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A UPS is adapted to receive power from a polyphase AC source. The UPS includes power conversion circuitry adapted to convert the power supplied by the polyphase AC source to DC power. The power conversion circuitry includes phase conductors. The UPS also includes a neutral coupled to an output of th

A UPS is adapted to receive power from a polyphase AC source. The UPS includes power conversion circuitry adapted to convert the power supplied by the polyphase AC source to DC power. The power conversion circuitry includes phase conductors. The UPS also includes a neutral coupled to an output of the UPS, a plurality of input capacitors coupling the phase conductors to the neutral, a DC system including a positive DC bus with a positive DC bus voltage and a negative DC bus with a negative DC bus voltage, and a control system. The control system is adapted to control a difference between a magnitude of the positive DC bus voltage and a magnitude of the negative DC bus voltage by controlling a DC voltage across the input capacitors.

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What is claimed is: 1. An uninterruptible power supply adapted to receive power from a polyphase AC source, the uninterruptible power supply comprising: power conversion circuitry adapted to convert the power supplied by the polyphase AC source to DC power, the power conversion circuitry including

What is claimed is: 1. An uninterruptible power supply adapted to receive power from a polyphase AC source, the uninterruptible power supply comprising: power conversion circuitry adapted to convert the power supplied by the polyphase AC source to DC power, the power conversion circuitry including phase conductors; a neutral coupled to an output of the uninterruptible power supply; a plurality of input capacitors coupling the phase conductors to the neutral; a DC system including a positive DC bus having a positive DC bus voltage and a negative DC bus having a negative DC bus voltage; and a control system adapted to control a difference between a magnitude of the positive DC bus voltage and a magnitude of the negative DC bus voltage by controlling a DC voltage across the input capacitors. 2. The uninterruptible power supply of claim 1, wherein the control system controls the DC voltage across the input capacitors based, at least in part, on a difference between a potential of the neutral and a potential of a neutral point of the polyphase AC source. 3. The uninterruptible power supply of claim 1, wherein the control system controls the DC voltage across the input capacitors based, at least in part, on a maximum allowable DC offset. 4. The uninterruptible power supply of claim 1, wherein the power conversion circuitry includes at least two controllers per phase of the polyphase AC source. 5. The uninterruptible power supply of claim 1, wherein the power conversion circuitry includes a plurality of controllers, wherein the control system includes a reference waveform generator that generates a reference waveform for at least one of the plurality of controllers, and wherein an amplitude of the reference waveform is based, at least in part, on a combined magnitude of the positive DC bus and the negative DC bus. 6. The uninterruptible power supply of claim 5, wherein a signal based, at least in part, on a difference between the magnitude of the positive DC bus voltage and the magnitude of the negative DC bus voltage is combined with the reference waveform. 7. The uninterruptible power supply of claim 5, wherein a signal based, at least in part, on a difference between a potential of the neutral and a potential of a neutral point of the AC source is combined with the reference waveform. 8. The uninterruptible power supply of claim 5, wherein a signal based, at least in part, on both a difference between the magnitude of the positive DC bus voltage and the magnitude of the negative DC bus voltage, and a difference between a potential of the neutral and a potential of a neutral point of the AC source is combined with the reference waveform. 9. The uninterruptible power supply of claim 5, wherein the reference waveform generator generates a separate reference waveform for each of the plurality of controllers. 10. The uninterruptible power supply of claim 1, wherein the control system further includes a first regulator that includes an input and an output, wherein a signal corresponding to the difference between the magnitude of the positive DC bus voltage and the magnitude of the negative DC bus voltage is supplied to the input, and wherein a reference signal corresponding to an adjustment to eliminate the difference is generated at the output of the first regulator. 11. The uninterruptible power supply of claim 10, wherein the control system further includes a second regulator that includes an input and an output, wherein the input of the second regulator is in communication with the output of the first regulator, and wherein a signal is generated at the output of the second regulator based on the reference signal and a difference between a potential of the neutral and a potential of a neutral point of the polyphase AC source. 12. The uninterruptible power supply of claim 11 wherein a control element is coupled to the output of the first regulator, wherein the control element compares a maximum reference signal with the reference signal, and wherein a signal less than or equal to the maximum reference signal is provided at the input of the second regulator when the reference signal is greater than the maximum reference signal. 13. The uninterruptible power supply of claim 11, wherein the control system further includes a digital signal processor and a memory, and wherein a first regulator operation and a second regulator operation are included in the memory. 14. The uninterruptible power supply of claim 11, wherein a bandwidth of the second regulator is at least ten times greater than a bandwidth of the first regulator. 15. The uninterruptible power supply of claim 11, wherein the first regulator and the second regulator employ proportional-integral control. 16. A method of controlling an operation of a polyphase uninterruptible power supply comprising power conversion circuitry, a neutral, and a DC system including the neutral, where the neutral is coupled to the power conversion circuitry and an output of the uninterruptible power supply, the method comprising acts of: determining an imbalance of the DC system; and responding to the imbalance by controlling a DC voltage across input capacitors coupling each of a plurality of phase conductors included in the power conversion circuitry to the neutral. 17. The method of claim 16, wherein the act of determining comprises an act of determining a difference in magnitude between a voltage of a positive DC bus and a voltage of a negative DC bus. 18. The method of claim 16, further comprising an act of eliminating the imbalance. 19. The method of claim 16, further comprising acts of: generating a first reference signal based, at least in part, on a combined magnitude of a positive DC bus voltage and a negative DC bus voltage; generating a second reference signal based, at least in part, on a difference in magnitude between the voltage of the positive DC bus and the voltage of the negative DC bus; and generating a third reference signal by combining the first reference signal and the second reference signal; and using the third reference signal to control operation of the power conversion circuitry. 