VOLTAGE-STIFF CONVERTER WITH HIGH VOLTAGE CABLES OF DIFFERENT POLES ARRANGED CLOSE TO EACH OTHER

Abstract: A device in voltage-stiff converter devices, which has at least one phase leg having at least two current valves, has lines for interconnecting said current valves at the end of the phase leg with two different pole conductors (18, 19) of different polarity of a direct voltage side of the converter device (1). The lines extend between the respective end of the phase leg (2-4) for connection thereof to said pole conductors and a capacitor defining the direct voltage of the direct voltage side. The lines comprise for connection to each pole conductor at least one separate high voltage cable having an insulating layer of polymer base surrounding the conductor thereof. Such cables belonging to different poles are along at least parts of the extension thereof arranged tightly side by side.

Description: VOLTAGE-STIFF CONVERTER WITH HIGH VOLTAGE CABLES OF DIFFERENT POLES ARRANGED CLOSE TO EACH OTHER FIELD OF THE INVENTION AND PRIOR ART The present invention relates to a device in voltage-stiff converter devices, which have at least one phase leg with at least two current valves, for interconnecting said current valves at one end of the phase leg through lines with two different pole conductors of different polarity at a direct voltage side of the converter device, in which the lines extend between the respective end of a phase leg for connection thereof to said pole conductors and at least one capacitor defining the direct voltage of the direct voltage side. Such voltage-stiff converter devices may be voltage-stiff inverters, DC/DC-converters, converters in SVCs or VSC-con- verters being used for transmitting power between a direct voltage network and an alternating voltage network, which application has recently become known through the thesis "P M and control of two and three level High Power Voltage Source Converters" by Anders Lindberg, Kungliga Tekniska Hogskolan, Stockholm, 1995, in which thesis a plant for transmitting electric power through a direct voltage network for High Voltage Direct Current (HVDC) while utilising such converters is described. However, it is pointed out that the invention is not in any way restricted to exactly this application, although this case will be discussed more thor- oughly hereinafter for illuminating the invention and the problems thereof. When a voltage-stiff converter device is used a problem not being there for current-stiff such devices occurs, which will now be explained through making reference to Fig 1, which by way of example shows such a converter device in the form of a VSC-converter 1 having three phase legs 2, 3, 4 with each two current valves 5-10, which each consists of at least one semiconductor element of turn-off type, such as an IGBT 11, and a rectifying member in the form of a diode 12 connected in anti-parallel therewith. The phase legs are connected to a phase 13, 14, 15 each of a three phase alternating voltage network 16. The opposite end of the converter is connected to a direct voltage network 17 with a positive pole conductor 18 and a negative pole conductor 19. Thus, the converter is formed by a so called six-pulse bridge. If we now consider for example the phase 15 and assume that the semiconductor element 11 belonging to the current valve 10 is turned on and a current flows from the alternating voltage network through this semiconductor element and to the direct voltage network, a turning-off of the semiconductor element 11 of the current valve 10 will result in a firstly continued current from the phase 15 in the direction towards the direct voltage network, but now through the diode of the current valve 7. The commutation times are short and com- paratively large time differential coefficients for the current results, which makes it desirable to lower the inductance in the commutation circuit, i.e. in the very phase leg and in the lines being connected to the different pole conductors of the direct voltage network and to the capacitors defining a direct voltage, to such a low level as possible so as to avoid unnecessarily high over voltages and by that losses in said commutations. The present invention is more exactly occupied with the problem to reduce the inductance exactly in said lines of the commutation circuit. In low or moderate voltages at the direct voltage side of the converter device air isolation may be used, possibly in combination with insulating material in discs and still a comparatively low inductance be obtained. However, the air distances between said lines connected to different pole conductors have to be that large for preventing flash-overs when the voltages are high, that it will be nearly impossible to obtain a sufficiently low inductance. A possibility to solve this problem consists in using insulation of mica base of the type utilised in for example big generators, but this solution is too expensive. SUMMARY OF THE INVENTION The object of the present invention is to provide a device of the type defined in the introduction, which makes it possible to interconnect the current valves in question at the end of the phase leg through said lines and said pole conductors and capacitors while obtaining a low overall induc- tance of these lines also at high voltages between the pole conductors of the direct voltage side of the converter device without the result that the device gets unacceptably costly. This object is according to the invention obtained by in such a device design the lines for connection to each pole conductor and said capacitor so that they comprises at least one separate high voltage cable with an insulating layer of polymer base surrounding the conductor thereof, and by ar- ranging such cables belonging to different poles along at least parts of the extension thereof closely beside each other. By utilising such a high voltage