Patent Publication Number: US-2015061408-A1

Title: Method for operating an electrical circuit and electrical circuit

Description:
FIELD OF THE INVENTION 
     Embodiments of the invention relate to a method for operating an electrical circuit and a corresponding electrical circuit. 
     BACKGROUND OF THE INVENTION 
     From publication DE 10 2010 046 142 A1 an electrical circuit has been known, said circuit being composed of a plurality of modular switches. As a result of an appropriate arrangement and activation of the power semiconductor components of the modular switches, it is possible to embody the electrical circuit as a converter, i.e., for the conversion of a direct voltage into an alternating voltage, or vice versa. Consequently, the electrical circuit can be used, in particular, for the transmission of energy with high direct voltages. 
     Referring to DE 10 2010 046 142 A1, the current can flow across the modular switches in only one direction. Therefore, if the known electrical circuit is used, for example in high-voltage direct current (HVDC) transmission, this has the result that a reversal of the direction of energy transmission can be achieved only in that the direct voltage is reversed. However, in the case of a unipolar undersea cable this is possible only within considerable constraints. 
     It is the object of the present invention to improve the known electrical circuit. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The electrical circuit in accordance with an embodiment of the invention comprises at least one modular switch, wherein the modular switch is provided with a first series circuit comprising a first controllable power semiconductor component and a first diode, and with a second series circuit comprising a second diode and a second controllable semiconductor component; wherein the connecting point of the first power semiconductor component and the first diode form a first connection, and the connecting point of the second diode and the second power semiconductor component form a second connection of the modular switch; wherein, in the first series circuit, the first power semiconductor component is connected in parallel to a third diode, and the first diode is connected in parallel to a third controllable power semiconductor element; wherein, in the second series circuit, the second power semiconductor component is connected in parallel to a fourth diode, and the second diode is connected in parallel to a fourth controllable power semiconductor component; wherein the conducting directions of the third diode and the third power semiconductor component correspond to the conducting directions of the first diode and the first power semiconductor component, and the conducting directions of the fourth diode and the fourth power semiconductor component correspond to the conducting directions of the second diode and the second power semiconductor; wherein the modular switch is provided with a capacitor; and wherein the first series circuit and the second series circuit and the capacitor of the modular switch are connected in parallel relative to each other. 
     A method for operating an electrical circuit according to an embodiment. The electrical circuit comprises at least one modular switch, wherein the modular switch is provided with a first series circuit comprising a first controllable power semiconductor component and a first diode, and with a second series circuit comprising a second diode and a second controllable semiconductor component. The connecting point of the first power semiconductor component and the first diode form a first connection, and the connecting point of the second diode and the second power semiconductor component form a second connection of the modular switch. In the first series circuit, the first power semiconductor component is connected in parallel to a third diode, and the first diode is connected in parallel to a third controllable power semiconductor element. In the second series circuit, the second power semiconductor component is connected in parallel to a fourth diode, and the second diode is connected in parallel to a fourth controllable power semiconductor component. The conducting directions of the third diode and the third power semiconductor component correspond to the conducting directions of the first diode and the first power semiconductor component, and the conducting directions of the fourth diode and the fourth power semiconductor component correspond to the conducting directions of the second diode and the second power semiconductor. The modular switch is further provided with a capacitor, wherein the first series circuit and the second series circuit and the capacitor of the modular switch are connected in parallel relative to each other. The first and the second power semiconductor components are switched, individually or together, so as to be conducting, and both the third and the fourth power semiconductor components are switched so as to be blocking, so that a current flows from the first connection across the first power semiconductor component, across the capacitor and across the second power semiconductor component to the second connection, or that both the third and fourth power semiconductor components are switched so as to be conducting, and both the first and the second power semiconductor components are switched so as to be blocking, so that the current flows in reverse direction from the second connection across the fourth power semiconductor component, across the capacitor and across the third power semiconductor component to the first connection. 
