Abstract:
Methods and apparatus for delivering uninterruptible power to loads are provided. One aspect on the invention is directed to a system for providing power to a load. The system includes a first input to receive AC power from a first AC power source, a second input to receive AC power from a second AC power source, a third input to receive DC power from a first DC power source, an output that provides output AC power to the load, and converter circuitry, coupled to the first, second and third inputs, adapted to provide the output power derived from at least one of the first AC power source, the second AC power source and the first DC power source.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to a system and method for providing redundant power to critical loads.  
       BACKGROUND OF THE INVENTION  
       [0002]     The use of an uninterruptible power system (UPS) to provide power to a critical load is well known. Known uninterruptible power systems include on-line UPS&#39;s and off-line UPS&#39;s. On-line UPS&#39;s provide conditioned AC power as well as back-up AC power upon interruption of a primary source of AC power. Off-line UPS&#39;s typically do not provide conditioning of input AC power, but do provide back-up AC power upon interruption of the primary AC power source.  FIG. 1  shows a block diagram of one type of on-line UPS  10 . Other on-line UPS&#39;s are described in U.S. Pat. No. 5,982,652, and U.S. Pat. No. 5,686,768, both of which are incorporated herein by reference. On-line UPS&#39;s of the type described in the referenced patents are available from American Power Conversion Corporation, West Kingston, R.I. under the trade names Symmetra and Silcon. The UPS  10  of  FIG. 1  includes an input circuit breaker/filter  12 , a rectifier  14 , a control switch  15 , a controller  16 , a battery  18 , an inverter  20 , an isolation transformer  22 , and a bypass switch  23 . The UPS also includes an input  24  for coupling to an AC power source, and an outlet  26  for coupling to a load.  
         [0003]     The UPS  10  operates as follows. The circuit breaker/filter  12  receives input AC power from the AC power source through the input, filters the input AC power and provides filtered AC power to the rectifier  14 . The rectifier rectifies the input voltage. The control switch  15  receives the rectified power and also receives DC power from the battery  18 . The controller  16  determines whether the power available from the rectifier is within predetermined tolerances, and if so, controls the control switch to provide the power from the rectifier to the inverter  20 . If the power from the rectifier is not within the predetermined tolerances, which may occur because of “brown out” or “black out” conditions, or due to power surges, then the controller controls the control switch to provide the DC power from the battery to the inverter  20 .  
         [0004]     The inverter  20  of the UPS  10  receives DC power and converts the DC power to AC power and regulates the AC power to predetermined specifications. The inverter  20  provides the regulated AC power to the isolation transformer  22 . The isolation transformer is used to increase or decrease the voltage of the AC power from the inverter and to provide isolation between a load and the UPS. The isolation transformer is typically an optional device, the use of which is typically dependent on UPS output power specifications. Depending on the capacity of the battery and the power requirements of the load, the UPS  10  can provide power to the load during brief power source dropouts or for extended power outages. The bypass switch  23  is used to provide a bypass of UPS circuitry to provide the input power directly to the output. The bypass switch may be controlled by the controller  16  to provide bypass of the UPS circuitry upon a failure condition of the UPS.  
         [0005]     To provide further power redundancy, it is known to supply power to a bypass switch of a UPS from a second source of AC power as shown in  FIG. 2 . One problem with this approach is that in bypass mode, the load receives unconditioned power and the source and the load must be able to handle transients that may occur when the load is transferred to the second AC source. In some systems, to minimize transients, the first and second AC sources are required to be substantially synchronous and have substantially the same voltage. To at least partially overcome these problems, a second UPS may be installed in line with the second AC source, but such a solution can be quite expensive since two UPS&#39;s are used.  
       SUMMARY OF THE INVENTION  
       [0006]     Embodiments of the present invention provide improved high power availability solutions.  
         [0007]     A first aspect of the present invention is directed to a system for providing power to a load. The system includes a first input to receive AC power from a first AC power source, a second input to receive AC power from a second AC power source, a third input to receive DC power from a first DC power source, an output that provides output AC power to the load, and converter circuitry, coupled to the first, second and third inputs, adapted to provide the output power derived from at least one of the first AC power source, the second AC power source and the first DC power source.  
