Intelligent level transition systems and methods for transformerless uninterruptible power supply

1. An uninterruptible power supply (UPS) system comprising:

a neutral-point-clamped inverter to supply power to a load; and

a controller configured to control the inverter to operate in a three-level mode of operation or higher when the power to the load is substantially balanced and to operate in a two-level mode of operation when the power to the load is substantially unbalanced, wherein the controller is configured to determine when the load is substantially balanced and substantially unbalanced based at least in part on a difference between voltage differences of capacitors, wherein the controller is configured to control the inverter to operate in a normal three-level mode when the difference between the voltage differences of the capacitors exceeds a first threshold and to operate in a modified three-level mode when the difference between the voltage differences of the capacitors exceeds the first threshold but not a second threshold.

2. The system of claim 1 , wherein the inverter is configured to connect to the load via a three-wire configuration.

3. The system of claim 1 , wherein the inverter is configured to connect to the load via a four-wire configuration.

4. The system of claim 3 , wherein the inverter comprises four legs, a first, second, and third of which provide power to the load and a fourth of which regulates a neutral current from the load when the load is substantially unbalanced.

5. The system of claim 4 , wherein the controller is configured to control the fourth leg to reduce the difference between the voltage differences of the capacitors disposed on opposite sides of a neutral point in the inverter.

6. The system of claim 4 , wherein the inverter comprises a reverse blocking IGBT or a static switch module, or both, between the fourth leg and a neutral point coupled to the first, second, and third legs and the controller is configured to control the reverse blocking IGBT or the static switch module, or both, to be open when the load is substantially balanced and to be closed when the load is substantially unbalanced.

7. The system of claim 1 , wherein the inverter comprises three legs and each of the three legs comprises four switches, wherein the controller is configured to variously switch the four switches of each leg on or off while the inverter is operating in the three-level mode and to variously switch only two of the four switches of each leg on and off while the remaining two of the four switches of each leg are maintained on while the inverter is operating in the two-level mode.

8. An uninterruptible power supply (UPS) system comprising:

a neutral-point-clamped inverter to supply power to a load; and

a controller configured to control the inverter to operate in a three-level mode or higher when the load is substantially balanced and to operate in a two-level mode when the load is substantially unbalanced, wherein the inverter comprises at least two capacitors disposed on opposite sides of a neutral point and the controller is configured to determine when the load is substantially balanced and substantially unbalanced based at least in part on a difference between voltage differences of the capacitors, wherein the controller is configured to control the inverter to operate in a normal three-level mode when the difference between the voltage differences of the capacitors exceeds a first threshold and to operate in a modified three-level mode when the difference between the voltage differences of the capacitors exceeds the first threshold but not a second threshold, and wherein the load is substantially unbalanced when the difference between voltage differences of the capacitors exceeds the second threshold.

9. The system of claim 8 , wherein the controller is configured to control the inverter to operate in a modified three-level mode when the difference between the voltage differences of the capacitors exceeds some threshold but the load otherwise remains substantially balanced, wherein the inverter is configured to control the inverter to operate in the modified three-level mode by selecting redundant space vector pulse width modulation states that charge or discharge the capacitors so as to reduce the difference between the voltage differences of the capacitors.

10. The system of claim 1 , wherein the controller is configured to determine that the load is substantially unbalanced when a difference between voltage differences of capacitors disposed on opposite sides of a neutral point in the inverter cannot be reduced by selecting redundant space vector pulse width modulation states that charge or discharge the capacitors so as to reduce the difference between the voltage differences of the capacitors.

11. The system of claim 1 , wherein the controller is configured to cause the inverter to operate in the two-level mode according to space vector pulse width modulation using only states defined as outer corner points of a space vector pulse width modulation hexagon.

12. An article of manufacture comprising:

one or more tangible, non-transitory machine-readable media at least collectively comprising machine-executable instructions, the instructions comprising:

instructions to receive a value of a first voltage difference associated with a first capacitor in a neutral-point-clamped inverter;

instructions to receive a value of a second voltage difference associated with a second capacitor in the neutral-point-clamped inverter;

instructions to determine a value of a difference between the first voltage difference and the second voltage difference;

instructions to determine, when the difference between the first voltage difference and the second voltage difference does not exceed a first threshold but not a second threshold, to operate the neutral-point-clamped inverter according to a three-level mode or higher; and

instructions to determine, when the difference between the first voltage difference and the second voltage difference exceeds the first threshold, to operate the neutral-point-clamped inverter according to a two-level mode.

13. The article of manufacture of claim 12 , wherein the instructions comprise:

instructions to pass the difference between the first voltage difference and the second voltage difference through a P I controller to obtain a DC offset value; and

instructions to modify the three-level mode so as to reduce the DC offset value.

14. The article of manufacture of claim 13 , wherein the instructions to modify the three-level mode comprise instructions to select from among redundant space vector pulse width modulation states so as to reduce the DC offset value.

15. The article of manufacture of claim 13 , wherein the instructions to modify the three-level mode comprise instructions to select between space vector pulse width modulation states POO and ONN, PPO and OON, OOP and NNO, POP and ONO, OPO and NON, and OPP and NOO so as to reduce the DC offset value.

16. The article of manufacture of claim 12 , wherein the instructions comprise instructions to determine, when the difference between the first voltage difference and the second voltage difference exceeds the first threshold, to operate a fourth leg of the neutral-point-clamped inverter to regulate a neutral current and increase utilization of a direct current (DC) bus supplying power to the neutral-point-clamped inverter.

17. The article of manufacture of claim 12 , wherein the instructions comprise instructions to control a reverse blocking IGBT or a static switch module, or both, to be open when the difference between the first voltage difference and the second voltage difference does not exceed the first threshold and to be closed when the difference between the first voltage difference and the second voltage difference exceeds the first threshold, wherein the reverse blocking IGBT or the static switch module, or both, are disposed between a fourth leg and a neutral point coupled to the first, second, and third legs of the neutral-point-clamped inverter.

18. The system of claim 1 , wherein the load is substantially unbalanced when the difference between voltage differences of the capacitors exceeds the second threshold.

19. The system of claim 1 , wherein the inverter comprises at least two capacitors disposed on opposite sides of a neutral point of the inverter.