Abstract:
A medium or high voltage multi-inverter system is described, in which multiple inverter cells are included in a phase line to increase the voltage level supported by the system, and support higher voltage loads such as AC three-phase motors. In one configuration, five cells are used: two each in two phase lines, and one in a third phase line. In one of the phase lines having two cells, a second cell is series connected in reverse polarity, generates a phase matching the phase of the third line, while the other cell in the phase line generates an output with a different phase. One or more failure switches may be included to allow for short-circuiting of the output poles of one or more of the cells, so that in case of cell failure, the system can continue operation. For example, switches may be employed for the second cells in the two phase lines having two cells, and those cells may be available as spare cells in case of a cell failure.

Description:
BACKGROUND  
       [0001]     Since before Ben Franklin&#39;s historic kite-flying experiment in 1752, humans have been unlocking and unraveling the many mysteries surrounding electricity. Today, nearly every gadget and piece of machinery uses electricity to operate, spanning from the very small (e.g., nano-robots) to the very large (e.g., industrial drives and other high-power machinery). The present application relates to the latter. In particular, this application relates generally to medium- and high-voltage motors, such as three-phase AC (alternating current) motors.  
         [0002]     Today&#39;s power plants generate three-phase AC electricity, and that electricity is stepped down and/or rectified to provide the specific level and type of power needed for a given application. In the case of driving larger motors, this may be done using inverter cells. For example, FIG. I depicts an example configuration for driving a three-phase AC motor. As shown in the figure, three-phase electricity may be supplied by the local power company to an input side of a transformer  101 . The output side of the transformer  101  may include secondary windings  102   a - f,  each of which may provide three-phase AC input to three power cells  103   a - c.  In some situations, the same pair of secondary windings (e.g.,  102   a - b ) may supply inputs to all three cells  103 . The transformer  101  serves to-isolate the power cells  103  from the power source, and may also be used to step up or down the voltage level and/or adjust the phase output.  
         [0003]     The power cells  103   a - c  receive the two sets of three-phase power inputs, and each provides two output terminals (e.g., Uo and Vo). One of these terminals (Vo) is tied to the corresponding terminal in the other cells, while the other terminal (Uo) provides an output from the cell to a phase input on a three-phase load, such as motor  104 . These outputs of the three cells  103  may be identical in amplitude, and may be offset from one another by  120  degrees of phase.  
         [0004]     The highest power level supportable by the  FIG. 1  configuration depends on the circuit components used in the power cells  103 , and their various voltage ratings. Higher rated components will support higher voltage levels, but such components are more expensive, and the output voltage required by some applications can even exceed the highest-rated components. Accordingly, there is a need for higher power level configurations that can perhaps minimize the cost by not requiring these higher power level cells.  
       SUMMARY  
       [0005]     The following summary generally addresses many of the features described herein, but is not intended to limit the scope of this disclosure or identify features of greater importance to the claims herein.  
         [0006]     The systems and features described herein relate generally to an improved circuit design in which multiple single-phase inverters may be coupled to provide support for higher voltages. In some aspects, multiple inverters receive two isolated three-phase power inputs, and are series-connected in a given phase line to support higher voltage levels.  
         [0007]     In some aspects, a three-phase motor may have non-identical configurations in each of its phase input lines. In a first input line, two single-pole NPC inverter cells may be serially connected such that their voltage amplitudes stack and their phases are coincident. The output of that phase line is provided to the first phase input of the motor. In a third input line, a single cell is used to supply the third phase input to the motor.  
         [0008]     In the second input line, two inverter cells may be placed in the line, but may be connected in reverse polarity such that they have corresponding, connected output poles, and where the other output of the second cell supplies the second phase input to the motor. In this second input line, the second cell may also supply a different phase voltage from the first cell in the line. In particular, the second cell may supply a phase that is coincident with the phase of the single cell used in the third input line. In some aspects, a circuit having five single phase inverters supplied with three-phase power at 120 degrees phase separation and interconnected with the reverse-connected second phase line may support the same 6.6 kV application otherwise supported by a six-inverter configuration.  
