Apparatus and a method for voltage conversion

• US 6,507,503 B2
• Filed: 08/07/2001
• Issued: 01/14/2003
• Est. Priority Date: 05/17/2001
• Status: Expired due to Term

• Current Assignee: ABB AB (ABB Limited)
• Original Assignee: ABB AB (ABB Limited)
• Inventors: Norrga, Staffan
• Primary Examiner(s): HAN, YOUNGHUIE JESSICA
• Application Number:
  US09/922,649
• Publication Number:
  US 20020176261A1
• Time in Patent Office: 525 Days
• Field of Search: 363/17, 363/81, 363/95, 363/98, 363/125, 363/131, 363/132, 363/34, 363/35, 363/37
• US Class Current: 363/17
• CPC Class Codes: H02M 7/758 with automatic control of output waveform or frequency

Abstract

An apparatus for converting direct voltage into alternating voltage and conversely comprises a VSC-converter (8) having a direct voltage intermediate link (9) and at least one phase leg (12, 13). Each current valve (14-17) of the phase legs has at least one semiconductor device of turn-off type and a rectifying member connected in anti-parallel therewith. A transformer (19) has two opposite ends of a first winding (20) thereof connected to an output (21, 22) each of the VSC-converter and a second winding (23) connected to a direct converter having at least one phase leg. Each of the current valves of the direct converter being able to conduct current and block voltage in both directions and to turn on by gate control. A midpoint (27) of the phase leg of the direct converter is provided with a phase output for forming a terminal for the alternating phase voltage between this output and a further phase output (28).

Claims

1. An apparatus for converting direct voltage into alternating voltage and conversely comprising a VSC-converter having a direct voltage intermediate link with a positive and a negative pole and at least one phase leg interconnecting the two poles and having at least two current valves connected in series, each current valve having at least one semiconductor device of turn-off type and a rectifying member connected in anti-parallel therewith, the apparatus further comprising a transformer with two opposite ends of a first winding thereof connected to an output each of the VSC-converter and with a second winding thereof connected to an arrangement adapted to form voltage pulses for forming an alternating phase voltage, the apparatus also comprising a unit adapted to control the VSC-converter and said arrangement for obtaining said voltage conversion, wherein the VSC-converter comprises at least one snubber capacitor connected to said current valves thereof, said arrangement comprises a direct converter having at least one phase leg connected through the opposite ends thereof to opposite ends of said second winding of the transformer and having at least two current valves connected in series, each of these current valves being able to conduct current and block voltage in both directions and to turn on by gate control, and a midpoint of said phase leg of the direct converter being provided with a phase output for forming a terminal for said alternating phase voltage between this output and a further phase output of the direct converter.

2. An apparatus according to claim 1, wherein said current valves of the VSC-converter each comprises said snubber capacitor connected in parallel with said semiconductor device and rectifying member.

3. An apparatus according to claim 2, wherein starting from a switching state in which the midpoint of said phase leg of the VSC-converter is connected to a first pole of the direct voltage intermediate link said control unit is adapted to turn the semiconductor device of the current valve connecting the output to said first pole off for charging the snubber capacitor in parallel therewith and connecting said midpoint through the other current valve to the other, second pole of the direct voltage intermediate link for changing the sign of the voltage across said first transformer winding.

4. An apparatus according to claim 1, wherein said VSC-converter has two of said phase legs and said outputs connected to the ends of said first transformer winding are formed by a midpoint between current valves of phase leg each.

5. An apparatus according to claim 4, wherein said control unit is adapted to commutate one phase leg of the VSC-converter at a time starting from a state in which the two midpoints are connected to different poles of the direct voltage intermediate link for obtaining an intermediate state in which said midpoints are connected to the same pole for applying a zero-voltage to the first winding of the transformer.

6. An apparatus according to claim 5, wherein said control unit is adapted to varying the order in which the phase legs of the VSC-converter are commutated.

7. An apparatus according to claim 4, wherein said control unit is adapted to control the semiconductor devices of the current valves of the VSC-converter for commutating both phase legs at the same time starting from the state in which the two midpoints are connected to different poles of the direct voltage intermediate link through a conducting semiconductor device each by turning these semiconductor devices of both said valves off.

