Bidirectional VSC converter with a resonant circuit

The invention relates to a VSC-converter for converting direct voltage into auxiliary voltage and vice versa, which comprises a series connection of at least two current valves (5, 6) arranged between two poles (7, 8), a positive and a negative, of a direct voltage side of the converter, each current valve comprising several series connected circuits (12), each of which circuits comprising a semiconductor component (13) of turn-off type and a rectifying component (14) connected in anti-parallel therewith, an alternating voltage phase line (16) being connected to a midpoint (15), denominated phase output, of the series connection of current valves (5, 6) between two of said current valves while dividing the series connection into two equal parts. Each of the series connected circuits (12) of the respective current valve comprises, in order to make possible a good voltage distribution between the semiconductor components (13) of turn-off type included in the respective current valve, a snubber capacitor (17) connected in parallel with the semiconductor component (13) of turn-off type included in the circuit. The converter (1) further comprises a resonance circuit (18) for recharging the snubber capacitors (17) of the current valves.

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a VSC-converter according to the preamble of the subsequent claim1.

A VSC-converter for connection between a direct voltage net-work and an alternating voltage network is previously known e.g. from the thesis “PWM and control of two and three level High Power Voltage Source Converters” by Anders Lindberg, Royal Institute of Technology, 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 creation of this thesis, plants for transmitting electric power between a direct voltage network and an alternating voltage network have been based upon the use of network commutated CSC(Current Source Converter)-converters in stations for power transmission. However, in this thesis a totally new concept is described, which is based on instead using VSC(Voltage Source Converter)-converters for forced commutation for transmitting electric power between a direct voltage network being voltage stiff therethrough, in the case in question for high-voltage direct current, and alternating voltage networks connected thereto, which offers several considerable advantages as compared to the use of network commutated CSC-converters in HVDC, among which it may be mentioned that the consumption of active and reactive power may be controlled independently of each other and that there is no risk of commutation faults in the converters and thereby no risk of commutation faults being transmitted between different HVDC-links, as may occur with network commutated CSC:s. Furthermore, it is possible to feed a weak alternating voltage network or a network without any generation of its own (a dead alternating voltage network). There are also further advantages.

The inventional VSC-converter may be included in a plant for transmitting electric power through a direct voltage network for high-voltage direct current (HVDC), in order to e.g. transmit the electric power from the direct voltage network to an alternating voltage network. In this case, the converter has its direct voltage side connected to the direct voltage network and its alternating voltage side connected to the alternating voltage network. The inventional VSC-converter may however also be directly connected to a load, such as a high-voltage generator or motor, in which case the converter has either its direct voltage side or its alternating voltage side connected to the generator/motor. The invention is not limited to these applications; on the contrary the converter may just as well be used for conversion in a SVC (Static Var Compensator) or a Back-to-back station. The voltages on the direct voltage side of the converter are with advantage high, 10-400 kV, preferably 130-400 kV. The inventional converter may also be included in other types of FACTS-devices (FACTS=Flexible Alternating Current Transmission) than the ones mentioned above.

VSC-converters are known in several designs. In all designs, a VSC-converter comprises a number of so-called current valves, each of which comprising a semiconductor element of turn-off type, such as an IGBT (Insulated Gate Bipolar Transistor) or a GTO (Gate Turn-Off Thyristor), and a rectifying member in the form of a diode, normally a so-called free wheeling diode, connected in anti-parallel therewith. Each semiconductor element of turn-off type is normally in high voltage applications built up of several series connected, simultaneously controlled semiconductor components of turn-off type, such as several separate IGBT:s or GTO:s. In high-voltage applications a comparatively high number of such semiconductor components is required in order to hold the voltage to be held by each current valve in the blocking state. In the corresponding manner, each rectifying member is built up of several series connected rectifying components. The semiconductor components of turn-off type and the rectifying components are in the current valve arranged in several series connected circuits, each of which circuits comprising i.a. a semiconductor component of turn-off type and a rectifying component connected in anti-parallel therewith.

In the manufacturing of VSC-converters intended for high-voltage applications, it is in the current valves of the converter desirable to use semiconductor components of turn-off type that are dimensioned to stand a voltage as high as possible. Hereby, the conduction losses in the current valves can be reduced. The use of high-voltage components also entails that the number of components of the current valves, and thereby the manufacturing costs for the converter, can be limited. High-voltage semiconductor components designed for low conduction losses do however have the characteristic that they during the conduction process will build up a conductive plasma that will cause a relatively large reverse recovery amount when the semiconductor component is turned off or the rectifying component connected in anti-parallel therewith is commutated off. Since this reverse recovery amount, on grounds of manufacturing technique and due to temperature variations between different components during operation, varies from component to component, it is in practise not possible to achieve a completely synchronous turn-off of the current through all the semiconductor components. This entails that the voltage distribution between the semiconductor components of turn-off type in one and the same current valve will be unequal. Such an unequal voltage distribution will i.a. result in an unequal degradation of the semiconductor components of turn-off type included in the current valves, and is therefore not desired.