         [0009]     In another aspect, one or more failure switches may be added to circuit configurations having more than one cell in a given phase line. Failure switches may close across output terminals of one or more of the inverter cells, and may be closed in the event of a failure in one of the cells. In a cell failure in a configuration having more than three cells, the circuit may be dynamically reconfigured to operate at a three-cell level (e.g., one cell per phase line) by closing the failure switches to result in a three-cell configuration. The cells whose terminals are shorted together by the closed switch may then be removed (if they failed) or used as a spare to replace another cell.  
         [0010]     Additional features described herein will be addressed in greater detail below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  illustrates an example configuration in which a three-phase AC motor is controlled by three single-phase inverters in a wye connection.  
         [0012]      FIG. 2  illustrates a single-phase neutral point clamp inverter cell that may be used in the  FIG. 1  configuration.  
         [0013]      FIG. 2   a  and  2   b  illustrate tables showing typical transistor switching patterns of a single-phase neutral point inverter of  FIG. 2 .  FIG. 2   a  illustrates a table of eight switching modes of transistors to generate square wave output, and  FIG. 2   b  illustrates a table of switching of eight transistors for PWM wave output  
         [0014]      FIG. 2   c  illustrates a single-phase neutral point inverter cell that may reduce input harmonic current by using additional rectifier bridges and additional transformer windings.  
         [0015]      FIG. 3  illustrates a multi-inverter system that may. be used to double the output voltage afforded by the  FIG. 1  system.  
         [0016]      FIG. 4  illustrates a booster voltage inverter system using two additional single-phase inverter cells.  
         [0017]      FIG. 5  illustrates a vector configuration for the output of a system as shown in  FIG. 3 .  
         [0018]      FIGS. 6   a  and  6   b  illustrate a vector configuration for the output of a system as shown in  FIG. 4 .  
         [0019]      FIG. 7  illustrates an alternative configuration for the  FIG. 4  system, employing backup circuitry, and  FIG. 7   a  illustrates an example method using this backup circuitry.  
         [0020]      FIG. 8  illustrates a table showing example power output levels that may be achieved using the  FIG. 4  and/or  FIG. 7  configuration.  
         [0021]      FIG. 9  illustrates example wave form diagrams for the  FIG. 4  and/or  FIG. 7  configuration, using a square wave single cell output.  
         [0022]      FIG. 10  illustrates example wave form diagrams for the  FIG. 4  and/or  FIG. 7  configuration, using a pulse-width modulated (PWM) cell output. 
     
    
     DETAILED DESCRIPTION  
       [0023]     The configuration in  FIG. 1 , which may be referred to as a three-phase inverter single-pole wye-connection system, may be used to drive the three-phase AC motor  104 . Different types of power cells  103  may be used, such as single-phase neutral point clamp (NPC) cells and six-step  3 -level single-phase inverter cells. For higher output voltages, single-phase NPC cells may be used.  
         [0024]      FIG. 2  illustrates an example single pole NPC inverter cell that may be used in a configuration as shown in  FIG. 1 . As shown in  FIG. 2 , the inverter cell  201  may be provided with two sets of three-phase AC input voltages, such as from secondary windings  102   a - b  shown in  FIG. 1 . The U, V and W input phases may be separated by a phase angle, such as 120 degrees, and the two groups of isolated inputs may be supplied to separate rectifier bridges  202   a - b  (REC 1  and REC 2 ). The rectifier bridges convert the two isolated received AC powers into DC (direct current) powers.  
         [0025]     The DC output from the rectifier bridges may contain unwanted current ripples, and smoothing capacitors  203   a - b  (C 1  and C 2 ) may smooth out the DC powers by removing such unwanted spikes. The smoothed DC power is then supplied to an inverter stage, which may include inverter transistors  204   a - h  (GTR 1 A, GTR 1 B, GTP 2 A, GTR 2 B, GTR 3 A, GTR 3 B, GTR 4 A and GTR 4 B), neutral clamp diodes  205   a - d  (D 1 , D 2 , D 3  and D 4 ) and free-wheeling diodes  206   a - h  (D 1 A, D 1 B, D 2 A, D 2 B, D 3 A, D 3 B, D 4 A and D 4 B) as shown, for conversion back into AC power. This conversion is done under the control of a control circuit (not shown), which supplies control signals to the various transistors  204   a - h  in the inverter stages to turn them on and off in a timed sequence to cause the desired output.  FIG. 2A  is an example of an on-off timing sequence for the inverter transistors used in the  FIG. 2  configuration to generate five-level square-wave output, and  FIG. 2B  is an example of an on-off timing sequence that can be used to generate five-level PWM output. As referenced, V(Uo-Vo) is the voltage between terminal  207   a  (Uo) and  207 b (Vo), and “edc” is the voltage of capacitors  203   a  (C 1 ) and  203   b  (C 2 ). This output is available from each inverter cell  201  in  FIG. 2  at its two poles, shown as a first pole  207   a  (Uo) and a second pole  207   b  (Vo), which may also be referred to as the cell&#39;s left and right poles. These poles may be referred to as “opposite” one another as a convenient way to differentiate them, although the term “opposite” does not necessarily refer to or define differences in voltage amplitude or phase angle between the poles.  
