WO2010069399A1

WO2010069399A1 - A voltage source converter

Info

Publication number: WO2010069399A1
Application number: PCT/EP2008/068048
Authority: WO
Inventors: Gunnar Asplund; Björn JACOBSON; Per-Olof Hedblad; Lennart Harnefors
Original assignee: Abb Technology Ag
Priority date: 2008-12-19
Filing date: 2008-12-19
Publication date: 2010-06-24

Abstract

A Voltage Source Converter having at least one phase leg connected to opposite poles of a direct voltage side of the converter and comprises a series connection of switching cells (7a) including at least one energy storing capacitor (20) and configured to obtain two switching states, namely a first switching state and a second switching state, in which the voltage across said at least one energy storing capacitor and a zero voltage, respectively, is applied across the terminals of the switching cell, has said switching cells arranged in stacks comprising at least two semiconductor assemblies. The converter comprises an arrangement configured to apply a pressure to opposite ends of each stack.

Description

A Voltage Source Converter

TECHNICAL FIELD OF THE INVENTION AND BACKGROUND ART

The present invention relates to a Voltage Source Converter having at least one phase leg connecting to opposite poles of a direct voltage side of the converter and comprising a series connection of switching cells, each said switching cell having at least two current paths between the terminals thereof with a first current path comprising a semiconductor assembly having a semiconductor device of turn-off type and a free-wheeling diode connected in parallel therewith, and a second current path comprising a series connection of on one hand a said semiconductor assembly having a semiconductor device of turn-off type and a free-wheeling diode connected in parallel therewith and on the other at least one energy storing capacitor, a mid point of said series connection of switching cells forming a phase output being configured to be connected to an alternating voltage side of the converter, each switching cell being configured to obtain two switching states by control of said semiconductor devices of each switching cell, namely a first switching state in which said first path is in a non-conducting state and the voltage across said at least one energy storing capacitor is applied across the terminals of the switching cell, and a second switching state, in which said first path is closed and a zero voltage is applied across the terminals of the switching cell, for obtaining a determined alternating voltage on said phase output.

Such converters with any number of said phase legs are comprised, but they have normally three such phase legs for having a three phase alternating voltage on the alternating voltage side thereof. A Voltage Source Converter of this type may be used in all kinds of situations, in which direct voltage is to be converted into alternating voltage and conversely, in which examples of such uses are in stations of HVDC-plants (High Voltage Direct Current), in which direct voltage is normally converted into a three- phase alternating voltage or conversely, or in so-called back-to- back stations in which alternating voltage is firstly converted into direct voltage and this is then converted into alternating voltage, as well as in SVCs (Static Var Compensator), in which the direct voltage side consists of capacitors hanging freely. However, the present invention is not restricted to these applications, but other applications are also conceivable, such as in different types of drive systems for machines, vehicles etc.

A Voltage Source Converter of this type is known through for example DE 101 03 031 A1 and WO 2007/023064 A1 and is as disclosed there normally called a multi-cell converter or M2LC. Reference is made to these publications for the functioning of a converter of this type. Said switching cells of the converter may have other appearances than those shown in said publications, and it is for instance possible that each switching cell has more than one said energy storing capacitor, as long as it is possible to control the switching cell to be switched between the two states mentioned in the introduction.

Another Voltage Source Converter of this type is known through US 5 642 275 used in a Static Var Compensator, in which the switching cells have a different appearance in the form of so- called full bridges.