OBJECT OF THE INVENTION

The object of the present invention is to achieve a VSC-converter according to the preamble of claim1, in which the above-described problem with unequal voltage distribution of the semiconductor components of turn-off type included in the current valves is reduced.

SUMMARY OF THE INVENTION

According to the invention, said object is achieved by means of a VSC-converter having the features indicated in the characterizing part of claim1.

In the inventional VSC-converter, a good voltage distribution between the semiconductor components of turn-off type included in a current valve is achieved by means of snubber capacitors, each of which being connected in parallel with a semiconductor component of turn-off type. By using a resonance circuit for recharging these snubber capacitors, it will also be possible to avoid high turn-on losses in connection with the turn-on of the semiconductor components of turn-off type of the current valves. The resonance circuit is adapted to achieve recharge of the snubber capacitors in connection with turn-on of the semiconductor components of turn-off type in the current valves of the converter so that said semiconductor components can be turned on at low voltage instead of high voltage, whereby the turn-on losses in the semiconductor components of the current valves are limited. The resonance circuit is also used in connection with turn-off of the semiconductor components in the current valves when the phase current is so low that the switching time for the voltage in the phase output otherwise would be unreasonable long. The solution according to the invention consequently implies that semiconductor components dimensioned for very high voltages can be used in the current valves of the converter while maintaining a good voltage distribution in the current valves and low turn-on losses.

In this description and the subsequent claims, the expression resonance circuit refers to the circuit by means of which the recharge of the snubber capacitors is effected. In its proper sense, the expression resonance circuit also embraces said snubber capacitors, but for the sake of simplicity and clarity it will here be made a distinction between the components denominated snubber capacitors and the other parts of the resonance circuit. Consequently it is these “other parts of the resonance circuit” which here and in the following will be denominated “resonance circuit”.

The solution according to the invention will give particularly large advantages in VSC-converters connected to high-voltage networks, with a network voltage of about 130-400 kV, but will also give advantages at lower network voltages, for instance in the order of 10-130 kV.

According to a preferred embodiment of the invention, the resonance circuit is an ARCP-circuit (ARCP=Auxilary Resonant Commutation Pole). A resonance circuit of this type has proven to be very suitable for the application here in question.

According to a further preferred embodiment of the invention, the ARCP-circuit comprises an auxiliary valve comprising several series connected sets of auxiliary valve circuits, where each set comprises two series connected auxiliary valve circuits, each of which comprising a semiconductor component of turn-off type and a rectifying component connected in anti-parallel therewith, the semiconductor components of turn-off type in the two auxiliary valve circuits in one and the same set being arranged in opposite polarity in relation to each other. By making the ARCP-circuit comprising several sets of auxiliary valve circuits in this manner, instead of one single set as conventional, it will in a simple manner be possible to adapt the ARCP-circuit in dependence of the desired characteristics of the converter.

According to a further preferred embodiment of the invention, the ARCP-circuit comprises several series connected partial circuits, each of which comprising a set of auxiliary valve circuits connected in series with an inductor, the ARCP-circuit further comprising a number of capacitors, which are connected in parallel with a respective partial circuit. By dividing the inductance of the resonance circuit on several separate inductors and arranging a capacitor connected in parallel with each of said partial circuits in the indicated manner, the problem with unequal voltage distribution between the auxiliary valve circuits included in the resonance circuit caused by stray capacitances to ground is reduced. This embodiment is particularly advantageous in VSC-converters dimensioned for very high voltages with a voltage on the direct voltage side amounting to 100 kV or more.

According to a further preferred embodiment of the invention, each set of auxiliary valve circuits of the ARCP-circuit comprises a voltage dividing circuit connected in parallel with the auxiliary valve circuits included in the set. Hereby, a good voltage distribution between the auxiliary valve circuits included in the resonance circuit is secured.

According to a further preferred embodiment of the invention, the respective current valve comprises such a number of series connected circuits, where each circuit comprises a semiconductor component of turn-off type and a rectifying component connected in anti-parallel therewith, that at least one of these circuits is redundant with respect to the voltage that the current valve is intended to hold in the blocking state. Hereby the current valve can continue to function even in case of a breakdown in one component in one of the series connected circuits, which is of very great importance with respect to the safety of operation of the VSC-converter. In this case, it is preferred that each of said series connected circuits is so designed that the circuit is short-circuited in case of an electric fault therein. Thereby, it is secured that a breakdown in a circuit will get as little influence as possible on the operation of the current valve, since the current relatively unimpedeately is allowed to pass the broken down circuit in both directions.

