Abstract:
A method for converting a multi-phase alternating voltage into a high-voltage direct voltage and then into a second multi-phase alternating voltage. The method utilizes first and second cascades of power converter cells, with each individual cell having respective first and second current valves. The method includes offsetting the clocking of individual power converter cells by a predetermined factor; cyclically switching off the first current valves in counterpoint with the second current valves, so that only one set of current valves are “on” at any given time while the other set of current valves is “off” at that time; and, in response to a signal indicating that an individual power cell is malfunctioning, shunting out the individual malfunctioning power cell.

Description:
Cross-reference to Related Applications 
       [0001]    This application is a divisional application of prior-filed and co-pending application Ser. No. 11/825,336, filed Jul. 6, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention is directed to a power converter circuit for converting a first, three-phase alternating voltage from a plurality of sources into a high-voltage direct voltage for transmission to a high-voltage direct voltage connection, and for further converting the voltage into a further three-phase alternating voltage. Such high-voltage direct voltage connections are used for instance in wind power plants; wherein, the output power and the output voltage of the generators both vary dynamically. When a plurality of wind power plants are arranged as so-called wind farms in the prior art, the individual wind power plants are connected to a high-voltage direct voltage connection by a common power inverter that serves to feed current into an electrical power system. 
         [0004]    2. Description of the Related Art 
         [0005]    In the prior art, three phase alternating current generators of the medium-voltage category are commonly used. The three phases of the outputs of these generators are connected to a transformer, which transforms the medium voltage into a high voltage on the order of magnitude of 100,000 volts. This alternating voltage generated is then rectified by means of a high-voltage diode rectifier and fed into a high-voltage direct voltage connection. 
         [0006]    Following the high-voltage direct voltage connection, the direct voltage is converted by means of a power inverter into a suitable alternating voltage and fed into an electrical power system. Such high-voltage direct voltage connections are known in the form of “HVDC” or “HVDC light” technology made by ABB. 
         [0007]    It is a particular disadvantage of such prior art systems that the input filters are extremely expensive. In this respect, power inverters are known that are embodied as a serial arrangement of a multiplicity of bipolar transistors of special construction and with special connections. A disadvantage of this prior art is that with this embodiment of the rectifier, all the transistors are switched simultaneously, and very large voltage changes per unit of time occur in the lines. To control these voltage changes, correspondingly large filters and also special, complicated and expensive intermediate circuit capacitors are necessary. The effort and expense in terms of circuitry, for instance in the form of these high-voltage capacitors, and for simultaneously switching all the transistors is a disadvantage. 
         [0008]    German Patent DE 101 14 075 B4 discloses a power converter comprising a rectifier circuit for converting an alternating current, generated in an alternating voltage generator, into a direct current, a direct current connection from the rectifier circuit to a cascaded power inverter, a downstream medium-voltage transformer for feeding a high-voltage power system, and a primary controller. The power inverter comprises a cascaded, serial arrangement of a plurality of power inverter cells, whose inputs are connected in series, and each of these power inverter cells can be switched by the overriding controller to be active or, by shunting their inputs, to be inactive. 
       SUMMARY OF THE INVENTION 
       [0009]    The object of the present invention is to provide a power converter circuit for converting a first alternating voltage into a high-voltage direct voltage and, after suitable transmission, to convert that voltage into a second alternating voltage, in which the power converter circuit is meant to be error-tolerant with regard to the failure of individual semiconductor switches; the changes in voltage per unit of time should also be reduced considerably compared to the prior art, and the construction should be possible using standard power semiconductor modules. 
         [0010]    A power converter circuit in accordance with the invention serves to convert at least a first multi-phase alternating voltage into a high-voltage direct voltage. The multi-phase alternating voltage may for instance be generated by a plurality of generators in the context of a decentralized energy supply, as in a wind farm. The power converter likewise serves to convert this high-voltage direct voltage into a second multi-phase alternating voltage for feeding into an electrical power system, such as a medium-voltage power system. 
         [0011]    The power converter of the invention comprises first and second cascades of power converter cells. 
         [0012]    The first cascade is formed of a serial arrangement of first power converter cells, and the second cascade is formed of a serial arrangement of second power converter cells. 
         [0013]    Every other power converter cell has first terminals on the transformer side and second terminals on the direct voltage side. The first terminals serve to connect the associated windings of a transformer to the respective center points of a three-phase bridge circuit. This bridge circuit in turn is connected to an intermediate circuit, which in at least one branch has a second current valve and, connected parallel to the three-phase bridge circuit, a first current valve. This current valve is connected to the first terminals of the power converter cell. 
         [0014]    The first power converter cells are either identical to the second power converter cells or have a three-phase rectifier circuit, whose respective center points are connected to the associated windings of a transformer. The second terminals, on the direct voltage side, are likewise suitably connected to the three-phase rectifier circuit. 
         [0015]    The aforementioned cascades are embodied as a serial connection of the second terminals of adjacent power converter cells. 
         [0016]    Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    In the drawings: 
           [0018]    The concept of the invention is described below with reference to preferred embodiments of the invention, in conjunction with  FIGS. 1 and 2 . 
