Transformer-less unified power flow controller

• US 9,294,003 B2
• Filed: 02/22/2013
• Issued: 03/22/2016
• Est. Priority Date: 02/24/2012
• Status: Active Grant

• Current Assignee: The Board of Trustees of Michigan State University (Michigan State University)
• Original Assignee: The Board of Trustees of Michigan State University (Michigan State University)
• Inventors: Peng, Fang Zheng
• Primary Examiner(s): Dole, Timothy J
• Assistant Examiner(s): Tiku, Sisay G
• Application Number:
  US14/380,631
• Publication Number:
  US 20150029764A1
• Time in Patent Office: 1,124 Days
• Field of Search: 363/37, 363/136, 363/137, 363/132, 363/65, 363/71, 363/72
• US Class Current: 1/1
• CPC Class Codes: H02J 3/1814 wherein al least one reactive element is actively controlled by a bridge converter, e.g. unified power flow controllers [UPFC]
• CPC Class Codes: H02J 3/1857 wherein such bridge converter is a multilevel converter
• CPC Class Codes: H02M 5/458 using semiconductor devices only
• CPC Class Codes: H02M 5/4585 having a rectifier with controlled elements
• CPC Class Codes: H02M 7/4835 comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage
• CPC Class Codes: H02M 7/49 Combination of the output voltage waveforms of a plurality of converters
• CPC Class Codes: Y02E 40/10 Flexible AC transmission systems [FACTS]
• CPC Class Codes: Y02E 40/20 Active power filtering [APF]

Abstract

A power control device includes a first terminal, a second terminal connected to a transmission line, a first cascade multilevel inverter (CMI) and a second CMI. The first CMI is connected to the second terminal. The second CMI connected in series between the first terminal and the second terminal.

Claims

1. A power control device comprising:

a first terminal;

a second terminal connected to a transmission line;

a first cascade multilevel inverter (CMI) connected to the second terminal;

a second CMI connected in series between the first terminal and the second terminal; and

a control module that controls the first CMI to cause a voltage phasor of the first CMI and a current phasor of the first CMI to approach a perpendicular angle to each other.

2. The power control device of claim 1 wherein the control module controls the first CMI to regulate average real power into and out from the second CMI.

3. The power control device of claim 2 wherein the control module controls the first CMI to apply a current to the second terminal to regulate average real power into and out from the second CMI.

4. The power control device of claim 3 wherein the control module (i) calculates a compensation current to regulate the average real power into and out from both the second CMI and the first CMI and (ii) controls the first CMI to apply the compensation current.

5. The power control device of claim 2 wherein the control module controls the second CMI to provide a commanded real power and a commanded reactive power from the first terminal to the second terminal.

6. The power control device of claim 2 wherein the control module controls the second CMI to provide a commanded real power and a commanded reactive power from the second terminal to the first terminal.

7. The power control device of claim 1 wherein the control module controls the first CMI to cause (i) the voltage phasor of the first CMI and the current phasor of the first CMI to approach a perpendicular angle to each other and (ii) a voltage phasor of the second CMI and a current phasor of the second CMI to approach a perpendicular angle to each other.

8. The power control device of claim 1 wherein the control module (i) determines a reference voltage based on a commanded real power and a commanded reactive power and (ii) controls the second CMI to create the reference voltage across the second CMI.

9. The power control device of claim 8 wherein the control module controls the second CMI in synchronization with an electrical grid to which the power control device is connected.

10. The power control device of claim 8 wherein the control module (i) determines a desired compensation current and (ii) controls the first CMI to generate the desired compensation current.

11. The power control device of claim 10 wherein the control module measures a current through the first CMI and controls the first CMI to reduce a difference between the measured current through the first CMI and the desired compensation current.

12. The power control device of claim 10 wherein the control module determines the desired compensation current based on a current through the transmission line and a current through the second CMI.

13. The power control device of claim 12 wherein the control module calculates the current through the second CMI based on the reference voltage, a voltage at the first terminal, and the current through the transmission line.

14. The power control device of claim 1 wherein the first CMI includes a plurality of bridge modules connected in series, wherein each of the plurality of bridge modules (i) has a first terminal and a second terminal and (ii) is controllable to produce one of a plurality of voltages between the first and second terminals of the bridge module.

15. The power control device of claim 14 wherein each of the plurality of bridge modules is controllable to produce one of two voltages between the first and second terminals of the bridge module.

16. The power control device of claim 1 wherein the second CMI includes a plurality of bridge modules connected in series, wherein each of the plurality of bridge modules has a first terminal and a second terminal and is controllable to produce one of a plurality of voltages between the first and second terminals of the bridge module.

17. The power control device of claim 1 wherein the first CMI is controlled to source zero current to the second terminal and sink zero current from the second terminal.

18. The power control device of claim 1 wherein the control module controls the first CMI to generate a desired reactive power into the second terminal.

19. A method of operating a power control device, the method comprising:

controlling a first cascade multilevel inverter (CMI) to (i) regulate average real power dissipated into and out from the first CMI and (ii) regulate average real power into and out from a second CMI;

controlling the second CMI to provide a commanded real power and a commanded reactive power from a first terminal of the power control device to a second terminal of the power control device, wherein the second CMI is connected in series between the first terminal and the second terminal, and wherein the first CMI is connected to the second terminal;

determining a reference voltage based on the commanded real power and the commanded reactive power; and

controlling the second CMI to create the reference voltage across the second CMI.