         [0008]     The system may further include a first bypass device coupled to the first input and the output and controllable to operate in a bypass mode to couple the first input to the output to provide AC power from the first AC power source directly to the output, bypassing the converter circuitry. The system may include a second bypass device coupled to the second input and the output and controllable to operate in a bypass mode to couple the second input to the output to provide AC power from the second AC power source directly to the output, bypassing the converter circuitry. The converter circuitry can include a plurality of controllable switches, each of the controllable switches being coupled to one of the first, second and third inputs to control current draw by the converter circuitry from the first AC power source, the second AC power source and the DC power source; wherein, in a first power source transition mode, the converter circuitry is adapted to detect an input AC voltage waveform period of the first AC power source and to control the controllable switches such that the converter circuitry draws current from the first AC power source during a first portion of the period and the converter circuitry draws current from the first DC power source during a second portion of the period for multiple periods. The system may operate in a second power source transition mode, and the converter circuitry may be adapted to detect an input AC voltage waveform period of the second AC power source and to control the controllable switches such that the converter circuitry draws current from the second AC power source during a first portion of the period and the converter circuitry draws current from the first DC power source during a second portion of the period for multiple periods.  
         [0009]     The system can still further include a fourth input to receive DC power from a second DC power source, and the converter circuitry can be coupled to the fourth input through a controllable switch that is controlled in the first transition mode to draw current from the second DC power source during the first portion of the period for multiple periods. The converter circuitry can include a first DC regulator circuit having an input and having an output that produces a first DC voltage, and a second DC regulator circuit having an input and having an output that produces a second DC voltage, and the input of the first DC regulator circuit can be coupled to the first input of the system through a first one of the plurality of controllable switches, the input of the first DC regulator circuit can be coupled to the second input through a second one of the plurality of controllable switches, the input of the first DC circuit can be coupled to the third input through a third one of the plurality of controllable switches, the input of the second DC regulator can be coupled to the first input of the system through a fourth one of the controllable switches, the input of the second DC regulator can be coupled to the second input of the system through a fifth one of the controllable switches, and the input of the second DC regulator can be coupled to the fourth input of the system through a sixth one of the plurality of controllable switches.  
         [0010]     In the system, the first DC voltage may be approximately the same magnitude and opposite polarity of the second DC voltage. The system can further include a DC to AC converter coupled to the outputs of the first and second DC regulator circuits and the output of the system to provide output AC power derived from the first DC voltage and the second DC voltage. Each of the controllable switches can include a thyristor. The system can further include the first DC source and the second DC source with a source voltage of the first DC source being of approximately equal magnitude and opposite polarity of a source voltage of the second DC source.  
         [0011]     Another aspect of the invention is directed to a system for providing power to a load. The system includes a first input to receive AC power from a first AC power source, a second input to receive AC power from a second AC power source, a third input to receive DC power from a first DC power source, an output that provides output AC power to the load; and converter means, coupled to the first, second and third inputs, for providing output power derived from at least one of the first AC power source, the second AC power source and the first DC power source.  
         [0012]     In implementations, the system may further include bypass means for selectively providing AC power from the first power source directly to the output, bypassing the converter means. The bypass means may include means for selectively providing AC power from the second power source directly to the output, bypassing the converter means. The converter means may include means for transitioning a draw of input current by the converter means from the first AC power source at the first input to the first DC power source at the third input, such that during a first transition period, input current is drawn by the converter means alternately from the first AC power source and the first DC power source. The converter means may include means for transitioning a draw of input current by the converter means from the first DC power source at the third input to the second AC power source at the second input, such that during a second transition period, input current is drawn by the converter means alternately from the first DC power source and the second AC power source.  
         [0013]     The system may further include a fourth input to receive DC power from a second DC power source, and the converter means may include means for transitioning a draw of input current by the converter means from the first AC power source at the first input to the second DC power source at the fourth input, such that during the first transition period, input current is drawn by the converter means alternately from the first AC power source and the second DC power source. The converter means may include means for transitioning a draw of input current by the converter means from the second DC power source at the fourth input to the second AC power source at the second input, such that during the second transition period, input current is drawn by the converter means alternately from the second DC power source and the second AC power source. The converter means may include regulator means for producing a first regulated DC voltage, and a second regulated DC voltage. The first regulated DC voltage may be approximately the same magnitude and opposite polarity of the second regulated DC voltage. The system may further include means for converting the first regulated DC voltage and the second regulated DC voltage to an AC voltage to provide output AC power. The system may further include the first DC power source and the second DC power source with a source voltage of the first DC power source being of approximately equal magnitude and opposite polarity of a source voltage of the second DC power source.  