         [0026]      FIG. 2C  illustrates an example optional configuration  251  for cell  201  that can be used to reduce input harmonic current of a three phase power supply. The  FIG. 2C  arrangement resembles the  FIG. 2  cell  201 , with inverter transistors  204   a - h  (GTR 1 A, GTR 1 B, GTR 2 A, GTR 2 B, GTR 3 A, GTR 3 B, GTR 4 A and GTR 4 B), neutral clamp diodes  205   a - d  (D 1 , D 2 , D 3  and D 4 ) and free-wheeling diodes  206   a - h  (D 1 A, D 1 B, D 2 A, D 2 B, D 3 A, D 3 B, D 4 A and D 4 B) arranged in the same configuration. The  FIG. 2C  configuration, however, has four rectifier bridges  252   a   1 , a 2 , b 1 , b 2  (REC 1 A, REC 1 B, REC 2 A and REC 2 B), instead of just two bridges as used in  FIG. 2 . These four bridges are given four group isolated inputs as four sets of three-phase AC input voltages. These voltages may be provided by secondary windings  152   a   1 ,a 2 ,b 1 ,b 2  of transformer  101 , whose voltages may be separated in phase by 15 degrees as shown. To supply such voltages, transformer  101  may include twelve (12) three-phase isolated windings, as compared to the six (6) windings used in  FIGS. 1 and 2 .  
         [0027]      FIG. 3  illustrates an example configuration that can be used to support higher output voltage levels than the  FIG. 1  configuration, using inverters such as that shown in  FIG. 2 . In the  FIG. 3  configuration, six single-phase inverters are connected, or stacked, in pairs to the phase lines of a three-phase AC motor. An input transformer  101  has twelve isolated three-phase secondary windings, and two three-phase isolated windings are connected to each single-phase inverter. The voltages of the secondary windings of transformer  101  may be separated in phase by 15 degrees among four windings for each line. For example, winding  102   a   1 ,  102   b   1 ,  102   a   2  and  102   b   2  for two single-phase inverters,  301   a  and  302   a,  for the U-phase line are illustrated as having phases separated by 15 degrees. Each phase line of the motor has two inverters connected in series. The phase line&#39;s first cells  301   a - c  (SPIu 1 , SPIv 1  and SPIw 1 ) have one of their output poles, such as their second respective poles (Vo), tied or short-circuited together.  
         [0028]     The other output pole (Uo) of each first inverter is tied to the opposite output pole of a second inverter  302   a - c  (SPIu 2 , SPIv 2 , SPIw 2 ) in the phase line, creating a forward polarity connection in which the phases positively combine. For example, as shown in each phase line of  FIG. 3 , the phase lines&#39; first cells  301  have their first output pole (Uo) connected to the second, or opposite, output pole (Vo) of the phase lines&#39; second cells  302 . This connection effectively doubles the supported phase line voltage level, since the voltage outputs of the two cells may combine with one another, and the connection of opposite poles allows the two cells to combine at the same phase angle. The remaining output poles (Uo) of the second inverters  302   a - c  are then connected to the three phase inputs of a three-phase AC motor  303 . In this configuration, the two inverter cells in each phase line generate the same AC voltage level and phase angle, thereby doubling the available voltage level for the line at the same phase. For example, the two inverters ( 301   a  and  302   a,  or SPIu 1  and SPIu 2 ) in the U-phase input each generate the same AC voltage level and the same phase as the U phase input to the three-phase AC motor  303 . Similarly, inverters  301   b  and  302   b  (SPIv 1  and SPIv 2 ) each generate the same AC voltage level and generate the same phase as the V-phase input; and inverters  301   c  and  302   c  (SPIw 1  and SPIw 2 ) generate the same AC voltage and phase as the W phase input.  
