Abstract:
A device has a converter which is connected to a direct voltage circuit through a short-circuit protection unit. The short-circuit protection unit is arranged at least partially in the direct voltage circuit and is provided in the direct voltage circuit to suppress short-circuit current flowing through the converter. The device contains one or more controllable power semiconductors, wherein a protection element is arranged in parallel to at least one of the controllable power semiconductors. The device prevents the negative effects of a short circuit occurring in the direct voltage network in a particularly reliable manner. For this purpose, the protection element is an energy store.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an apparatus having
         a converter which is connected to a DC voltage circuit via a short-circuit protection device,   which is arranged at least partially in the DC voltage circuit and is designed to suppress a short-circuit current flowing via the converter in the DC voltage circuit, and   has one or more controllable power semiconductors,   wherein a protection element is arranged in parallel with at least one of the controllable power semiconductors.       

     An apparatus such as this has already been disclosed in DE 698 37 414 T2. This document describes an apparatus having a self-commutated converter (so-called voltage source converter), which is connected between an AC voltage grid system and a DC voltage grid system and is designed to transmit electrical power between the DC voltage grid system and the AC voltage grid system. For this purpose, the converter converts DC voltage to AC voltage, or vice versa. The apparatus also has a short-circuit protection device, which is arranged in the DC voltage grid system and has a plurality of series-connected controllable power semiconductors, with a surge arrester in each case being arranged in parallel with each of the controllable power semiconductors. 
     In order to prevent a short circuit occurring in the DC voltage grid system from destroying any components of the converter, the short-circuit protection device limits the current in the DC voltage circuit to an acceptable level. For this purpose, at least one of the controllable power semiconductors is switched off. A voltage rise then quickly occurs across the power semiconductor that has been switched off. A surge arrester is provided as a protection element in parallel with the switched-off power semiconductor and reacts with a low impedance above a voltage threshold value. Since the short-circuit current is passed via the surge arrester, which then has a low impedance, this limits the current in the DC voltage circuit, protecting the converter. 
     Since the short-circuit current level cannot be predicted, the number of power semiconductors to be controlled, and possibly also the control frequency, must be matched to the respective circumstances within the scope of this prior art. The relative closed-loop control effort for controlling this apparatus conceals the risk that short-circuit currents which occur suddenly will not be adequately limited sufficiently quickly. This can lead to destruction of converter components. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the invention is therefore to provide an apparatus of the type mentioned initially which reliably prevents the negative effects of a short circuit occurring in the DC voltage grid system. 
     The invention achieves this object by the protection element being an energy store. 
     According to the invention, the protection element is used for safe interruption of the short-circuit current, by being an energy store. In contrast to this, the protection element according to the prior art is used to limit the short-circuit current, by being a surge arrester. This means that the short-circuit protection device according to the invention interrupts a short-circuit current safely and quickly without any risk of destroying one of the switched-off power semiconductors in the short-circuit protection device. In contrast to this, according to the prior art, the short-circuit current is limited in steps by means of the known short-circuit protection device, thus resulting in a risk of destruction of converter components during the current limiting process. 
     The total number of the one or more, possibly series-connected, controllable power semiconductors in the short-circuit protection device according to the invention is chosen such that no voltage which exceeds the withstand voltage of the power semiconductor occurs across any of the simultaneously switched-off power semiconductors when short-circuit voltages occur. In other words, the one or more controllable power semiconductors in the short-circuit protection device are designed to block the maximum short-circuit voltage that occurs. The energy store connected in parallel with at least one of the controllable power semiconductors is used to ensure that minor time differences in controlling the power semiconductors and therefore minor differences in the switching off of the power semiconductors do not lead to an incorrect distribution of the voltage dropped across the components, which would be damaging to the power semiconductors. 
     By way of example, the apparatus having a converter is part of a high-voltage direct-current transmission installation (HVDCT installation). HVDCT installations such as these can transport electrical power, which is in the form of AC voltage, over long distances via a DC voltage grid system, by transformation of the AC voltage to DC voltage in a converter, with the electrical power being converted back to AC voltage at the arrival location via a further converter. The two converters are in this case of identical design and, according to the invention, each of these converters can be protected by at least one short-circuit protection device against short circuits occurring in the DC voltage grid system. However, the apparatus having a converter could also be a part of the electrical drive apparatus of a rail vehicle. The apparatus according to the invention is, of course, not restricted to the exemplary embodiments mentioned. 