         [0014]     Another aspect of the invention is directed to a method of providing power to a load using an uninterruptible power supply (UPS) having a first input that can be selectively coupled to a first AC source, a second AC source, and a first DC source. The method includes coupling the first input of the UPS to the first AC source and providing output power to a load based on AC power from the first AC source, detecting a loss of the first AC source, coupling the first input of the UPS to the first DC source and providing output power from the UPS based on DC power from the first DC source, and transitioning a draw of input current at the first input from the first DC source to the second AC source by alternately coupling the first DC source and the second AC source to the first input of the UPS. In implementations, the method may further include further comprising detecting return of the first AC Source, transitioning a draw of input current at the first input from the second AC source to the first DC source by alternately coupling the second AC source and the first DC source to the first input of the UPS, and transitioning a draw of input current at the first input from the first DC source to the first AC source by alternately coupling the first DC source and the first AC source to the first input of the UPS. The UPS may have a second input that can be selectively coupled to the first AC source, the second AC source, and a second DC source, and the method can further include coupling the second input of the UPS to the first AC source and providing output power to a load based on AC power from the first AC source, after detecting the loss of the first AC source, coupling the second input of the UPS to the second DC source and providing output power from the UPS based on DC power from the second DC source, and transitioning a draw of input current at the second input from the second DC source to the second AC source by alternately coupling the second DC source and the second AC source to the second input of the UPS. The method can further include detecting the return of the first AC Source, transitioning a draw of input current at the second input from the second AC source to the second DC source by alternately coupling the second AC source and the second DC source to the second input of the UPS, and transitioning a draw of input current at the second input from the second DC source to the first AC source by alternately coupling the second DC source and the first AC source to the second input of the UPS. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings:  
         [0016]      FIG. 1  is a functional block diagram of a first prior art UPS system;  
         [0017]      FIG. 2  is a functional block diagram of a second prior art UPS system;  
         [0018]      FIG. 3  is a functional block diagram of a power supply system in accordance with one embodiment of the invention;  
         [0019]      FIG. 4  is a state diagram showing operational states of systems of embodiments of the present invention;  
         [0020]      FIG. 5  is a functional block diagram of a UPS system that may be used in embodiments of the invention;  
         [0021]      FIG. 6  is a graph of voltage and current waveforms in UPS systems of embodiments of the invention;  
         [0022]      FIG. 7  is a functional block diagram of a power supply system in accordance with another embodiment of the invention; and  
         [0023]      FIG. 8  is a schematic diagram of a UPS used in the embodiment of  FIG. 7 .  
     
    
     DETAILED DESCRIPTION  
       [0024]     This invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing”, “involving”, and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.  
         [0025]     Embodiments of the present invention provide cost-effective, high availability power solutions. One embodiment of a high availability power system  100  in accordance with the present invention will now be described with reference to  FIG. 3 . The system  100  shown in  FIG. 3  includes a single UPS  108  having an input  118  to receive AC power and an output  106  that provides AC power. The system  100  further includes first and second inputs  102  and  104  for coupling to independent sources of AC power, a first input stage  110 , a second input stage  112 , a first bypass switch  114  and a second bypass switch  116 .  
         [0026]     In the system  100 , the UPS  108  is an on-line UPS and may be implemented using, for example, one of a number of UPS&#39;s available from American Power Conversion, Corporation, West Kingston, R.I. including the Symmetra UPS&#39;s and the Silcon UPS&#39;s. The input stages  110  and  112  may be implemented using a number of known switches such as mechanical switches, static switches or the switching function may be integrated into a rectifier stage of the UPS that controls the source of power flow into the UPS. In addition, although shown as two separate devices, the two input stages may be implemented using one device that selects one of the two input power sources as the output power source. The bypass switches  114  and  116  and the input stages  110  and  112  may be controlled by a controller contained within the UPS, and as shown in  FIG. 3 , may have control lines coupled to the UPS to allow control by a controller within the UPS. In addition to the bypass switches  114  and  116 , the UPS may have an internal bypass device as described above with reference to the UPS  10  of  FIG. 1 .  