According to a further preferred embodiment of the invention the auxiliary valve of the ARCP-circuit comprises such a number of said sets of auxiliary valve circuits that at least one of the sets is redundant with respect to the voltage that the auxiliary valve is intended to hold in the blocking state. Thereby, the ARCP-circuit can continue to operate even in case of a breakdown in the component in one of the auxiliary valve circuits. It is also here preferred that each of the auxiliary valve circuits is so designed that the auxiliary valve circuit is short-circuited in case of an electric fault therein. Thereby, it is secured that a break-down in an auxiliary valve circuit will get as little influence as possible on the operation of the ARCP-circuit, since the current relatively unimpededly is allowed to pass the broken-down auxiliary valve circuit in both directions.

Further preferred embodiments of the inventional VSC-converter will appear from the dependent claims and the subsequent description.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2schematically illustrate two alternative embodiments of a VSC-converter1according to the invention. The converter is here provided with three so-called phase legs2-4and is consequently adapted for a three-phase alternating voltage network. This type of assemblage with three-phase legs is normally denominated three-phase bridge. The VSC-converter could however also be adapted for an alternating voltage network having more as well as fewer phases than three. It could for instance be adapted for a single phase alternating voltage network and would in such a case only have one phase leg of the type in question.

Each phase leg2-4of the VSC-converter illustrated inFIGS. 1 and 2has two current valves5,6connected in series between the two poles7,8of a direct voltage side of the converter. A capacitor circuit9comprising at least one so-called intermediate link capacitor is arranged between the two poles7,8. In the converter illustrated inFIGS. 1 and 2the capacitor circuit9comprises two series connected intermediate link capacitors10a,10b. A point11between these capacitors10a,10bis here, as customary, connected to ground so as to provide the potentials +U/2 and −U/2, respectively, at the respective pole, U being the voltage between the two poles7,8. The grounding point11may however be excluded, for instance in SVC-applications.

The respective current valve5,6comprises, in accordance with the above indicated, several series connected circuits12, each of which circuit comprising a semiconductor component13of turn-off type, such as an IGBT, a GCT or a GTO, and a rectifying component14, in the form of a diode, such as a free wheeling diode, connected in anti-parallel therewith. In the embodiments shown inFIGS. 1 and 2each current valve5,6comprises three series connected circuits12of the type described above, but the series connected circuits12may be larger as well as smaller in number. Depending i. a. on the voltage for which the converter is designed, the number of said series connected circuits12in the respective current valve5,6may extend from two up to several hundred.

A midpoint15of the series connection between the two current valves5and6, which constitutes the phase output of the converter, is connected to an alternating voltage phase line16. In this manner, said series connection is divided into two equal parts with one current valve5and6, respectively, in each such part. In the embodiment with three phase legs2-4, the converter consequently comprises three phase outputs15,15′,15″, which are connected to a respective alternating voltage phase line16,16′,16″ of a three-phase alternating voltage network. The phase outputs are normally connected to the alternating voltage network via electric equipment in the form of breakers, transformers etc.

According to the invention, each of the series connected circuits12of the respective current valve5,6is provided with a capacitor17, here denominated snubber capacitor, connected in parallel with the semiconductor component13of turn-off type included in the circuit. The capacitance of the respective snubber capacitor17must be so high that a good voltage distribution between the semiconductor components14of turn-off type included in the respective current valve is made possible in connection with turn-off of the semiconductor components of turn-off type of a current valve. The choice of capacitance of the snubber capacitors17is adapted from case to case and depends i.a. on the voltage-blocking capacity and the current-handling capacity of the semiconductor components13of turn-off type and the rectifying components14and the manufacturing tolerance of these components with respect to the reverse recovery amount. The snubber capacitors17also help to limit the turn-off losses, i.e. the losses in the semiconductor components of turn-off type when these are turned off.

When the semiconductor components13of a current valve are turned off, the snubber capacitors17that are connected across these semiconductor components13will be charged. If the snubber capacitors17keep this charge when the semiconductor components13subsequently are turned on, turn-on losses will ensue in the semiconductor components13. The relatively high capacity snubber capacitors17that will come into question in this connection will in this case cause very high turn-on losses, which turn-on losses make the use of high switching frequencies impossible. In order to eliminate or at least reduce these turn-on losses, and make possible the use of high switching frequencies, the converter according to the invention is provided with a resonance circuit18for recharging the snubber capacitors17of the current valves. This resonance circuit is intended to accomplish discharge of the snubber capacitors17of a current valve when the semiconductor components13of the current valve are to be turned on, so that the voltage across the respective semiconductor component13is equal to or close to zero when it is turned on, whereby the turn-on losses are limited.