     As has already been described, the converter is used to convert AC voltage to DC voltage, and vice versa. The design of converters such as these is known. By way of example, this may be a so-called self-commutated converter with a DC voltage link circuit using two-point or three-point technology, or a self-commutated converter with a DC voltage link circuit and multilevel topology (so-called modular multilevel voltage source converter). The design of converters such as these is known and comprises series circuits of submodules which have one or more controllable power semiconductors, each having a freewheeling diode arranged back-to-back with them. When a short circuit occurs in the DC voltage grid system, the AC voltage grid system drives a current via the freewheeling diodes in the converter to the fault location that caused the short circuit. During the process, the freewheeling diodes, which carry the short-circuit current, are destroyed. In order to prevent this, according to the invention, the short-circuit protection device which interrupts the short-circuit current is arranged in the DC voltage link circuit. In contrast, comparatively fast interruption of the short-circuit current would be impossible using a conventional, mechanically switching, circuit breaker. 
     The energy store can advantageously be a capacitor. 
     The capacitor connected in parallel with the controllable power semiconductor is in this case designed such that, when the power semiconductor is switched off, this does not lead to a sudden rise in the voltage dropped across the power semiconductor and therefore, with short time differences between the power semiconductors being switched off, to any incorrect distribution of the voltage within the series circuit that would be damaging to these power semiconductors. 
     It may also be considered advantageous to connect a diode in each case back-to-back in parallel with each controllable power semiconductor. 
     This so-called freewheeling diode is additionally used for protection against overvoltages when the controllable power semiconductor is switched off. The overvoltages occur because of inductances in the short-circuit circuit to be switched off. The freewheeling diode prevents such voltage peaks. 
     It may also be considered advantageous to arrange at least two controllable power semiconductors, which follow one another in a series circuit, back-to-back, and to bridge them by a common capacitor. 
     According to this exemplary embodiment, the short-circuit protection device interrupts a short-circuit current independently of its current direction. This makes it possible to use the apparatus according to the invention in installations in which the current can flow in both directions in the DC voltage link circuit. 
     It is advantageously also possible for the converter to have power semiconductor valves which each have a series circuit of submodules, and the short-circuit protection device has a series circuit of submodules, wherein the submodules of the short-circuit device and the submodules of the converter are of identical design. 
     The use of standard components reduces the procurement costs for the apparatus according to the invention. 
     According to one advantageous refinement of the invention, the submodule has two controllable power semiconductors which are connected in the same sense in series and are bridged by a common capacitor. 
     According to a further advantageous refinement of the invention, the apparatus has a detection device for detection of a short-circuit current, and a control apparatus for controlling at least one of the power semiconductors in the short-circuit protection device, wherein the detection device is connected to the control device via a communication line. 
     A further advantageous refinement of the invention provides that the controllable power semiconductor is a bipolar transistor with an insulated gate electrode (IGBT). 
     It can also be considered advantageous for the controllable power semiconductor to be a thyristor. 
     Further advantages and refinements of the invention are the subject matter of the description of exemplary embodiments of the invention with reference to the figures of the drawings, in which the same reference symbols refer to components having the same effect, and in which: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows a short-circuit current path in a self-commutated converter (VSC) using two-point technology, illustrated schematically, 
         FIG. 2  shows a short-circuit current path in a self-commutated converter (VSC) using so-called multilevel topology, illustrated schematically, 
         FIG. 3  shows a first exemplary embodiment of the apparatus according to the invention, illustrated schematically, 
         FIG. 4  shows a detail of a second exemplary embodiment of the short-circuit protection device according to the invention, illustrated schematically, 
         FIG. 5  shows a detail of a third exemplary embodiment of the short-circuit protection device according to the invention, illustrated schematically, and 
         FIG. 6  shows a detail of a fourth exemplary embodiment of the short-circuit protection device according to the invention, illustrated schematically. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a self-commutated converter  3  (VSC) using two-point technology, which is designed for power transmission between an AC voltage grid system  1  and a DC voltage grid system  2 . For this purpose, the three-phase AC voltage grid system  1  is connected by a respective AC voltage connection  5   a ,  5   b ,  5   c  to one of three phase module branches  4   a ,  4   b ,  4   c  of the converter  3 . Each of the three phase module branches  4   a ,  4   b ,  4   c  has two DC voltage connections  6   a   1 ,  6   a   2 ,  6   b   1 ,  6   b   2 ,  6   c   1 ,  6   c   2  for connection to the DC voltage grid system  2 . The phase module branches  4   a ,  4   b ,  4   c  have a series circuit formed by submodules  8 , of which in each case two are illustrated for each valve branch in  FIG. 1 . However, there may be any desired number of submodules within the scope of the invention, and the number may even be greater than 100. Each submodule  8  is provided by a parallel circuit formed by a controllable power semiconductor  9  and a freewheeling diode  10  connected back-to-back to it. An intermediate-circuit capacitor  7  in the converter  3  is connected in parallel with the three phase module branches  4   a ,  4   b ,  4   c.    
