VSC-converter

Details VSC-converter VSC-converter

2000-05-16

2001-07-10

Nguyen, Matthew (Department: 2838)

Electric power conversion systems

Current conversion

Using semiconductor-type converter

Reexamination Certificate

active

06259616

ABSTRACT:

FIELD OF THE INVENTION AND PRIOR ART
The present invention relates to a VSC-converter for converting direct voltage into alternating voltage and conversely and which comprises at least one phase leg with an NPC-connection, i.e. four current valves connected in series, which consist of at least one semiconductor device of turn-off type and a first rectifying member connected in anti-parallel therewith, in which a point on the phase leg between two inner valves of the series connection is intended to be connected to a phase of an alternating voltage network and the opposite ends of the phase leg are intended to be connected to a pole conductor each of a direct voltage network or a direct voltage intermediate link, and in which a series connection of two second so called clamping rectifying members directed in the same direction with respect to said series connection as the first rectifying members are connected between on one hand a point between one of the outer valve of the series connection and the adjacent inner valve and on the other a point between the other of the outer valves in the series connection and the adjacent inner valve, and a midpoint between the two clamping rectifying members is connected to a zero potential defined by capacitors connected in series between said pole conductors.
Such a VSC-converter for connection between a direct voltage network and an alternating voltage network has recently become known through the thesis “PWM and control of two and three level High Power Voltage Source Converters” by Anders Lindberg, Kungliga Tekniska Högskolan, Stockholm, 1995, in which publication a plant for transmitting electric power through a direct voltage network for high voltage direct current (HVDC) while utilizing such converters is described. Before the issuance of said thesis plants for transmitting electric power between a direct voltage network and an alternating voltage network have been based upon the use of line-commutated CSC(Current Source Converters)-converters in stations for power transmission. However, in this thesis a totally new concept is described, which is based on the use of VSC(Voltage Source Converter)converters for forced commutation instead for transmitting electric power between a direct voltage network being voltage-stiff therethrough, in the present case for high voltage direct current, and alternating voltage networks connected thereto, which offers several important advantages with respect to the use of line-commutated CSC-converters in HVDC, of which it may be mentioned that the consumption of active and reactive power may be controlled in dependency of each other and there is no risk of commutation failures in the converter and thereby no risk of commutation failures between different HVDC links, which may take place in line-commutated CSC's. Furthermore, there is a possibility to feed a weak alternating voltage network or a network without a generation of its own (a dead alternating voltage network). There are also further advantages.
The invention is not restricted to this application, but the converter may just as well be intended for conversion in a SVC, in which case the direct voltage network is replaced by a DC-intermediate link. “Network”is also to be given a very broad sense, and it does not have to be a question about any such networks in the real meaning of that word. However, the problems of the invention will now be illuminated for exactly this application, although the problem is common to all conceivable applications within the scope of the invention. When using a converter of the type mentioned in the introduction instead of a current-stiff CSC-converter a new problem not present in the latter is created, which will now be explained by reference to
FIGS. 1
,
2
and
3
. Firstly, it may be mentioned that the advantage of using a so called NPC(Neutral Point Clamped)-connection, which accordingly means that the phase terminal may be provided with positive and negative voltage from the respective pole conductor as well as zero voltage, with respect to a so called two-pulse bridge, or in the case of three phases a so called six-pulse bridge, is that a lower switching frequency may used, which results in lower losses and a higher efficiency. However, this still suffers from said problems not yet described. A VSC-converter
1
of the type defined in the introduction is shown in FIG.
1
and it has a phase leg
2
with four current valves
3
-
6
connected in series, which each consists of at least one semiconductor device of turn-off type, such as an IGBT
7
, and a first rectifying member in the form of a diode
8
connected in anti-parallel therewith. A point on the phase leg between the two inner valves
4
,
5
in the series connection is intended to be connected to a phase
9
of an alternating voltage network. A series connection of two second so called clamping rectifying members
10
,
11
in the form of diodes directed in the same direction with respect to said series connection as the first rectifying members are connected between on one hand a point
12
between one outer valve
3
in the series connection and the inner valve
4
adjacent thereto and on the other a point
13
between the outer valve
6
of the series connection and the inner valve
5
adjacent thereto with a midpoint
14
between the two clamping rectifying members connected to a zero potential
15
defined by capacitors
19
,
20
connected in series between a positive pole conductor
16
and a negative pole conductor
17
of the direct voltage network
18
. It is illustrated in
FIGS. 2 and 3
what is happening upon commutation, i.e. when the voltage output on the phase terminal
9
is changed. It is imagined that in the case shown in
FIG. 2
current flows from the negative pole conductor
17
of the direct voltage network through the diodes of valves
5
and
6
to the phase terminal
9
, which then gets a voltage −U
d
of the pole conductor
17
. The semiconductor devices
7
of the current valves
3
and
4
are turned off. If a commutation now takes place, so that the semiconductor device in the current valve
4
is turned on, the current will then be commutated to go through the clamping diode
10
and the current valve
4
to the phase terminal
9
, which then receives zero potential. This will result in a commutation current according to the loop
21
, which is shown in FIG.
2
. It is illustrated in
FIG. 3
what is happening if the first semiconductor devices of the current valves
5
and
6
are turned on and thereby the current flows from the phase terminal
9
to the pole conductor
17
and thereby the phase terminal has the potential −U
d
and then a commutation takes place, so that the semiconductor device in the current valve
6
is turned off and the current is instead lead through the current valve
5
and the clamping diode
11
to the point
14
and through the capacitor
20
to the pole conductor
17
, so that the phase output
9
receives zero potential. A commutation current according to a smaller commutation loop
22
is then resulting. The commutation current will flow in the respective commutation loop in the opposite direction should the commutation take place in the opposite order to the one described. Similar commutation loops, i.e. a large and a small loop also exist upon commutation through the upper capacitor
19
. The commutation times of such a VSC-converter are short and comparatively large time differential coefficients of the current are created, which makes it desired to lower the inductance in the commutation circuit, i.e. in the very phase leg and the line connected between the poles of the direct voltage network with capacitors for defining the direct voltage, to a level being as low as possible so as to avoid unnecessarily high over-voltages and thereby losses upon said commutation.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a VSC-converter of the type defined in the introduction, in which the problem mentioned above has been solved in a satisfying wa

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