The resonance circuit18is suitably of a so-called quasi-resonant type, which implies that the resonance only is initiated when the current is to be commutated between two current valves, i.e. when the voltage on the phase output of the converter is to be changed-over. Resonance circuits of this type are known in several designs and have i.a. been proposed for use in converters where each current valve is provided with a snubber capacitor connected in parallel across the entire current valve. The use of a quasi-resonant so-called ARCP-circuit is for instance described in U.S. Pat. No. 5,047,913.

Two alternative locations of a resonance circuit for recharging the snubber capacitors17of the current valves are illustrated inFIGS. 1 and 2. According to the embodiment shown inFIG. 1, the resonance circuit18is connected between the phase output15and the midpoint11of the resonance circuit9, which midpoint11can be either grounded or ungrounded as previously mentioned. Each phase leg2-4is in this case provided with a separate resonance circuit. In the converter illustrated inFIG. 1, a first resonance circuit18is consequently connected between a first phase output15and said midpoint11, a second resonance circuit18′ is connected between a second phase output15′ and said midpoint11and a third resonance circuit18″ is connected between a third phase output15″ and said midpoint11. The respective resonance circuit18,18′,18″ here suitably consists of a so-called ARCP-circuit (ARCP=Auxilary Resonant Commutation Pole). This ARCP-circuit may for instance be of the type shown in said U.S. Pat. No. 5,047,913, which comprises an auxiliary valve consisting of one set of two series connected auxiliary valve circuits, each of which comprising a semiconductor component of turn-off type and a rectifying component connected in anti-parallel therewith, the semiconductor components of turn-off type of the two auxiliary valve circuits being arranged in opposite polarity in relation to each other. The ARCP-circuit further comprises an inductor connected in series with said set of auxiliary valve circuits. ARCP-circuits with a design particularly suitable for the inventional application will be described more closely below with reference toFIGS. 3 and 4.

In this description and the subsequent claims, the expression auxiliary valve refers to the current valves included in a resonance circuit18.

An advantage with arranging the resonance circuit in the manner illustrated inFIG. 1is that symmetry in relation to ground is automatically obtained, i.e. the resonance circuits will not cause any direct voltages on the alternating voltage side of the converter. Furthermore, each phase is treated separately, which is advantageous with respect to supervision.

According to the embodiment shown inFIG. 2, the resonance circuit18is connected between the two pole connections19,20of the capacitor circuit and the two pole connections21,22of the current valves. In the embodiment with three phase legs2-4, the current valves of all three phase legs have a common connection21to one7of the poles and a common connection22to the other pole. The resonance circuit18is here adapted to inject a resonance current of the direct voltage side of the converter, between the negative pole7and the positive pole8, in order to shortly bring the pole voltage to zero without discharging the intermediate link capacitors10a,10b. Normally, a current that is superposed the load current is in this case shortly forced in the reverse direction through all the rectifying components14of the converter. The commutation of the load current is performed when all the rectifying components14are conducting and the snubber capacitors17have been discharged, i.e. when the voltage across all the current valves5,6is zero or close to zero. Hereby, it will be possible to turn on or turn off a current valve5,6when the voltage across the current valve is zero or close to zero. In order to prevent the intermediate link capacitors10a,10bfrom being discharged, the resonance circuit18comprises in this case either inductors85,87or auxiliary valves71,72or both in order to insulate the intermediate link capacitors from the pole connections21,22of the current valves, i.e. in this case from the three-phase bridge. The resonance circuit18in the embodiment illustrated inFIG. 2should be so designed that the insulation of the intermediate link capacitors10a,10bfrom the current valves takes place in a symmetric manner. Hereby it is avoided that all the phases are connected to one or the other of the two poles in connection with commutation, which is of importance for the avoidance of direct voltage components on the alternating voltage side of the converter.