The present invention is primarily, but not exclusively, directed to such Voltage Source Converters configured to transmit high powers, and the case of transmitting high powers will for this reason mainly be discussed hereinafter for illuminating but not in any way restricting the invention thereto. When such a Volt- age Source Converter is used to transmit high powers this also means that high voltages are handled, and the voltage of the direct voltage side of the converter is determined by the voltages across said energy storing capacitors of the switching cells. This means that a comparatively high number of such switching cells are to be connected in series for a high number of semiconductor devices, i.e. said semiconductor assemblies, are to be connected in series in each said switching cell, and a Voltage Source Converter of this type is particularly interesting when the number of the switching cells in said phase leg is comparatively high. A high number of such switching cells connected in series means that it will be possible to control these switching cells to change between said first and second switching state and by that already at said phase output obtain an alternating voltage being very close to a sinusoidal voltage. This may be obtained already by means of substantially lower switching frequencies than typically used in known Voltage Source Converters of the type shown in Fig 1 in DE 101 03 031 A1 having switching cells with at least one semiconductor device of turn-off type and at least one free- wheeling diode connected in anti-parallel therewith. This makes it possible to obtain substantially lower losses and also considerably reduces problems of filtering and harmonic currents and radio interferences, so that equipment therefor may be less costly.

In a Voltage Source Converter of this type, where several switching cells may be connected in series in order to handle high voltages, reliability may be reduced since a failure in a single switching cell or semiconductor assembly thereof may jeop- ardize the operation of the entire converter. In known converters of this type problems with reliability are stemming from potential violent capacitor discharge through said semiconductor device of turn-off type, such as an IGBT, in case of a fault of this semiconductor device, a control fault of a semiconductor device of a semiconductor assembly of a switching cell or an insulation fault across the blocking semiconductor device of a switching cell. In the first case there will be a short circuit of the capacitor of the switching cell resulting in a very high current through the semiconductor device in question, which will then be destroyed. In the second case of a control fault, which means that the semi- conductor devices of both semiconductor assemblies of the switching cell will be turned on simultaneously, the capacitor will also be abruptly discharged and the semiconductor devices destroyed. The same will happen if an insulation fault appears across the blocking semiconductor device resulting in a con- ducting state in both semiconductor devices simultaneously.

It is necessary to ensure continued operation of the converter in spite of the appearance of any of these fault cases, and WO 2007/023064 discloses a solution to this problem by achieving redundancy. This is made by short-circuiting a failing switching cell by arrangement of a by-pass switch. However, this puts high demands on the reliability of the means, i.e. the switch, used for short-circuiting the switching cell and it also requires provision of a reliable control of said means.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a Voltage Source Converter of the type defined in the introduction ad- dressing the problem of reliability and by simple means obtaining continued operation of the converter in spite of a fault in connection to a semiconductor device in a switching cell thereof.

This object is according to the invention obtained by providing a Voltage Source Converter of the type defined in the introduction, in which said semiconductor assemblies of said switching cells are arranged in stacks comprising at least two said semiconductor assemblies, that the converter comprises an arrangement configured to apply a pressure to opposite ends of each said stack for pressing said semiconductor assemblies towards each other so as to obtain electric contact between semiconductor assemblies in said stack while ensuring that a said semiconductor assembly goes into a permanently closed state in case of a failure of the semiconductor device thereof, and each of said first and second current paths of each switching cell comprises at least two said semiconductor assemblies connected in series.

By the combination of a series connection of at least two semiconductor assemblies and by that semiconductor devices of turn-off type, such as IGBTs, in each current path of each switching cell instead of one semiconductor assembly therein and a use of the so-called press pack technique known through US patent 5 705 853 for interconnecting the semiconductor assemblies connected in series a fault in one of the semiconductor devices will not cause capacitor short circuit and any need for protective hardware is removed, which results in a saving of cost and losses and also results in increased reliability of the converter. Thus, when a said semiconductor device fault occurs it will thanks to the so-called press pack technique be ensured that a semiconductor assembly having the failing semiconductor device and by that the failing semiconductor device will go into a permanently closed state and by that be by-passed, while the other semiconductor assembly/assemblies of the current path in question will ensure continued appropriate operation of the switching cell in question. Another advantage of this solution to the above reliability problems is that the diodes of said semiconductor assemblies in each said current path of a switching cell now connected in series will have an increased ability to withstand ground faults on the DC-side of the converter.

