U.S. patent application number 10/046720 was filed with the patent office on 2003-07-17 for apparatus and a method for voltage conversion.
Invention is credited to Norrga, Staffan.
Application Number | 20030133317 10/046720 |
Document ID | / |
Family ID | 21945004 |
Filed Date | 2003-07-17 |
United States Patent
Application |
20030133317 |
Kind Code |
A1 |
Norrga, Staffan |
July 17, 2003 |
APPARATUS AND A METHOD FOR VOLTAGE CONVERSION
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.
Inventors: |
Norrga, Staffan; (Stockholm,
SE) |
Correspondence
Address: |
Dykema Gossett, P.L.L.C.
Suite 300 West
1300 I Street, N.W.
Washington
DC
20005-3306
US
|
Family ID: |
21945004 |
Appl. No.: |
10/046720 |
Filed: |
January 17, 2002 |
Current U.S.
Class: |
363/127 |
Current CPC
Class: |
Y02B 70/10 20130101;
H02M 5/297 20130101; H02M 7/4807 20130101; H02M 7/219 20130101;
H02M 7/483 20130101; H02M 7/2195 20210501; H02M 1/0048
20210501 |
Class at
Publication: |
363/127 |
International
Class: |
H02M 007/217 |
Claims
1. An apparatus for converting direct voltage into a three-phase
alternating voltage and conversely comprising a VSC-converter (8)
having a direct voltage intermediate link (9) with a positive (10)
and a negative (11) pole and at least one phase leg (12, 13)
interconnecting the two poles and having at least two current
valves (14-17) connected in series, each current valve having at
least one semiconductor device (18) of turn-off type and a
rectifying member (42) connected in anti-parallel therewith, the
apparatus further comprising a transformer (19) with two opposite
ends of a first winding (20) thereof connected to an output (21,
22, 30) each of the VSC-converter and with a second winding (23)
thereof connected to an arrangement adapted to form voltage pulses
for forming a three-phase alternating voltage, the apparatus also
comprising a unit (7) adapted to control the VSC-converter and said
arrangement for obtaining said voltage conversion, characterized in
that the VSC-converter comprises at least one snubber capacitor
(33-36, 50) connected to said current valves thereof, that said
arrangement comprises a direct converter (26) having at least three
phase legs (24, 24', 24") connected through the opposite ends
thereof to opposite ends of said second winding (23) of the
transformer (19) and having at least two current valves (37, 38)
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, characterized in that said
current valves of the VSC-converter each comprises a said snubber
capacitor (33-36) connected in parallel with said semiconductor
device and rectifying member.
3. An apparatus according to claim 1, characterized in that said
VSC-converter has two said phase legs (12, 13) and that said
outputs connected to the ends of said first transformer winding
(20) are formed by a midpoint (21, 22) between current valves of a
phase leg each.
4. An apparatus according to claim 1, characterized in that the
VSC-converter has one said phase leg (12), that one of said outputs
connected to the ends of said first transformer winding is formed
by a midpoint (21) 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 (30) of the direct
voltage intermediate link separated from both said positive and
negative poles (10, 11) by at least one capacitor (31, 32).
5. An apparatus according to claim 3 or 4, characterized in that
the VSC-converter comprises one said snubber capacitor (50)
connected in parallel to said first transformer winding (20).
6. An apparatus according to any of claims 1-5, characterized in
that said direct converter (26) has a three-phase terminal for said
alternating phase voltage.
7. An apparatus according to claim 6, characterized in that said
direct converter has three said phase legs (24, 24', 24") and that
said two phase outputs forming said terminal are formed by a
midpoint (27, 27', 27") between the current valves of a phase leg
each.
8. An apparatus according to any of claims 1-7, characterized in
that the semiconductor device (18) and the rectifying member (42)
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.
9. An apparatus according to any of the preceding claims,
characterized in that the valves (37, 38) 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, characterized in that said
current valves (37, 38) 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, characterized in that each
said current valve (37, 38) of the direct converter comprises two
reverse-blocking controllable second valves connected in
anti-parallel.
12. An apparatus according to claim 11, characterized in that each
said second valve comprises a single reverse-blocking controllable
semiconductor device, e.g. a thyristor.
13. An apparatus according to claim 12, characterized in that said
single reverse-blocking controllable semiconductor device has
silicon carbide as base material.
14. An apparatus according to claim 11, characterized in that 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.
15. An apparatus according to claim 14, characterized in that
interconnection points between the semiconductor device and the
rectifying member in the two second valves are directly
interconnected.
16. An apparatus according to claim 14 or 15, characterized in that
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, characterized in that said
diodes (42) are based on silicon carbide.
18. An apparatus according to claim 14 or 15, characterized in that
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 any of the preceding claims,
characterized in that 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 any of the preceding claims,
characterized in that the current valves (37, 38) 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 any of the preceding claims,
characterized in that said control unit (7) is adapted to control
the semiconductor devices (18) of the VSC-converter for changing
the switching state of this converter, by changing the connection
of at least one of said outputs (21, 22) thereof from one pole of
said direct voltage intermediate link to the other while charging
and discharging said snubber capacitor(-s) (33-36, 50) for lowering
the voltage derivatives during turn-off of a semiconductor
device.
22. An apparatus according to claim 21, characterized in that said
control unit (7) is adapted to turn on semiconductor devices of the
current valves (14-17) of the VSC-converter when a current flows
through the rectifying member (42) of the valve in question for
turning the semiconductor device (18) on at substantially
zero-voltage thereacross and zero-current therethrough.
23. An apparatus according to claims 3 and 21 or 22, characterized
in that said control unit (7) is adapted to commutate one phase leg
(12, 13) of the VSC-converter at a time starting from a state in
which the two midpoints (21, 22) 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 (20) of
the transformer.
24. An apparatus according to claim 23, characterized in that said
control unit (7) is adapted to varying the order in which the phase
legs (12, 13) 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 claims 3 and 21, characterized in
that said control unit (7) is adapted to control the semiconductor
devices (18) of the current valves of the VSC-converter for
commutating both phase legs (12, 13) at the same time starting from
the state in which the two midpoints (21, 22) 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 claims 2, 4, 5 and 21, characterized
in that starting from a switching state in which the midpoint (21,
22) of said phase leg of the VSC-converter is connected to a first
pole of the direct voltage intermediate link said control unit (7)
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 (20).
27. An apparatus according to any of the preceding claims,
characterized in that said control unit (7) is adapted to commutate
the phase legs (24, 24', 24") 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 each phase leg for changing the connection of the output thereof
from one end of said second transformer winding (23) 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, characterized in that
starting from a state in which the output of a phase leg (24, 24',
24") of said direct converter is connected to a first end of the
second transformer winding (23) 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, characterized in that said
control unit (7) is adapted to control the conducting current
valves of the phase legs (12, 13) 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 (23) 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(-s) (33-36, 50) 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(-s).
30. An apparatus according to claim 29, characterized in that the
control unit (7) 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(-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 that the
control unit (7) is adapted to then, after the rectifying members
(42) 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.
31. An apparatus according to claim 29 or 30, characterized in that
it comprises an additional inductor (40) connected in series with
said transformer for increasing the inductance of said resonance
circuit.
32. An apparatus according to claim 7, characterized in that a
midpoint of the second winding (23) of the transformer is provided
with a connection (25) for grounding purposes.
33. An apparatus according to claim 7, characterized in that a
midpoint (25) of the second transformer winding (23) 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, characterized in that said
control unit (7) is adapted to control the current valves of the
direct converter (26) for obtaining a desired pulse width
modulation pattern for said alternating phase voltage on said
terminal (29).