Two different designs of resonance circuits suitable for being included in a converter according to the invention is illustrated in FIG.2. One18aof these resonance circuits comprises a first71, a second72and a third73auxiliary valve, each of which comprising a semiconductor element74of turn-off type, such as an IGBT, a GCT or a GTO, and a rectifying member75in the form of a diode, such as a free wheeling diode, connected in antiparallel therewith. The first auxiliary valve71has one76of its ends connected to a first19of the pole connections of the capacitor circuit and its other end77connected to the corresponding first pole connection21of the current valves. The second auxiliary valve72has one78of its ends connected to a second20of the pole connections of the capacitor circuit and its other end79connected to the corresponding second pole connection22of the current valves. The third auxiliary valve73forms a series connection together with a capacitor80and an inductor81, which series connection has one82of its ends connected between the second end77of the first auxiliary valve75and the first pole connection21of the current valves, and its other end83connected between the second end79of the second auxiliary valve72and the second pole connection22of the current valves. The resonance circuit18afurther comprises a rectifying member84connected in series with the inductor81and in parallel with a series connection formed by the third auxiliary valve73and said capacitor80.

The auxiliary vales71and72, which are conducting in the normal state, are used in order to electrically insulate the intermediate link capacitors10a,10bfrom the so-called three-phase bridge, i.e. the current valves5,6of the three phase legs2-4, when the voltage across the three-phase bridge is shortly brought to zero in connection with commutation. The two auxiliary valves71,72are each able to take up half the pole voltage and insulate the intermediate link capacitors from the three-phase bridge in a symmetrical manner, which is advantageous when the capacitor circuit9is grounded in its midpoint. The auxiliary valve73is used in order to start the resonance that discharges the snubber capacitors17in the three-phase bridge. The capacitor80and the inductor81are used as energy storing components in order to, when the voltage across the three-phase bridge is equal to zero, shortly store the energy that existed in the resonance capacitors17of the three-phase bridge before the starting of the commutation process. This energy can subsequently be returned to the resonance capacitors17of the three-phase bridge without being lost.

The resonance circuit18awill operate as follows. Initially the voltage across the capacitor80is close to zero. By shortly turning on the semiconductor elements74of the auxiliary valves71,72and73, energy is fed into the capacitor80and the inductor81from the intermediate link capacitors10a,10b. When the semiconductor elements74in the auxiliary valves71and72are subsequently turned off, the current through the inductor81will force the voltage across the three-phase bridge down to zero, whereupon all the rectifying members14in the three-phase bridge will start to conduct and the snubber capacitors17will be discharged. A commutation can now take place in any of the three phase legs2-4of the three-phase bridge. When the current in the inductor81has returned to zero, the capacitor80will be charged with an energy approximately corresponding to the energy that was supplied from the intermediate link capacitors10a,10bat the start of the process plus the energy that the snubber capacitors17in the three-phase bridge had before they began to discharge. During the commutation process energy from the alternating voltage side of the converter has also been fed into or out of the capacitor80during the time the semiconductor elements74of the auxiliary valves71and72have been blocked. The energy in the capacitor80is then used in order to bring back energy to the resonance capacitors17of the three-phase bridge. The remaining energy is brought back to the intermediate link capacitors10a,10b, via the rectifying members73and84.

The second18bof the resonance circuits illustrated inFIG. 2comprises a first inductor85and a second inductor86connected in series with each other and in series with the capacitor circuit9and the current valves5,6between a first19of the pole connections of the capacitor circuit and the corresponding first pole connection21of the current valves. The resonance circuit18bfurther comprises a third inductor87and a fourth inductor88connected in series with each other and in series with the capacitor circuit9and the current valves5,6between a second20of the pole connections of the capacitor circuit and the corresponding second pole connection22of the current valves. The resonance circuit18bfurther comprises a series connection of a first auxiliary valve89, a second auxiliary valve90and a first capacitor91, each of which auxiliary valves comprising a semiconductor element92of turn-off type, such as an IGBT, a GCT or a GTO, and a rectifying member93in the form of a diode, such as a free wheeling diode, connected in antiparallel therewith. Said series connection has one94of its ends connected between the first inductor85and the second inductor86and its other end95connected between the third inductor87and the fourth inductor88. The resonance circuit18bfurther comprises a second capacitor96connected in series with the first auxiliary valve89in parallel with a series connection formed by the second auxiliary valve90and the first capacitor91. Said first capacitor91constitutes a clamp capacitor.