8. An apparatus according to claim 4, wherein the VSC-converter comprises one said snubber capacitor interconnecting said midpoints between current valves of the two phase legs.

9. An apparatus according to claim 1, wherein the VSC-converter has one said phase leg, that one of said outputs connected to the ends of said first transformer winding is formed by a midpoint between current valves of said phase leg, and the output connected to the opposite end of the first transformer winding is formed by a midpoint of the direct voltage intermediate link separated from both said positive and negative poles by at least one capacitor.

10. An apparatus according to claim 9, wherein the VSC-converter comprises one said snubber capacitor interconnecting said midpoint between the current valves and said midpoint of the direct voltage intermediate link.

11. An apparatus according to claim 1, wherein the valves of the direct converter comprise semiconductor devices adapted to be turned off and thereby turn off the valve by zero-crossing of the current through the semiconductor devices.

12. An apparatus according to claim 11, wherein said current valves of the direct converter are adapted to turn off upon forcing the current through these valves down to zero as a result of events in an external circuit to which these valves are adapted to be connected.

13. An apparatus according to claim 12, wherein each said current valve of the direct converter comprises two reverse-blocking controllable second valves connected in anti-parallel.

14. An apparatus according to claim 13, wherein each said second valve comprises a single reverse-blocking controllable semiconductor device, e.g. a thyristor.

15. An apparatus according to claim 14, wherein said single reverse-blocking controllable semiconductor device has silicon carbide as base material.

16. An apparatus according to claim 13, wherein each said second valve comprises a series connection of a semiconductor device that can be turned on by gate control and a rectifying member, such as a diode.

17. An apparatus according to claim 16, interconnection points between the semiconductor device and the rectifying member in the two second valves are directly interconnected.

18. An apparatus according to claim 16, wherein said rectifying members of the valves of the direct converter are diodes based on a material having a wide energy gap between the valence band and the conduction band, i.e. a band gap exceeding 2 eV.

19. An apparatus according to claim 18, said diodes are based on silicon carbide.

20. An apparatus according to claim 16, wherein said semiconductor device in each second valve is one of an insulated gate bipolar transistor (IGBT), a gate turn-off thyristor (GTO), a gate commutated thyristor (GCT), an integrated gate commutated thyristor (IGCT), a MOS controlled thyristor (MCT), a MOSFET and a JFET.

21. An apparatus according to claim 1, wherein the VSC-converter and/or the direct converter comprises at least one phase leg having a plurality of current valves on each side of said midpoint of the phase leg for together holding a voltage to be held in a blocking state the phase leg part they belong to.

22. An apparatus according to claim 1, wherein the current valves of the direct converter are equipped with appropriate snubber circuits in order to prevent overvoltages due to reverse recovery processes during turn off of said valves.

23. An apparatus according to claim 1, wherein said control unit is adapted to control the semiconductor devices of the VSC-converter for changing the switching state of this converter, by changing the connection of at least one of said outputs thereof from one pole of said direct voltage intermediate link to the other while charging and discharging said at least one snubber capacitor for lowering the voltage derivatives during turn-off of a semiconductor device.

24. An apparatus according to claim 23, wherein said control unit is adapted to turn on semiconductor devices of the current valves of the VSC-converter when a current flows through the rectifying member of the valve in question for turning the semiconductor device on at substantially zero-voltage thereacross and zero-current therethrough.

25. An apparatus according to claim 23, wherein said control unit is adapted to control the conducting current valves of the phase legs of the VSC-converter to turn off for commutating the output of those phase legs and at the same time, starting from a state in which the output of a phase leg of the direct converter is connected to a first end of the second transformer winding through a conducting first current valve, control the other, second current valve of that phase leg to turn on for short-circuiting the second transformer winding through that phase leg for opening a current path through that winding of the transformer in the direction of the voltage across the transformer so as to form a resonance-circuit by the capacitance of the at least one snubber capacitor of the VSC-converter and the leakage inductance of the transformer making the current through said first transformer winding increasing for assisting the commutation of said phase leg of the VSC-converter by charging and discharging said snubber capacitor(-s).

