Apparatus and a method for voltage conversion

• US 6,603,675 B1
• Filed: 01/17/2002
• Issued: 08/05/2003
• Est. Priority Date: 01/17/2002
• Status: Active Grant

• Current Assignee: ABB AB (ABB Limited)
• Original Assignee: ABB AB (ABB Limited)
• Inventors: Norrga, Staffan
• Primary Examiner(s): BERHANE, ADOLF D
• Application Number:
  US10/046,720
• Publication Number:
  US 20030133317A1
• Time in Patent Office: 565 Days
• Field of Search: 363/131, 363/132, 363/137
• US Class Current: 363/132
• CPC Class Codes: H02M 1/0048 Circuits or arrangements for reducing losses using snubbers H02M1/34
• CPC Class Codes: H02M 5/297 for conversion of frequency
• CPC Class Codes: H02M 7/219 in a bridge configuration
• CPC Class Codes: H02M 7/2195 the switches being synchronously commutated at the same frequency of the AC input voltage
• CPC Class Codes: H02M 7/4807 having a high frequency intermediate AC stage
• CPC Class Codes: H02M 7/483 Converters with outputs that each can have more than two voltages levels
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

An apparatus for converting direct voltage into three-phase 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 three phase legs. 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. The midpoints (27, 27′, 27″) of the phase legs of the direct converters are provided with phase outputs for forming a terminal for the alternating phase voltage between these phase outputs.

Claims

1. An apparatus for converting direct voltage into a three-phase alternating voltage and conversely comprising:

a VSC-converter having a direct voltage intermediate link with a positive pole 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 a three-phase alternating 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, that said arrangement comprises a direct converter having at least three phase legs 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 that the midpoints of said phase legs of the direct converter are provided with phase outputs for forming a terminal for said alternating voltage between these phase outputs.

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

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

4. 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 that 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.

5. An apparatus according to claim 4, wherein the VSC-converter comprises one said snubber capacitor connected in parallel to said first transformer winding.

6. An apparatus according to claim 1, wherein said direct converter has a three-phase terminal for said alternating phase voltage.

7. An apparatus according to claim 6, wherein said direct converter has three said phase legs and that said two phase outputs forming said terminal are formed by a midpoint between the current valves of a phase leg each.

8. 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, having an inherent body diode.

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

10. An apparatus according to claim 9, 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.

11. An apparatus according to claim 10, wherein said current valve of the direct converter comprises two reverse-blocking controllable second valves connected in anti-parallel.

12. An apparatus according to claim 11, wherein said second valve comprises a single reverse-blocking controllable semiconductor device, e.g. a thyristor.

13. An apparatus according to claim 12, wherein said single reverse-blocking controllable semiconductor device has silicon carbide as base material.

14. An apparatus according to claim 11, wherein 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.

15. An apparatus according to claim 14, wherein interconnection points between the semiconductor device and the rectifying member in the two second valves are directly interconnected.

16. An apparatus according to claim 11, 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.

17. An apparatus according to claim 16, wherein said diodes are based on silicon carbide.

18. An apparatus according to claim 14, 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.

19. An apparatus according to claim 1, wherein the phase legs of the VSC-converter and/or the direct converter comprise a plurality of current valves connected in series on each side of said midpoint of the phase leg for together holding a voltage to be held in a blocking state of the phase leg part they belong to.

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

21. 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 snubber capacitor for lowering the voltage derivatives during turn-off of a semiconductor device.

22. An apparatus according to claim 21, 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.

23. An apparatus according to claim 3, 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.

24. An apparatus according to claim 23, wherein said control unit is adapted to varying the order in which the phase legs of the VSC-converter are commutated in such a fashion that the conduction losses of the valves of the VSC-converter are evenly distributed.

25. An apparatus according to claim 3, 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.

26. 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.

27. An apparatus according to claim 1, wherein said control unit is adapted to commutate the 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, by controlling the current valves of each phase leg 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.

28. An apparatus according to claim 27, 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.

29. An apparatus according to claim 21, wherein said 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 snubber capacitor of the VSC-converter and the leakage inductance of the transformer making the current through the transformer increasing for assisting the commutation of said phase leg of the VSC converter by charging and discharging said snubber capacitor.

30. An apparatus according to claim 29, 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 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 antiparallel to the latters on at zero-voltage and zero-current conditions.

31. An apparatus according to claim 29, wherein it comprises an additional inductor connected in series with said transformer for increasing the inductance of said resonance circuit.

32. An apparatus according to claim 7, wherein a midpoint of the second winding of the transformer is provided with a connection for grounding purposes.

33. An apparatus according to claim 7, wherein a midpoint of the second transformer winding is provided with a connection for connecting loads between this connection and any of the alternating voltage side phase outputs.

34. An apparatus according to claim 27, 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 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 21, 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 phase leg before the change of switching state of the VSC-converter has been completed, when there is a desire to avoid the state the system appears in after a commutation of the VSC-converter.

37. An apparatus according to claim 36, further comprising means for detecting the voltage across said transformer, and that the control unit 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.

38. An apparatus according to claim 21, 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 avoid the state the system appears in after a commutation of all of the phase legs of the direct converter.

39. An apparatus according to claim 38, further comprising means for detecting the current through one of the transformer windings, and that the control unit 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.

40. A method for converting direct voltage into three-phase alternating voltage and conversely through an apparatus employing 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 a threephase alternating 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 has at least one snubber capacitor connected to said current valves thereof, said arrangement comprising a direct converter having at least three phase legs 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 the midpoints of said phase legs of the direct converter being provided with phase outputs for forming a terminal for said alternating voltage between these phase outputs, 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 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

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

41. A method according to claim 40, wherein 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.

42. A method according to claim 40, wherein it is carried out for a said apparatus in which the VSC-converter comprises one said snubber capacitor connected in parallel to said first transformer winding.

43. A method according to claim 40, in which th e 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, and wherein the VSC-converter comprises one said snubber capacitor interconnecting said midpoints between current valves of the two phase legs.

44. A method according to claim 40, 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.

45. A method according to claim 40, 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 for lowering the voltage derivates during turn-off of a semiconductor device.

46. A method according to claim 45, 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.

47. A method according to claim 45, 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, and wherein 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.

48. A method according to claim 45, 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, and 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.

49. A method according to claim 42, 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 alternately.

50. A method according to claim 41, in which 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.

51. A method according to claim 40, wherein the phase legs of the direct converter are commutated by controlling the current valves of 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.

52. A method according to claim 51, 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, 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.

53. A method according to claim 45, 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 one or several phase legs 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 said phase leg or phase legs of the direct converter is controlled to turn on for short-circuiting the second transformer winding through said phase leg or phase legs 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 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.

54. A method according to claim 53, 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 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.

55. A method according to claim 50, 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.

56. A method according to claim 45, wherein the semiconductor devices of the VSC-converter are controlled for changing the switching state thereof and it is started to commutate one or more 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 avoid the state the system appears in after a commutation of the VSC-converter.

57. A method according to claim 56, wherein the voltage across the transformer is detected and the commutation of the direct converter is started 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.

58. A method according to claim 45, 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 avoid the state the system appears in after a commutation of all of the phase legs of the direct converter.

59. A method according to claim 58, wherein the current through one of the transformer windings is detected, and that 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 legs of the direct converter commenced, the current through the second transformer winding has changed direction and exceeded a certain threshold current value.

60. A method according to claim 41, 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.

61. 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 40 when said product is run on a computer.

62. A computer program product according to claim 61 provided at least partially through a network as the Internet.

63. A computer readable medium having a program recorded thereon including software code portions adapted to make a computer control the steps of claim 40.