of the local printed circuit boards described hereinabove. For reasons of simplicity, the circuit  410  will be described as a printed circuit board in the context of its use in the system  390  of  FIG. 12 .  
     The circuit board  410  includes an on-board DC control power supply  412 . The power supply  412  includes series limiting impedance components Z 1 , Z 2 , Z 3  and a rectifier  414 . The impedance components Z 1 , Z 2 , Z 3  are respectively connected to three input lines which are connected to input terminals A, B, C of a power cell (e.g., power cell  210 ). The impedance components Z 1 , Z 2 , Z 3  may be embodied, for example, to include capacitors, and may be sized such that if one fails, the other two can continue to operate to limit the available current. According to various implementations, the impedance components Z 1 , Z 2 , Z 3  may also be embodied to include resistors in series with the capacitors. According to various embodiments, the rectifier  414  is a six-pulse diode rectifier. The printed circuit board  410  also includes a capacitor C 1  connected to the power supply  412 , a group of switching devices  416  connected to the capacitor C 1 , and another regulator  418  connected to the capacitor C 1 . As shown in  FIG. 13 , the group of switching devices  416  is also connected to a coil (e.g., coil  400  of the bypass device  392  of  FIG. 12 ). 
     The capacitor C 1  is sized to be able to store the amount of energy needed to cause the plunger and/or the movable portions of the contacts to change positions when such energy is applied to the coil. Capacitor C 1  may be embodied as, for example, an electrolytic capacitor, an ultra capacitor, a film type capacitor, etc. Although the capacitor C 1  is shown as a single capacitor in  FIG. 13 , it will be appreciated that capacitor C 1  may be embodied as multiple capacitors (e.g., three capacitors) connected in series or parallel. 
     The group of switching devices  416  is configured to apply either a positive or a negative current pulse to the coil. The individual switching devices may be  embodied as, for example, mosfets, insulated gate bipolar transistors, etc. Although each coil described herein is described in the context of a single winding for purposes of simplicity, one skilled in the art will appreciate that according to other embodiments, a given coil can comprise two redundant windings, with one winding being connected to receive the positive current pulse and the other winding connected to receive the negative current pulse. Thus, although the group of switching devices  416  is shown in  FIG. 13  as having four individual switching devices, according to other embodiments, the group of switching devices  416  may include a different number of switching devices (e.g., two switching devices). The regulator  418  may be configured to condition the voltage of the power supply  412  to operate fiber optic and control circuits. 
     In operation, the power supply  412  receives AC power from the input lines connected to the input terminals A, B, C of the power cell. The AC power flows through the series limiting impedance components Z 1 , Z 2 , Z 3  to the rectifier  414 . The rectifier  414  converts the AC power to DC power, and the DC power charges capacitor C 1  to the voltage at V 1 . The voltage at V 1  is applied to the group of switching devices  416 , and depending on the respective states (e.g., on, off) of the individual switching devices, the group of switching devices  416  may deliver a positive or a negative current pulse to the coil. The positive or negative current pulses create a magnetic field which is utilized to change the position of the plunger and/or the movable portions of the contacts which are connected to the input and output terminals of the power cell. 
     When the capacitor C 1  is discharging, the group of switching devices  416  may employ pulse-width modulation (PWM) to maintain a substantially constant average voltage (or constant current) on the coil. In general, when the capacitor C 1  is  discharging, the voltage across the capacitor C 1  is equal to or greater than approximately one-half of its worst case initial voltage. When the coil is a latching coil and none of the switching devices are in the “ion” state, the plunger and/or the movable portions of the contacts may maintain their existing positions by magnetic latching. During the time that the plunger and/or the movable portions of the contacts maintain their existing positions by magnetic latching, the power supply  412  can recharge the capacitor C 1 . 
     In general, the capacitor C 1 , the group of switching devices  416 , the coil connected to the group of switching devices  416 , and the regulator  418  should each be rated for the maximum voltage expected to be delivered to the power cell (i.e., the peak of the input AC line-to-line voltage delivered to the power cell). Additionally, as the voltage at V 1  can vary from the maximum voltage delivered to the power cell down to approximately one-half of the lowest voltage delivered to the power cell (i.e., when capacitor C 1  is discharging), the coil should also be designed to control the position of the plunger and/or the movable portions of the contacts even when approximately one-half of the lowest voltage delivered to the power cell is applied to the coil. 
     With no DC load, the rectifier  414  may generate a DC output voltage at V 1  which is substantially equal to the peak of the input line-to-line AC voltage feeding the power cell. In some embodiments, a power supply may function with power cells having a range of nominal voltage ratings from about 630 VAC to about 750 VAC. The power supply may also need to tolerate a range of utility voltage from 110% to 70% of nominal. Thus, the peak of the input AC line-to-line voltage (and hence the no-load DC voltage at V 1 ) may be as high as 1167 volts, and as low as 686 volts. According to other embodiments, other values are possible.  
     With a short (i.e., line-to-neutral) on the DC output, the DC output current is effectively limited to approximately 0.55 times the peak of the input AC line-to-line voltage, divided by the impedance of the impedance components Z 1 , Z 2 , Z 3 . For example, if the impedance components Z 1 , Z 2 , Z 3  are embodied as 0.1 μfd capacitors and the peak AC voltage is 1167 volts at 60 Hertz, the maximum available DC current would be approximately 0.024 amperes.  FIGS. 14 and 15  show the per-unit DC current and power available, for various per-unit DC voltages at V 1 . The per-unit values are based on open-circuit and short-circuit values defined above. 