26. An apparatus according to claim 25, wherein the control unit is adapted to turn one or two semiconductor devices in the VSC-converter that are carrying current off when the current through the transformer has increased to a certain value as a consequence of the opening of said current path for initiating a resonant process recharging the at least one snubber capacitor and by that transferring the potential of the phase terminal of the phase leg or the phase legs to an opposite direct voltage pole, and that the control unit is adapted to then, after the rectifying members that initially blocked the direct voltage have taken over the current, turn the semiconductor devices being anti-parallel to the latters on at zero-voltage and zero-current conditions.

27. An apparatus according to claim 25, comprising an additional inductor connected in series with said transformer for increasing the inductance of said resonance circuit.

28. An apparatus according to claim 23, wherein the control unit is adapted to control the semiconductor devices of the VSC-converter for changing the switching state thereof and start commutating one or several phase legs of the direct converter by controlling a current valve of that (those) phase leg(s) before the change of switching state of the VSC-converter has been completed, when there is a desire to have a power flow through the apparatus from the direct voltage side to the alternating voltage side, i.e. to have the same polarity of the voltage pulses on said terminal of the alternating voltage side as the current there.

29. An apparatus according to claim 28, wherein it comprises means for detecting the voltage across said transformer, and that the control unit, assuming there is a desire to have a power flow through the apparatus from the direct voltage side to the alternating voltage side, is adapted to start the commutation of the direct converter based upon information from said voltage detecting means when, as a consequence of the change of switching state of the VSC-converter commenced, the voltage across the first transformer winding has changed sign and exceeded a certain threshold voltage value.

30. An apparatus according to claim 23, wherein the control unit is adapted to commutate all phase legs of the direct converter by controlling the current valves of the phase legs and start controlling the semiconductor devices of the VSC-converter for changing the switching state thereof before the commutation of all phase legs of the direct converter has been completed, when there is a desire to have a power flow through the apparatus from the alternating voltage side to the direct voltage side, i.e. to have a polarity of the voltage pulses on said terminal of the alternating voltage side being opposite to the polarity of the current there.

31. An apparatus according to claim 30, comprising means for detecting the current through one of the transformer windings, and that the control unit, assuming there is a desire to have a power flow through the apparatus from the alternating voltage side to the direct voltage side, is adapted to start the control of the VSC-converter for changing the switching state thereof based upon information from said current detecting means when, as a consequence of the commutation of the phase leg or the phase legs of the direct converter commenced, the current through the second transformer winding has changed direction and exceeded a certain threshold current value.

32. An apparatus according to claim 1, wherein said control unit is adapted to commutate one or several phase legs of the direct converter when the power flow in the apparatus is directed from the alternating voltage side to the direct voltage side, i.e. from the direct converter to the VSC-converter, by controlling the current valves of that phase leg or those phase legs for changing the connection of the output thereof from one end of said second transformer winding to the other for changing the direction of the current through said second transformer winding enabling a change of the switching state of the VSC-converter.

33. An apparatus according to claim 32, wherein starting from a state in which the output of a phase leg of said direct converter is connected to a first end of the second transformer winding through a conducting first current valve said control unit is adapted to turn the other, second current valve of that phase leg on for short-circuiting the phase leg for opening a current path through the winding of the transformer in the direction of the voltage across the transformer, so that the second current valve gradually takes over the current through the transformer and the first current valve turns off as the current through it goes to zero.

34. An apparatus according to claim 32, wherein said control unit is adapted to control the current valves of the direct converter for obtaining a desired pulse width modulation pattern for said alternating phase voltage on said terminal.

35. An apparatus according to claim 1, wherein said control unit is adapted to a) control the semiconductor devices of the VSC-converter for changing the switching state of this converter by changing the connection of at least one of said outputs thereof from one pole of said direct voltage intermediate link to the other for changing the sign of the voltage across said first transformer winding and b) commutate the phase leg or phase legs of the direct converter for changing the end of the second transformer winding to which the respective phase output is connected in such a sequence and at such delays that desired voltage pulses are obtained on said terminal and do this until the current through the second transformer winding has changed direction, and then start over with controlling the VSC-converter to change switching state again.

36. An apparatus according to claim 1, wherein said direct converter has only one said terminal for said alternating phase voltage.

37. An apparatus according to claim 36, wherein said direct converter has two said phase legs and said two phase outputs forming said terminal are formed by a midpoint between the current valves of a phase leg each.

38. An apparatus according to claim 37, wherein a midpoint of the second winding of the transformer is provided with a connection for grounding purposes.