         [0029]      FIG. 4  illustrates a booster voltage inverter configuration that uses five similarly-rated (e.g., same voltage level) inverter cells, instead of the six used in the  FIG. 2  configuration. In the  FIG. 4  configuration, a three-phase AC motor  403  receives power from three phase input lines, one for each phase. Two of these lines use two inverter cells each, while the third line has just one inverter cell. Furthermore, the two pairs of cells in the first two phase lines are coupled differently from that shown in  FIG. 3 , as will be explained in greater detail below.  
         [0030]     In this configuration, the first inverters  401   a - c  (SPIu 1 , SPIv 1  and SPIw 1 ) in each phase input line receive two isolated three-phase inputs from the transformer  101 . This much resembles the configuration shown in  FIG. 3 .  
         [0031]     The first phase line, having cells  401   a  (SPIu 1 ) and  402   a  (SPIu 2 ), also has a similar configuration with the first phase line in  FIG. 3 . Specifically, the first cell  401   a  has one output (Vo) tied in common with the corresponding outputs of the other first phase line cells  401   b,c,  and the other output (Uo) tied to the opposite output (the second output, Vo) of the second cell  402   a  in the first phase line, creating a forward polarity connection between the cells in the first phase line. For example, the two cells  401   a,    402   a  both supply a common phase of output. The output of the first phase line is provided by second cell  402   a  (SPIu 2 ), which has its first output (Uo) connected to a first phase input of the motor  403  (terminal U in  FIG. 4 ).  
         [0032]     The third phase input line has just one cell,  401   c  (SPIw 1 ). The cell  401   c  generates a voltage having a third phase, and the cell&#39;s first output (Uo) is tied to the third phase input of the motor  403 .  
         [0033]     The second phase input line has two cells,  401   b  and  402   b  (SPIv 1  and SPIw 2 ), but the two are connected differently from the two in the first phase input line. In particular, the line&#39;s second cell  402   b  (SPIw 2 ) is connected in reverse polarity, having an output pole (Uo) tied with the corresponding pole (Uo) of the line&#39;s first cell  401   b  (SPIv 1 ). Furthermore, instead of generating an output voltage with the same phase as the line&#39;s first cell  401   b,  the line&#39;s second cell  402   b  generates the phase generated by the single cell  401   c  (SPIw 1 ) in the third phase line (e.g., the single-cell phase line, or the W phase in  FIG. 4 ). The second phase line provides its output via an output pole (Vo) of the second cell  402   b,  which is connected to the second phase input line of the three-phase motor  403 .  
         [0034]     Accordingly, in the  FIG. 4  configuration, cells  401   a  and  402   a  (SPIu 1  and SPIu 2 ) generate voltages at the same phase as one another; cell  401   b  (SPIv 1 ) generates voltages at a second phase (120 degrees different from first phase,  401   a  and  402   a ); and cells  401   c  and  402   b  (SPIw 1  and SPIw 2 ) generate voltages at a third phase (120 degrees different from first phase and second phase). Furthermore, these cells may all generate the same voltage amplitude. Using this configuration allows some cost savings as compared to the six-cell configuration in  FIG. 3 , since fewer cells are used, and yet this configuration can still support the 6.6 kV standard voltage level supported by the  FIG. 3  configuration. These benefits will be explained in greater detail below.  
         [0035]      FIG. 5  illustrates the vector configuration for the system shown in  FIG. 3 . As shown, point Nu represents a neutral point that is a common point connected with the first cells ( 301   a - c,  or SPIu 1 , SPIv 1  and SPIw 1 ) in each phase input line, and Eu 1 , Ev 1 , Ew 1 , Eu 2 , Ev 2  and Ew 2  are phase voltage vectors output by each of the cells  301   a - c  and  302   a - c,  respectively (e.g., SPIu 1 , SPIv 1 , SPIw 1 , SPIu 2 , SPIv 2  and SPIw 2 , respectively). Vectors Eu-v, Ev-w and Ew-u are phase-to-phase voltage vectors at terminals U, V and W respectively. When all six cells generate the same voltage amplitude (denominated, ‘e’), and the three phases generated by cells  301   a - c  and  302   a - c  are 120 degrees out of phase with one another, then the resulting phase-to-phase output voltages Eu-v, Ev-w and Ew-u are 2√{square root over (3)} times the individual cell voltage e. For example, if inverter cells  301  and  302  are rated at 2.5 kV, then the  FIG. 3  configuration can support (2.5 k)×(2√{square root over (3)})=8.6 kV.  