35. An apparatus according to any of the preceding claims,
haracterized in that said control unit (7) is adapted to a) control
the semiconductor devices of the VSC-converter (8) for changing the
switching state of this converter by changing the connection of at
least one of said outputs (21, 22) thereof from one pole of said
direct voltage intermediate link (9) to the other for changing the
sign of the voltage across said first transformer winding (20) and
b) commutate phase legs (24, 24', 24") of the direct converter for
changing the end of the second transformer winding (23) 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 claims 21 and 27, characterized in
that the control unit (7) is adapted to control the semiconductor
devices of the VSC-converter (8) 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) (24, 24', 24") 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, characterized in that it
comprises means (39) for detecting the voltage across said
transformer, and that the control unit (7) is adapted to start the
commutation of the direct converter (26) 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 (20) has changed sign
and exceeded a certain threshold voltage value.
38. An apparatus according to claims 21 and 27, characterized in
that the control unit (7) is adapted to commutate all phase legs
(24, 24', 24") of the direct converter (26) by controlling the
current valves of the phase legs and start controlling the
semiconductor devices (18) 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, characterized in that it
comprises means (41) for detecting the current through one of the
transformer windings (20, 23), and that the control unit (7) is
adapted to start the control of the VSC-converter (8) 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 comprising
a VSC-converter (8) having a direct voltage intermediate link (9)
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 three-phase alternating voltage, said
VSC-converter and said arrangement being controlled for obtaining
said voltage conversion, characterized in that the control is
carried out for an apparatus in which the VSC-converter comprises
at least one snubber capacitor (33-36, 50) connected to said
current valves thereof, said arrangement comprising a direct
converter (26) having at least three phase legs connected through
the opposite ends thereof to opposite ends of said second winding
(23) 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 (27, 27', 27") 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 (18) 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 (20), b)
commutate the phase legs (24, 24', 24") of the direct converter for
changing the end of the second transformer winding (23) 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, characterized in that it is
carried out for a said apparatus in which said current valves
(14-17) of the VSC-converter each comprises a said snubber
capacitor (33-36) connected in parallel with said semiconductor
device and rectifying member.
42. A method according to claim 40, characterized in that it is
carried out for a said apparatus in which the VSC-converter
comprises one said snubber capacitor (50) connected in parallel to
said first transformer winding (20).
43. A method according to claim 40, in which the VSC-converter (8)
has two phase legs (12, 13) and said outputs connected to the ends
of said first transformer winding are formed by a midpoint (21, 22)
between current valves of a phase leg each, characterized in that
the VSC-converter comprises one said snubber capacitor (50)
interconnecting said midpoints (21, 22) between current valves of
the two phase legs.
44. A method according to any of claims 40-43, characterized in
that the semiconductor devices in the valves of the direct
converter (26) are controlled to turn off by zero-crossing of the
current through the semiconductor devices.
45. A method according to any of claims 40-44, characterized in
that the semiconductor devices of the VSC-converter (8) are
controlled for changing the switching state of this converter by
changing the connection of at least one of said outputs (21, 22)
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.
46. A method according to claim 45, characterized in that the
semiconductor devices (18) 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 (8)
has two phase legs (12, 13) and said outputs connected to the ends
of said first transformer winding are formed by a mid-point (21,
22) between current valves of a phase leg each, characterized in
that 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 (20) of
the transformer.
48. A method according to claim 45, in which the VSC-converter has
two said phase legs (12, 13) and said outputs connected to the ends
of said first transformer winding are formed by a mid-point (21,
22) between current valves of a phase leg each, characterized in
that 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 (21,
22) 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 47 or 48, characterized in that 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 claims 41 and 45, in which the
VSC-converter has one said phase leg (12), one of said outputs
connected to the ends of said first transformer winding (20) is
formed by a midpoint between current valves of said phase leg and
the output (30) 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 (31, 32), characterized in that
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 (33-36) 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 any of the claims 40-50, characterized in
that the phase legs (24, 24', 24") of the direct converter are
commutated by controlling the current valves (37, 38) of those
phase legs for changing the connection of the output (27, 27', 27")
thereof from one end of said second transformer winding (23) 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 (8).
52. A method according to claim 51, characterized in that starting
from a state in which the output of a phase leg (24, 24', 24") of
said direct converter is connected to a first end of the second
transformer winding (23) 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, characterized in that a
conducting current valve of a phase leg (12, 13) of the
VSC-converter (8) 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 (26) is connected to a first end of the second
transformer winding (23) 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(-s) (33-36, 50) 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).
54. A method according to claim 53, characterized in that 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 (42) 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 51, characterized in that the
current valves (37, 38)of the direct converter are controlled for
obtaining a desired pulse width modulation pattern for said
alternating phase voltage on said terminal (29).
56. A method according to claims 45 and 51, characterized in that
the semiconductor devices (18) of the VSC-converter (8) are
controlled for changing the switching state thereof and it is
started to commutate one or more phase legs of the direct converter
(26) 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, characterized in that the
voltage across the transformer is detected and the commutation of
the direct converter (26) 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 claims 45 and 51, characterized in that
all phase legs (24, 24', 24") of the direct converter (26) are
commutated by controlling the current valves (37, 38) of the phase
legs of that converter and it is started to control the
semiconductor devices (18) of the VSC-converter (8) 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, characterized in that the
current through one of the transformer windings (23) is detected,
and that it is started to control the VSC-converter (8) 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 (26) commenced, the current
through the second transformer winding has changed direction and
exceeded a certain threshold current value.
60. A method according to any of claims 40-59, characterized in
that 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 any of claims 40-60 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 any of claims 40-60.
Description
FIELD OF THE INVENTION AND PRIOR ART
[0001] The present invention is occupied with the problem to
convert direct voltage into three-phase alternating voltage and
conversely in all types of applications, such as in plants for
transmission of electric power and for connecting three-phase
electric machines to DC voltage loads and sources. Any ranges of
voltages, currents and powers are conceivable.
[0002] The invention relates more particularly to such an apparatus
comprising a VSC-converter (VSC=Voltage Source 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, and a method for converting
direct voltage into alternating voltage and conversely according to
the preamble of the appended independent method claim.
[0003] It is pointed out that "first winding" and "second winding"
are to be interpreted as a primary and a secondary winding of a
transformer used for voltage transformation, although it is here
not indicated which one is which.
[0004] "Rectifying member" is here and in the entire disclosure,
including the appended claims, to be interpreted broadly, and it
may be any member with ability to take a voltage and block current
in at least one direction therethrough, and it does not have to be
a diode, but it could for example also be controllable, such as a
thyristor (see for example FIG. 3 of this disclosure). Furthermore,
the rectifying member and the semiconductor device may also be
integrated in one single semiconductor device or switching device.
This means for the VSC-converter a semiconductor device with
reverse conducting property, such as a MOSFET with an inherent
"body diode".
[0005] An apparatus of this type may be used for converting direct
voltage into alternating voltage and conversely in applications
where it is important to obtain a galvanic isolation between the
direct voltage side and the alternating voltage side. Furthermore,
it is possible to obtain a voltage with variable frequency and
amplitude on the alternating voltage side, a bilateral power flow
and voltage as well as current transformation by an apparatus of
this type.
[0006] A known such apparatus comprises a transformer that operates
at the alternating voltage side frequency, which generally means a
low frequency and thereby a heavy and bulky transformer. This
results in a considerably lower efficiency of the transformer and
thereby of the apparatus than would the transformer be able to
operate at higher frequencies.
[0007] An apparatus of this type enabling operation of the
transformer at higher frequencies than the alternating voltage
frequency is known through "Power loss reduction techniques for
three-phase high frequency link DC-AC converter" 24.sup.th Annual
IEEE Power Electronics Specialists Conference, PESC '93 Record. pp.
663-668, 1993 by I. Yamato and N. Tokunaga, and through
"High-frequency link DC/AC converter with suppressed voltage clamp
circuits-naturally commutated phase angle control with self
turn-off devices", IEEE Transactions on Industry Applications, vol.