     Referring to the method in accordance with an embodiment of the invention, either both the first and second power semiconductor components are connected so as to be conducting, and both the third and fourth power semiconductor components are controlled so as to be blocking, so that a current from the first connection flows across the first power semiconductor component, across the capacitor and across the second power semiconductor component to the second connection, or both the third and fourth power semiconductor components are connected so as to be conducting, and both the first and second power semiconductor components are controlled so as to be blocking, so that the current flows in reverse direction from the second connection across the fourth power semiconductor component, across the capacitor and across the third power semiconductor component to the first connection. 
     Embodiments of allow current to flow through the modular switches in both directions. This may be achieved with an appropriate activation of the modular switches. In doing so, it is possible for electrical energy in the form of a direct current to be carried in both directions across power converters that comprise the modular switches. 
     Referring to the electrical circuit in accordance with an embodiment of the invention, a voltage reversal of the direct voltage is not necessary. Among other things, this allows unipolar cables to be used in direct-voltage transmission. 
     If embodiments of the invention are applied, for example, in the energy transmission of high direct voltages within a meshed direct-voltage network, it is possible to freely adjust the direct voltages that are used for energy transmission. In this manner, it is possible—even in the case of an error situation—to limit the direct voltage to one transmission section and to thus be able to respond to the error situation. 
     Furthermore, embodiments of the invention substantial limit errors and short circuit situations. Therefore, if, in a meshed direct voltage network, as many as possible or all current converters are capable of changing the direct voltage and thus limit the direct current, it is possible—after an error or a short circuit has been detected—to first limit the error or short circuit current at the error or short circuit location with the use of embodiments of the invention in order to subsequently, for example, completely break and galvanically separate the error current or short circuit current, for example with the help of common, already commercially available, circuit breakers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Additional features, possibilities of use, and advantages of the invention can be inferred from the description of the exemplary embodiments of the invention hereinafter, the exemplary embodiments being illustrated in the related figures. In doing so, the object of the invention is represented by each of the described or illustrated examples, individually or in any combination, and independently of their summarization or their citation or illustration in the description, or in the figures. In the drawings: 
         FIG. 1  a schematic block circuit diagram of an exemplary embodiment of an electrical circuit; 
         FIGS. 2A ,  2 B,  3 A, and  3 B show sections of the electrical circuit of  FIG. 1 ; 
         FIG. 4A  is a schematic block circuit diagram of an application of the electrical circuit of  FIG. 1 ; and 
         FIG. 4B  is a schematic time-dependency diagram of current and voltage characteristics as in  FIG. 4A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an electrical circuit  10  that can preferably be used within the framework of a so-called high-voltage direct current (HVDC) transmission. In particular, the circuit  10  may be used for connecting two existing electrical power supply networks in order to transmit electrical energy between the power supply networks in both directions. Hereinafter, follows a description of the direction of the current flow during normal operation, i.e., for the operation in which the current flows through clocked power semiconductor components and not in their anti-parallel diodes. Other current flows in opposite directions are possible. However, they will not be specifically described here. 
     The circuit  10  comprises a first converter  11  and a second converter  12 . The first converter  11  is connected to a first transformer  13  on its alternating-voltage side, and the second converter  12  is connected to a second transformer  14  on its alternating voltage side. Each of the converters  11 ,  12 , the transformers  13 ,  14  and their electrical connections are three-phased in the present exemplary embodiment. 
     On their direct-voltage side, the two converters  11 ,  12  are connected to each other by way of two electrical lines  15 ,  16 . Inductances  17 ,  18  may exist between the converters  11 ,  12  and the lines  15 ,  16 . 
     Each of the two converters  11 ,  12  is disposed to convert a direct voltage into an alternating voltage, or vice versa. The two transformers  13 ,  14  are disposed to adapt the voltage on the alternating-voltage side of the respectively associate converter  11 ,  12  to the existing boundary conditions. 
     A direct voltage is applied between the two electrical lines  15 ,  16 . Specifically, this is a high voltage, for example 320 kV. The length of the two lines  15 ,  16  may be several kilometers, for example 100 km. One of the two lines  15 ,  16 , for example line  16 , may be grounded. Preferably, a high-voltage direct current (HVDC) transmission can be implemented by way of the two lines  15 ,  16 . 