           [0019]      FIG. 1  shows a first embodiment of a power converter circuit arrangement of the invention. 
           [0020]      FIG. 2  shows a second embodiment of the power converter circuit arrangement of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]      FIG. 1  shows a first embodiment of a power converter circuit  1  according to the invention. What is shown here is a fully symmetrical embodiment, not taking the embodiment of the transformers, which are not shown, into account. 
         [0022]    Each power converter  20 ,  30  comprises a three-phase bridge circuit  22 ,  32  (respectively), which is embodied in turn with one upper power switch  220 ,  320  (respectively) and one lower power switch  230 ,  330  (respectively) per phase. Each power switch  220 ,  230 ,  320 ,  330  in turn comprises a parallel circuit of a plurality of bipolar transistors  224 ,  234 ,  324 ,  334  (respectively), only one of each of which is labeled. Connected antiparallel to these transistors  224 ,  234 ,  324 ,  334  is a plurality of free-wheeling diodes  222 ,  232 ,  322 ,  332  (respectively), and once again only one of each is labeled. The transistors and diodes are for instance semiconductor components with a rating of 1700 V, of the kind used in manifold ways in standard power semiconductor modules. The direct voltage side of the three-phase bridge circuit  22 ,  32  is connected to an intermediate circuit  24 ,  34  (respectively), which has at least one capacitor  240 ,  340 . The alternating voltage side of each three-phase bridge circuit  22 ,  32  is connected centrally to a respective associated winding of a transformer. 
         [0023]    A first current valve  26 ,  36  is connected parallel to three-phase bridge circuit  22 ,  32  (respectively) and to capacitors  240 ,  340  (respectively), respectively. In this embodiment of power converter circuit  1 , first current valve  26 ,  36  is embodied as an antiparallel circuit of at least one bipolar transistor  260 ,  360  and at least one diode  262 ,  362 . First current valve  26 ,  36  serves to shunt out the respective power converter cell  20 ,  30 . Thus, on the one hand a cascade  2 ,  3  with a redundant number of power converter cells  20 ,  30  can be embodied. On the other hand, a defective power converter can be shunted out, and therefore the functionality of the entire power converter circuit  1  can be preserved even if a failure occurs in any individual component. 
         [0024]    Intermediate circuit  24 ,  34  also, in one branch, has a second current valve  28 ,  38 , which is likewise embodied as an antiparallel circuit of at least one bipolar transistor  280 ,  380  and at least one diode  282 ,  382 . 
         [0025]    In the embodiment shown of power converter circuit  1 , cascades  2 ,  3  are formed of six power converter cells  20 ,  30  each, but this is merely an example. A high-voltage direct current transmitter operates for instance with a voltage of 100 kV, for which purpose, depending on the voltage ratings of the semiconductor components used, the number of serially connected power converter cells  20 ,  30  is on the order of magnitude of 100. 
         [0026]    In such an embodiment of the power converter circuit  1 , it is advantageous for insulation regions to connect each cascade  2 ,  3  at its center point to ground potential  12 ,  14 . Thus the high-voltage direct current transmitter  10 , with a voltage of 100 kV, for example, is advantageously connected at its center point to ground potential, and as a result the potential of the individual line toward ground amounts to only 50 kV each, making its insulation simpler. 
         [0027]    A further advantage of this embodiment of the power converter circuit  1  is that by means of offset clocking of the individual power converter cells  20 ,  30 , the voltage changes per unit of time are substantially lower in comparison to the prior art discussed above. As a result, the input filters still needed can be substantially less complicated. 
         [0028]    It is also advantageous that in the embodiment of the power converter circuit  1  of the invention, as a result of offset clocking of the individual power converter cells  20 ,  30 , the current ripple in the power system is less, compared to the prior art, by a factor that corresponds to the number of power converter cells  20 ,  30 . At a clock frequency of 1 kHz, for instance, and an offset of 0.1 ms, the current ripple in the power system is equivalent to a virtual switching frequency of 10 kHz. 
         [0029]    In this symmetrical embodiment of the power converter circuit  1 , energy can be transmitted in both directions, that is, from first cascade  2  to second cascade  3 , but also from second cascade  3  to first cascade  2 . In transmission from first cascade  2  to second cascade  3 , all the transistors  260  of first current valve  26  are switched to be nonconducting; thus only the current path via diodes  262  is conducting. The various transistors  280  of second current valve  28  of those power converter cells that are to be added are in this case made conducting. By this kind of addition of power converter cells  20 , the generated direct voltage of first cascade  2  is increased, or upon subtraction is reduced. 
         [0030]    Transistors  380  of second current valves  38  of second cascade  3  are all switched to be nonconducting, and as a result only the current path through diode  382  is conducting. Depending on the direct voltage transmitted, transistors  380  of first current valve  36  are made conducting. By cyclical switching of these transistors  360 , all the power converter cells  30  are may be loaded uniformly. It is equally possible for individual defective cells to be fundamentally switched off. This purpose can also be served by an additional mechanical switch  39 . 