According to an embodiment of the invention each of said first and second current paths of each switching cell comprises at least four said semiconductor assemblies connected in series. By the use of as many as four said semiconductor assemblies connected in series in each current path of the switching cells the reliability of proper operation of each switching cell of the converter will be very high, since theoretically more than one semiconductor device in each said current path of a switching cell may fail and the switching cell still function perfectly. This number of semiconductor assemblies connected in series also means a smaller increase of the voltage to be taken by each remaining semiconductor device in the blocking state thereof when one semiconductor device has failed putting less stress on these remaining semiconductor devices.

According to another embodiment of the invention each of said first and second current paths of each switching cell comprises at least eight said semiconductor assemblies connected in series, which further strengthen the advantages of the previous embodiment.

According to another embodiment of the invention said arrangement comprises means adapted to apply a spring loaded pressure to each said stack urging the semiconductor assemblies of each stack into said electric contact while releasing potential energy stored in members of said means, and these members storing potential energy are advantageously springs acting on at least one end of each said stack. These springs may be mechanical springs as well as other types of springs, such as gas springs. This means that electric contact between the semiconductor assemblies in said stack may be obtained with a high reliability irrespectively of irregularities in the dimensions thereof, such as for instance in the case of parallel connection of semiconductor assemblies in said stack. There is also no risk that the interconnection of the adjacent semiconductor assemblies will be destroyed by the over-current resulting upon occurrence of a said failure making a faulty semiconductor assembly permanently conducting.

According to another embodiment of the invention the number of the switching cells of said phase leg is >4, >12, >30 or >50. A converter of this type is, as already mentioned above, particularly interesting when the number of switching cells of a said phase leg is rather high resulting in a high number of possible levels of the voltage pulses delivered on said phase output.

According to another embodiment of the invention said semi- conductor device of the switching cell assemblies are IGBTs (Insulated Gate Bipolar Transistor), IGCTs (Integrated Gate Corn- mutated Thyristor) or GTOs (Gate Turn-Off Thyristor). These are suitable semiconductor devices for such converters, although other semiconductor devices of turn-off type are also conceiv- able.

According to another embodiment of the invention said converter is configured to have said direct voltage side connected to a direct voltage network for transmitting High Voltage Direct Current (HVDC) and the alternating voltage side connected to an alternating voltage phase line belonging to an alternating voltage network. This is due to the high number of semiconductor assemblies required a particularly interesting application of a converter of this type.

According to another embodiment of the invention the converter is a part of a SVC (Static Var Compensator) with a direct voltage side formed by said energy storing capacitors of the switching cells and the alternating voltage phase output connected to an alternating voltage network. Thus, when a failure occurs in a semiconductor device of a switching cell in the form of a so- called full bridge of this type of converter the series connection of said semiconductor assemblies will ensure that the switching cell in question will continue to function as before.

According to another embodiment of the invention the converter is configured to have a direct voltage across said poles being 10 kV - 1200 kV, 20 kV - 1200 kV or 100 kV - 1200 kV. The invention is the more interesting the higher said voltage is. The invention also relates to a plant for transmitting electric power according to the appended claim therefor. The stations of such a plant may be given attractive dimensions and a high reliability to a low cost.

Further advantages as well as advantageous features of the invention will appear from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a description of embodiments of the invention cited as examples.

In the drawings:

Fig 1 is a very simplified view of a Voltage Source Converter of the type according to the present invention,

Figs 2 and 3 illustrates two different known switching cells, which may be a part of the Voltage Source Converter according to the invention,

Fig 4 is a simplified view very schematically illustrating a Voltage Source Converter according to the present invention,

Fig 5 is a view of a switching cell of the type shown in Fig 2 of a Voltage Source Converter according to an em- bodiment of the present invention,

Fig 6 is a very simplified view illustrating the principle of connecting semiconductor assemblies in series in a said current path of a switching cell according to the present invention, Fig 7 is a view of a switching cell of the type shown in Fig 3 in a converter according to an embodiment of the invention, and

Fig 8 illustrates very schematically a converter according to the present invention used in a Static Var Compensator.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Fig 1 illustrates very schematically the general construction of a Voltage Source Converter 1 of the type to which the present invention relates. This converter has three phase legs 2-4 con- nected to opposite poles 5, 6 of a direct voltage side of the converter, such as a direct voltage network for transmitting high voltage direct current. Each phase leg comprises a series connection of switching cells 7 indicated by boxes, in the present case 16 to the number, and this series connection is divided into two equal parts, an upper valve branch 8 and a lower valve branch 9, separated by a mid point 10-12 forming a phase output being configured to be connected to an alternating voltage side of the converter. The phase outputs 10-12 may possibly through a transformer connect to a three phase alternating volt- age network, load, etc. Filtering equipment is also arranged on said alternating voltage side for improving the shape of the alternating voltage on said alternating voltage side.