         [0027]     The system  100  provides considerable options that enable power to be provided to the load upon failure of the UPS, upon failure of either of the input AC power sources, and upon failure of both of the input AC sources. A description of the operation of the system  100  will now be provided with reference to Tables 1 and 2 and  FIG. 4 . Table 1 includes a summary of operational modes of system  100 ,  FIG. 4  provides a state diagram for the system  100 , and Table 2 provides a description of the conditions for transfer in the state diagram of  FIG. 4 .  
                                             TABLE 1                           Operational States of System 100                    Input   Input   Bypass   Bypass           State   State description   stage 110   stage 112   switch 114   Switch 116   UPS 108               S1   Normal operation   Active   Inactive   Inactive   Inactive   Active           Both supplies           But ready       on mains           present           AC Mains 1           operation           preferred       S2   Normal operation   Inactive   Active   Inactive   Inactive   Active           Both AC supplies               But ready   on mains           present           AC Mains 2           operation           preferred       S3   Normal operation   Active   Inactive   Inactive   Inactive   Active           AC Mains 2 not               But ready   on mains           present           AC Mains 1           operation       S4   Normal operation   Inactive   Active   Inactive   Inactive   Active           AC Mains 1 not           But ready       on mains           present           AC Mains 2           operation       S5   DC operation   Inactive   Inactive   Inactive   Inactive   Active           (stored energy                   on           mode)                   backup           AC Mains 1 and 2                   power           not present       S6   Bypass operation   Inactive   Inactive   Active   Inactive   Inactive           Both AC supplies               But ready   or failed           present           AC mains 1           operation           preferred       S7   Bypass operation   Inactive   Inactive   Inactive   Active   Inactive           Both AC supplies           But ready       or failed           present           AC mains 2           operation           preferred       S8   Bypass operation   Inactive   Inactive   Active   Inactive   Inactive           AC Mains 2 not                   or failed           present           AC Mains 1           operation       S9   Bypass operation   Inactive   Inactive   Inactive   Active   Inactive           AC Mains 1 not                   or failed           present           AC Mains 2           operation       S10   Load disconnect   Inactive   Inactive   Inactive   Inactive   Inactive           AC Mains 1 and 2                   or failed           not present                  
 
         [0028]     In Table 1, in states S1 through S4, the system  100  is operating in normal operation with the load supplied with conditioned power and with backup power available. In state S5, the system is in DC/stored energy mode of operation, and the load is supplied with conditioned power. In states S6 through S9, the system is operating in an overload or fault condition with the load being powered through a bypass switch from one of the input power sources. In state S10, the system is in a fault condition or missing AC sources with the batteries having been fully drained. In Tables 1 and 2 and in  FIG. 4 , reference is made to a preferred AC source. In embodiments of the present invention, one of multiple AC sources may be designated as a preferred source that will be used when multiple sources are available. The system  100  is configured to transfer between states to continue to provide power to a load. The state diagram of  FIG. 4  shows transfers between states that can occur, and Table 2 provides further description of the transfers shown in  FIG. 4 .  