32, No. 2, pp. 293-300, March-April 1996. by M. Matsui, M. Nagai,
M. Mochizuki and A. Nabae, the latter being shown in the appended
FIG. 1. The reference numerals used there are as follows: direct
voltage intermediate link 1, voltage source converter 2,
transformer 3, arrangement 4, direct voltage side 5 and alternating
voltage side 6. The arrangement on the alternating voltage side of
the transformer is here a cycloconverter operating with natural
commutation and converting the high frequency alternating voltage
from the voltage source converter into an alternating voltage of
the desired frequency. However, the voltage source converter still
operates with forced commutation and hard switching resulting in
comparatively high stresses on the semiconductor devices of the
current valves resulting in comparatively high switching losses.
Furthermore, the current valves of the voltage source converter are
in the apparatus according to the latter reference controlled by a
control unit 7 according to a method resulting in square voltage
pulses with no zero-voltage interval thereby indirectly increasing
the content of harmonics in the alternating voltage side output
voltage. Besides the fact that the power lost in the form of heat
results in considerable costs the semiconductor devices of the
current valves have to either be dimensioned to be able to
withstand high thermal stresses and thereby be costly or a lower
frequency of the VSC-converter has to be applied resulting in a
more bulky transformer and a degraded curve shape for the
alternating current curves.
[0008] It is also known to utilize so called soft switching for
reducing switching losses in apparatuses for converting direct
voltage into alternating voltage and conversely, and these concepts
generally incorporate additional semiconductor devices that do not
take part in the power conversion itself. These additional
(auxiliary) semiconductor devices and the control circuitry
associated therewith add to the costs and complexity of such an
apparatus. Moreover, they often involve a derating of the main
semiconductor devices in the current valves either in terms of the
maximum current or voltage.
SUMMARY OF THE INVENTION
[0009] The object of the present invention is to provide an
apparatus of the type defined in the introduction having improved
properties with respect to such apparatuses already known.
[0010] This object is according to the invention obtained by
providing such an apparatus, in which the VSC-converter comprises
at least one snubber capacitor connected to said current valves
thereof, in which 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 in which the midpoints of said phase legs of the
direct converter are provided with phase outputs for forming a
terminal for said alternating phase voltage between these phase
outputs.
[0011] The use of at least one such snubber capacitor in an
apparatus of this type including a VSC-converter, a transformer and
a direct converter results in a possibility to obtain soft
switching of the semiconductor devices in the VSC-converter. This
capacitor/these capacitors will be used as energy storing means and
be discharged and recharged when changing the switching state of
the VSC-converter remarkably reducing the voltage derivatives when
the valves are switched and the direct converter commutating the
current gives rise to further advantages with respect to switching
losses and stresses for the semiconductor devices and rectifying
members, and the former may also be turned on at zero-voltage and
low current derivatives. The rectifying members, e.g. diodes, may
be turned on at low voltage derivatives and turned off at
zero-voltage and at low current derivatives. In the direct
converter no hard turn-off capability is needed, but the valves may
very well turn off at a current zero-crossing similar to the
turn-off process in a conventional thyristor converter.
Accordingly, the losses may be reduced in an apparatus of this type
with respect to such apparatuses already known and thereby costs be
saved. Less costly semiconductor devices may also be used thanks to
the reduced thermal stresses thereon. The basic functionality of an
apparatus of this type in the form of voltage conversion with
variable frequency on the alternating voltage side, the bilateral
power flow, galvanic isolation by a magnetic transformer and
voltage and current transformation may of course still be obtained.
Furthermore, this design of the apparatus enables a variety of
different control regimes for adapting the operation of the
apparatus to the conditions prevailing. The basic principle of the
operation of the apparatus is that the switching state of the
VSC-converter determines the sign of the voltage across the
transformer and the switching state of the direct converter
determines the direction of the current through the transformer.
Fundamentally, it is necessary to commutate the VSC-converter, i.e.
changing the sign of the transformer voltage, for being able to
commutate the direct converter, which is necessary for being able
to commutate the VSC-converter again and so on. The VSC-converter
also has to be commutated on a regular basis for limiting the
transformer flux, whereas the direct converter is modulated for
obtaining an alternating voltage pulse pattern on said terminal.
"Direct converter" is here defined as a converter having no energy
storing means, such as a direct voltage intermediate link.
[0012] According to a preferred embodiment of the invention said
current valves of the VSC-converter each comprises a said snubber
capacitor connected in parallel with said semiconductor device and
rectifying member. An alternative to provide the function of said
at least one snubber capacitor is provided by the fact that
according to another preferred embodiment the VSC-converter
comprises one said snubber capacitor connected in parallel to said
first transformer winding.
[0013] According to a preferred embodiment of the invention said
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 according to
another preferred embodiment of the invention 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.
The embodiment with a VSC-converter having two phase legs has the
advantage of making it possible to obtain zero-voltage intervals
across said first transformer winding. However, the embodiment with
only one phase leg has the advantage of a smaller number of
components with respect to the two-phase legs design.
[0014] According to a preferred embodiment of the invention, which
has already been indicated above, the valves of the direct
converter comprise a semiconductor device adapted to be turned off
and thereby turn off the valve by zero-crossing of the current
through the semiconductor devices resulting in soft switching
properties.
[0015] According to another preferred embodiment of the invention
the rectifying members of the valves of the direct converter are
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, and
are preferably of silicon carbide. Especially when the switching
devices turn off at current zero-crossing the reverse recovery of
the diodes may cause overvoltages across the valves and increased
switching losses if traditional silicon diodes are used. However,
this problem is solved by using diodes of such a material,
especially of silicon carbide, which exhibit nearly ideal behaviour
in terms of reverse recovery.
[0016] According to another preferred embodiment of the invention
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(-s) for lowering the voltage derivatives during turn-off
of a semiconductor device. In an embodiment, in which the
VSC-converter has two said phase legs, 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. Accordingly, this way of changing the switching state
of the VSC-converter makes it possible to obtain zero-voltage
intervals also at said alternating phase voltage terminal.
Moreover, according to another preferred embodiment of the
invention said control unit is adapted to varying in an appropriate
fashion the order in which the phase legs of the VSC-converter are
commutated, which results in a possibility to distribute the losses
in the diodes and semiconductor switches equally over several
switching cycles.
[0017] According to another preferred embodiment of the invention
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. This control regime has the advantage of being somewhat
simpler than the regime for commutating one phase leg at the
time.
[0018] According to another preferred embodiment of the invention
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, i.e.
from the direct converter to the VSC-converter, 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. All phase legs of the direct converter have
to be commutated in this way for changing the direction of the
transformer current. A desired voltage pulse width modulation
pattern may be achieved on the alternating phase voltage terminal
by such a control.
[0019] According to another preferred embodiment of the invention
constituting a further development of the embodiment just mentioned
the control unit is, 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, 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 by natural
commutation as the current through it goes down to zero.
[0020] According to another preferred embodiment of the invention
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 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 legs
of the VSC-converter by charging and discharging said snubber
capacitor(-s). This embodiment takes care of a problem that may be
severe under certain conditions, namely when the current on the
alternating voltage side of the apparatus is low, since it may then
be impossible to commutate the VSC-converter in the normal way. The
current through the transformer may then be insufficient for
recharging the snubber capacitor(-s) regardless of the switch state
of time direct converter. The recharge of the snubber capacitors
may take too long time or in the extreme case when i.sub.AC,i=0,
i=1, 2, 3 will not occur at all. By forming the resonance circuit
in this way a resonance process governed by the snubber
capacitances and the leakage inductance is initiated. Through this
process the snubber capacitor(-s) are recharged so that the
potential of the phase outputs of the phase legs of the
VSC-converter swing to the opposite pole of the direct voltage
intermediate link. This also means that the transformer voltage
changes direction.
[0021] According to another preferred embodiment of the invention
the apparatus comprises an additional inductor connected in series
with said first transformer winding for increasing the inductance
of said resonance circuit. This means that the time required for
changing the switching state of the VSC-converter may be
pro-longed.