20. The method of claim 19, further comprising an act of generating the second reference signal based, at least in part, on a difference between a potential of the neutral and a potential of a neutral point of the polyphase AC source. 21. The method of claim 20, wherein the act of determining the difference between the potential of the neutral and the potential of the neutral point of the polyphase AC source includes an act of adding instantaneous line voltages of the polyphase AC source and dividing the result of the addition by a quantity of lines provided by the polyphase AC source. 22. The method of claim 19, wherein the power conversion circuitry includes a plurality of controllers, the method further comprising acts of: generating a separate first reference signal for each of the plurality of controllers; generating a plurality of third reference signals by combining, for each third reference signal, one of the separate first reference signals with the second reference signal; and supplying each of the plurality of controllers with a different one of the plurality of third reference signals. 23. The method of claim 22, wherein each of the plurality of third reference signals is a reference waveform. 24. The method of claim 19, further comprising an act of determining whether the second reference signal is greater than a maximum value. 25. The method of claim 24, further comprising an act of adjusting the second reference signal when the second reference signal exceeds the maximum value. 26. The method of claim 19, wherein the uninterruptible power supply includes a first regulator with an input and an output and a second regulator with an input and an output, and wherein the method further comprises acts of: generating a signal at the output of the first regulator based, at least in part, on the difference in magnitude between the voltage of the positive DC bus and the voltage of the negative DC bus; communicating the signal generated at the output of the first regulator to the input of the second regulator; and generating the second reference signal at the output of the second regulator. 27. The method of claim 26, further comprising an act of combining the signal generated at the output of the first regulator with a signal corresponding to the difference between the potential of the neutral and the potential of the neutral point of the polyphase AC source before the act of communicating. 28. The method of claim 27, further comprising an act of determining whether the signal generated at the output of the first regulator exceeds a predetermined setpoint before the act of combining. 29. An uninterruptible power supply adapted to receive power from a polyphase AC source, the uninterruptible power supply comprising: power conversion circuitry adapted to convert the power supplied by the polyphase AC source to DC power at both a positive DC bus having a positive DC bus voltage and a negative DC bus having a negative DC bus voltage, the power conversion circuitry including phase conductors; a neutral coupled to an output of the uninterruptible power supply; a plurality of input capacitors coupling the phase conductors to the neutral; means for controlling a difference between a magnitude of the positive DC bus voltage and a magnitude of the negative DC bus voltage by controlling a DC voltage across the plurality of input capacitors. 30. The uninterruptible power supply of claim 29, wherein the means for controlling controls the DC voltage across the plurality of input capacitors based, at least in part, on a difference between a potential of the neutral and a potential of a neutral point of the polyphase AC source. 31. The uninterruptible power supply of claim 29, wherein the means for controlling controls the DC voltage across the plurality of input capacitors based, at least in part, on a maximum allowable DC offset. 32. The uninterruptible power supply of claim 29, wherein the power conversion circuitry includes a plurality of controllers, wherein the means for controlling includes a reference waveform generator that generates a reference waveform for at least one of the plurality of controllers, and wherein an amplitude of the reference waveform is based, at least in part, on a combined magnitude of the positive DC bus and the negative DC bus. 33. The uninterruptible power supply of claim 32, wherein a signal based, at least in part, on a difference between the magnitude of the positive DC bus voltage and the magnitude of the negative DC bus voltage is combined with the reference waveform. 34. The uninterruptible power supply of claim 32, wherein a signal based, at least in part, on a difference between a potential of the neutral and a potential of a neutral point of the AC source is combined with the reference waveform. 35. The uninterruptible power supply of claim 32, wherein a signal based, at least in part, on both a difference between the magnitude of the positive DC bus voltage and the magnitude of the negative DC bus voltage, and a difference between a potential of the neutral and a potential of a neutral point of the AC source is combined with the reference waveform. 36. The uninterruptible power supply of claim 32, wherein the reference waveform generator generates a separate reference waveform for each of the plurality of controllers. 37. The uninterruptible power supply of claim 29, wherein the means for controlling further includes a first regulator that includes an input and an output, wherein a signal corresponding to the difference between the magnitude of the positive DC bus voltage and the magnitude of the negative DC bus voltage is supplied to the input, and wherein a reference signal corresponding to an adjustment to eliminate the difference is generated at the output of the first regulator. 38. The uninterruptible power supply of claim 37, wherein the means for controlling further includes a second regulator that includes an input and an output, wherein the input of the second regulator is in communication with the output of the first regulator, and wherein a signal is generated at the output of the second regulator based on the reference signal and a difference between a potential of the neutral and a potential of a neutral point of the polyphase AC source. 39. The uninterruptible power supply of claim 38 wherein the means for controlling further includes a control element that is coupled to the output of the first regulator, wherein the control element compares a maximum reference signal with the reference signal, and wherein a signal less than or equal to the maximum reference signal is provided at the input of the second regulator when the reference signal is greater than the maximum reference signal. 40. The uninterruptible power supply of claim 38, wherein the means for controlling further includes a digital signal processor and a memory, and wherein a first regulator operation and a second regulator operation are included in the memory. 41. The uninterruptible power supply of claim 38, wherein a bandwidth of the second regulator is at least ten times greater than a bandwidth of the first regulator. 42. The uninterruptible power supply of claim 38, wherein the first regulator and the second regulator employ proportional-integral control.