39. An apparatus according to claim 36, wherein the direct converter has one said phase leg, one of said phase outputs is formed by a midpoint between the current valves of said phase leg, and the other phase output is formed by a midpoint of said second transformer winding located between said opposite ends of that winding.

40. An apparatus according to claim 36, wherein the control unit is adapted to control the current valves of the direct converter so as to short-circuit these two phase outputs, i.e. connecting them to the same end or ends of said second transformer winding, and letting them stay in this state for forming a zero-voltage interval at the terminal for the alternating phase voltage.

41. An apparatus according to claim 1, wherein the semiconductor device and the rectifying member of the respective valve of the VSC-converter are integrated in one and the same semiconductor device, e.g. a MOSFET with an inherent body diode.

42. An apparatus for converting a direct voltage into direct voltage comprising a VSC-converter having a direct voltage intermediate link with a positive and a negative pole and at least one phase leg interconnecting the two poles and having at least two current valves connected in series, each current valve having at least one semiconductor device of turn-off type and a rectifying member connected in anti-parallel therewith, the apparatus further comprising a transformer with two opposite ends of a first winding thereof connected to an output each of the VSC-converter and with a second winding thereof connected to an arrangement adapted to form a direct voltage, the apparatus also comprising a unit adapted to control the VSC-converter and said arrangement for obtaining said voltage conversion, wherein the VSC-converter comprises at least one snubber capacitor connected to said current valves, said arrangement comprises a direct converter having at least one phase leg connected through the opposite ends thereof to opposite ends of said second winding of the transformer and having at least two current valves connected in series, each of these current valves being able to conduct current and block voltage in both directions and to turn on by gate control, and a midpoint of said phase leg of the direct converter is provided with an output for forming a terminal for said direct voltage between this output and a further output of the direct converter.

43. A method for converting direct voltage into alternating voltage and conversely through an apparatus comprising a VSC-converter having a direct voltage intermediate link with a positive and a negative pole and at least one phase leg interconnecting the two poles and having at least two current valves connected in series, each current valve having at least one semiconductor device of turn-off type and a rectifying member connected in anti-parallel therewith, the apparatus further comprising a transformer with two opposite ends of a first winding thereof connected to an output each of the VSC-converter and with a second winding thereof connected to an arrangement adapted to form voltage pulses for forming an alternating phase voltage, said VSC-converter and said arrangement being controlled for obtaining said voltage conversion, wherein the control is carried out for an apparatus in which the VSC-converter comprises at least one snubber capacitor connected to said current valves thereof, said arrangement comprising a direct converter having at least one phase leg connected through the opposite ends thereof to opposite ends of said second winding of the transformer and having at least two current valves connected in series, each of these current valves being able to conduct current and block voltage in both directions and to turn on by gate control, a midpoint of said phase leg of the direct converter being provided with a phase output for forming a terminal for said alternating phase voltage between the output and a further phase output of the direct converter, and that the control comprises the steps of:

a) controlling the semiconductor devices of the VSC-converter for changing the switching state of this converter by changing the connection of at least one of said outputs thereof from one pole of the direct voltage intermediate link to the other for changing the sign of the voltage across said first transformer winding,

b) commutate the phase leg or phase legs of the direct converter for changing the end of the second transformer winding to which the respective phase output is connected in such a sequence and at such delays that desired voltage pulses are obtained on said terminal and doing this until the current through the second transformer winding has changed direction, and

c) starting over with controlling the VSC-converter to change switching state again.

44. A method according to claim 43, it is carried out for a said apparatus in which said current valves of the VSC-converter each comprises a said snubber capacitor connected in parallel with said semiconductor device and rectifying member.

45. A method according to claim 44, wherein the VSC-converter has one said phase leg, one of said outputs connected to the ends of said first transformer winding is formed by a midpoint between current valves of said phase leg and the output connected to the opposite end of the first transformer winding is formed by a midpoint of the direct voltage intermediate link separated from both said positive and negative poles by at least one capacitor, and wherein starting from a switching state in which the midpoint of said phase leg of the VSC-converter is connected to a first pole of the direct voltage intermediate link the semiconductor device of the current valve connecting the output to said first pole is turned off for charging the snubber capacitor in parallel therewith and connecting said midpoint through the other current valve to the other, second pole of the direct voltage intermediate link for changing the sign of the voltage across said first transformer winding.

