Abstract:
One aspect of the present invention concerns an inverter for converting a direct current into an alternating current. In one aspect, in order to provide an inverter which can be repaired more quickly and more reliably at the location of use, an inverter is of an at least partially modular structure, with releasably installed modules and connecting lines releasably mounted to the modules.

Description:
RELATED INFORMATION 
     The present invention concerns an inverter (dc-ac converter) for converting a direct current into an alternating current. Such inverters have long been known and are used in many different situations, inter alia in wind power installations. In that respect modern inverters use semiconductor switching elements such as IGBTs which are suitably actuated to produce the alternating current. Those semiconductor switching elements are admittedly overall relatively reliable, but failure thereof is not out of the question. 
     In the case of inverters of a conventional structure, upon failure of such a semiconductor switching element it is replaced on site. In that situation, unintended and unwanted fouling and/or damage and/or fitting errors can occur, which remain unnoticed and which in a short time result in (renewed) destruction of the semiconductor switching element. The repair procedure which is thus required once again involves expenditure which would have been avoidable. 
     SUMMARY OF THE INVENTION 
     Therefore the object of an aspect of the present invention is to provide an inverter which reduces or avoids those disadvantages. 
     In one aspect, in an inverter of the kind set forth in the opening part of this specification, that object is attained by an at least partly modular structure, with releasably installed modules and connecting lines releasably mounted to the modules. In that respect, an aspect of the invention is based on the realization that repair on site is naturally more susceptible to error than repair in a workshop which is specifically set up for that purpose. Therefore repair on site is limited to identifying the defective module, removing it, and replacing it by a faultless module. Besides the reduced level of susceptibility to error of the repair operation in itselt, replacement is to be effected, if the modules are of a suitable design configuration, more quickly than repair, so that the inverter can be more rapidly brought back into operation again. 
     In a preferred embodiment of an aspect of the invention modularization is directed to the function of a module so that, upon the occurrence of a fault, it is already possible to infer from the fault a specific module and at least a small number of modules which are then still to be checked. 
     In a particularly preferred development of an aspect of the invention the inverter further includes an intermediate storage means which is formed from a plurality of capacitors and to which at least some of the modules are connected. If those modules which are connected to the intermediate storage means are the semiconductor switching element modules, the intermediate storage means is in a position to compensate for fluctuations as a consequence of the switching procedures of the semiconductor switching elements, and thus to permit stable operation of the semiconductor switching elements. 
     In a particularly advantageous development of an aspect of the invention the connection between the intermediate storage means and at least some of the modules is capacitive. That capacitive configuration of the connection provides that it is possible for the influences of parasitic inductances which are inevitable in connecting lines to be reduced to a minimum. In that way it is also possible to avoid operational faults and disturbances which are to be attributed to the effect of such inductances. 
     In order to eliminate inductive influences to the greatest possible extent, the capacitive connection in an inverter according to an aspect of the invention particularly preferably includes at least one plate capacitor and the modules are mechanically connected to the plates of that plate capacitor. 
     In a preferred development of an aspect of the invention the plates of a plate capacitor form a mechanical and electrical connection between the intermediate storage means formed from a plurality of capacitors, and the connected modules. It is possible in that way to reduce the influence of inductances to the greatest possible extent. 
     In a particularly advantageous development of an aspect of the invention the spacing of the output terminals of the semiconductor switching elements of a module does not exceed a predetermined magnitude. In that way the lines connected thereto are also at a corresponding spacing. In that fashion those adjacent lines can pass through a common measuring transducer and thus the complication and expenditure involved for detecting the current flowing in the lines is limited. 
     Advantageous developments of the invention are recited in the appendant claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of aspects of the invention is described in greater detail hereinafter with reference to the Figures in which: 
         FIG. 1  shows a simplified view of an inverter according to one aspect of the invention, 
         FIG. 2  shows a selection of modules from  FIG. 1 , 
         FIG. 3  shows a detail on an enlarged scale from  FIG. 2 , 
         FIG. 4  shows a simplified view of a semiconductor module, 
         FIG. 5  shows a side view of a capacitor plate, 
         FIG. 6  shows a view on to a further capacitor plate, 
         FIG. 7  shows a detail of the capacitor plate, and 
         FIG. 8  shows an alternative embodiment to  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a simplified view of an inverter according to one aspect of the invention. This Figure does not show components such as chokes, relays, safety devices and the like but also cables, in order to improve the clarity of the drawing. The inverter is disposed in a cabinet  1  and has a plurality of modules. Those modules perform different functions. Thus, the modules  2 ,  3  and  4  are semiconductor modules which produce an alternating current from a direct current. In that respect, provided for each phase is a specific module at  2 ,  3  and  4 . A further module  5  is provided for controlling all modules of the inverter according to the aspect of the invention. A module  6  is a voltage balancing device. Further modules  7  can also be provided. They can perform the function of a chopper, a step-up booster device or the like. 