A control arrangement 13 is arranged for controlling the switch- ing cells 7 and by that the converter to convert direct voltage into alternating voltage and conversely.

The Voltage Source Converter has switching cells 7 of the type having on one hand at least two semiconductor assemblies with each a semiconductor device of turn-off type, and a free-wheeling diode connected in parallel therewith and on the other at least one energy storing capacitor, and two examples of such switching cells are shown in Fig 2 and Fig 3. The terminals 14, 15 of the switching cell are adapted to be connected to adjacent switching cells in the series connection of switching cells form- ing a phase leg. The semiconductor devices 16, 17 are in this case IGBTs connected in parallel with diodes 18, 19. Although only one semiconductor device and one diode is shown per assembly these may stand for a number of semiconductor devices and diodes, respectively, connected in parallel for sharing the current flowing through the assembly. An energy storing capacitor 20 is connected in parallel with the respective series connection of the diodes and the semiconductor devices. One terminal 14 is connected to the mid point between the two semiconductor devices as well as the mid point between the two di- odes. The other terminal 15 is connected to the energy storing capacitor 20, in the embodiment of Fig 2 to one side thereof and in the embodiment according to Fig 3 to the other side thereof. It is pointed out that each semiconductor device and each diode as shown in Fig 2 and Fig 3 may be more than one connected in series for being able to handle the voltages to be handled, and the semiconductor devices so connected in series may then be controlled simultaneously so as to act as one single semiconductor device.

The switching cells shown in Fig 2 and Fig 3 may be controlled to obtain one of a) a first switching state and b) a second switching state, in which for a) the voltage across the capacitor 20 and for b) a zero voltage is applied across the terminals 14, 15. For obtaining the first state in Fig 2 the semiconductor de- vice 16 is turned on and the semiconductor device 17 turned off and in the embodiment according to Fig 3 the semiconductor device 17 is turned on and the semiconductor 16 is turned off. The switching cells are switched to the second state by changing the state of the semiconductor devices, so that in the em- bodiment according to Fig 2 the semiconductor device 16 is turned off and 17 turned on and in Fig 3 the semiconductor device 17 is turned off and 16 turned on.

Fig 4 shows a little more in detail how a phase leg of the con- verter according to Fig 1 is formed by switching cells of the type shown in Fig 3, in which totally ten switching cells have been left out for simplifying the drawing. The control arrangement 13 is adapted to control the switching cells by controlling the semiconductor devices thereof, so that they will either deliver a zero voltage or the voltage across the capacitor to be added to the voltages of the other switching cells in said series connection. A transformer 21 and filtering equipment 22 are here also indicated. It is shown how each valve branch is through a phase reactor 50, 51 connected to the phase output 10, and such phase reactors should also be there in Fig 1 for the phase outputs 10, 1 1 and 12, but have there been left out for simplifying the illustration.

Fig 5 illustrates very schematically the design of a switching cell 7a of the type shown in Fig 2 of a Voltage Source Converter according to a first embodiment of the invention and how such switching cells are connected in series in a phase leg of that converter. Each switching cell 7a has a first current path 23 comprising two semiconductor assemblies 24, 25 connected in series and each having a semiconductor device 26, 27 of turn- off type and a free wheeling diode 28, 29 connected in parallel therewith. A second current path 30 of the switching cell comprises a series connection of on one hand two said semiconductor assemblies 31 , 32 each having a semiconductor device 33, 34 of turn-off type and a free wheeling diode 35, 36 connected in parallel therewith and on the other at least one energy storing capacitor 20.