                                     TABLE 2                           UPS 100 State Transfers            Trans-       Start-               fer   Condition for   ing   End           State   transfer   state   state   Output effect               T1a   Internal failure in   S1   S6   None except that load no       T1b   UPS   S2   S7   longer supplied with                       conditioned power       T2a   The non-   S1   S3   None       T2b   preferred AC   S2   S4           mains disappears       T3a   The preferred AC   S1   S5   None       T3b   mains disappears   S2   S5       T4a   The non-   S3   S1   None       T4b   preferred AC   S4   S2           mains reappears       T5a   The remaining   S3   S5   None if either mains reappear       T5b   AC mains   S4   S5   before DC source is drained           disappears       T6a   Internal failure in   S3   S8   None except that load no       T6b   UPS or overload   S4   S9   longer supplied with                       conditioned power       T7a   The preferred AC   S5   S3   None except that load is once       T7b   mains reappear   S5   S4   again supplied with                       conditioned power       T8a   Internal failure in   S5   S8   None except that load no       T8b   UPS or DC   S5   S9   longer supplied with           backup drained           conditioned power       T9   DC backup   S5   S10   Load dropped           drained       T10a   Overload   S8   S3   None except that load is once       T10b   disappears   S9   S4   again supplied with                       conditioned power       T11a   The remaining   S8   S5   None if either mains reappear       T11b   AC mains   S9   S5   before DC source is drained           disappears and           DC backup           possible       T12a   The remaining   S8   S10   Load dropped       T12b   AC mains   S9   S10           disappears and           DC backup not           possible       T13a   The preferred AC   S8   S7   None       T13b   mains reappears   S9   S6       T14a   The non-   S8   S6   None       T14b   preferred AC   S9   S7           mains disappears       T15a   The non-   S6   S8   None       T15b   preferred AC   S7   S9           mains reappears       T16a   The preferred AC   S6   S9   None       T16b   mains disappears   S7   S8       T17a   Failure in UPS   S6   S1   None except that load       T17b   corrected   S7   S2   supplied with                       conditioned power                  
 
         [0029]     For example, with reference to  FIG. 4  and Table 2, when the system is in steady state S1, and the preferred power source disappears, the system will perform transfer T3a to state S5 to operate in battery mode, and will then perform transfer T7b to operate in state S4 using the secondary AC source. As discussed below in greater detail, in some embodiments of the present invention, when transitioning from one AC input source to another, the DC mode (state S5) is used to provide a gradual transfer between power sources that may not be synchronous and may have different output voltage levels. The combination of transfers T3a and T7b can be used to accomplish this gradual transfer. In other embodiments of the present invention, transfers may occur directly between states S1 and S4 and between states S2 and S3.  
         [0030]     One of the most significant advantages of embodiments of power systems of the present invention is the flexibility to provide power under a number of fault conditions. In addition to the operating modes described above with reference to Tables 1 and 2, other modes of operation may be used with systems of the present invention. For example, when operating in normal mode (S1), and the UPS goes down, rather than transferring to state S6 as shown in the diagram, the system can transfer to state S7, operating in bypass mode with the second AC source. Similarly, when operating in normal mode (S2) and the UPS goes down, the system could transfer to state S6 operating in bypass mode with the first AC source.  
         [0031]     In the embodiment of the invention described above, input stages and the bypass switches of system  100  are shown as being devices separate from the UPS  108 . In other embodiments of the present invention, the input stages and the bypass switches may be incorporated within the chassis of a UPS that has multiple power inputs for receiving AC and/or DC power.  
         [0032]     In typical UPS systems when the input power source is changed from an AC power source to a DC power source or from a DC source back to an AC source, the change is usually an abrupt change that results in power being drawn from only one source before and after the changeover. In one UPS system  200 , described in copending U.S. patent application Ser. No. 10/470,124, titled Combined AC-DC to DC Converter, filed Jul. 25, 2003, assigned to the assignee of the present invention, and incorporated herein by reference, a smooth gradual transition of power between an AC source and a DC source can be achieved.  FIG. 5  shows a schematic diagram of an input portion of the UPS system  200 . The UPS system  200  has two input DC ports  203 ,  205 , two AC input ports  201 ,  207  and has two output DC ports  209  and  211 . The input DC ports may be internal to the UPS  200  and be coupled to a battery or batteries contained within the UPS  200 . The AC ports are designed to receive power from an external AC power source The output DC ports couple to a DC to AC converter (not shown) that provide output AC power from the UPS.  
         [0033]     The UPS  200  includes two diodes  202  and  204 , two thyristors  222  and  224 , a positive boost converter  213 , a negative boost converter  215 , and output capacitors  218  and  220 . The positive boost converter includes an inductor  206 , a transistor  208  and a diode  210 , and similarly, the negative boost converter includes an inductor  212 , a transistor  214  and a diode  216 .  