46. A method according to claim 43, wherein one or several phase legs of the direct converter are commutated by controlling the current valves of that phase leg or those phase legs for changing the connection of the output thereof from one end of said second transformer winding to the other for changing the direction of the current through said second transformer winding enabling a change of the switching state of the VSC-converter.

47. A method according to claim 46, wherein starting from a state in which the output of a phase leg of said direct converter is connected to a first end of the second transformer winding through a conducting first current valve with a power flow from said terminal into the phase leg, the other second current valve of that phase leg is turned on for short-circuiting the phase leg for opening a current path through the winding of the transformer in the direction of the voltage across the transformer, so that the second current valve gradually takes over the current through the transformer and the first current valve turns off as the current through it goes to zero.

48. A method according to claim 46, in which said direct converter has only one said terminal for said alternating phase voltage and two phase legs, and said two phase outputs forming said terminal are formed by a midpoint between the current valves of a phase leg each, wherein the current valves of the direct converter are controlled so as to short-circuit these two phase outputs, i.e. connecting them to the same end or ends of said second transformer winding and letting them stay in this state for forming a zero-voltage interval on the terminal for the alternating phase voltage.

49. A method according to claim 46, wherein the current valves of the direct converter are controlled for obtaining a desired pulse width modulation pattern for said alternating phase voltage on said terminal.

50. A method according to claim 43, wherein the VSC-converter has two phase legs and said outputs connected to the ends of said first transformer winding are formed by a midpoint between current valves of a phase leg each, wherein the VSC-converter comprises one said snubber capacitor interconnecting said midpoints between current valves of the two phase legs.

51. A method according to claim 43, wherein the semiconductor devices in the valves of the direct converter are controlled to turn off by zero-crossing of the current through the semiconductor devices.

52. A method according to claim 43, wherein the semiconductor devices of the VSC-converter are controlled for changing the switching state of this converter by changing the connection of at least one of said outputs thereof from one pole of said direct voltage intermediate link to the other while charging and discharging said snubber capacitor(-s) for lowering the voltage derivates during turn-off of a semiconductor device.

53. A method according to claim 52, wherein the semiconductor devices of the current valves of the VSC-converter are turned on at substantially zero-voltage thereacross and zero-current therethrough when a current flows through the diode of the valve in question.

54. A method according to claim 52, in which the VSC-converter has two phase legs and said outputs connected to the ends of said first transformer winding are formed by a midpoint between current valves of a phase leg each, one phase leg of the VSC-converter is commutated at a time from a state in which the two midpoints are connected to different poles of the direct voltage intermediate link for obtaining an intermediate state in which said midpoints are connected to the same pole for applying a zero-voltage to the first winding of the transformer.

55. A method according to claim 54, wherein for the VSC-converter a control regime of commutating one phase leg at a time and a control regime of commutating both phase legs at the same time are used alternatingly.

56. A method according to claim 52, wherein a conducting current valve of a phase leg of the VSC-converter is controlled to turn off for commutating the output of that phase leg and at the same time, starting from a state in which the output of a phase leg of the direct converter is connected to a first end of the second transformer winding through a conducting first current valve, the other, second current valve of that phase leg of the direct converter is controlled to turn on for short-circuiting the second transformer winding through that phase leg for opening a current path through that winding of the transformer in the direction of the voltage across the transformer so as to form a resonance circuit by the capacitance of the snubber capacitor(-s) of the VSC-converter and the leakage inductance of the transformer making the current through said first transformer winding increasing for assisting the commutation of said phase leg of the VSC-converter by charging and discharging said snubber capacitor(-s).

57. A method according to claim 56, wherein one or two semiconductor devices in the VSC-converter that are carrying current are turned off when the current through the transformer has increased to a certain value as a consequence of the opening of said current path for initiating a resonant process recharging the snubber capacitor(-s) and by that transferring the potential of the phase terminal of the phase leg or the phase legs to an opposite direct voltage pole, and after the rectifying members that initially blocked the direct voltage have taken over the current the semiconductor devices being anti-parallel to the latters are turned on at zero-voltage and zero-current conditions.