The two semiconductor assemblies connected in series in each said current path are arranged in a stack, and an arrangement is configured to apply a pressure to opposite ends of each said stack for pressing said semiconductor assemblies towards each other so as to obtain electric contact between semiconductor assemblies in said stack while ensuring that a said semiconductor assembly goes into a permanently closed state in case of a failure of a semiconductor device thereof. How this is obtained in the case of series connection of four such semiconductor assemblies in each said current path is schematically illustrated in Fig 6. The stack 38 so formed has metal plates 39 upon each of which a number of said semiconductor assemblies 24, 24', 25, 25' are arranged and pressed into electric contact with through a spring member 40 acting between an adjacent plate and a disc 41 on top of the semiconductor assembly. Electrical connection between a plate in the stack to semiconductor assemblies on a plate thereunder is realized by flexible conductors 42, 43. Rods 44 extending through the plates and nuts received thereon and tightened against the outer plates of the stack 38 achieve in cooperation with said springs 40 a series connection of the semiconductor assemblies by the so-called press pack technique, which means that when a fault occurs in a semiconductor device of a semiconductor assembly the chip constituting the semiconductor assembly will be short circuited and said semiconductor assembly will be by-passed by being permanently conducting.

It is here shown that the symbol for a semiconductor assembly as shown in Fig 5 may stand for a larger number of semiconductor assemblies connected in parallel on a said plate 39 for sharing the current through the switching cell.

Thus, a fault of a semiconductor device in any of the semicon- ductor assemblies 24, 25, 31 , 32 in a switching cell according to

Fig 5 will not cause any short circuit of the capacitor 20, but the semiconductor assembly having the faulty semiconductor device or in the case of a parallel connection of a number of such semiconductor assemblies all the semiconductor assemblies will be by-passed and the operation of the switching cell will be maintained. Thus, the need for protective hardware is removed, so that costs and losses may be saved and the reliability of the converter will be very high. This reliability is of course further improved by connecting more than two semiconductor assemblies in series in each said current path 23', 30' as shown in Fig 7, in which it is shown how eight such semiconductor assemblies and by that semiconductor devices 61 -68, 71 -78 are connected in series in each current path of a switching cell 7b of the type shown in Fig 3. Furthermore, the risk that a current path that shall not be conducting may be accidently transferred into a conducting state resulting in a capacitor short circuit by a control fault will in the practise be eliminated when a plurality of semiconductor assemblies are connected in series in the current path in question, especially when the number is as high as shown in Fig 7. It is obvious that a switching cell having the ap- pearance shown in Fig 7 may have a higher voltage between the terminals thereof when said capacitor is connected thereto than a switching cell shown in Fig 2 or 3, such as for instance 20 kV in the case of a use of IGBTs dimensioned to block a voltage of 4.5 kV, which also means that a lower number of such switching cells will be connected in series in the phase leg of the converter. Such a switching cell 7b or any other switching cell shown in the Figures may have a plurality of capacitors, although they are only symbolized by one in these Figures.

Fig 8 illustrates the general construction of a Voltage Source Converter according to the present invention used in a Static Var Compensator for reactive power compensation. A direct voltage side of this converter is formed by said energy storing capacitors of the switching cells 7c, and the switching cells 7c of this converter are so-called full bridges with semiconductor assemblies having a semiconductor device of turn-off type and a free-wheeling diode connected in parallel therewith as disclosed in US patent 5 642 275.

In the present invention each symbol 45-48 for a semiconductor assembly stands for a series connection of a plurality of such semiconductor assemblies having each a semiconductor device of turn-off type and a free wheeling diode connected in parallel therewith resulting in reliably ensured continued operation of the switching cell in case of the occurrence of the faults discussed above in connection with a said semiconductor device in a semiconductor assembly thereof.

The invention is of course not in any way restricted to the embodiments described above, but many possibilities to modifica- tions thereof will be apparent to a person with ordinary skill in the art without departing from the scope of the invention as defined in the appended claims.