     In the event of a short circuit, as indicated by the reference symbol  12 , in the DC voltage circuit, a short-circuit current fed from the AC voltage grid system  2  is created, inter alia along a short-circuit current path  13 . This short-circuit current path  13  is, for example, selected from a multiplicity of possible short-circuit current paths. The freewheeling diodes  14 ,  15 ,  16 ,  17  located on the short-circuit current path  13 , as well as the intermediate-circuit capacitor  7  will be destroyed or damaged after even a short time because of the high current along the short-circuit current path  13 . 
       FIG. 2  shows a short-circuit current path in a self-commutated converter  18  (VSC), in which, in contrast to  FIG. 1 , the converter  18  is designed using so-called multilevel topology. The phase module branches  19   a ,  19   b ,  19   c  are once again respectively connected by an AC voltage connection  5   a ,  5   b ,  5   c  to one phase of an AC voltage grid system  1 , and via two DC voltage connections  6   a   1 ,  6   a   2 ,  6   b   1 ,  6   b   2 ,  6   c   1 ,  6   c   2  to a DC voltage grid system  2 . With reference to the converter  3  in  FIG. 1 , the series-connected submodules  20  in the phase module branches  19   a ,  19   b ,  19   c  are, however, designed differently. In particular, the capacitance of the central capacitor shown in  FIG. 1  is distributed between a multiplicity of submodules in the exemplary embodiment shown in  FIG. 2 . Each of the submodules therefore has its own capacitor. Furthermore, a series circuit of two controllable power semiconductors  22 , arranged in the same sense, is connected in parallel with each capacitor  21 , and each of these power semiconductors  22  has a freewheeling diode  23 ,  25  arranged back-to-back in parallel with it. 
     In the event of a short circuit  12  occurring in the DC voltage grid system  2 , a short-circuit current fed from the AC voltage grid system  2  is once again created, inter alia along the short-circuit current path  24 . By way of example, this short-circuit current path  24  is also selected from a multiplicity of possible short-circuit current paths. The freewheeling diodes  23 ,  26 ,  27 ,  28  which are located on the short-circuit current path  24  will be destroyed or damaged in a short time because of the high current along the short-circuit current path  24 . 
       FIG. 3  shows a first exemplary embodiment of the apparatus according to the invention having a converter  29  which is connected by AC voltage connections  5   a ,  5   b ,  5   c  to an AC voltage grid system  1 , and by DC voltage connections  6   a   1 ,  6   a   2 ,  6   b   1 ,  6   b   2 ,  6   c   1 ,  6   c   2  to a DC voltage circuit  2 . The converter  29  is a self-commutated converter (VSC) using multilevel topology, and has a series circuit of bipolar submodules, which each have an associated energy store, for example in the form of a capacitor. A power semiconductor circuit is connected in parallel with the energy store and can be used to produce the voltage dropped across the energy store or a zero voltage at the submodule connection. Within the scope of the invention, the converter may, however, in principle have any desired topology, which means that two-stage, three-stage or five-stage VSCs are also possible. In  FIG. 3 , a capacitor is shown between the poles of the DC voltage link circuit, as typically occurs only in the case of two-stage VSCs. This is intended to indicate that any desired converter topology may in principle be used. A short-circuit protection device  30  is arranged in the DC voltage circuit  2  with a series circuit, arranged in the DC voltage circuit  2 , of submodules  31  which have at least one controllable power semiconductor, and a capacitor connected in parallel with it. The controllable power semiconductors in the submodules  31  are connected to a control apparatus  33  via a control line  32 . In addition to the submodules  31 , a detection device  34  is arranged in the DC voltage grid system  2 , and is connected to the control apparatus  33  via a communication line  35 . 
