Methods and apparatus for three-phase inverter with reduced energy storage

1. A circuit, comprising:

first and second energy sources coupled in series, the first energy source including a first DC-DC converter that generates a first modulated DC waveform on a first energy storage device coupled to the first energy source, the second energy source including a second DC-DC converter that generates a second modulated DC waveform on a second energy storage device coupled to the second energy source, wherein the first DC-DC converter is operably coupled in series with the second DC-DC converter to form a positive rail, a negative rail, and a neutral; and

a single multi-level inverter coupled to the positive rail, negative rail, and neutral, the multi-level inverter configured to unfold and sum the first and second modulated DC waveforms to produce three-phase sinusoidal output voltages.

2. The circuit according to claim 1 , wherein the first and second DC-DC converters comprise respective first and second current source DC-DC converters.

3. The circuit according to claim 2 , wherein the first and second energy storage devices are capacitive; the first energy storage device is coupled end-to-end across the positive rail and the neutral; and the second energy storage device is coupled end-to-end across the negative rail and the neutral.

4. The circuit according to claim 1 , wherein the three-phase output signals are at a frequency of about 60 Hz.

5. The circuit according to claim 1 , wherein the three-phase output signals are at a frequency of about 50 Hz.

6. The circuit according to claim 1 , wherein the three-phase output signals are at a frequency of about 400 Hz.

7. The circuit according to claim 1 , wherein the frequency of the three-phase output signals varies from zero to a predetermined maximum frequency.

8. The circuit according to claim 1 , wherein the inverter includes switches to commutate once per cycle of the output frequency.

9. The circuit according to claim 3 , wherein the voltages of the first and second capacitive energy storage devices are described as follows:

ωt

0-60°

60°-120°

120°-180°

180°-240°

240°-300°

300°-360°

V C1

V m sin(ωt + 120°)

V m sin(ωt − 60°)

V m sin(ωt)

−V m sin(ωt)

V m sin(ωt − 120°)

V m sin(ωt + 60°)

V C2

V m sin(ωt)

V m sin(ωt + 60°)

V m sin(ωt − 120°)

V m sin(ωt − 60°)

V m sin(ωt + 120°)

−V m sin(ωt)

where ω is the angular frequency of the fundamental output, V C1 is a voltage across the first capacitive energy storage and V C2 , is a voltage across the second capacitive energy storage, and V m , is an amplitude of the inverter output line-to-line voltages.

10. The circuit according to claim 1 , wherein the multi-level inverter output phase voltages are described as follows:

0-60°

60°-120°

120°-180°

180°-240°

240°-300°

300°-360°

V A

0

V C1

V C1

0

−V C2

−V C2

V B

−V C2

−V C2

0

V C1

V C1

0

V C

V C1

0

−V C2

−V C2

0

V C1

where ω is the angular frequency of the fundamental output, V A , V B , V C are multi-level inverter output phase voltages, V C1 is a voltage across the first capacitive energy storage and V C2 , is a voltage across the second capacitive energy storage, and V m is an amplitude of the inverter output phase voltages.

11. The circuit according to claim 1 , wherein the first and second energy sources are bidirectional.

12. The circuit of claim 1 , wherein the inverter operates at an output frequency and includes switches to commutate once per cycle of the output frequency according to the table that follows:

Q1

Q2

Q3

Q4

Q5

Q6

Q7

Q8

Q9

Q10

Q11

Q12

0-

X

X

X

X

X

X

60°

60°-

X

X

X

X

X

X

120°

120°-

X

X

X

X

X

X

180°

180°-

X

X

X

X

X

X

240°

240°-

X

X

X

X

X

X

300°

300°-

X

X

X

X

X

X

360

where an X in the table indicates a switch that is commutated.

13. A method comprising:

coupling first and second energy sources in series, the first energy source including a first DC-DC converter and the second energy source including a second DC-DC converter, wherein the first DC-DC converter is operably coupled in series with the second DC-DC converter to form a positive rail, a negative rail, and a neutral;

coupling first and second energy storage devices to the first and second energy sources, such that the first energy storage device is coupled end-to-end across the positive rail and the neutral, and the second energy storage device is coupled end-to-end across the negative rail and the neutral;

generating a first modulated DC voltage waveform on the first energy storage device and a second modulated DC voltage waveform on the second energy storage device;

coupling a single three-level inverter to the positive rail, negative rail, and neutral; and

unfolding and summing the first and second modulated DC waveforms to produce three-phase sinusoidal output voltages at outputs of the three-level inverter.

14. The method according to claim 13 , wherein the first and second energy sources are current sources.

15. The method according to claim 13 , wherein the first and second energy storage devices are capacitors.

16. The method according to claim 13 , wherein the three-phase output signals are at a frequency of about 60 Hz.

17. The method according to claim 13 , wherein the three-phase output signals are at a frequency of about 50 Hz.

18. The method according to claim 13 , wherein the three-phase output signals are at a frequency of about 400 Hz.

19. The method according to claim 13 , wherein the frequency of the three-phase output signals varies from zero to a predetermined maximum frequency.