     Each of the converters  11 ,  12  is composed of a plurality of modular switches  21 ,  22 . Due to the exemplary three-phase embodiment, the modular switches  21 ,  22  in each of the two converters  11 ,  12  are arranged in three groups. Each of the groups of each converter  11 ,  12  includes the same number of modular switches  21 ,  22 . As will still be explained hereinafter, a three-step embodiment of the respective converter requires, respectively, two modular switches  21 ,  22  per group, a five-step embodiment requires respectively four modular switches  21 ,  22 , and so on. 
     It is to be understood that the number of phases of the circuit  10  may also be greater or smaller than three. Likewise, the number of phases of the two converters  11 ,  12  or the associate transformers  13 ,  14  may also be different. Likewise, the number of modular switches  21 ,  22  per group in the two converters  11 ,  12  may also be different. Instead of a transformer, it is also possible to use a throttle for a solution not using a transformer. 
       FIG. 2A  shows the modular switch  21  that is provided in the converter  11 . 
     The modular switch  21  has a first series circuit comprising a first controllable power semiconductor component V 1  and a first diode D 1 , as well as a second series circuit comprising a second diode D 2  and a second controllable power semiconductor component V 2 . 
     In a first series circuit, the collector of the first power semiconductor component V 1  and the anode of the first diode D 1  are connected to each other. This connecting point is referred to as the first connection  24 . In the second series circuit, the emitter of the second power semiconductor component V 2  and the cathode of the second diode D 2  are connected to each other. This connecting point is referred to as the second connection  25 . 
     The two series circuits are connected in parallel relative to each other. Consequently, the cathode of the first diode D 1  is connected to the collector of the second power semiconductor component V 2 , and the emitter of the first power semiconductor component V 1  is connected to the anode of the second diode D 2 . 
     In the first series circuit, a third diode D 3  is connected in parallel to the first power semiconductor component V 1 , and the first diode D 1  is connected in parallel to a third power semiconductor component V 3 . The conducting directions of the third diode D 3  and of the third power semiconductor component V 3  correspond to the conducting directions of the first diode D 1  and the first power semiconductor components V 1 . Correspondingly, the second power semiconductor component V 2  is connected in parallel to a fourth diode D 4 , and the second diode D 2  is connected in parallel to a fourth power semiconductor component V 4 . 
     A capacitor C is connected in parallel to the two series circuits that are connected in parallel. 
     A direct voltage u dc  is applied to the capacitor C, and a connecting voltage u a  exists between the two connections  24 ,  25 . The direction of the aforementioned voltages is indicated in  FIG. 2A . Furthermore, a current i flows from the first connection  24  in the direction to the second connection  25 . 
     Referring to the power semiconductor components V 1 , V 2 , V 3 , V 4 , these are controllable switches, for example, transistors, and in particular, field effect transistors, or thyristors with an optionally required auxiliary protective element, in particular gate turn-off (GTO) thyristors or insulated gate bipolar transistors (IGBTs), or comparable electronic components. Depending on the embodiment of the power semiconductor components V 1 , V 2 , V 3 , V 4 , their connections may be identified in different ways. The aforementioned terms collector and emitter relate to the exemplary use of IGBTs. The capacitor C may be configured so as to be unipolar. 
     The modular switch  21  is able to assume the following states, which are numbered for clarity and are in no way meant to be limiting. 
     (1) If the power semiconductor components V 1 , V 2 , V 3 , V 4  are switched off (blocking), the current i can flow either from the first connection  24  across the diode D 1 , across the capacitor C and across the diode D 2  to the second connection  25 , or in the reverse direction, i.e., from the second connection  25  across the diode D 4 , across the capacitor C, and across the diode D 3 , to the first connection  24 . In both cases, the capacitor C is charged by the flowing current i or by the reversely flowing current i so that the direct voltage u dc  becomes higher. Apart from the voltage drops on the diodes D 1 , D 2  and D 3 , D 4 , respectively, the connecting voltage u a  is equal to the negative direct voltage −u dc , therefore u a =−u dc , or equal to the positive direct voltage u dc . Therefore, u a =u dc . 