[0022] According to another preferred embodiment of the invention
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 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. This is a preferred generic commutation strategy to be used,
in which it is assumed that the power initially flows from the
direct voltage side to the alternating voltage side, and in the
opposite case it is started by step b) followed by step a) and then
by step b) again.
[0023] According to another preferred embodiment of the invention
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 avoid the state the system
appears in after a commutation of the VSC-converter. By using such
an interlaced commutation of the two converters an interval of each
switching cycle during which the power flow will be of the opposite
direction with regard to the desired direction may be reduced and
the overall commutation speed can be increased. The apparatus has
then preferably means for detecting the voltage across said first
transformer winding, and 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 predetermined threshold voltage value. It is thereby ensured that
a complete commutation of both converters takes place, since it is
for that necessary that the polarity of the transformer voltage is
reversed and have reached a certain magnitude before the
commutation of the direct converter is initiated.
[0024] According to another preferred embodiment of the invention,
also relating to interlaced commutation, 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. When
a power flow in that direction is desired it is also preferred to
provide the apparatus with means for detecting the current through
the second transformer winding, and 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 legs
of the direct converter commenced, the current through the second
transformer winding has changed direction and exceeded a
predetermined threshold current value. A complete commutation of
both converters is ensured when the direction of the transformer
current has been reversed and reached a certain magnitude before
the commutation of the VSC-converter is initiated.
[0025] The invention also relates to a method for converting direct
voltage into alternating voltage and conversely according to the
independent method claim. The advantages of such methods and of
methods according to preferred embodiments of the invention defined
in the dependent method claims appear without any doubt from the
above discussion of the apparatus according to the preferred
embodiments of the invention.
[0026] The invention also relates to a computer program product and
a computer readable medium according to the corresponding appended
claims. It is easily understood that the method according to the
invention defined in the appended said method claims is well suited
to be carried out through program instructions from a processor
adapted to be influenced by a computer program provided with the
program steps in question.
[0027] Further advantages as well as advantageous features of the
invention appear from the following description and the other
dependent claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] With reference to the appended drawings, below follows a
specific description of preferred embodiments of the invention
cited as examples:
[0029] In the drawings:
[0030] FIG. 1 is a circuit diagram schematically illustrating an
apparatus according to the prior art,
[0031] FIG. 2 is a circuit diagram illustrating an apparatus
according to a first preferred embodiment of the invention,
[0032] FIG. 3 illustrates schematically different options of
designing a valve of the direct converter in an apparatus according
to the invention,
[0033] FIGS. 4a, 4b and 4c are circuit diagrams illustrating a part
of an apparatus according to further preferred embodiments of the
invention,
[0034] FIGS. 5a-d are circuit diagrams of the direct voltage side
of the apparatus according to FIG. 2 in different states during a
procedure for commutating the two phase legs simultaneously,
[0035] FIGS. 6a-f are views corresponding to those of the FIGS.
5a-d for a procedure of commutating one phase leg at a time,
[0036] FIGS. 7a-c are simplified circuit diagrams of one phase leg
of the direct converter of an apparatus according to the invention
in different states during a procedure for commutating this phase
leg,
[0037] FIGS. 8a-f are circuit diagrams of an apparatus according to
the invention having a VSC-converter with one phase leg in
different states during a procedure for resonantly assisted
commutation of said VSC-converter,
[0038] FIG. 9 is a graph illustrating voltages and currents versus
time for the procedure according to FIGS. 8a-f,
[0039] FIGS. 10a-f are circuit diagrams of an apparatus according
to the invention in different states during a procedure for
resonantly assisted commutation of the VSC-converter thereof, in
which the two phase legs of the VSC-converter are commutated
simultaneously,
[0040] FIG. 11 is a graph illustrating voltages and currents versus
time for the procedure according to FIGS. 10a-f,
[0041] FIGS. 12a-h are circuit diagrams of an apparatus according
to the invention in different states during a procedure for
resonantly assisted commutation of the VSC-converter thereof in
which the two phase legs thereof are commutated one at a time,
[0042] FIG. 13 is a graph illustrating voltages and currents versus
time for the procedure according to FIGS. 12a-h,
[0043] FIG. 14 is a schematical view of a possible Pulse Width
Modulation pattern for the alternating phase voltage of an
apparatus according to the invention,
[0044] FIGS. 15a-i are circuit diagrams of an apparatus according
to the invention in different states during an example of a
commutation cycle not involving resonant commutation of the voltage
source converter,
[0045] FIG. 16 is a graph corresponding to FIG. 9 for the procedure
illustrated in FIGS. 15a-i,
[0046] FIGS. 17a-k are circuit diagrams of an apparatus according
to the invention in different states of an example of a
commmutation cycle involving resonantly assisted commutation of the
voltage source converter, and
[0047] FIG. 18 is a graph corresponding to FIG. 9 for the procedure
illustrated in FIGS. 17a-k.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
[0048] FIG. 2 illustrates an apparatus according to a preferred
embodiment of the invention having a VSC-converter 8 with a direct
voltage intermediate link 9 with a positive 10 and a negative 11
pole and two phase legs 12, 13 (1 and 2, respectively, in the
formulas following) interconnecting the two poles and having two
current valves 14-17 connected in series. Each current valve has
one semiconductor device 18 of turn-off type, such as an IGBT, and
a rectifying member 42, such as a rectifying diode, connected in
anti-parallel therewith. A snubber capacitor 33-36 is connected in
parallel with each said semiconductor device 18 and diode 42. A
transformer 19 is with two opposite ends of a first winding 20
connected to an output 21, 22 each of the VSC-converter and with a
second winding 23 (having a connection 25 to the midpoint thereof
for grounding purposes) thereof with the opposite ends connected to
the opposite ends of three phase legs 24, 24' and 24" (1, 2 and 3,
respectively, in the formulas following) of a direct converter 26.
The phase legs of the direct converter have each two current valves
connected in series, which each has at least one semiconductor
device and a rectifying member connected in anti-parallel therewith
making it able to conduct current and block voltage in both
directions and making it possible to control the valve to turn on.
A midpoint 27, 27', 27" of each phase leg of the direct converter
is provided with a phase output for forming a terminal 29, 29', 29"
for an alternating phase voltage between the respective phase
output and the connection 25. The alternating voltage side currents
i.sub.AC,i are defined to be positive as shown here. The same is
valid for the voltages u.sub.AC,i.
[0049] It is pointed out that each current valve shown in the
figures may be substituted by a plurality of current valves
connected in series, which will then have the same function and be
controlled in the same way as one single such current valve. When
high voltages are to be handled it may be necessary to connect a
plurality of current valves in series in that way, since the
semiconductor device and the diode of each valve may not alone
block a voltage being high enough with respect to the voltages to
be handled by the apparatus.
[0050] FIG. 3 illustrates three possibilities of designing a
current valve for the direct converter. The left one is constituted
by a connection of two thyristors in anti-parallel with each other,
whereas the other two are formed by a series connection of on one
hand an IGBT connected in anti-parallel with a first diode and on
the other an IGBT having the opposite conduction direction to the
IGBT first mentioned connected in anti-parallel with a second
diode. In one of them the emitters and in the other one the
collectors of the IGBTs are connected to each other.
[0051] FIG. 4a illustrates a further possibility to modify an
apparatus according to FIG. 2 by providing a VSC-converter having
only one phase leg, so that one output of this converter is formed
by a midpoint 30 of the direct voltage intermediate link separated
from both said positive and negative poles by at least one
capacitor 31, 32. The semiconductor devices of the apparatuses
according to these embodiments are controlled by a control unit 7
schematically indicated only in FIG. 2.
[0052] FIG 4b illustrates a still further possibility to modify an
apparatus according to FIG. 2 by providing a VSC-converter in which
the snubber capacitors of each current valve have been replaced by
one single capacitor 50 interconnecting the midpoints of the phase
legs 12, 13. An advantage of this embodiment is that only one
capacitor is needed instead of four.