20. The method of claim 19 further comprising:

calculating a compensation current to regulate the average real power into and out from both the second CMI and the first CMI; and

controlling the first CMI to apply the compensation current to the second terminal.

21. The method of claim 19 further comprising selectively controlling the second CMI to provide the commanded real power and the commanded reactive power from the second terminal to the first terminal.

22. The method of claim 19 further comprising controlling the first CMI to cause a voltage phasor of the first CMI and a current phasor of the first CMI to approach a perpendicular angle to each other.

23. The method of claim 22 further comprising controlling the first CMI to cause (i) the voltage phasor of the first CMI and the current phasor of the first CMI to approach a perpendicular angle to each other and (ii) a voltage phasor of the second CMI and a current phasor of the second CMI to approach a perpendicular angle to each other.

24. The method of claim 19 further comprising controlling the second CMI in synchronization with an electrical grid to which the power control device is connected.

25. The method of claim 19 further comprising:

determining a desired compensation current; and

controlling the first CMI to generate the desired compensation current.

26. The method of claim 25 further comprising:

measuring a current through the first CMI; and

controlling the first CMI to reduce a difference between the measured current through the first CMI and the desired compensation current.

27. The method of claim 25 further comprising determining the desired compensation current based on (i) a current through a transmission line connected to the second terminal and (ii) a current through the second CMI.

28. The method of claim 27 further comprising calculating the current through the second CMI based on the reference voltage, a voltage at the first terminal, and the current through the transmission line.

29. The method of claim 19 wherein:

the first CMI includes a plurality of bridge modules connected in series, wherein each of the plurality of bridge modules has a first terminal and a second terminal and is controllable to produce one of a plurality of voltages between the first and second terminals of the bridge module; and

controlling the first CMI includes selecting one of the plurality of voltages for each of the plurality of bridge modules.

30. The method of claim 19 wherein:

the second CMI includes a plurality of bridge modules connected in series, wherein each of the plurality of bridge modules has a first terminal and a second terminal and is controllable to produce one of a plurality of voltages between the first and second terminals of the bridge module; and

controlling the second CMI includes selecting one of the plurality of voltages for each of the plurality of bridge modules.

31. The method of claim 19 further comprising controlling the first CMI to generate a desired reactive power into the second terminal.

32. A power control device comprising:

a first cascade multilevel inverter (CMI) connected in series between a first terminal of a first 3-phase line and a first terminal of a second 3-phase line;

a second CMI connected in series between a second terminal of the first 3-phase line and a second terminal of the second 3-phase line;

a third CMI connected in series between a third terminal of the first 3-phase line and a third terminal of the second 3-phase line;

a fourth CMI having a first terminal connected to the first terminal of the second 3-phase line;

a fifth CMI having a first terminal connected to the second terminal of the second 3-phase line; and

a sixth CMI having a first terminal connected to the third terminal of the second 3-phase line, wherein;

the fourth CMI has a second terminal that is connected to the first terminal of the fifth CMI,

the fifth CMI has a second terminal that is connected to the first terminal of the sixth CMI, and

the sixth CMI has a second terminal that is connected to the first terminal of the fourth CMI.

33. A power control device comprising:

a first cascade multilevel inverter (CMI) connected in series between a first terminal of a first 3-phase line and a first terminal of a second 3-phase line;

a second CMI connected in series between a second terminal of the first 3-phase line and a second terminal of the second 3-phase line;

a third CMI connected in series between a third terminal of the first 3-phase line and a third terminal of the second 3-phase line;

a fourth CMI having a first terminal connected to the first terminal of the second 3-phase line;

a fifth CMI having a first terminal connected to the second terminal of the second 3-phase line; and

a sixth CMI having a first terminal connected to the third terminal of the second 3-phase line,

wherein the fourth CMI has a second terminal that is connected to a second terminal of the fifth CMI and a second terminal of the sixth CMI.

34. The power control device of claim 33 wherein the second terminals of the fourth, fifth, and sixth CMIs are connected to a reference potential.

35. A power control device comprising:

a first cascade multilevel inverter (CMI) connected in series between a first terminal of a first 3-phase AC line and a first terminal of a second 3-phase AC line;

a second CMI connected in series between a second terminal of the first 3-phase AC line and a second terminal of the second 3-phase AC line;

a third CMI connected in series between a third terminal of the first 3-phase AC line and a third terminal of the second 3-phase AC line;

a fourth CMI having a first terminal connected to the first terminal of the second 3-phase AC line;

a fifth CMI having a first terminal connected to the second terminal of the second 3-phase AC line; and

a sixth CMI having a first terminal connected to the third terminal of the second 3-phase AC line,

wherein each CMI of the first CMI, the second CMI, the third CMI, the fourth CMI, the fifth CMI, and the sixth CMI comprises;

a plurality of bridge modules connected in series,

wherein each of the plurality of bridge modules has a first terminal and a second terminal and is controllable to produce one of a plurality of voltages between the first and second terminals of the bridge module.