     Those modules  2 ,  3 ,  4 ,  5 ,  6 ,  7  are installed in a cabinet  1  which is already prepared in a particular manner for the installation of modules. Extending between the modules  2 ,  3 ,  4 ,  5 ,  6 ,  7  are cable connections (not shown) which preferably connect the individual modules together by way of plug connections. The modules  2 ,  3 ,  4 ,  5 ,  6 ,  7  themselves are fixed releasably in the cabinet  1 , for example with screws. 
     Therefore, to exchange a module  2 ,  3 ,  4 ,  5 ,  6  or  7 , it is only necessary to withdraw the cable at that module and to release the screw connections of the module. Then, each individual one of the modules  2 ,  3 ,  4 ,  5 ,  6 ,  7  can be separately replaced in that way. In the case of a fault therefore the service engineer only has to fit a suitable replacement module in place of the module which has been recognised as being defective, and the inverter can be brought back into operation again after a short repair time. 
       FIG. 2  shows in particular the connection of the modules  2 ,  3 ,  4  and  7  to the dc circuit of the inverter. It has already been explained with reference to  FIG. 1  that the modules  2 ,  3  and  4  are semiconductor modules which produce for a respective phase an alternating current from a direct current. The switching elements used can be for example thyristors or IGBTs or other semiconductors. In order to feed direct current to those modules  2 ,  3 ,  4 ,  7 , the arrangement has connecting plates  12 ,  13 ,  16 ,  17  to which the modules  2 ,  3 ,  4 ,  7  are connected. That connection is made by way of bars  10 ,  11 ,  14 ,  15  which are mounted on the one hand to the connecting plates  12 ,  13 ,  16 ,  17  and on the other hand to the modules  2 ,  3 ,  4 ,  7  by screws  21 . The bars  10 ,  11 ,  14 ,  15 , like the connecting plates  12 ,  13 ,  16 ,  17 , are separated in accordance with potentials. In the present embodiment for example plates  12  and  16  can be anodes and thus involve a positive potential and the plates  13  and  17  can be cathodes and accordingly involve a negative potential or also ground potential. It will be appreciated that in that case the bars  10  and  14  are correspondingly at positive potential and the bars  11  and  15  are at negative or ground potential. 
     The use of bars  10 ,  11 ,  14 ,  15  for feeding electrical power to the modules  2 ,  3 ,  4 ,  7  means that correspondingly high currents can flow as the cross-section of the bars can be correspondingly large. To improve handleability, the bars  10 ,  11 ,  14 ,  15  can be divided so that a respective separate bar  10 ,  11 ,  14 ,  15  extends from a connecting plate to a module  2 ,  3 ,  4 ,  7 . 
     As can be seen from  FIG. 2  disposed behind the connecting plates  12 ,  13 ,  16 ,  17  are further plates  18 ,  19  in mutually parallel relationship. The arrangement and the mode of operation of those plates  18 ,  19  will be described in greater detail with reference to  FIG. 3 . In that respect  FIG. 3  is a view on an enlarged scale of the portion enclosed by a circle in  FIG. 2 . 
       FIG. 3  shows a 3-layer structure. That 3-layer structure is formed from two capacitor plates  18 ,  19  and a dielectric  20 . Accordingly that structure forms a plate capacitor. In this case the plates  18 ,  19  of the capacitor can be produced for example from aluminium and may be of a thickness of several millimeters. The dielectric  20  can be formed by a plastic film or sheet and can be of a thickness of some tenths of a millimeter. With this structure, one of the plates  18 ,  19  is necessarily the anode plate and the other plate is then inevitably the cathode plate. The plates  18 ,  19  of the capacitor are used to feed direct current to the modules  2 ,  3 ,  4 ,  7 . By virtue of the fact that the feed means is in the form of a plate capacitor, that feed is purely capacitive and the influence of unwanted inductances is avoided. If the capacitor plate  18  is the cathode plate, the connecting plates  13  and  17  are connected to that capacitor plate  18 . In a corresponding manner the connecting plates  12  and  16  are connected to the capacitor plate  19  acting as an anode plate. 