The inductors85and87are used in order to insulated the intermediate link capacitors10a,10bfrom the three-phase bridge so that the voltage across the three-phase bridge shortly can be brought to zero in connection with commutation. The two inductors85,87are each able to take up half the pole voltage and consequently insulate the intermediate link capacitors10a,10bfrom the three-phase bridge in a symmetrical manner, which is advantageous in case the capacitor circuit9is grounded in its midpoint. The capacitor91is a clamp capacitor, which is charged to a voltage that in the normal case is about 20% higher than the pole voltage U. The energy in the clamp capacitor91is used in order to start the commutation process. At the initiation of a commutation process, the semiconductor elements92of the auxiliary valves89and90are turned on, whereupon the capacitor91begins to force a current through the inductors85and87. When the semiconductor element92of the auxiliary valve90is subsequently turned off, the current through the inductors85and87will force the voltage across the capacitor96and the voltage across the three-phase bridge down to zero, whereupon all the rectifying components14in the three-phase bridge begin to conduct and the snubber capacitors17discharges. When the snubber capacitors17have been discharged, a commutation may take place in any of the three phase legs2-4of the three-phase bridge. The energy that is discharged from the capacitor96and the snubber capacitors17during this process is shortly transmitted to the intermediate link capacitors10a,10b. When the current in the inductors85and87reverses, the capacitor96and the snubber capacitors17will begin to recharge, the energy being brought back from the intermediate link capacitors10a,10bto the snubber capacitors17and to the capacitor96and finally also to the clamp capacitor91. The inductors86and88represent small inductances that limit the surge from the clamp capacitor91and the capacitor96to the resonance capacitors17of the three-phase bridge when the semiconductor elements92of the auxiliary valves89and90are being turned on. These small inductances may in the extreme case consist of the leakage inductance of the circuit, in which case the inductors86and88consequently can be omitted.

Each semiconductor element74,92of turn-off type included in the auxiliary valves71-73,89,90of the above described resonance circuits18aand18bis suitably built-up of several series connected, simultaneously controlled semiconductor components of turn-off type, such as several separate IGTB:s or GTO:s. In high-voltage applications a comparatively large number of such semiconductor components is namely required in order to hold the voltage to be held by each auxiliary valve in the blocking state. In the corresponding manner, each rectifying member75,84,93is suitably built-up of several series connected rectifying components. The semiconductor components of turn-off type and the rectifying components are in the respective auxiliary valve, in the same manner as in the current valves5,6, arranged in several series connected circuits, each of which circuit comprising i.a. a semiconductor component of turn-off type and a rectifying component connected in anti-parallel therewith.

FIGS. 3 and 4schematically illustrate VSC-converters according to preferred embodiments of the invention, which are provided with a resonance circuit in the form of an ARCP-circuit connected between the phase output15and ground. The ARCP-circuit is suitably, as illustrated inFIGS. 3 and 4, connected to the midpoint11between the intermediate link capacitors10a,10b, that are arranged on the direct voltage side of the converter.

InFIGS. 3 and 4, only the part of the converter that is connected to one phase of an alternating voltage phase line is shown, the number of phases normally being three, but it is also possible that this constitutes the entire converter when the converter is connected to a single phase alternating voltage network. The shown part of the converter constitutes a phase leg2, and a VSC-converter adapted for a three-phase alternating voltage network comprises three phase legs of the type shown. Just like the embodiments according toFIGS. 1 and 2, the phase leg2of the converter illustrated inFIGS. 3 and 4, respectively, comprises two series connected current valves5,6, each of which comprising, in accordance with the above indicated, several series connected circuits12, where each circuit comprises a semiconductor component13of turn-off type and a rectifying component14of previously described type connected in anti-parallel therewith. InFIGS. 3 and 4, only two series connected circuits of the respective current valve are shown, but the number of such circuits may be considerably larger than that. A midpoint15of the series connection between the two current valves5and6, which constitutes the phase output of the converter, is connected to an alternating voltage phase line16. In this manner, said series connection of current valves is divided into two equal parts with a current valve5and6, respectively, in each such part. Each of these series connected circuits12of the respective current valve5,6is provided with a snubber capacitor17connected in parallel with the semiconductor component13of turn-off type included in the circuit.

In the embodiment illustrated inFIG. 3, the ARCP-circuit comprises an auxiliary valve37comprising several series connected sets30of auxiliary valve circuits, where each set comprises two series connected auxiliary valve circuits31,32, each of which comprising a semiconductor component33of turn-off type, such as an IGBT or a GTO, and a rectifying component34in the form of a diode, such as a free wheeling diode, connected in anti-parallel therewith. The semiconductor components33of turn-off type of the two auxiliary valve circuits in one and the same set are arranged in opposite polarity in relation to each other. The ARCP-circuit further comprises an inductor35connected in series with said set of auxiliary valve circuits. This auxiliary valve37constitutes a bidirectional valve that can be made to conduct in one or the other direction.

InFIG. 3, only two series connected sets30of auxiliary valve circuits in the auxiliary valve37are shown, but the number of such sets can be considerably larger than that. The number of sets of auxiliary valve circuits in the auxiliary valve37may be optimized independently of the number of series connected circuits12in the current valves5,6, and depends i.a. on the voltage the resonance circuit is to be able to hold in the blocking state and the characteristics of the individual semiconductor components33that are being used. Generally, it can be observed that the auxiliary valve37in the blocking state only has to hold half the pole voltage, i.e. U/2, in contrast to the current valves5,6which each has to be dimensioned so as to be able to hold the entire pole voltage U in the blocking state.