cable known per se within other applications, such as for transmitting electric power between different stations of an electric power net- work, it will be possible to arrange cables belonging to different poles closely together without any risk of flash- overs even at comparatively high voltages, so that the magnetic fields rotating in different directions which may be generated by the current in these cables counteract each other and an acceptable low inductance in this part of said commutation circuit is obtained. According to a preferred embodiment of the invention said cable is an extruded cable. Such a cable has a good voltage withstand capability proven in electric power transmitting situations and has turned out to be well suitable for this totally new use thereof, where it is important to be able to arrange it close to other such cables with a comparatively high potential difference therebetween. According to another preferred embodiment of the invention each line comprises for connection to a pole conductor a plurality of said cables arranged substantially in parallel with each other. It has turned out that it is possible to lower the inductance further by dividing each line into a plurality of such cables, in which these then may be arranged in different patterns so as to lower the overall inductance of said lines. According to another preferred embodiment of the invention, which constitutes a further development of the embodiment last mentioned, said cables are arranged in patterns beside each other with alternating cables for connection to the one and the other of said pole conductors. A very low inductance is by this obtained, since the magnetic fields generated by adjacent cables counteract each other. According to another preferred embodiment of the invention, which constitutes a further development of the embodiment last mentioned, said cables are arranged beside each other in at least two superimposed layers so that the cables located directly above each other are intended for connection to a different one of said two pole conductors. By arranging the cables in such layers in this alternating way with respect to the polarity thereof, a very low total inductance of the lines may be obtained. According to another preferred embodiment of the invention the cables belonging to different conductors are along at least parts of the extension thereof held tightly together by being inserted in a common channel. The utilisation of such a channel is advantageous for ensuring that said cables are held tightly together and a low inductance results. According to another preferred embodiment of the invention the device is intended for converter devices in the form of VSC-converters, where it is particularly interesting to use it. According to another preferred embodiment of the invention the device is designed for connection to a direct voltage network for High Voltage Direct Current (HVDC). It is in such a device extremely important that the inductance may be kept down at a low level in said commutation circuit so as to avoid large losses in transmitting electric power in the stations in which said voltage-stiff converters are in- eluded. Further advantages as well as advantageous features of the invention will appear from the following description and the other dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a description of preferred embodiments of the invention cited as examples. In the drawings: Fig 1 is a schematic diagram illustrating the structure of a VSC-converter, to which the device according to the invention is applicable, Fig 2 is a view corresponding to Fig 1 illustrating the structure of a device according to a preferred embodiment of the invention, Fig 3 is a view of the device according to the invention shown in Fig 2 being more detailed than that according to Fig 2, Fig 4-6 are schematical sectioned views illustrating different arrangements of cables in lines for connection of the ends of the phase legs of a converter to pole conductors of a direct voltage side thereof. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig 1 has already been discussed above, and it may here be noticed that it illustrates schematically the structure of a VSC-converter connected between a direct voltage network for High Voltage Direct Current (HVDC = High Voltage Direct Current) and a three phase alternating voltage network. Each phase leg of the converter has two so called current valves, which consist of units of breakers 11 of turn-on and turn- off type connected in series, preferably in the form of IG- BTs, and diodes 12 connected in anti-parallel therewith. A great number of such units and by that IGBTs may be connected in series in one single valve so as to be turned on and turned off simultaneously so as to function as one single breaker, wherethrough the voltage across the valve is distributed among the different breakers connected in series and high voltages, for example within the interval 50-300 kV, may in this way be held by such a current valve. The control of the breakers takes place by pulse width modula- tion (PWM). The inductance problems existing in such a converter has, with respect to the lines 20 and 21, which connect the respective end of the phase legs with the different pole conductors of the direct voltage network connected to the converters and with the capacitors defining the direct voltage of the direct voltage side, carefully been discussed in the introduction of the description, and some of many possible preferred embodiments of the invention will now be described with reference to the other figures 2-6. It is schematically illustrated in Fig 2 how said lines 20, 21 may extend, in which the lines extend over large parts substantially in parallel with and close to each other, as is more in detail shown in Fig 3. The different phase legs are omitted in Fig 3 and only the members 22 for connection