         [0034]     The UPS  200  operates as follows. In AC mode, AC voltage is rectified by diodes  202  and  204  to provide positive voltage at the input of the positive boost converter and negative voltage at the input of the negative boost converter. The positive boost converter generates a positive DC voltage across capacitor  218 , and the negative boost converter generates a negative DC voltage across capacitor  220 . A controller is used to control the boost converters to provide regulated output voltages. Upon interruption of the AC voltage, the controller provides a signal to thyristors  222  and  224  to allow positive and negative backup DC voltages from, for example, batteries to be applied to the inputs of the boost converters. Under control of the controller, the boost converters generate the regulated voltages across the output capacitors from the DC voltages.  
         [0035]     Upon return of the AC power source, as the AC voltage ramps up to the full voltage value, the UPS  200  provides a gradual transition from DC power back to AC power.  FIG. 6  shows current waveforms  251  and  252  through inductors  206  and  212  for different modes of operation of the UPS  200 . Waveform  253  represents the total current supplied by the AC source. During the period of time  254 , current is provided solely from the batteries. During the period of time  255 , a transition occurs from battery to the AC source, with the amount of current supplied by the batteries slowly decreasing, and the amount of current supplied by the AC source, slowly increasing. The controller, in conjunction with detection circuits incorporated into the UPS  200  ensures that the total current drawn is sufficient to meet load requirements while providing the transition from the batteries to the AC source. During the transition period  255 , the thyristors are controlled to allow battery current to be supplied to an inductor when there is no current from the AC source. The inductor  206  only receives AC current during the positive portion of the input AC waveform, and inductor  212  only receives AC current during the negative portion of the input AC waveform. DC battery current is supplied, as shown in  FIG. 6 , when AC current is not present. During period  256 , current is provided only from the AC source and not from the batteries.  
         [0036]     A second embodiment of the present invention of a system  300  for providing redundant power to loads includes input stages, incorporated within a UPS, that provide selective coupling to a plurality of power sources. The system  300  will now be described with reference to  FIGS. 7 and 8 .  FIG. 7  shows a block diagram of the system  300 , and  FIG. 8  shows a schematic diagram of a UPS  308  used in the system  300 . The system  300  includes the UPS  308 , AC inputs  302  and  304  for coupling to AC sources  303  and  305 , a system AC output  306 , and bypass switches  314  and  316 . The UPS and the bypass switch may be installed in a common chassis or rack designed as item  303  in  FIG. 7 . The bypass switches  314  and  316  are similar to bypass switches  114  and  116  discussed above, and in some embodiments may be incorporated within the UPS  308 . System  300  operates in a manner similar to system  100  described above, using the operational modes shown in Tables 1 and 2 and in  FIG. 4 . In system  300 , input stages are incorporated within the UPS  308 , while in system  200  the input stages are shown as discrete devices located external to the UPS  208 .  
         [0037]     The UPS  308  is coupled to the AC inputs  302  and  304  and to the AC output  306  and is designed to provide output power at the AC output derived from either one of the AC inputs or batteries within the UPS. The UPS  308  includes input thyristors  322 ,  324 ,  326 ,  328 ,  330  and  332 , a positive battery  321 , a negative battery  323 , a positive boost converter  307 , a negative boost converter  309 , output capacitors  318  and  320 , a DC to AC converter  319 , and a controller  334 . The controller may have several inputs to receive voltage and current signals representative of voltage and current levels at monitoring points in the system. In addition, the controller may be coupled to control bypass switches  314  and  316 . The positive boost converter includes an inductor  306 , a transistor  318  and a diode  310 . The negative boost converter includes an inductor  315 , a transistor  317 , and a diode  319 . Input thyristors  322 ,  324 ,  326 , and  328  under control of the controller  334  provide the function of the input stages discussed above to selectively couple the AC sources at the AC inputs to the boost converters of the UPS  308 . Input thyristor  330  is used to selectively couple battery  321  to the positive boost converter, and input thyristor  332  is used to selectively couple the negative battery  323  to the negative boost converter.  