       FIG. 16  illustrates various embodiments of a circuit  430  for controlling a bypass device (e.g., bypass device  392  of  FIG. 12 ). The circuit  430  is similar to the circuit  410  of  FIG. 13 , and further includes a shunt regulator  432  connected to the rectifier  414  and a diode D 1  connected to the shunt regulator  432 . The shunt regulator  432  operates to limit the voltage on capacitor C 1  to a particular voltage (e.g., 400 volts). For example, whenever the voltage at V 1  exceeds a particular voltage (e.g., 400 volts), the shunt regulator  432  may short out the rectifier  414 . The diode D 1  prevents capacitor C 1  from discharging into the shunt regulator  432 . 
     In general, for such embodiments, the capacitor C 1 , the group of switching devices  416 , the coil connected to the group of switching devices  416  (e.g., coil  400  of the bypass device  392  of  FIG. 12 ), and the regulator  418  could each be rated for the particular voltage (e.g., 400 volts) which is less than the maximum voltage expected to be delivered to the power cell (i.e., the peak of the input AC line-to-line voltage delivered to the power cell). If the minimum no-load voltage available from the diode D 1  is, for example, 686 volts, the voltage at V 1  would always reach 400 volts for  a nominal cell voltage rating as low as 630 VAC, even with utility variations from 110% to 70% of nominal. If the maximum short-circuit current available from the rectifier  414  is limited to, for example, 0.024 amperes (see  FIGS. 14 and 15 ), no harm occurs when the rectifier  414  is shorted out by the shunt regulator  432 . For such embodiments, as the voltage at V 1  can vary from the maximum voltage (e.g., 400 volts) down to approximately one-half of the maximum voltage (e.g., 200 volts), the coil should also be designed to control the position of the plunger and/or the movable portions of the contacts even when the lowest voltage at V 1  is applied to the coil. 
       FIG. 17  illustrates various embodiments of a circuit  440  for controlling a bypass device (e.g., bypass device  392  of  FIG. 12 ). The circuit  440  is similar to the circuit  410  of  FIG. 13 , and further includes a step-down regulator  442  connected to the capacitor C 1 , and a capacitor C 2  connected to the step-down regulator  442 . The step-down regulator  442  operates to step-down the variable voltage at V 1  to a lower fixed voltage at V 2 . As the voltage applied to the group of switches  416 , the coil connected to the group of switches  416  (e.g., coil  400  of the bypass device  392  of  FIG. 12 ), and the regulator  418  is the lower voltage at V 2 , the group of switching devices  416 , the coil, and the regulator  418  may each be rated for the lower voltage. According to various embodiments, the voltage at V 2  may be low enough to allow for the use of integrated circuits for the group of switches  416  and the regulator  418 . Even if the voltage at V 1  sags significantly while the capacitor C 1  discharges its stored energy, the voltage at V 2  could be held nearly constant, provided that the voltage provided by the step-down regulator  442  is less than the minimum voltage that appeared at V 1  during the sag. For  such embodiments, pulse width modulation control of the switching devices of the H-bridge  416  is not necessary. 
       FIG. 18  illustrates various embodiments of a circuit  450  for controlling a bypass device (e.g., bypass device  392  of  FIG. 12 ). The circuit  450  is similar to the circuit  410  of  FIG. 13 , and further includes a shunt regulator  432  connected to the rectifier  414 , a diode D 1  connected to the shunt regulator  432 , a step-down regulator  442  connected to the capacitor C 1 , and a capacitor C 2  connected to the step-down regulator  442 . The added components operate as described in  FIGS. 16 and 17 , and such operation can result in the voltage at V 1  being, for example, between 200 volts DC and 400 volts DC. For such embodiments, the lower DC voltages at V 1  would reduce the peak voltage stress on the shunt regulator  432 , the capacitor C 1 , and the step-down regulator  442 . For example, the peak voltage stress on the shunt regulator  432 , the capacitor C 1 , and the step-down regulator  442  may be reduced from 1167 volts DC to 400 volts DC. The voltage at V 2  could be held nearly constant, at a low value. For such embodiments, pulse width modulation control of the group of switching devices  416  is not necessary, and integrated circuits can be utilized for the group of switches  416  and the regulator  418 . Also, the amount of insulation needed for the coil connected to the group of switching devices  416  (e.g., coil  400  of the bypass device  392  of  FIG. 12 ) could be reduced. 
       FIG. 19  illustrates various embodiments of a circuit  460  for controlling a plurality of bypass devices (e.g., the bypass devices of  FIG. 11 ). The circuit  460  is similar to the circuit  450  of  FIG. 18 , but includes three groups of switching devices  416 ,  462 ,  464  which are each connected to the capacitor C 2 . The additional groups of switching devices  462 ,  464  are similar to the group of switching devices  416  as  described hereinabove. Each of the groups of switching devices  416 ,  462 ,  464  are also connected to a different coil (e.g., coils  378 ,  380 ,  382  of  FIG. 11 ) which forms a portion of a different bypass device. Thus, one skilled in the art will appreciate that the circuit  460  may he utilized, for example, to provide the functionality of the local printed circuit board  384  of  FIG. 11 , which controls the three coils  378 ,  380 ,  382  which control the respective positions of the plunger and/or the movable portions of contacts  372 ,  374 ,  376 . For such embodiments, the capacitor C 1  is sized to be able to store the amount of energy needed to cause the plunger and/or the movable portions of all the contacts to concurrently change positions when such energy is applied to all the coils. 
     While several embodiments of the invention have been described herein by way of example, those skilled in the art will appreciate that various modifications, alterations, and adaptions to the described embodiments may be realized without departing from the spirit and scope of the invention defined by the appended claims.