[0053] FIG. 4c illustrates another possibility to modify an
apparatus according to FIG. 2 by providing a VSC-converter
differing from that in the embodiment according to FIG. 4a by the
fact that the snubber capacitors of each current valve have been
replaced by one single capacitor 51 interconnecting the midpoint of
the phase leg 12 and the midpoint of the direct voltage
intermediate link. This results in the same properties of the
VSC-converter as for the one in FIG. 4a, and it will be controlled
in the same way, but an advantage is that only one capacitor is
needed instead of two.
[0054] It is pointed out that the present invention also covers
embodiments having snubber capacitors both in parallel with the
current valves as shown in for instance FIG. 4a and between
midpoints as shown in FIGS. 4b and 4c.
[0055] The different properties and differences in operation
behaviour of these embodiments will be described further below.
[0056] We will now make some definitions to be used when explaining
different phenomena below.
[0057] The coupling functions for the AC side converter may be
written:
u.sub.AC,i=N.sub.tru.sub.trk.sub.AC,i
i.sub.tr=N.sub.tr(k.sub.AC,1i.sub.AC,1+k.sub.AC,2i.sub.AC,2+k.sub.AC,3i.su-
b.AC,3)
[0058] where k.sub.AC,i equals -1/2 if phase leg i. connects the
corresponding AC side terminal to the lower end of the second
transformer winding and +1/2 if it connects the AC terminal to the
upper end of the second transformer winding. N.sub.tr is the turns
ratio of the transformer.
[0059] Correspondingly, for the DC side converter for the case with
two phase legs the following relation applies:
u.sub.tr=U.sub.d(k.sub.DC,1-k.sub.DC,2)
[0060] and for the case with one phase leg:
u.sub.tr=U.sub.dk.sub.DC,1
[0061] where k.sub.DC,i equals -{fraction (1/2)} if phase leg i.
connects the corresponding transformer terminal to the lower DC
link pole (negative) and +1/2 if it connects the transformer
terminal to the upper DC link pole (positive).
[0062] The switching states of the VSC-converter and the direct
converter may be changed by commutation of the phase legs thereof,
which means for the VSC-converter that the output of a phase leg
thereof is moved from being connected to one pole of the direct
voltage intermediate link to the other pole thereof. For the direct
converter the phase output of the phase leg is moved from being
connected to one end of the second transformer winding to be
connected to the other end of that winding. The assumption is made
that the inductances of the line filter 50 are much larger than the
leakage inductance of the transformer and large enough to keep the
currents on the alternating voltage side, i.sub.AC,i essentially
constant during the interval between two commutations of the
converters in the system. Likewise, the capacitance of the direct
voltage link is assumed to be much larger than the snubber
capacitances of the valves in the VSC-converter and large enough to
keep the direct voltage, U.sub.d, essentially constant during the
interval between two commutations of the converters in the system.
Under these assumptions the following is valid:
[0063] The direction of the current i.sub.tr through the
transformer is determined by the switch state of the direct
converter and the directions of the alternating voltage side
currents i.sub.AC,i, whereas the sign of the voltage across the
transformer u.sub.tr is determined by the switch state of the
VSC-converter.
[0064] The condition that has to be fulfilled for enabling
commutation of the VSC-converter is u.sub.tri.sub.tr>0, i.e. the
power flow is directed out of that converter towards the AC
side.
[0065] FIGS. 5a-d illustrates a procedure for changing the
switching state of the VSC-converter. In this and the following
circuit diagram figures the instantaneous current path is indicated
by thicker lines. It is in FIGS. 5a-d assumed that the switching
state of the direct converter is unchanged, which means that the
current through the transformer i.sub.tr will be constant. The two
semiconductor devices in the current valves that carry the current
are firstly turned off (FIG. 5b) thus diverting the current to the
snubber capacitors 33-36. As the capacitors are recharged u.sub.tr
changes from +U.sub.d to -U.sub.d. The voltage derivatives and thus
the stresses on the valves will be remarkably reduced thanks to the
existence of the capacitors. Finally, the diodes of the opposite
valves take over the current and the commutation is completed. At
this stage the semiconductor devices (IGBTs) that are anti-parallel
to the conducting diodes are turned on sit zero-voltage and
zero-current conditions (FIG. 5d).
[0066] FIGS. 6a-f show an alternative way of commutating the
VSC-converter, in which one phase leg is commutated at a time.
After the commutation of the first phase leg u.sub.tr and thereby
also u.sub.AC becomes zero as the current freewheels (FIG. 6c). It
is obvious that the commutation processes described could have been
carried out analogously if u.sub.tr and i.sub.tr were both
negative. However, when arriving to the switching states according
to FIGS. 5d and 6f it is not possible to go back to the switching
state according to FIGS. 5a and 6a, respectively, without first
changing the direction of the current i.sub.tr through the
transformer by changing the switching state of the direct
converter.
[0067] For commutation of a phase leg of the direct converter to be
possible the following condition has to be fulfilled:
k.sub.AC,ii.sub.AC,iu.sub.tr<0
[0068] The effect of commutation of one phase leg of the direct
converter is that the output thereof is shifted from being
connected to one end of the second transformer winding to the other
end thereof. This corresponds to a sign reversal of the coupling
function k.sub.AC,i. FIGS. 7a-c illustrate how the commutation of a
phase leg may be carried out. Initially the upper valve conducts
the current, i.e. k.sub.AC,i=1/2. To start the commutation the
semi-conductor switch in the lower valve 37 that blocks the voltage
applied to the phase leg is turned on. Thereby the phase leg is
short-circuited and the voltage instead appears across the leakage
inductance of the transformer. The current in the transformer
starts changing and correspondingly the lower valve takes over the
current from the upper valve 38. Finally, the current through the
upper valve reaches zero and the diode that initially carried the
current turns off. After this the semiconductor switch that
initially carried the current is turned off at zero-current. It
should be noted that two or more phase legs may be commutated
simultaneously. In some cases this can be of great advantage in
order to speed up the commutation sequences.
[0069] As already briefly discussed above it may not be possible to
commutate the VSC-converter in the fashion described with reference
to FIGS. 5 and 6. The current through the transformer may be
insufficient for recharging the snubber capacitors regardless of
the switch state of the direct converter. The recharge of the
snubber capacitors may take too long time or in the extreme case
when i.sub.AC,ii=1, 2, 3, all equal zero it will not occur at all.
In these situations a method based on resonantly assisted
commutation may be used. This method will now be described with
reference to FIGS. 8a-f. In short, the method consists in switching
both converters simultaneously in order to form a resonance circuit
between the snubber capacitances and the leakage inductance of the
transformer. FIGS. 8a-f show a simplified model of the system that
can be used for analysing the resonantly assisted commutation for
the case where the direct voltage side is equipped with one phase
leg. In the first step (FIG. 8b) one or several phase legs of the
alternating voltage side converter are switched so as to provide a
path for the current in the direction of u.sub.tr. The current
through the transformer starts increasing linearly. In this state
the current is allowed to increase by a certain amount, denoted
enhancement current, i.sub.enh. The required duration is equal to:
1 t enh = 2 L i enh N tr 2 U d
[0070] L.sub..lambda. is the leakage inductance of the transformer
expressed with respect to the second winding. When t.sub.enh has
elapsed the semi-conductor device in the VSC-converter that is
carrying current is turned off. Thereby a resonant process (FIG.
8c) governed by the snubber capacitances and the leakage inductance
of the transformer is initiated. Through this process the snubber
capacitances are recharged so that the potential of the phase
terminal of the phase leg swings to the opposite direct voltage
rail. This also means that u.sub.tr goes from +U.sub.d/2 to
-U.sub.d/2 or vice versa. When this is completed the diode that
initially blocked the direct voltage takes over the current and the
semiconductor device that is anti-parallel to it is turned on at
zero-voltage and zero-current conditions. The current i.sub.tr is
forced down linearly until it reaches the initial level. At this
stage the valves of the alternating voltage side converter, that
were turned on initially, turn off by natural commutation and the
process is completed. Note that the enhancement current can be used
to compensate for losses in the resonant circuit to ensure that the
snubber capacitors are completely recharged before the free current
path on the alternating voltage side is broken. It can also be used
to compensate for variations in the alternating voltage side
currents, i.sub.AC,i, during the commutation process.