     In the event of a short circuit occurring in the DC voltage circuit  2 , the detection device  34  detects a short-circuit current and sends a detection signal to the control apparatus  33 , in response to which the control apparatus  33  switches the power semiconductors in the submodules  31  to a state in which they block the current. The short-circuit protection device  30  therefore leads to the short-circuit current being interrupted in the DC voltage circuit  2 , thus protecting the components of the converter  29 . 
       FIG. 4  illustrates the design of a submodule according to a second exemplary embodiment. The illustration shows two of a plurality of series-connected submodules  38  in the short-circuit protection device. The submodules  38  have a controllable power semiconductor  39  and, in parallel with it, a capacitor  36  and a freewheeling diode  37 , with the freewheeling diode  37  being connected back-to-back in parallel with the power semiconductor  39 . 
     A current flowing through the submodules  38  in one direction is blocked by operating the power semiconductors  39  simultaneously. In this case, the capacitors  36  result in short time differences in the operation of the power semiconductors  39 , and therefore short time differences between the power semiconductors  39  being switched off, not leading to an incorrect distribution of the voltage dropped across the series circuit, which would be damaging for the power semiconductors  39 . 
       FIG. 5  shows a further embodiment  44  of the submodules illustrated in  FIG. 4 , according to a third exemplary embodiment. The submodules  44  are designed using multilevel topology, corresponding to the submodules in a self-commutated converter, and comprise a series circuit of two controllable power semiconductors  45   a ,  45   b  which block the current in the same direction. A freewheeling diode  41   a ,  41   b  is respectively connected back-to-back in parallel with the power semiconductors  45   a ,  45   b . A capacitor  40  is arranged in parallel with the series circuit. Each of the bipolar submodules  44  is connected, starting from a connecting terminal  42 , to a connecting terminal  43  of an adjacent submodule  44 , with the connecting terminal  43  being conductively connected to the potential point between the power semiconductors  45   a ,  45   b  in the adjacent submodule  44 . 
     The illustrated series circuit of submodules  44  makes it possible to block a current in a current direction which is in the opposite direction to the freewheeling diodes  41   a ,  41   b.    
       FIG. 6  shows a further embodiment  47  of the submodules illustrated in  FIG. 4 , according to a fourth exemplary embodiment. In contrast to submodules  44  in  FIG. 5 , the submodules  47  have power semiconductors  48   a ,  48   b  which block current in the opposite direction. The submodules are connected, starting from a connecting terminal  42 , to a connecting terminal  46  of an adjacent submodule  47 . 
     The series circuit of the submodules  47  makes it possible to block a current in both current directions. 
     LIST OF REFERENCE SYMBOLS 
     
         
           1  AC voltage grid system 
           2  DC voltage grid system 
           3  Converter 
           4   a ,  4   b ,  4   c  Phase module branch 
           5   a ,  5   b ,  5   c  AC voltage connection 
           6   a   1 ,  6   a   2 ,  6   b   1 , DC voltage connection 
           6   b   2 ,  6   c   1 ,  6   c   2   
           7  Intermediate-circuit capacitor 
           8  Submodule 
           9  Controllable power semiconductor 
           10  Freewheeling diode 
           12  Short circuit 
           13  Short-circuit current path 
           14 ,  15 ,  16 ,  17  Freewheeling diode 
           18  Converter 
           19   a ,  19   b ,  19   c  Phase module branch 
           20  Submodule 
           21  Capacitor 
           22  Controllable power semiconductor 
           23  Freewheeling diode 
           24  Short-circuit current path 
           25 ,  26 ,  27 ,  28  Freewheeling diode 
           29  Converter 
           30  Short-circuit protection device 
           31  Submodule 
           32  Control line 
           33  Control apparatus 
           34  Detection device 
           35  Communication line 
           36  Capacitor 
           37  Freewheeling diode 
           38  Submodule 
           39  Controllable power semiconductor 
           40  Capacitor 
           41   a ,  41   b  Freewheeling diode 
           42  Connecting terminal 
           43  Connecting terminal 
           44  Submodule 
           45   a ,  45   b  Controllable power semiconductor 
           46  Connecting terminal 
           47  Submodule 
           48   a ,  48   b  Controllable power semiconductor