58. A method according to claim 52, in which the VSC-converter has two said phase legs and said outputs connected to the ends of said first transformer winding are formed by a midpoint between current valves of a phase leg each, wherein the semiconductor devices of the current valves of the VSC-converter are controlled for commutating both phase legs at the same time starting from the state in which the two midpoints are connected to different poles of the direct voltage intermediate link through a conducting semiconductor device each by turning these semiconductor devices of both said valves off.

59. A method according to claim 52, wherein the semiconductor devices of the VSC-converter are controlled for changing the switching state thereof and it is started to commutate one or two phase legs of the direct converter by controlling a current valve of that phase leg or those phase legs before the change of switching state of the VSC-converter has been completed, when there is a desire to have a power flow through the apparatus from the direct voltage side to the alternating voltage side, i.e. to have the same polarity of the voltage pulses on said terminal of the alternating voltage side as the current there.

60. A method according to claim 59, wherein the voltage across the transformer is detected and, assuming there is a desire to have a power flow through the apparatus from the direct voltage side to the alternating voltage side, the commutation of the direct converter is stated based upon information from said voltage detection when, as a consequence of the change of switching state of the VSC-converter commenced, the voltage across the first transformer winding has changed sign and exceeded a certain threshold voltage value.

61. A method according to claim 52, wherein all phase legs of the direct converter are commutated by controlling the current valves of the phase legs of that converter and it is started to control the semiconductor devices of the VSC-converter for changing the switching state thereof before the commutation of all phase legs of the direct converter has been completed, when there is a desire to have a power flow through the apparatus from the alternating voltage side to the direct voltage side, i.e. to have a polarity of the voltage pulses on said terminal of the alternating voltage side being opposite to the polarity of the current there.

62. A method according to claim 61, wherein the current through one of the transformer windings is detected, and that, assuming there is a desire to have a power flow through the apparatus from the alternating voltage side to the direct voltage side, it is started to control the VSC-converter for changing the switching state thereof based upon information from the current detection when, as a consequence of the commutation of the phase-leg or phase legs of the direct converter commenced, the current through the second transformer winding has changed direction and exceeded a certain threshold current value.

63. A method according to claim 43, wherein the order of steps carried out for obtaining commutations of phase legs according to a commutation cycle is optionally changed in case the direction of the alternating voltage side current, i.e. the current on the direct converter side of the apparatus, changes during operation of the apparatus.

64. A computer readable medium having a program recorded thereon including software code, portions adapted to make a computer control the steps of claim 43.

65. A computer program product directly loadable into the internal memory of a digital computer, comprising software code portions for performing the steps according to claim 43 when said product is run on a computer.

66. A computer program product according to claim 65 provided at least partially through a network as the Internet.

67. A method for converting a direct voltage into direct voltage through an apparatus comprising a VSC-converter having a direct voltage intermediate link with a positive and a negative pole and at least one phase leg interconnecting the two poles and having at least two current valves connected in series, each current valve having at least one semiconductor device of turn-off type and a rectifying member connected in anti-parallel therewith, the apparatus further comprising a transformer with two opposite ends of a first winding thereof connected to an output each of the VSC-converter and with a second winding thereof connected to an arrangement adapted to form a direct voltage, said VSC-converter and said arrangement being controlled for obtaining said voltage conversion, wherein the control is carried out for an apparatus in which the VSC-converter comprises at least one snubber capacitor connected to said current valves, said arrangement comprising a direct converter having at least one phase leg connected through the opposite ends thereof to opposite ends of said second winding of the transformer and having at least two current valves connected in series, each of these current valves being able to conduct current and block voltage in both directions and to turn on by gate control, a midpoint of said phase leg of the direct converter being provided with an output for forming a terminal for said direct voltage between the output and a further output of the direct converter, and that the control comprises the steps of:

a) con trolling the semiconductor devices of the VSC-converter for changing the switching state of this converter by changing the connection of at least one of said outputs thereof from one pole of the direct voltage intermediate link to the other for changing the sign of the voltage across said first transformer winding,

b) commutate the phase leg or phase legs of the direct converter for changing the end of the second transformer winding to which the respective phase output is connected in such a sequence and at such delays that a constant desired voltage is obtained on said terminal and doing this until the current through the second transformer winding has changed direction, and

c) starting over with controlling the VSC-converter to change switching state again.