         [0031]    Cells capable of functioning that are switched off can furthermore provide a necessary reactive power for the alternating voltage power system connected, and this reactive power can have arbitrary capacitive or inductive components. 
         [0032]    The power converter circuit  1  of the invention, because of its cascaded construction comprising cascades  2 ,  3  of power converter cells  20 ,  30  with standard components, has the advantage that even high-voltage direct current transmitters for small outputs, for instance from 1 MW up, can be produced economically. The scalable construction thus has the advantage that the effort and expense are also scaled approximately linearly with the power. This makes the economical use of high-voltage direct current transmitters between asynchronous separate power systems, for instance, possible. 
         [0033]      FIG. 2  shows a second exemplary embodiment of a power converter circuit  1 ′ of the invention. In this case, an asymmetrical embodiment is shown, with first power converter cells that are embodied as rectifiers and second power converter cells whose fundamental embodiment is equivalent to the power converter cells of  FIG. 1 . 
         [0034]    Each first power converter cell  20 ′ comprises a three-phase rectifier circuit  21 , each having one upper current valve  210  and one lower current valve  212  per phase; each current valve  210 ,  212  is embodied as a diode, or preferably as a plurality of parallel-connected diodes, for example with a rating of 1700 V, and as a result standard power semiconductor modules can be used as components of cascade  2 ′. 
         [0035]    The direct voltage side of three-phase rectifier circuit  21  forms second terminals on the direct voltage side, while the alternating voltage side is connected, in each case centrally, to an associated winding  42  of transformer  40 . 
         [0036]    In this respect, it is especially advantageous if the first terminals, on the transformer side, of two adjacent first power converter cells  20  are each connected to respective associated windings  42  of a transformer  40 . 
         [0037]    Second power converter cells  30 ′ of power converter circuit  1 ′ are embodied in two different embodiments. In each embodiment, second power converter cell  30 ′ comprises a three-phase bridge circuit  32 ′, which in turn is embodied with one upper power switch  320 ′ and one lower power switch  330 ′ per phase. Each power switch  320 ′,  330 ′ in turn comprises a parallel circuit of a plurality of bipolar transistors  324 ′,  334 ′, only one of each of which is labeled. A free wheeling diode  322 ′,  332 ′ is connected antiparallel to each of transistors  324 ′,  334 ′ respectively, and once again only one of each is labeled. The transistors and diodes here are for instance a semiconductor component with a rating of 1700 V, of the kind used in manifold ways in standard power semiconductor modules. The direct voltage side of three-phase bridge circuit  32 ′ is connected to an intermediate circuit  34 ′, which has at least one capacitor  340 ′. The alternating voltage side of the three-phase bridge circuit  32 ′ is connected, in each case centrally, to an associated winding of a transformer, not shown. 
         [0038]    A first current valve  36 ′ is connected parallel to three-phase bridge circuit  32 ′ and to capacitor  340 ′. Current valve  36 ′, in the first embodiment of second power converter cells  30 ′, is embodied as an antiparallel circuit of at least one bipolar transistor  360 ′ and at least one diode  362 ′. This current valve  36 ′ serves to shunt out the respective power converter cell  30 ′. Thus, on the one hand, a cascade  3 ′ with a redundant number of power converter cells  30 ′ can be embodied. On the other hand, a defective power converter cell can thus be shunted out, and hence the functionality of the entire power converter circuit  1 ′ can be preserved even if individual component failures occur. 
         [0039]    Intermediate circuit  34 ′ moreover, in one branch, has a second current valve  38 ′, which is likewise embodied as an antiparallel circuit of at least one bipolar transistor  380 ′ and at least one diode  382 ′. 
         [0040]    In the second embodiment of second power converter cells  30 ′, the second current valve  38 ′ includes at least one diode  384 . If there is a plurality of such diodes, then they are understood to be connected in parallel. First current valve  36 ′ is embodied as a thyristor  364 . 
         [0041]    In the embodiment shown of power converter circuit  1 ′, cascades  2 ′,  3 ′ are once again formed of six power converter cells  20 ′,  30 ′ each, and this is again merely an example, for the sake of simplicity. The precise number is determined by the voltage of the high-voltage direct current transmitter  10 ′, the voltage categories of the power semiconductor components used, and the desired redundance in terms of power converter cells  20 ′,  30 ′. One of ordinary skill in the art would recognize the appropriate number of components to be used in any given application without undue experimentation. 
         [0042]    In such an embodiment of power converter circuit  1 ′, it is advantageous for insulation reasons to connect each cascade at its center point to ground potential  12 ,  14 . 
         [0043]    In this asymmetrical embodiment of the power converter circuit  1 ′, energy can be transmitted solely from first cascade  2 ′ to second cascade  3 ′. 
         [0044]    The connection to an associated transformer  40  is shown, taking two first power converter cells  20 ′ as an example. It is especially advantageous that the first terminals, on the transformer side, of two adjacent power converter cells  20 ′ are each connected to respective associated windings  42  of transformer  40 . 
         [0045]    Thus, while there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.