Control of four-leg transformerless uninterruptible power supply

1. A system for managing a four-leg transformerless Uninterrupted Power Supply (UPS) comprising:

a rectifier modulation signal generator generating rectifier modulation signals;

a 3-leg inverter modulation signal generator generating inverter modulation signals;

a conversion module receiving said rectifier modulation signals and said inverter modulation signals and determining compensated rectifier modulated signals and 4 leg inverter modulation signals based on at least one of a measured rectified DC voltage and a frequency of said input AC signal;

a rectifier PWM signal generator receiving said compensated rectifier modulated signals generating PWM rectifier signals;

a 4-leg inverter PWM signal generator receiving said 4 leg inverter modulation signals generating PWM inverter signals, and a UPS comprising:

a rectifier section receiving an input AC signal and rectifying said input signal based on said Pulse Width Modulated (PWM) rectifier signals;

an inverter section receiving and inverting the rectified DC signals based on said Pulse Width Modulated inverter signals.

2. The system of claim 1 , wherein said conversion module comprises:

means for determining an input frequency of signals received from said input AC signal;

means for determining a 4 th leg modulation signal based on said input frequency and said 3 leg inverter modulation signals;

means for determining said 4 leg inverter modulation signals based on said 4 th modulation signal and said 3 leg inverter modulation signal.

3. The system of claim 2 , wherein said conversion module further comprises:

means for determining said compensated rectifier modulated signals based on received rectifier modulation signals and said determined 4 th leg modulation signal.

4. The system of claim 2 , wherein said means for determining a 4 th modulation signal comprises:

means for determining whether said input frequency is within a known range about a nominal frequency; and

means for setting said 4 th leg modulation signal to said nominal frequency and a level corresponding to a known proportion of a DC link voltage when said input is outside said known range about said nominal frequency.

5. The system of claim 2 , wherein said means for determining a 4 th leg modulation signal comprises:

means for determining whether said input frequency is within a known range about a nominal frequency; and

means for determining a level of said 4 th leg modulation signal based on a weighted percentage of said 3 leg inverter modulation signals.

6. The system of claim 5 , wherein said 4 th leg modulation signal is determined based on a minimum and a maximum of said 3 leg inverter modulation signals and a DC link voltage.

7. The system of claim 2 , wherein said 4 th modulation signal is used to generate a neutral from inverter's fourth leg.

8. The system of claim 1 , wherein said rectifier modulation signal generator receives a current reference, said current reference being determined based on a measured DC voltage being within a known range of a DC Link voltage.

9. The system of claim 1 , further comprising a neutral associated with said input AC signal.

10. A method for managing a four-leg transformerless Uninterrupted Power Supply (UPS) a rectifier section and a 4 leg inverter section, the rectifier section receiving compensated rectifier control signals and said 4 leg inverter receiving inverter control signals, said method comprising:

determining an input frequency of reference signals received from said AC input section;

determining a 4 th leg modulation signal based on said input frequency, and determining said inverter control signals based on said 3 leg inverter modulation signals and said determined 4 th leg modulation signal.

11. The method of claim 10 , further comprising:

determining said compensated rectifier control signals based on received rectifier modulation signals and said determined 4 th leg modulation signal.

12. The method of claim 10 , wherein said determining a 4 th modulation signal comprises:

determining whether said input frequency is within a known range about a nominal frequency; and

setting said modulation signal to said nominal frequency and a level corresponding to a known proportion of a DC link voltage when said input is outside said known range about said nominal frequency.

13. The method of claim 10 , wherein said determining a 4 th leg modulation signal comprises:

determining whether said input frequency is within a known range about a nominal frequency; and

determining a level of said 4 th leg modulation signal based on a weighted percentage of said 3 leg inverter modulation signals.

14. The method of claim 13 , wherein said 4 th leg modulation signal is determined based on a minimum and a maximum of said 3 leg inverter modulation signals and a DC link voltage.

15. The method of claim 10 , wherein said rectifier modulation signal generator receives a current reference, said current reference being determined based on a measured DC voltage being within a known range of a DC Link voltage.

16. The method of claim 10 , wherein said 4 th modulation signal is used to generate a neutral from inverter's fourth leg.

17. An apparatus for managing a four-leg transformerless Uninterrupted Power Supply (UPS) a rectifier section and a 4 leg inverter section, the rectifier section receiving compensated rectifier control signals and said 4 leg inverter receiving inverter control signals, said apparatus comprising:

a processor in communication with a memory, the memory including code which when accessed by the processor causes the processor to:

determine an input frequency of reference signals received from said AC input section;

determine a 4 th leg modulation signal based on said input frequency, and

determine said inverter control signals based on said 3 leg inverter modulation signals and said determined 4 th leg modulation signal.

18. The apparatus of claim 17 , said processor further accessing code to:

determine said compensated rectifier control signals based on received rectifier modulation signals and said determined 4 th leg modulation signal.

19. The apparatus of claim 17 , wherein said determining a 4 th leg modulation signal comprises:

determining whether said input frequency is within a known range about a nominal frequency; and

setting said modulation signal to said nominal frequency and a level corresponding to a known proportion of a DC link voltage when said input is outside said known range about said nominal frequency.

20. The apparatus of claim 17 , wherein said determining a 4 th leg modulation signal comprises:

determining whether said input frequency is within a known range about a nominal frequency; and

determining a level of said 4 th leg modulation signal based on a weighted percentage of said 3 leg inverter modulation signals.

21. The apparatus of claim 20 , wherein said 4 th leg modulated signal is determined based on a minimum and a maximum of said 3 leg inverter modulation signals and a DC link voltage.

22. The apparatus of claim 17 , wherein said rectifier modulation signal generator receives a current reference, said current reference being determined based on a measured DC voltage being within a known range of a DC Link voltage.

23. The apparatus of claim 17 , wherein said 4 th leg modulation signal is used to generate a neutral from inverter's fourth leg.