Operation of a three level converter

1. A method of operating a three level converter comprising:

when a DC link voltage of the three level converter is lower than one-half of a rated DC link voltage of the converter, controlling the output voltage of the converter by, for each leg of the converter, alternately connecting an output terminal of the leg to the positive terminal or the negative terminal of the DC link; and

when the DC link voltage is at least one-half of the rated DC link voltage, controlling the output voltage of the converter by, for each leg, selectively connecting the output terminal of the converter to the positive terminal, the negative terminal, or a mid-point of the DC link;

wherein controlling the output voltage of the converter when the DC link voltage is lower than one-half the rated DC link voltage further comprises maintaining outer switching devices of the three level converter in an ON state while alternately switching inner switching devices of the three level converter to ON and OFF states.

2. The method of claim 1 , wherein switching pulses for the switching devices are generated by a pulse width technique (PWM) technique.

3. The method of claim 2 , wherein the PWM technique comprises sine triangular PWM or space vector PWM.

4. The method of claim 1 , further comprising providing DC power to the three level converter from a photovoltaic module, a fuel cell, or a battery.

5. A method of delivering solar power from a power generation system comprising a photovoltaic (PV) module, the method comprising:

providing DC power from the PV module to a three level converter;

when a DC link voltage of a three level converter is less than a one-half of a rated DC link voltage of the converter, controlling the output voltage of the converter by, for each leg of the converter, alternately connecting an output terminal of the leg to the positive terminal or the negative terminal of the DC link; and

when the DC link voltage is at least one-half of the rated DC link voltage controlling the output voltage of the converter by, for each leg, selectively connecting the output terminal of the converter to the positive terminal, the negative terminal, or a mid-point of the DC link;

wherein controlling the output voltage of the converter when the DC link voltage is lower than one-half the rated DC link voltage further comprises maintaining outer switching devices of the three level converter in an ON state while alternately switching inner switching devices of the three level converter to ON and OFF states.

6. A method of operating a three-level neutral point clamped (NPC) converter comprising first and second outer switching devices with first and second mechanical switches connected thereacross, first and second inner switching devices, first and second diodes and a split DC link having a first mid point connected across the two outer switching devices wherein the first outer and the first inner switching devices are connected in series at a second mid point, the first inner and the second inner switching devices are connected in series at a third mid point; the second inner and the second outer switching devices are connected in series at a fourth mid point, the first diode is connected between the first mid point and second mid point, and the second diode is connected in between the first mid point and the fourth mid point, the method comprising:

maintaining the outer switching devices and mechanical switches in an ON state while alternately switching the inner switching devices to ON and OFF states when a DC link voltage of a leg of the NPC converter is less than one-half of a rated DC link voltage of the NPC converter;

generating a positive output voltage when a DC link voltage of a leg of the NPC converter is at least one-half of the rated DC link voltage of the NPC converter by maintaining mechanical switches in OFF state and alternatively switching the first outer switching device and the second inner switching device to ON and OFF states while the first inner switching device and the second outer switching device are in ON state and OFF state respectively; and

generating a negative output voltage when a DC link voltage of a leg of the NPC converter is at least one-half of the rated DC link voltage of the NPC converter by maintaining mechanical switches in OFF state and alternatively switching the first inner switching device and second outer switching device to ON and OFF states while the first outer switching device and the second inner switching device are in OFF state and ON state respectively for negative output voltage.

7. The method of claim 6 , wherein switching pulses for at least some of the switching devices are generated by a pulse width technique (PWM) technique.

8. The method of claim 7 , wherein the PWM technique comprises sine triangular PWM or space vector PWM.

9. The method of claim 6 , wherein the three level converter comprises a single phase three level converter.

10. The method of claim 6 , wherein the three level converter comprises a three phase three level converter.

11. The method of claim 6 , wherein the switching devices are selected from the group consisting of IGBTs, IGCTs, MCTs, MTOs, MOSFETs and silicon carbide devices.

12. A three-level neutral point clamped (NPC) converter comprising

first and second outer switching devices and first and second inner switching devices, wherein the first inner and the second inner switching device are connected in series at a first mid point; the second inner and the second outer switching devices are connected in series at a second mid point and the first outer and the first inner switching device are connected in series at a third mid point;

first and second diodes and a split DC link having a fourth mid point connected across first and second outer switching devices wherein the first diode is connected between the fourth mid point and the third mid point, and the second diode is connected in between the fourth mid point and the second mid point;

a controller configured to provide control signals to maintain the outer switching devices in an ON state while alternately switching the inner switching devices to ON and OFF states when a DC link voltage of a leg of the NPC converter is less than one-half of a rated DC link voltage of the NPC converter;

wherein the controller is further configured to provide control signals when a DC link voltage of a leg of the NPC converter is at least one-half of the rated DC link voltage of the NPC converter to alternatively switch the first outer switching device and second inner switching device to ON states while the first inner switching device and the second outer switching device are in ON state and OFF state respectively to generate positive output voltage.

13. The three level NPC converter of claim 12 , wherein the controller is further configured to provide control signals when a DC link voltage of a leg of the NPC converter is at least one-half of the rated DC link voltage of the NPC converter to maintain the mechanical switches in OFF state and to alternatively switch the first inner switching device and the second outer switching device to ON and OFF states while the first outer switching device and the second inner switching device are in OFF state and ON state respectively to generate negative output voltage.

14. The three level NPC converter of claim 12 further comprising first and second mechanical switches connected across first and second outer switching devices.

15. The three level NPC converter of claim 14 , wherein the controller is configured to provide control signals to maintain the mechanical switches in an ON state when the DC link voltage of a leg of the NPC converter is less than one-half of the rated DC link voltage of the NPC converter.

16. The three level NPC converter of claim 14 , wherein the controller is configured to provide control signals to maintain the mechanical switches in an OFF state when the DC link voltage of a leg of the NPC converter is at least one-half of the rated DC link voltage of the NPC converter.

17. The three level NPC converter of claim 12 , wherein the three level converter comprises a single phase three level converter.

18. The three level NPC converter of claim 12 , wherein the three level converter comprises a three phase three level converter.

19. The three level NPC converter of claim 12 , wherein the switching devices are selected from the group consisting of IGBTs, IGCTs, MCTs, MTOs, MOSFETs and silicon carbide devices.