         [0036]     Using these same inverter cells in the  FIG. 1  configuration would support half of that voltage, or 4.3 kV. Accordingly, in the United States, the  FIG. 1  configuration can use 2.5 kV-rated cells and support a standard 4160V system, while the  FIG. 3  configuration can use 2.5 kV-rated cells and support a standard 6.6 kV system.  
         [0037]      FIGS. 6   a  and  6   b  illustrate the vector configuration for the arrangement shown in  FIG. 4 . Here, Eu 1 , Ev 1 , Ew 1 , Eu 2  and Ew 2  are phase voltage vectors of the output voltages of the cells  401   a - c  and  402   a - b  (SPIu 1 , SPIv 1 , SPIw 1 , SPIu 2  and SPIw 2 ), respectively, shown in  FIG. 4 . E′u-v, E′v-w and E′w-u are voltage vectors for the phase-to-phase voltages at terminals U, V and W in  FIG. 4 , and Nx is again a common connection point of the first cells in each of the phase lines (cells  401   a - c,  or SPIu 1 , SPIv 1  and SPIw 1 ). As apparent in these figures, the use of the sole cell  401   c  (SPIw 1 ) in the third phase line results in a shortened amplitude on that phase line (e.g., the vector Ew 1 ), while the reverse polarity connection of the second line&#39;s second cell  402   b  (SPIw 2 ) causes a reversal of phase when the cell&#39;s output (e.g., the vector “-Ew 2 ”) is combined with the output of the first cell  401   b  (SPIv 1 ).  FIG. 6   b  illustrates the same vector relationship from  FIG. 6   a,  but with trigonometric notations showing the supportable voltages in the  FIG. 4  configuration. As will be explained, when the  FIG. 4  configuration uses the 120 degree phase separation between cells  401   a - c,  and the same types of cells (e.g., voltage-rated ‘e’), the  FIG. 4  configuration supports a phase-to-phase voltage of √{square root over (7)} (or 2.6457) times the voltage e supported by each individual cell. The following calculations bear this out, where U-V, V-W and W-U are the vector lengths of the phase-to-phase voltage between terminals U and V, V and W, and W and U respectively, and V-Nx is the vector length of the voltage between terminal V and the common point Nx:  
                 V   -   Nx     _     =         e   2     +     e   2     -     2   ⁢     e   ·   e     ⁢           ⁢   cos   ⁢           ⁢     (     120   ⁢   °     )                       =         e   2     +     e   2     -     2   ⁢       e   2     ⁡     (       -   1     /   2     )                         =       3     ⁢   e                                 U   -   V     _     =           (     2   ⁢   e     )     2     +       (       3     ⁢   e     )     2     -     2   ⁢     (     2   ⁢   e     )     ⁢     (       3     ⁢   e     )     ⁢   cos   ⁢           ⁢     (     90   ⁢   °     )                       =         4   ⁢     e   2       +     3   ⁢     e   2       -     4   ⁢       3     ·   0                       =       7     ⁢   e                                 V   -   W     _     =           (       3     ⁢   e     )     2     +     e   2     -     2   ⁢       (       3     ⁢   e     )     ·   e     ⁢           ⁢     cos   ⁡     (     150   ⁢   °     )                         =         3   ⁢     e   2       +     e   2     -     2   ⁢     3     ⁢       e   2     ⁡     (     -       3     2       )                         =         3   ⁢     e   2       +     e   2     +     3   ⁢     e   2                       =       7     ⁢   e                                 W   -   U     _     =           (     2   ⁢   e     )     2     +     e   2     -     2   ⁢     (     2   ⁢   e     )     ⁢   e   ⁢           ⁢   cos   ⁢           ⁢     (     120   ⁢   °     )                       =         4   ⁢     e   2       +     e   2     -     4   ⁢       e   2     ⁡     (       -   1     /   2     )                         =       7     ⁢   e                 
         [0038]     As shown in these calculations, the  FIG. 4  embodiment can be used to support voltage levels of √{square root over (7)} times the voltage provided by an individual cell. If the same 2.5 kV-rated cells are used as discussed above, then the  FIG. 4  system can support √{square root over (7)} (2.5 kV), or 6.614 kV. Accordingly, the  FIG. 4  configuration can support the U.S.A.&#39;s standard 6.6 kV voltage using one fewer cell than the six-cell system shown in  FIG. 3 . Of course, the  FIG. 4  configuration can also support the 4160V standard as well.  