Claims

1 . A Voltage Source Converter having at least one phase leg (2-4) connecting to opposite poles (5, 6) of a direct voltage side of the converter and comprising a series connection of switching cells (7a, 7b, 7c), each said switching cell having at least two current paths (23, 30) between the terminals (14, 15) thereof with a first current path (23) comprising a semiconductor assembly (24) having a semiconductor de- vice of turn-off type (26) and a free-wheeling diode (28) connected in parallel therewith, and a second current path (30) comprising a series connection of on one hand a said semiconductor assembly (31 ) having a semiconductor device (33) of turn-off type and a free-wheeling diode (35) connected in parallel therewith and on the other at least one energy storing capacitor (37), a mid point (10-12) of said series connection of switching cells forming a phase output being configured to be connected to an alternating voltage side of the converter, each switching cell being configured to obtain two switching states by control of said semiconductor devices of each switching cell, namely a first switching state in which said first path is in a non-conducting state and the voltage across said at least one energy storing capacitor is applied across the terminals of the switching cell, and a second switching state, in which said first path is closed and a zero voltage is applied across the terminals (14, 15) of the switching cell, for obtaining a determined alternating voltage on said phase output, characterized in that said semiconductor assemblies of said switching cells are arranged in stacks (38) comprising at least two said semiconductor assemblies, that the converter comprises an arrangement (39, 40, 44) configured to apply a pressure to opposite ends of each said stack for pressing said semiconductor assemblies towards each other so as to obtain electric contact between semiconductor assemblies in said stack while ensuring that a said semicon- ductor assembly goes into a permanently closed state in case of a failure of the semiconductor device thereof, and that each of said first and second current paths (23, 30) of each switching cell (7a, 7b, 7c) comprises at least two said semiconductor assemblies (24, 25, 31 , 32, 45-48) connected in series.

2. A converter according to claim 1 , characterized in that each of said first and second current paths (23, 30) of each switching cell comprises at least four said semiconductor assemblies connected in series.

3. A converter according to claim 1 , characterized in that each of said first and second current paths (23, 30) of each switching cell comprises at least eight said semiconductor assemblies (61 -68, 71 -78) connected in series.

4. A converter according to any of the preceding claims, characterized in that said arrangement comprises means adapted to apply a spring loaded pressure to each said stack urging the semiconductor assemblies of each stack (38) into said electric contact while releasing potential energy stored in members of said means.

5. A converter according to claim 4, characterized in that said members storing potential energy are springs (40) acting on at least one end of each said stack.

6. A converter according to any of the preceding claims, characterized in that the number of switching cells (7a, 7b,

7c) of said phase leg is > 4, > 12, > 30 or > 50.

7. A converter according to any of the preceding claims, characterized in that said semiconductor devices (26, 27, 33, 34) of the switching cell assemblies are IGBTs (Insu- lated Gate Bipolar Transistor), IGCTs (Integrated Gate Commutated Thyristor) or GTOs (Gate Turn-off Thyristor).

8. A converter according to any of the preceding claims, characterized in that it is configured to have said direct voltage side connected to a direct voltage network for transmitting High Voltage Direct Current (HVDC) and the alternating voltage side connected to an alternating voltage phase line belonging to an alternating voltage network.

9. A converter according to any of claims 1 -7, characterized in that it is a part of a SVC (Static Var Compensator) with a direct voltage side formed by said energy storing capacitors of the switching cells and the alternating voltage phase out- put connected to an alternating voltage network.

10. A converter according to any of the preceding claims, characterized in that it is configured to have a direct voltage across said poles being 10 kV-1200 kV, 20 kV - 1200 kV or 100 kV - 1200 kV.

1 1 . A plant for transmitting electric power comprising a direct voltage network and at least one alternating voltage network connected thereto through a station, said station being adapted to perform transmitting of electric power between the direct voltage network and the alternating voltage network and comprises at least one Voltage Source Converter adapted to convert direct voltage into alternating voltage and conversely, characterized in that said station of the plant comprises a voltage Source Converter according to any of claims 1 -10.