thereto are shown. Each line is made of at least a high voltage cable with an insulating layer 26 of polymer base surrounding the conductor 25 thereof, such as an extruded cable, for example a so called PEX-cable. The simplest variant consists in utilising only one such cable for each line 20 and 21, respectively, in which these cables are arranged close to each other for reducing the total inductance of the lines, and this embodiment is illustrated in Fig 4. However, it is also possible that each line is made of a plurality of thin cables of said type arranged substantially in parallel with each other, in which these cables then are at the connection to the respective phase leg, i.e. for ex- ample just before the connection members 22, brought together to a connection in common to the phase leg in ques- tion. A very preferred embodiment having lines consisting of a plurality of such cables is illustrated in Fig 5. Cables belonging to different lines have in this embodiment been mixed with each other along that part over which they are running substantially in parallel with each other, more exactly by arranging them side by side in two layers with the cables located directly above each other intended for connection to different of said both pole conductors. The inductance may be reduced to a very low level by such an ar- rangement. Another embodiment is illustrated in Fig 6, which differs from that according to Fig 5 by gathering the cables 23 in a channel 24 having walls of a material with a low electric resistivity, such as Cu or Al, which has as a consequence that the magnetic field generated around the cables when a current flows therethrough is compressed and reduced further, so that a low inductance may be obtained. The invention is of course not in any way restricted to the preferred embodiments described above, but many possibilities to modifications thereof would be apparent to a man skilled in the art without departing from the basic idea of the invention. It is easily understood that an amount of other combinations of the number of cables as well as the mutual arrangement thereof for obtaining a low overall inductance of the lines are conceivable, in which it however is essential that the cables are located comparatively close to each other without any large air gaps therebetween. It is pointed out that the expression "two different pole conductors of different polarity" is defined to comprise both the cases in which one pole of the direct voltage side is formed by ground and the other pole is on negative or positive potential and the case in which the poles are on positive and negative potential, respectively. In the case of a SVC no direct voltage network connected to the lines is there, but this has nevertheless a direct voltage side connected to a free capacitor. The converter device in question may also be a DC/DC-converter, in which the number of "phases" then is one and the converter device then accordingly has two direct voltage sides.

Claims: Claims 1. A device in voltage-stiff converter devices (1), which have at least one phase leg (2-4) with at least two current valves, for interconnecting said current valves (5-10) at one end of the phase leg through lines with two different pole conductors (18, 19) of different polarity at a direct voltage side of the converter device, in which the lines extend between the respective end of a phase leg for connec- tion thereof to said pole conductors and at least one capacitor defining the direct voltage of the direct voltage side, characterized in that the lines comprise for connection to each pole conductor and said capacitor at least one separate high voltage cable (23) with an insulating layer of polymer base surrounding the conductor thereof, and that such cables belonging to different poles are along at least parts of the extension thereof arranged closely beside each other. 2. A device according to claim 1, characterized in that said cable ( 23 ) is an extruded cable. 3. A device according to claim 1 or 2, characterized in that each line (20, 21) for connection to a pole conductor (18, 19) comprises a plurality of said cables (23) arranged substantially in parallel with each other. 4. A device according to claim 3, characterized in that said plurality of cables (23) of one and the same line (20, 21) is at the connection to the respective phase leg brought together to one common connection (22) to the phase leg in question. 5. A device according to claim 3 or 4, characterized in that said cables (23) are arranged in patterns beside each other with alternating cables for connec- tion to the one and the other of said pole conductors (18, 19). 6. A device according to claim 5, characterized in that said cables ( 23 ) are arranged beside each other in at least two superimposed layers so that the cables located directly above each other are intended for connection to a different one of said two pole conductors (18, 19). 7. A device according to any of the claims 1-6, characterized in that the cables (23) belonging to different pole conductors (18, 19) are kept tightly together by being inserted in a common channel ( 24 ) along at least parts of their extension. 8. A device according to claim 7, characterized in that said channel ( 24 ) is formed by walls of material having a low electric resistivity. 9. A device according to claim 8, characterized in that said wall material is Cu or Al. 10. A device according to any of the claims 1-9, characterized in that it is intended for converter devices in the form of VSC-converters ( 1 ) . 11. A device according to any of the claims 1-10, characterized in that it is intended for connection to a di- rect voltage network ( 17 ) for High Voltage Direct Current (HVDC) .

Inventors: EKWALL OLLE, SPJUT HENRIK, EKWALL, OLLE, SPJUT, HENRIK
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1998
Main classification: H02M1
IPC codes: H02J3, H02M7