20. The method according to claim 13 , wherein the voltages of the first and second energy storage devices are described as follows:

ωt

0-60°

60°-120°

120°-180°

180°-240°

240°-300°

300°-360°

V C1

V m sin(ωt + 120°)

V m sin(ωt − 60°)

V m sin(ωt)

−V m sin(ωt)

V m sin(ωt − 120°)

V m sin(ωt + 60°)

V C2

V m sin(ωt)

V m sin(ωt + 60°)

V m sin(ωt − 120°)

V m sin(ωt − 60°)

V m sin(ωt + 120°)

−V m sin(ωt)

where ω is the angular frequency of the fundamental output, V C1 is a voltage across the first energy storage and V C2 , is a voltage across the second energy storage, and V m is an amplitude of the inverter output line-to-line voltages.

21. The method according to claim 13 , wherein the inverter operates at an output frequency and includes switches to commutate once per cycle of the output frequency.

22. The method according to claim 13 , wherein the inverter operates at an output frequency and includes switches to commutate once per cycle of the output frequency according to the table that follows:

Q1

Q2

Q3

Q4

Q5

Q6

Q7

Q8

Q9

Q10

Q11

Q12

 0-60°

X

X

X

X

X

X

 60°-120°

X

X

X

X

X

X

120°-180°

X

X

X

X

X

X

180°-240°

X

X

X

X

X

X

240°-300°

X

X

X

X

X

X

300°-360°

X

X

X

X

X

X

where an X in the table indicates a switch is in the ON state.

23. The method according to claim 13 , wherein the multi-level inverter output phase voltages result from summation of two voltages V C1 , V C2 and are described as follows:

0-60°

60°-120°

120°-180°

180°-240°

240°-300°

300°-360°

V A

0

V C1

V C1

0

−V C2

−V C2

V B

−V C2

−V C2

0

V C1

V C1

0

V C

V C1

0

−V C2

−V C2

0

V C1

where ω is the angular frequency of the fundamental output, V A , V B, V C are multi-level inverter output phase voltages, V C1 is a voltage across the first energy storage and V C2 , is a voltage across the second energy storage, and V m is an amplitude of the inverter output phase voltages.

24. The method of claim 13 , further comprising:

configuring the single three-level inverter to unfold and sum the first and second modulated DC waveforms to produce the three-phase sinusoidal output voltages.

25. A method for generating a three-phase output voltage, the method comprising:

providing a first energy source comprising a first DC-DC converter that generates a first modulated DC waveform and a second energy source comprising a second DC-DC converter that generates a second modulated DC waveform, wherein each DC waveform comprises two or more segments, with each segment corresponding to a predetermined portion of a fundamental frequency sinusoid;

coupling the first energy source in series with the second energy source to form a positive rail, a neutral, and a negative rail;

coupling a first energy storage device between the positive rail and the neutral, such that the first energy source generates the first modulated DC waveform on the first energy storage device;

coupling a second energy storage device between the negative rail and the neutral, such that the second energy source generates the second modulated DC waveform on the second energy storage device;

coupling a multi-level inverter to the positive rail, neutral, and negative rail;

configuring the multi-level inverter to unfold and sum each of the first and second modulated DC waveforms in accordance with a predetermined switching scheme; and

generating three-phase sinusoidal output voltages at the multi-level inverter outputs.

26. The method of claim 25 , wherein the predetermined portion comprises a 60° segment of the fundamental frequency sinusoid.

27. The method of claim 25 , wherein the multi-level inverter operates at an output frequency and includes a plurality of switches configured to operate in accordance with the predetermined switching scheme, which involves commutating the switches once per cycle of the output frequency according to the following table:

Q1

Q2

Q3

Q4

Q5

Q6

Q7

Q8

Q9

Q10

Q11

Q12

0-

X

X

X

X

X

X

60°

60°-

X

X

X

X

X

X

120°

120°-

X

X

X

X

X

X

180°

180°-

X

X

X

X

X

X

240°

240°-

X

X

X

X

X

X

300°

300°-

X

X

X

X

X

X

360°

where an X in the table indicates a switch is in the ON state.

28. The method of claim 25 , further comprising distributing voltages associated with the first modulated DC waveform and the second modulated DC waveform between output phases of the multi-level inverter, to generate output voltages according to the table that follows:

0 -

60° -

120° -

180° -

240° -

300° -

60°

120°

180°

240°

300°

360°

V A

0

V C1

V C1

0

−V C2

−V C2

V B

−V C2

−V C2

0

V C1

V C1

0

V C

V C1

0

−V C2

−V C2

0

V C1

where ω is the angular frequency of the fundamental output, V A , V B , V C are the multi-level inverter output phase voltages, V C1 is a voltage across the first energy storage device and V C2 , is a voltage across the second energy storage device, and V m is an amplitude of the inverter output phase voltages.