[0071] It is illustrated in FIG. 9 how u.sub.AC,i, u.sub.tr and
i.sub.tr develop over time during the different states illustrated
in FIGS. 8a-f. N.sub.r has in this figure for simplicity been
considered to be 1.
[0072] FIG. 10a shows a simplified model of the system that can be
used for analysing the resonantly assisted commutation for the case
where the VSC-converter is equipped with two phase legs.
[0073] Similarly as for a normal commutation of the VSC-converter
there are principally two ways of performing the resonantly
assisted commutation. The first alternative, in which both phase
legs are commutated simultaneously, is shown in FIGS. 10a-f. In the
first step (FIG. 10b) one or several phase legs of the alternating
voltage side converter are switched so as to provide a path for the
current in the direction of u.sub.tr. The currents through the
transformer starts increasing linearly. In this state the current
is allowed to increase by a certain predefined amount, denoted
enhancement current, i.sub.enh. The required duration is equal to:
2 t enh = L i enh N tr 2 U d
[0074] When t.sub.enh has elapsed both switches in the
VSC-converter that are carrying current are turned off. Thereby a
resonant process (FIG. 10c) governed by the snubber capacitances
and the leakage inductance of the transformer is initiated. Through
this process the snubber capacitances are recharged so that the
potential of the phase terminals of both phase legs swing to the
opposite direct voltage rail. This also means that u.sub.tr goes
from +U.sub.d to -U.sub.d or vice versa. When this is completed the
diodes that initially blocked the direct voltage take over the
current and the switches that are anti-parallel to them are turned
on at zero-voltage and zero-current conditions. The current
i.sub.tr is forced down linearly until it reaches the initial
level. At this stage the valves of the direct converter, that were
turned on initially, turn off by natural commutation and the
process is completed. Note that the enhancement current can be used
to compensate for losses in the resonant circuit to ensure that the
snubber capacitors are completely recharged before the free current
path on the alternating voltage side is broken. It can also be used
to compensate for variations in the alternating voltage side
current, i.sub.AC,i, during the commutation process.
[0075] It is illustrated in FIG. 11 how u.sub.AC,i, u.sub.tr and
i.sub.tr develop over time during the different states illustrated
in FIGS. 10a-f. It is noted that the procedures of one phase leg
(FIG. 8a-f) and two phase legs (FIGS. 10a-f) are principally the
same with respect to the variables shown in FIGS. 9 and 11.
N.sub.tr has in this figure for simplicity been considered to be
1.
[0076] It should be noted that the above description represents a
full utilization of the resonant commutation method. Another
alternative is to switch the phase legs of the direct converter so
as to provide the said current path after the turn off of the
switch(-es) (semiconductor device(-s)) in the VSC-converter.
Thereby a process is obtained in which the snubber capacitors
firstly are partly recharged by the non-resonant method, thereafter
for a period of time the resonant current assists in the
commutation, and finally the commutation is completed without the
resonant current. This method has the advantage that the peak
current through the transformer is reduced which leads to lower
losses. The direct converter phase legs have to be switched before
u.sub.tr reaches zero in order for the resonant process to take
place.
[0077] In the second alternative, illustrated in FIGS. 12a-h, the
VSC phase legs are switched one at a time. Initially the direct
converter short-circuits the transformer in the same fashion as
described above in order to increase the transformer current by a
certain predefined amount. In the next step only one of the
conducting switches is turned off leading to a resonance between
the snubber capacitances of the concerned phase leg and the leakage
inductance of the transformer. After some time a diode in the
commutating phase leg takes over the current and the semiconductor
device anti-parallel to this diode is turned on at zero-voltage and
zero-current conditions. Thereby the system enters the state where
the transformer voltage and thereby also the alternating voltage
side voltage equal zero. The resonant current is still flowing
through the transformer. To complete the commutation the other
phase leg is commutated by turning off the remaining semiconductor
device that is carrying current. The resonance between the snubber
capacitors and the leakage inductance brings down the current and
brings the phase potential to the opposite direct voltage rail.
Again, after some time a diode in the commutating phase leg takes
over the current and the semiconductor device anti-parallel to this
diode is turned on at zero-voltage and zero-current conditions.
Finally, the current in the transformer is forced down to the
initial value and the alternating voltage side returns to its
initial state by a natural commutation. Also in this case the
enhancement current may be used as a means for ensuring that the
commutation of the VSC-converter is rapidly completed. It can also
be used to compensate for variations in the alternating voltage
side currents i.sub.AC,i, during the commutation process.
[0078] It is illustrated in FIG. 13 how u.sub.AC,i, u.sub.tr and
i.sub.tr develop over time during the different states illustrated
in FIGS. 12a-h. N.sub.tr has in this figure for simplicity been
considered to be 1.
[0079] It should be noted that the above description represents a
full utilization of the resonant commutation method. Another
alternative is to switch the phase legs of the direct converter so
as to provide the said current path after the turn off of the
switch in the first phase leg in the VSC-converter. Thereby a
process is obtained in which the snubber capacitors firstly are
partly recharged by the non resonant method, thereafter the
resonant current assists in the commutation, and finally the
commutation is completed without the resonant current. This method
has the advantage that the peak current through the transformer is
reduced which leads to lower losses. The direct converter phase
legs have to be switched before u.sub.tr reaches zero in order for
the resonant process to take place.
[0080] The converter system according to the present invention has
similar properties as voltage source bidirectional dc/ac converters
in the sense that it can provide a controllable voltage pulse train
on the alternating voltage side terminal. Regardless of the
polarity and magnitude of the alternating voltage side currents,
i.sub.AC,i, the alternating voltage side phase voltages.
u.sub.AC,i, can be made up of positive or negative voltage pulses.
The shape of the pulse train, i.e. the polarity and duration of the
pulses, is determined in such a way that certain objectives are
fulfilled. As examples a few such objectives are mentioned
below:
[0081] 1. That the alternating voltage side phase voltages,
u.sub.AC,i, are pulse width modulated to follow desired reference
signals.
[0082] 2. A certain desired power flow from the direct voltage side
to the alternating voltage side or vice versa.
[0083] 3. A certain harmonic content in the alternating voltage
side phase voltages or, indirectly, in the alternating voltage side
currents.
[0084] 4. A certain impedance as seen from the alternating voltage
side terminals.
[0085] The nature of the desired pulse pattern will in general be
heavily affected by the type of application and by the nature of
the circuitry connected to the alternating voltage side of the
converter system. Methods for determining the shape of the pulse
pattern in order to fulfil objectives such as those described above
are well known and have been described extensively in the
literature, see for example "Power Electronics-Converters,
Applications and Design", second edition, John Wiley, 1995, Mohan,
Undeland and Robbins. They will therefore not be treated here. An
example of a pulse train for one of the alternating voltage side
phase voltages for the case where the average voltage during a
switching interval should coincide with a certain reference voltage
is given in FIG. 14.
[0086] A detailed description of possible commutation sequences
will now be made. By commutation sequence it is here meant a
sequence, of arbitrary length, of commutations of the phase legs of
the two converters in the system, which is carried out in order to
achieve certain objectives. A generic commutation sequence consists
of alternating between commutation of all the direct voltage side
phase legs and commutation of all alternating voltage side phase
legs. The assumption is made that the converter initially is in a
state where the voltages of the alternating voltage side terminals
are of the same sign as the corresponding currents on these
terminals, i.e. that u.sub.AC,ii.sub.AC,i>0, or that
i.sub.AC,i=0, i=1, 2, 3.