         [0039]      FIG. 7  illustrates an alternative configuration, in which circuitry is added to accommodate potential failures in one or more of the cells used in the  FIG. 4  system. In the  FIG. 7  configuration, cells  701   a - c  and  702   a - b  (SPIu 1 , SPIv 1 , SPIw 1 , SPIu 2  and SPIw 2 ) may be the same as cells  401   a - c  and  402   a - b  (also SPIu 1 , SPIv 1 , SPIw 1 , SPIu 2  and SPIw 2 ) discussed above in  FIG. 4 , with the same series-connected cells  701   a,    702   a  (SPIu 1 , SPIu 2 ) in the first phase line, the reverse-connected (and supplying a different phase) cell  702   b  (SPIw 2 ) in the second phase line, and a single cell  701   c  (SPIw 1 ) in the third phase line.  FIG. 7  also illustrates a control circuit  703  (CTR), which may be an NPC inverter control circuit that sends switching signals to the various transistors in cells  701   a - c  and  702   a -b. Control circuit  703  may include a U phase switching signal circuit  703   a,  V phase switching signal circuit  703   b  and W phase switching signal circuit  703   c,  each of which may provide isolated switching signals to the cells in their corresponding phases. The isolated switching signals may help avoid effects of harmful interference experienced along the route from the control circuit  703  to the various cells, with optical signals as one example of a type of isolated switching signal that may be used. The switching signals are used to control the state of the various inverter transistors, and the switching signals may be converted at transistor drive circuits  708   a - c  (DRu 1 , DRv 1  and DRw 1 ) and  709   a - b  (DRu 2  and DRw 2 ) from a first isolated format (e.g., optical) to a second format (e.g., electric drive signals) suitable for controlling the transistors. For example, U phase switching signal circuit  703   a  may send isolated switching signals to transistor drive circuits  708   a  and  709   a,  which may in turn convert those signals to electric drive signals, and supply the resulting electric drive signals to cells  701   a  and  702   a  in the U phase. Similarly, V phase switching signal circuit  703   b  may send isolated switching signals to transistor drive circuit  708   b,  which may convert the switching signals to electric drive signals for cell  701   b  in the V phase; and W phase switching signal circuit  703   c  may send isolated switching signals to transistor drive circuits  708   c  and  709   b,  which may convert the switching signals to electric drive signals for cells  701   c  and  702   b  in the W phase.  
         [0040]     As with the  FIG. 4  configuration, the cells in the first phase line,  701   a  and  702   a  (SPIu 1  and SPIu 2 ), may generate the same voltage amplitude and phase as one another; and the cells  701   c  and  702   b  (SPIw 1  and SPIw 2 ), although located in different phase lines, may generate the same voltage amplitude and phase as one another. As with  FIG. 4 , the second line&#39;s second cell  702   b  (SPIw 2 ) may be connected in reverse polarity with the line&#39;s first cell. The third cell  701   c  (SPIw 1 ) may be alone in the third phase line, and may generate voltage at a third phase (e.g., the W phase), which is supplied to the motor&#39;s third phase line input.  
         [0041]      FIG. 7  also shows a number of additional components. Reactors  704  (Lu, Lv, Lw) and capacitors  705  (Cuv, Cvw, Cwu) may form a line filter to trap surge voltages generated by voltage changes (dV/dt) occurring with PWM switching of the main transistor devices in the single phase NPC cells  701   a - c,    702   a -b. Grounding capacitors  706  (Cwg, Cvg, Cug) may also be used to fix the neutral point of the three phase output voltage at the ground potential.  