Each set30of auxiliary valves circuits is suitably provided with a voltage dividing circuit36, such as illustrated inFIG. 3, connected in parallel with the auxiliary valve circuits31,32included in the set, in order to obtain a good voltage distribution between the series connected auxiliary valve circuits of the resonance circuit. InFIGS. 5-7, different alternative designs of such voltage dividing circuits are illustrated.

The voltage dividing circuit36may, as shown inFIG. 5, comprise a series connection of a capacitor40and a resistor41, which series connection is connected in parallel across both the auxiliary valve circuits31,32of the respective set30of auxiliary valve circuits.

The voltage dividing circuit36may also, as shown inFIG. 6, comprise to series connections, each of which comprising a capacitor42and a resistor43, these series connections being connected in parallel with a respective auxiliary valve circuit31,32of the respective set30. Each of the two series connections of the voltage dividing circuit may further, as illustrated inFIG. 7, comprise a rectifying member44in the form of a diode connected in parallel with the resistor43and in series with the capacitor42.

In order to further improve the voltage distribution between the auxiliary valve circuits31,32of the auxiliary valve, the voltage dividing circuit36should include a high-ohmic resistor23. In the voltage dividing circuit36illustrated inFIG. 5, a high-ohmic resistor45is in such a case connected in parallel with the series connection of the resistor41and the capacitor40. In the voltage dividing circuits36illustrated inFIGS. 6 and 7, a high-ohmic resistor45is in such a case connected in parallel with each of the auxiliary valve circuits31,32. The resistance of these resistors45is adapted in such a manner that disparities in leakage current in the different semiconductor components33in an auxiliary valve37, when this is in the blocking state, will not cause any inequality to speak of in the voltage distribution between the auxiliary valve circuits31,32of the auxiliary valve.

Each set30of auxiliary valve circuits in the auxiliary valve37is suitably, as illustrated inFIG. 3, provided with its own control unit50, which is adapted to control the turn-on and turn-off of the semiconductor components33of turn-off type included in the set, all control units of the auxiliary valve being connected to a common control member51, which is adapted to simultaneously send control signals to all these control units50. Hereby, a simultaneous control of all the auxiliary valve circuits31,32of the auxiliary valve is secured.

It is further preferred that each of the semiconductor components13of turn-off type included the current valves5,6of the converter, as illustrated inFIG. 3, is provided with its own control unit52, which is adapted to control turn-on and turn-off of the semiconductor component13, all control units52of the current valves being connected to a common control member51, which is adapted to simultaneously send control signals to all control units52included in a current valve5,6. Hereby, a simultaneous control of all the semiconductor components13of a current valve is secured. In this case, the control units50of the auxiliary valve and the control units52of the current valves are advantageously connected to one and the same control member51. The inventional VSC-converter is preferably controlled with PWM-technique (PWM=Pulse Width Modulation).

According to a further preferred embodiment of the invention, which is illustrated inFIG. 4, the ARCP-circuit comprises several series connected partial circuits60, each of which comprising a set30of auxiliary valve circuits31,32of the above described type connected in series with an inductor35. For the sake of simplicity, each such set30of auxiliary valve circuits is inFIG. 4indicated with the symbol for a switch. The ARCP-circuits comprises also here a number of capacitors61, which are connected in parallel with a respective partial circuit60. By dividing the inductance of the resonance circuit on several separate inductors35and arranging a capacitor61connected in parallel with each of said partial circuits60in this manner, the problem with unequal voltage distribution between the auxiliary valve circuits31,32included in the resonance circuit18, which unequal voltage distribution is caused by stray capacitances Csto ground, is reduced. For the capacitors61to be able to achieve this, it is required that the total capacitance across the series connected capacitors61of the resonance circuit is considerably higher than the total stray capacitance of the resonance circuit. Said stray capacitances Csbetween the auxiliary valve circuits and ground are inFIG. 4symbolised with broken lines.

Stray capacitances Csalso occur between the current valves5,6and ground, which stray capacitances also are symbolised with broken lines in FIG.4. The negative effect of these stray capacitances with respect to the voltage distribution of the semiconductor components13of turn-off type in the current valves is limited by means of the snubber capacitors17. For the snubber capacitors17to be able to achieve this, it is required that the total capacitance across the series connected snubber capacitors17of the respective current valve5,6is considerably higher than the total stray capacitance of the current valve.