[0087] I. Commutate the phase legs of the VSC-converter. This could
be made in a variety of ways. Firstly, the commutations could
either be of the non-resonant kind or the resonant kind. Secondly,
in case the VSC-converter is equipped with two phase legs, these
could either be commutated simultaneously or one at a time. In case
the phase legs are commutated one at a time the interval between
their respective commutations is chosen in order to obtain an
interval where the transformer voltage, u.sub.tr, and thus also the
alternating voltage side phase voltages, u.sub.AC,i, are zero.
After the commutation of all phase legs in the VSC-converter the
alternating voltage side phase voltages, u.sub.AC,i, are of
opposite sign as the corresponding alternating voltage side
currents, i.sub.AC,i i.e. the power flow in the system is directed
from the alternating voltage side to the direct voltage side.
[0088] II. Commutate the phase legs of the alternating voltage side
converter. The phase legs will be commutated in a certain order
with certain time intervals between the commutations. These time
intervals include an interval before the commutation of the first
phase leg as well as a time interval after the commutation of the
last phase leg. One or several time intervals may very well be
zero, i.e. the commutation of two or three phase legs may occur
simultaneously. It should be noted that the fractions of the
commutation cycle during which u.sub.ac,i=1/2N.sub.trU.s- ub.d and
u.sub.AC,i=-1/2N.sub.trU.sub.d can be controlled by properly
choosing the instant at which phase leg i. is commutated. After the
commutation of all phase legs in the alternating voltage side
converter the alternating voltage side phase voltages, u.sub.AC,i,
are of the same sign as the corresponding alternating voltage side
currents, i.sub.AC,i, i.e. the power flow in the system is directed
from the direct voltage side to the alternating voltage side.
Thereby the sequence can start over again at I.
[0089] The sequence that is represented by the steps I and II above
is hereafter referred to as a commutation cycle. In case the
initial condition u.sub.AC,ii.sub.AC,i>0 for i=1, 2, 3 does not
apply the cycle could as well begin with any other applicable step
A commutation sequence is made up of a number of commutation cycles
following on each other. Note that the commutation cycles in the
sequence may very well be different from each other. The time
intervals mentioned above between the commutations will be
determined based on a number of considerations such as:
[0090] 1. The desired alternating voltage side output voltage pulse
pattern, as described above.
[0091] 2. The need to achieve proper operation of the transformer
and avoid saturation of the transformer core.
[0092] It is possible to choose the time delays mentioned above in
such a fashion that the commutation cycle is always run through at
a constant frequency.
[0093] The algorithm for choosing the time intervals based on the
above mentioned considerations could include a correction for the
fact that the commutation does not alter the transformer voltage
and the alternating voltage side output voltages
instantaneously.
[0094] An example of a commutation cycle as described above is
schematically illustrated in FIG. 15a-i. The development of
u.sub.tr, i.sub.tr and u.sub.AC,i, i=1, 2, 3 during this process is
illustrated in FIG. 16.
[0095] The steps of this commutation cycle are as follows:
[0096] FIG. 15a: Initial state. The current flows through the
semiconductor switches in the VSC-converter and the power flows
from the direct voltage side to the alternating voltage side.
[0097] FIG. 15b: The commutation of one of the phase legs of the
VSC-converter is initiated by turning off one of the semiconductor
switches that carries current. Thereby u.sub.tr starts to decrease
linearly and finally reaches zero.
[0098] FIG. 15c: As u.sub.tr reaches zero the opposite diode in the
commutating VSC-converter phase leg takes over the current. The
switch that is antiparallel to this diode is turned on at
zero-voltage and zero-current conditions. The duration of this
interval is set to provide the zero-voltage interval commanded by
the modulator.
[0099] FIG. 15d: The commutation of the other VSC-converter phase
leg is initiated and u.sub.tr starts increasing in the opposite
direction compared to the initial state.
[0100] FIG. 15e: As the commutation of the second phase leg of the
VSC-converter is completed the commutation of phase leg 1. and 3.
of the direct converter begins.
[0101] FIG. 15f: The commutation of phase leg 1. and 3. of the
direct converter is completed and power again flows from he DC side
to the AC side.
[0102] FIG. 15g: Finally, phase leg 2. of the direct converter is
commutated.
[0103] FIGS. 15h-i: The commutation of phase leg 2. of the direct
converter principally returns the system to the initial state and
the commutation cycle can be repeated.
[0104] In the case the VSC-converter is equipped with two phase
legs and these phase legs are commutated one at a time for several
commutation cycles measures can be taken in order to achieve a
uniform loading of the valves in the phase legs. This can be done
by varying the order in which the phase legs are commutated in an
appropriate fashion, noting that this order does not affect the way
the converter couples the direct voltage capacitor to the
transformer.
[0105] By alternating voltage side commutation is here meant the
commutation of all phase legs, simultaneously or one at a time, in
the direct converter, whereas, by direct voltage side commutation
is here meant the commutation of all phase legs, simultaneously or
one at a time, in the VSC-converter.
[0106] A few means of achieving rapid transitions from alternating
voltage side commutation to direct voltage side commutation, or
vice versa, will be described.
[0107] As mentioned above all of the phase legs of the
VSC-converter have to be commutated for the commutation of the
direct converter to be possible. The state that the system appears
in after the direct voltage side commutation implies that each of
the AC side phase voltages are of the opposite sign as the
corresponding AC side phase currents. This state may not be
desirable with regard to the desired pulse pattern on the AC
terminal and in such a case the commutation of the direct converter
should start immediately after the commutation of the
VSC-converter. During the commutation of the VSC-converter the AC
side phase voltages will, however, for a short period of time be
such that u.sub.AC,ii.sub.AC,i<0 for i=1, 2, 3. This period of
time and the voltage-time area during the period may be reduced by
starting the alternating voltage side commutation prior to the
completion of the direct voltage side commutation. This is made by
turning on the relevant semiconductor device or semiconductor
devices of the alternating voltage side converter when the
transformer voltage, u.sub.tr, has changed sign, due to the direct
voltage side commutation, and risen to a certain level. The minimum
allowable value of this voltage level is determined by the demand
that the direct voltage side commutation should be completed before
the transformer current reaches zero as a consequence of the
alternating voltage side commutation.
[0108] As mentioned above all of the phase legs of the direct
converter have to be commutated for the commutation of the
VSC-converter to be possible. The state that the system appears in
after the alternating voltage side commutation implies that each of
the AC side phase voltages are of the same sign as the
corresponding AC side phase currents. This state may not be
desirable with regard to the desired pulse pattern on the AC
terminal and in such a case the commutation of the VSC-converter
should start immediately after the commutation of the direct
converter. During the commutation of the VSC-converter the AC side
phase voltages will however for a short period of time fulfil
u.sub.AC,ii.sub.AC,i>0 for i=1, 2, 3. This period of time and
the voltage time area during it may be reduced by starting the
direct voltage side commutation prior to the completion of the
alternating voltage side commutation. This is made by turning off
the relevant semiconductor device or semiconductor devices of the
direct voltage side converter when the transformer current i.sub.tr
has changed sign due to the alternating voltage side commutation,
and risen to a certain level. The minimum allowable value of this
current level is determined by the demand that the alternating
voltage side commutation should be completed before the transformer
voltage reaches zero as a consequence of the direct voltage side
commutation.
[0109] The two methods of altering the conventional commutation
described above are hereafter referred to as interlaced
commutation.
[0110] As an example the commutation cycle illustrated in FIGS.
15a-15i and FIG. 16 can be modified to incorporate interlaced
commutation by starting the commutation of the direct converter
phase legs 1 and 3 prior to the completion of the commutation of
phase leg 2 of the VSC. The earliest point at which the commutation
of the direct converter can be started is determined by the demand
that the commutation of VSC phase leg 2 should be completed before
the transformer current changes direction.