     (2) If both the power semiconductor components V 1 , V 2  are switched on (conducting) and both the power semiconductor components V 3 , V 4  are switched off (blocking), the current i—normal mode—flows from the first connection  24  across the first power semiconductor component V 1 , across the capacitor C, and across the second power semiconductor component V 2  to the second connection  25 . The capacitor C is discharged by this current i so that the direct voltage u dc  decreases. Apart from the voltage drops on the power semiconductor components V 1 , V 2 , the connecting voltage u a  is equal to the positive direct voltage u dc . Therefore, u a =u dc . 
     (3) If both the power semiconductor components V 3 , V 4  are switched on (conducting) and both the power semiconductor components V 1 , V 2  are switched off (blocking), the current i flows in the reverse direction, i.e., from the second connection  25  across the fourth power semiconductor component V 4 , across the capacitor C, and across the third power semiconductor component V 3  to the first connection  24 . The capacitor C is discharged by this current  1 , so that the direct voltage u dc  becomes lower. Apart from the voltage drops on the power semiconductor components V 3 , V 4 , the connecting voltage u a  is equal to the negative direct voltage −u dc . Therefore, u a =−u dc . 
     (4) If the first power semiconductor component V 1  is switched on (conducting) and the power semiconductor components V 2 , V 3 , V 4  are switched off (blocking), the current  1  flows from the first connection  24  across the first power semiconductor component V 1 , and across the second diode D 2  to the second connection  25 . The direct voltage u dc  on the capacitor C remains constant. Apart from the voltage drops on the first power semiconductor component V 1  and the second diode  2 , the connecting voltage u a  is equal to zero. Therefore, u a =0. 
     (5) If the power semiconductor components V 1 , V 3 , V 4  are switched off (blocking) and the second power semiconductor component V 2  is switched on (conducting), the current i flows from the first connection  24  across the first diode D 1 , and the second power semiconductor component V 2  to the second connection  25 . The direct voltage u dc  on the capacitor C remains constant. Apart from the voltage drops on the first diode D 1  and the second power semiconductor component V 2 , the connecting voltage u a  is equal to zero. Therefore, u a =0. 
     (6) If the third power semiconductor component V 3  is switched on (conducting) and the power superconductor components V 1 , V 2 , V 4  are switched off (blocking), the current i flows in the reverse direction from the second connection  25  across the fourth diode D 4 , and across the third power semiconductor component V 3  to the first connection  24 . The direct voltage u dc  on the capacitor C remains constant. Apart from the voltage drops on the third power semiconductor component V 3  and the fourth diode D 4 , the connecting voltage u a  is equal to zero. Therefore, u a =0. 
     (7) If the power semiconductor components V 1 , V 2 , V 3  are switched off (blocking) and the fourth power semiconductor component V 4  is switched on (conducting), the current i flows in reverse direction from the second connection  25  across the fourth power semiconductor component V 4  and the third diode D 3  to the first connection  24 . The direct voltage u dc  on the capacitor C remains constant. Apart from the voltage drops on the third diode D 3  and the fourth power semiconductor component V 4 , the connecting voltage u a  is equal to zero. Therefore, u a =0. 
     Consequently, the current through the modular switch  21  is able to flow in both directions. 
     In both cases, i.e., independent of the direction in which the current flows through the modular switch  21 , the connecting voltage u a  can essentially assume three values, i.e., u a =−u dc  or u a =u dc  or u a =0. In doing so, the direct voltage u dc  on the capacitor C may increase or decrease. 
       FIG. 2B  shows how the modular switch  21  of  FIG. 2A  is switched within one of the groups of the converter  11 . In doing so, the right group of the converter  11  of  FIG. 1  is shown as an example. The other groups of the converter  11  are configured accordingly. 
       FIG. 2B  shows two modular switches  21  per group as an example. In accordance with  FIG. 2B , the two modular switches  21  are connected in series. The connection  25  of the upper modular switch  21  is connected to a positive pole of the converter  11  on the direct-voltage side and thus connected to the line  15 . The connection  24  of the lower modular switch is connected to a negative pole of the converter  11  on the direct-voltage side and thus connected to the line  16 . The connecting point of the two modular switches  21  represents the associate phase of the converter  11  on the alternating-voltage side and is connected to the transformer  13 . 
     The described embodiment of the converter  11  is a three-phase converter  11 . The voltage of the associate alternating-voltage side phase of the converter  11  can thus essentially assume a positive state or a negative state, or a zero state. 