         [0042]     To accommodate failures of one or more of the cells, the  FIG. 7  configuration includes failure switches  707  (CTT-U, CTT-V). These switches are placed in a position to short-circuit one or more of the cells in a phase line, such as a line&#39;s secondary cells  702   a - b  (or cells  402   a - b ). The switches are kept open during normal operation, and they may be closed when one or more of the cells in the system experience a failure. Different configurations can be used. For example, the failure switches may be located across the secondary cells in the phase lines, and upon a cell failure, closing the switches shorts those secondary cells out, and converts the system back to a three-cell configuration, similar to that shown in  FIG. 1 . By shorting out the secondary cells (e.g., cells  702   a - b ), those cells become available for removal without stopping operation of the system. The system may have to run at a lower capacity when the failure switches are closed, but that is preferable to a complete shutdown. If the failure occurred in one of the primary cells (e.g., cells  701   a - c ), the shorted-out secondary cells may be removed and used to replace the failed primary cell. In this manner, the system can quickly recover from a failure in a primary cell, and can remain in operation however long it takes to obtain a replacement for the failed cell.  
         [0043]      FIG. 7   a  illustrates an example method when a failure occurs. In step  750 , a failure in one or more of the cells  701   a - c,    702   a - b  is detected. In response to the failure, in step  751 , the failure switches are both closed to short circuit the output poles of one or more of. the cells. With the closing of these switches, the system may operate as a three-cell system instead of a five-cell system. Then, in step  752 , the failed cell(s) are removed, and in step  753 , if one or two of the first cells in the phase lines (e.g., cells  701   a - c ) experienced a failure, then one or both of cells  702   a - b  are used as spares to replace those failed cells, so the system can continue operation as a three-cell system.  
         [0044]      FIG. 8  is a table showing one example voltage output range that can be supported by the  FIG. 7  configuration. As shown, the designations “e1” and “e2” refer to phase-to-phase voltages when the failure switches are closed and open, respectively. As described above, when 2.5 kV-rated cells are used, the supported phase-to-phase voltages are √{square root over (3)}e, or 4.3 kV, when the switches  707  are closed, and when the switches  707  are open, the configuration supports voltages of √{square root over (7)}e, or 6.6 kV. The table in  FIG. 8  also shows the allowable apparent power (kVA) when the cells are rated at 660 kVA. When the switches  707  are closed, the calculation yields 3×660 kVA=1980 kVA; and when the switches  707  are open, the calculation yields 1980 kVA×(6.6 kV/4.3 kV)=3039 kVA.  
         [0045]      FIG. 9  illustrates example waveforms showing the output when the cells  401   a - c,    402   a - b  (or  701   a - c,    702   a - b ) generate the same 5-level square wave forms. Phase-to-phase voltages E′u-v, E′v-w and E′w-u at the output terminals in  FIG. 4  are calculated from the vector relationship in  FIG. 6  as follows: 
   E′u - v=Eu 1+ Eu 2− Ev 1+ Ew 2    E′v - w=Ev 1− Ew 1− Ew 2    E′w - u=Ew 1− Eu 1 −Eu 2  
         [0046]      FIG. 10  illustrates example waveforms when the cells  401   a - c,    402   a - b  (or  701   a - c,    702   a - b ) generate the same 5-level simple PWM wave forms. The phase-to-phase vector relationships are as described above for  FIG. 9 . These wave forms are closer to a sine wave than the  FIG. 9  waves, although some harmonic distortion is still included because the PWM wave forms generated by single phase cells are simple PWM wave forms, and not sine-wave modulated wave forms.  
         [0047]     The various calculations provided herein have a degree of mathematical precision that may be approximated in systems employing the features described herein. For example, although inverter cells may be described above as generating the same voltage levels and at certain phase angles, engineering and manufacturing tolerances may adjust the values achieved in implementation, such that the actual values may slightly vary, with the voltages and phases being substantially as described.  
         [0048]     The various features, examples and embodiments described above are not intended to limit the scope of the present application, and many of the components may be divided, combined and/or subcombined with one another as desired. Accordingly, the scope of the present patent should only be defined by the following claims.