         [0038]     The system  300 , including the UPS  308  operates in the modes of operation described above in Tables 1 and 2. In addition, the UPS  308  provides an additional feature of allowing for smooth transition from operating in battery mode to operating in AC mode from either an AC source at the first AC input or an AC source at the second AC input. In normal mode of operation (state S1), power at the first AC input, is coupled to the positive boost converter  307  through thyristor  326  for positive AC input voltage, and power at the first AC input is coupled to the negative boost converter  309  through thyristor  330  for negative AC input voltage. The positive boost converter generates a positive DC voltage across capacitor  318 , and the negative boost converter generates a negative DC voltage across capacitor  320 . The DC to AC converter converts the DC voltage across capacitors  318  and  320  into the output AC voltage.  
         [0039]     Upon failure of the AC source at input  1 , while in normal mode of operation (state S1), the controller turns on thyristors  330  and  332  to generate the output AC power from the batteries  321  and  323  (state S5). If AC power is available from a second AC source at input  304 , then to save battery life, the controller  334  transitions the power draw from the batteries to the second AC source in the manner described above with reference to  FIGS. 3 and 5  (state S4). If the first AC source becomes available once again, then the controller can provide a smooth transition from the second AC source to battery power (state S5) and from battery power back to the first AC source (state S1). For transitions from DC power to AC power, the current through inductors  306  and  315  will respectively follow the waveforms  251  and  252  during period  255  of  FIG. 6 . For transition from a live AC source to DC power, the current transition is the reverse of that shown in waveforms  251  and  252  during period  255  as the current transforms from AC to DC, or alternatively the transition from AC to DC may be an abrupt change.  
         [0040]     There are several advantages to the multiple input stage systems of embodiments of the present invention described above. The systems allow independent AC input sources to be used and the systems can tolerate independent frequency and phase variations of the AC input sources. In addition, in the system  308 , the input voltages of the AC input sources do not have to be the same. The boost converters can be controlled to allow for some differences in the input voltage and still provide a regulated DC voltage across the output capacitors.  
         [0041]     As discussed above, the system  300  allows transfer from one AC source to another AC source via DC (battery operation). This allows, for example, a generator set to be used as the second source of AC power, with the ability to provide for a smooth transfer from a first source of AC power to a generator set. The input stages of the system  300  allow the generator set to be permanently coupled to the UPS  308  without the need for additional switch gear. This allows the load to be supplied with conditioned power and backup power while running from a generator, which can be a significant advantage, as the frequency of the output voltage from a generator is not typically very stable. Further, the output voltages of generators are often sensitive to sudden load changes when transfer switches are used.  
         [0042]     The system  300  and UPS  308  of the embodiment described above includes two AC inputs. In other embodiments additional AC inputs, with associated input thyristors, may be added to increase the power availability of the system. Further, additional DC sources may be added to the system and be coupled to the UPS using thyristors, or other switches, as described above. In some embodiments that use multiple DC sources, transistors are used in place of the thyristors to allow different DC voltage levels to be used for the sources. In the system  300 , bypass switches  314  and  316  are shown external to the UPS of the system, in other embodiments, the bypass switches may be located internal to the UPS, and whether internal to the UPS or external to the UPS may be controlled by a controller of the UPS.  
         [0043]     The system  300  described above uses a split battery configuration that produces both a positive and a negative DC voltage. In other embodiments, only one battery may be used with a converter, for example a buck-boost converter, to generate a negative DC voltage or in some embodiments, only a positive DC voltage may be used. Also sources other than batteries may be used to provide DC power at DC inputs in embodiments of the present invention, and the DC sources, as well as the batteries may be external to the UPS.  
         [0044]     In embodiments of the invention described above, power systems having improved power availability are described. In at least some embodiments of the present invention, modular, redundant UPS&#39;s, such as those described in U.S. Pat. No. 5,982,652 may be used as the UPS in the power systems to provide additional power availability. In addition, embodiments of the present invention may be used with UPS&#39;s like that shown in  FIG. 1 , as well as other types of UPS&#39;s.  
         [0045]     In the embodiments described above, the input AC voltage is a single phase AC input. In other embodiments, three phase inputs and/or three phase outputs may be accommodated.  
         [0046]     In embodiments described above, thyristors are used to provide controlled switching of power sources to UPS&#39;s, in other embodiments, other types of switches may be used to perform the switching function, including transistors and other semiconductor devices.  
         [0047]     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements are intended to be within the scope and spirit of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention&#39;s limit is defined only in the following claims and the equivalents thereto.