[0111] In case resonant commutation is used for the direct voltage
side converter and there is a desire to achieve rapid transitions,
without unnecessary delays, from the alternating voltage side
commutation to the direct voltage side commutation, this can be
made by initiating the direct voltage side resonant commutation, by
turning on the relevant semiconductor device or semiconductor
devices in the alternating voltage side converter, prior to the
completion of the alternating voltage side commutation. Note that
this does not imply that any current will flow through these
semiconductor devices before the alternating voltage side
commutation is completed, but only that delays between the
commutation of the two converters are avoided.
[0112] Likewise, in case resonant commutation is used for the
direct voltage side converter and there is a desire to achieve
rapid transitions, without unnecessary delays, from the direct
voltage side commutation to the alternating voltage side
commutation, this can be made by initiating the alternating voltage
side commutation, by turning on the relevant semiconductor device
or semiconductor devices in the alternating voltage side converter,
prior to the completion of the direct voltage side resonant
commutation. Note that this does not imply that any current will
flow through these semiconductor devices before the direct voltage
side commutation is completed, but only that delays between the
commutations of the two converters are avoided.
[0113] An example of a switching cycle involving resonant
commutation of the VSC-converter with a desired power flow from the
alternating voltage side to the direct voltage side is illustrated
in FIGS. 17a-17k. The development of u.sub.tr, i.sub.tr and
u.sub.AC,i, i=1, 2, 3 during this process is illustrated in FIG.
18. N.sub.tr has in this figure for simplicity been set to 1. It is
schematically illustrated in FIG. 17c that the apparatus comprises
means 41 for detecting the current though the second transformer
winding for ensuring that the current through the transformer has
increased by a predetermined enhancement current value before the
control of the VSC-converter for changing the switching state
thereof is started.
[0114] The steps of this commutation cycle are as follows:
[0115] FIG. 17a: This is the initial stage, in which the current
flows through switches in the VSC-converter and power flows from
the direct voltage side to the alternating voltage side. An active
voltage vector, (N.sub.trU.sub.d/2, -N.sub.trU.sub.d/2,
-N.sub.trU.sub.d/2), is applied to the ac output.
[0116] FIG. 17b: This is the enhancement state. The phase legs of
the direct converter are switched so as to provide a path for
current through the transformer in the direction of the transformer
voltage. This causes the transformer current to start increasing
linearly.
[0117] FIG. 17c: This is the first resonant stage. When the
transformer current has increased by a certain predetermined
amount, the so called enhancement current, indicated by the current
detecting means 41, the commutation of one of the phase legs of the
VSC-converter is started. This initiates a resonance process, which
recharges the snubber capacitors of the commutating phase leg and
thereby brings down u.sub.tr to zero. Simultaneously the
transformer current increases.
[0118] FIG. 17d: When the recharging of the snubber capacitors is
completed a diode takes over the current in the commutating phase
leg of the VSC-converter. The semiconductor switch that is
anti-parallel to this diode is turned on at zero-voltage and
zero-current conditions. The current through the transformer
freewheels through the VSC-converter. The transformer voltage as
well as the output AC voltages are all zero.
[0119] FIG. 17e: When the desired AC output zero voltage interval
has expired the commutation of the other VSC-converter phase leg is
initiated. Thereby a second resonant process begins which recharges
the snubber capacitors of this phase leg and brings down the
transformer current. Simultaneously (or previously) the commutation
of phase legs 1 and 3 of the direct converter is prepared by
turning on the relevant switches of the direct converter.
[0120] FIG. 17f: The commutation of the second VSC-converter phase
leg is completed and the switch that is anti-parallel to the diode
that takes over the current is turned on at zero-voltage and
zero-current conditions. The transformer current decreases
linearly.
[0121] FIG. 17g: When i.sub.tr reaches the level of i.sub.AC,1 the
switches in the direct converter that were turned on in order to
provide the resonance circuit turn off by natural commutation. The
commutation of phase leg 1 and 3 of the direct converter begins
without delay.
[0122] FIG. 17h: The commutation of phase leg 1 and 3 of the direct
converter is completed and a second active voltage vector,
(N.sub.trU.sub.d/2, N.sub.trU.sub.d/2, -N.sub.trU.sub.d/2), is
applied to the ac output during a desired period of time.
[0123] FIG. 17i: Finally, the commutation of phase leg 2 of the
direct converter is started.
[0124] FIG. 17j-k: When phase leg 2 of the direct converter is
completed the system is principally back in the initial state and
the commutation cycle can be started over again.
[0125] The commutation sequence is adapted to the operating
conditions and may for certain conditions be as follows. In case
the direction of the alternating voltage side currents, i.sub.AC,i,
changes during operation, for instance because the currents are of
the alternating type, some alterations to the commutation cycle may
be necessary. This means that the system will go to a new step in
the commutation cycle without following the order described
previously in such a fashion that it can proceed from the mentioned
new step as described previously.
[0126] To exemplify, a few methods of achieving this are
described.
[0127] In case the direction of i.sub.AC,i changes during step II
in the commutation cycle before the commutation of phase leg i. the
following procedures could be undertaken. Firstly, the commutation
of phase leg i. could be omitted in the commutation cycle in
question. In this case other alterations of the commutation
sequence could also be made in order to minimise the impact on the
PWM pattern of u.sub.AC,i that the exclusion of the commutation may
have. Secondly, if possible, the commutation of phase leg i. in the
direct converter could be carried out by hard switching, i.e. by
turning off a valve that carries current by gate control. Thereby
the commutation could be made to occur at the desired instant
despite the change of direction of the current. In this case it
should be noted that the current is likely to be small close to a
direction reversal which leads to less severe stress and lower
losses in the device that turns off by hard commutation.
[0128] In case the direction of i.sub.AC,i changes during step II
in the commutation cycle after the commutation of phase leg i. the
phase leg could preferably be commutated once more before the end
of step II. This should preferably be made at the end of step II
whereby the impact on the PWM pattern of u.sub.AC,i can be
minimised.
[0129] In case the direct voltage side converter is equipped with
two phase legs and these are commutated one at a time by
non-resonant commutation and i.sub.tr changes direction in step I
during the interval when the first of these phase legs has been
commutated, due to variations in the alternating voltage side
currents, the following procedure could be undertaken. The phase
leg that was commutated at the beginning of the cycle is commutated
again when the desired zero-voltage interval is completed and
thereafter the system proceeds to step II. The timing of the
subsequent commutations is properly adapted in order to minimise
the impact on the PWM pattern of u.sub.AC,i, i=1, 2, 3 and also in
order to ensure proper operation of the transformer.
[0130] It is also possible to use an algorithm for predicting the
zero crossings of i.sub.AC,i in advance and thereby be able to make
appropriate changes to the commutation sequence that may take
effect also prior to the zero crossings.
[0131] In case a zero crossing of i.sub.AC,i may occur this may be
prepared for in the sense that the valves of phase leg i. in the
direct converter that carry current are controlled to conduct in
both directions.
[0132] The current valves of the direct converter may be equipped
with appropriate snubber circuits in order to prevent overvoltages
due to reverse recovery processes during the turn off of these
valves. Reverse recovery in the diodes or thyristors of the direct
converter may give rise to high current derivatives and by that
high overvoltages. It is a general knowledge that snubber circuits
may, and mostly they must, be used in this context, although it
does not belong to the core of the present invention.
[0133] To conclude, the proposed converter concept combines the
best properties of a direct converter with those of a voltage
source converter to form a system with low component count,
significantly reduced switching losses and a very attractive set of
functionalities.
[0134] The invention is of course not in any way restricted to the
preferred embodiments described above, but many possibilities to
modifications thereof will be apparent to a person with ordinary
skill in the art without departing from the basic idea of the
invention as defined in the appended claims.
[0135] It is pointed out that "detecting" as used above for the
current detecting means 41 and in the corresponding appended claim
has to be understood also to comprise the case of indirect
detection of the current through the second transformer winding.
This means may very well be connected to measure the current in the
first transformer winding and use the transformation ratio of the
transformer for obtaining the current in the second transformer
winding.
* * * * *