       FIG. 4  shows a simplified view of a semiconductor module  2 ,  3 ,  4 , more specifically with the semiconductor switching elements  22 . Those semiconductor switching elements  22  for which for example IGTBs are used are actuated in a suitable manner known from the state of the art in order to produce a desired alternating current which is taken off by way of cables  25 . The mode of operation of such a semiconductor module can be found for example from patent specification DE 197 48 479 C1. 
     In order to limit the spacing between cables  25  provided with the cable terminal lugs  24  and therewith naturally also between the cables  25  to a predetermined dimension, semiconductor switching elements  22  can be provided with suitably arranged connecting terminals. Alternatively, bars  23  are provided from the output terminals of the semiconductor modules  22  to the cable connecting lugs  24 , the bars  23  passing the output current of the semiconductor element  22  to a corresponding cable  25 . The bars  23  are fixed to the output terminal of the semiconductor module  22  by screws  26  which are shown here in the form of screws with a hexagonal recess in the head, and the cables  25  are in turn fixed to the bars  23  by means of cable connecting lugs  24  and screws  26  which are illustrated in the form of cross-headed screws. That arrangement makes it possible for the two cables  25  to be passed for example jointly through a measurement value pick-up device such as a ferrite ring of a current transformer in order to monitor the flow of current in the cables  25 . 
     In accordance with one aspect of the invention the inverter has an intermediate storage means formed from a plurality of capacitors. The purpose of that intermediate storage means is inter alia to smooth the dc voltage applied to the semiconductor modules and to compensate for voltage fluctuations as a consequence of the switching procedures of the semiconductor switching elements  22 . For that purpose that intermediate storage means (not shown) is connected by way of the plate capacitor  18 ,  19 ,  20  to the modules. The plates  18 ,  19  of the plate capacitor are shown in  FIGS. 5 and 6 . In that respect  FIG. 5  shows the anode plate and  FIG. 6  shows the cathode plate. Those plates  18 ,  19  have bores  26  and recesses  29  extending around those bores. That is shown in detail in a cross-sectional view by way of the example of a bare/recess in  FIG. 7 . 
     By virtue of that arrangement, it is possible on the one hand to connect the capacitors (not shown) forming the intermediate storage means to the plates  18 ,  19  by screw means so that, if required, individual capacitors can also be replaced, and, by virtue of the sunk arrangement of the screw head in the recess  29 , to arrange the plates in mutually juxtaposed relationship separated from each other only by the spacing of the dielectric (reference  20  in  FIG. 3 ). A comparison of the plates shown in  FIGS. 5 and 6  already shows that the connecting plates  12 ,  13 ,  16 ,  17  for the connecting bars are in displaced relationship with each other in respect of height, thus affording the staggered arrangement already shown in  FIG. 2 . 
       FIG. 8  shows an alternative arrangement to the connecting bars  10 ,  11 ,  14 ,  15  shown in  FIG. 2 . For that purpose the bars are broadened in such a way that they cover over both connecting plates  12 ,  13 . These bars which are also shown in plate form in this Figure are identified here by references  31  and  32 . In order to make it clear that they are in mutually superposed relationship, they are illustrated as being displaced relative to each other. In situ they are disposed in substantially aligned relationship one above the other. 
     In this case, one of the plates is connected to the anode  12  and the other to the cathode  13 . These plates  31 ,  32  also form a capacitor, by virtue of a dielectric which is inserted between the plates  31 ,  32  but which is not shown in the Figure. In accordance with the view in this Figure, the plate  32  is the anode plate and the plate  31  is the cathode plate. It will be appreciated that these plates are once again suitably connected to the semiconductor modules which are also not shown in this Figure, in order to feed them with the required direct current. As therefore the connecting plates  12 ,  13  are also connected to the semiconductor modules by way of a plate capacitor, inductive influence is also prevented on this portion of the electrical connection. 
     This therefore involves a capacitive connection from the intermediate storage means to the input terminals of the semiconductor modules.