According to a preferred embodiment of the invention, the respective current valve5,6comprises such a number of series connected circuits12, where each circuit comprises a semiconductor component13of turn-off type and a rectifying component14connected in anti-parallel therewith, that at least one of these circuits12is redundant with respect to the voltage that the current valve5,6is intended to hold in the blocking state. I.e. the circuits12are so many in number and so dimensioned that the respective current valve5,6in the blocking state is able to hold the pole voltage U even if one of the circuits12of the current valve would drop out and not help to hold any voltage. Hereby, the current valve5,6can continue to operate even in case of a breakdown in a component in any of these series connected circuits12. In this connection, it is preferred that each of said series connected circuits12is so designed that the circuit is short-circuited in case of an electric fault in the circuit. Hereby, it is secured that a breakdown in a circuit will get as little influence as possible on the operation of the current valve, since the current relatively unimpededly is allowed to pass the broken-down circuit in both directions. In order to secure a short-circuiting of a circuit in case of an electric fault in the semiconductor component13of turn-off type or the rectifying component14, the semiconductor components13of turn-off type and the rectifying components14in the current valves should be of “press pack” type. Furthermore, the snubber capacitors17should be of so-called self-healing type, which implies that an internal spark-over in the capacitor is insulated without the capacitor being short-circuited. The capacitance of such a capacitor only decreases a little when the capacitor is degraded and in case of a complete degradation the capacitance will go towards zero so that the capacitor will get the character of an electrically insulating component.

According to a further preferred embodiment of the invention, the auxiliary valve37of the ARCP-circuit comprises such a number of said sets30of auxiliary valve circuits that at least one of the sets is redundant with respect to the voltage that the auxiliary valve37is intended to hold in the blocking state. I.e. the sets30of auxiliary valve circuits are so many in number and so dimensioned that the auxiliary valve37in the blocking state is able to hold half the pole voltage U/2 even if one of the auxiliary valve circuits31,32would drop out and not help to hold any voltage. Hereby, the auxiliary valve37can continue to operate even in case of a breakdown in a component in any of the auxiliary valve circuits. Also here, it is preferred that each of the auxiliary valve circuits is so designed that the auxiliary valve circuit is short-circuited in case of an electric fault therein. Hereby, it is secured that a breakdown in an auxiliary valve circuit31,32will get as little influence as possible on the operation of the auxiliary valve, since the current relatively unimpededly is allowed to pass the broken-down auxiliary valve circuit in both directions. In order to secure a short-circuiting of an auxiliary valve circuit31,32in case of an electric fault in the semiconductor component33of turn-off type or the rectifying component34, the semiconductor components33of turn-off type and the rectifying components34of the auxiliary valve circuits should be of “press pack” type. Furthermore, the capacitors42included in the voltage dividing circuits36should be of self-healing type.

In order to further improve the voltage distribution between the semiconductor components13of turn-off type of the respective current valve5,6, each of the series connected circuits12of the respective current valve should comprise an high-ohmic resistor23connected in parallel with the semiconductor component13of turn-off type included in the circuit and in parallel with the snubber capacitor17included in the circuit. The resistance of these resistors23is adapted in such a manner that disparities in leakage current of the different semiconductor components13in a current valve5,6, when this is in the blocking state, will not cause any inequality to speak of in the voltage distribution between the semiconductor components13of the current valve.

The semiconductor components of turn-off type that are intended to be included in the current valves and/or auxiliary valves of the inventional converter are intended to be designed for very low conduction losses combined with a high voltage-blocking capacity of 2 kV or higher. It is preferred that said semiconductor components are designed to be able to shortly block a voltage of 4 kV or more. The snubber capacitors17preferably have a capacitance of 1 μF or more.

The inventional VSC-converter is preferably designed for network voltages of 130-400 kV, but may also be designed for voltages for instance in the order of 10-130 kV.

The function of an ARCP-circuit of the type illustrated inFIGS. 3 and 4is well known to a person skilled in the art and is for instance described in U.S. Pat. No. 5,047,913, and will therefore not be more closely described here.

It is emphasized that the invention is in no way limited to VSC-converters having only two series connected current valves per phase leg, but is also intended to embrace converters having a larger number of current valves and where the current valves are arranged in another way than shown inFIGS. 1-4. It is also emphasized that the converter according to the invention may have its direct voltage side designed in another way than shown inFIGS. 1-4, and for instance may comprise more than two series connected intermediate link capacitors.

The invention is of course neither as to the rest in any way restricted to the preferred embodiments described above, on the contrary many possibilities to modifications thereof should be apparent to a person skilled in the art without departing from the basic idea of the invention as defined in the appended claims.