     Referring to  FIG. 3A , the modular switch  22  is shown comprising the converter  12 . 
     Considering its design, the modular switch  22  of  FIG. 3A  essentially corresponds to the modular switch  21  of  FIG. 2A . When visualized, the modular switch  22  of  FIG. 3A  represents a specular view of the modular switch  21  of  FIG. 2A  on plane A of  FIG. 2A . Therefore, considering the design and the function of the modular switch  22  of  FIG. 3A , reference is made to the explanations regarding the modular switch  21  of  FIG. 2A  hereinabove. 
       FIG. 3B  illustrates how the modular switch  22  of  FIG. 3A  is connected within one of the groups of the converter  12 . For example, the right group of the converter  12  of  FIG. 1  is shown. The other groups of the converter  12  are designed accordingly. 
       FIG. 3B  shows the provision of four modular switches  22  per group as an example. In accordance with  FIG. 3B , the four modular switches  22  are connected in series. The connection  25  of the uppermost modular switch  22  is connected to the positive pole of the converter  12  on the direct-voltage side and thus, connected to the line  15 . The connection  24  of the uppermost modular switch  22  is connected to the connection  25  of the modular switch  22  connected underneath. The connection  24  of the lowermost modular switch is connected to a negative pole of the converter  12  on the alternating-voltage side  12  and is thus connected to the line  16 . The connection  25  of the lowermost modular switch  22  is connected to the connection  24  of the modular switch  22  connected thereabove. The connecting point of the two middle modular switches  22  represents the associate phase on the alternating-voltage side of the converter  12  and is thus connected to the transformer  14 . 
     The described embodiment of the converter  12  is configured so as to have five phases. This means that the voltage of each alternating-voltage-side phase of the converter  12  can essentially assume a high positive state or a mean positive state, or a high negative state or a mean negative state, or a zero state. 
     The electrical circuit  10  of  FIG. 1  is associated with a not illustrated control device. This control device may be provided directly at the individual power semiconductor components or in a central location independent of the power semiconductor components. Likewise, it is possible for a plurality of control devices to be provided, said devices being locally distributed and, for example, hierarchically set up. 
     This (these) control device(s) activates (activate) the power semiconductor components of the electrical circuit  10  in a clocked manner such that each of the modular switches  21 ,  22  provided in the converters  11 ,  12  assumes one of the explained states. The selection of the respectively to be activated state of the individual modular switch  21 ,  22  is a function of the direction in which the current i is to flow through the respective modular switch  21 ,  22 , as well as of the connecting voltage u a  that is to exist on the respective modular switch  21 ,  22 . As a function of a change of the connecting voltage u a , the current i flowing across the modular switch  21 ,  22  also changes. 
     Considering the explained electrical circuit  10 , the power semiconductor components V 1 , V 2 , V 3 , V 4  of the modular switches  21 ,  22  are always activated only in pairs in a clocked manner. Consequently, depending on the direction of the current flow, the power semiconductor components V 1 , V 2  are controlled in a clocked manner in conducting mode, and the other two power semiconductor components remain switched off or blocked, or vice versa. This paired activation of either the two power semiconductor components V 1 , V 2  or the two power semiconductor components V 3 , V 4  is consistent with the second and third states, as has been described hereinabove regarding the power semiconductor components. When clocking a power semiconductor pair V 1 -V 2 , the power semiconductor components V 1  and V 2  are individually switched on and off. The power semiconductor components V 1  and V 2  may be conductive synchronously or asynchronously (possible states are: V 1  and V 2  Off, V 1  or V 2  Off, as well as V 1  and V 2  O). 
     With the clocked activation of the two power semiconductor components, as well as by switching off the respectively other two power semiconductor components, the direct current in the respective direction of the current flow can be controlled or regulated so as to meet the desired values. 
       FIG. 4A  shows a meshed network  30  that is used as an example of two electrical power supply networks  31 ,  32 —that are connected to each other—and that represents an example of the design of two electrical circuits  10 . It is to be understood that the meshed network  30  may also be designed differently, for example in the form of a star. Likewise, it is to be understood that the meshed network  30  may also comprise more or fewer converters, as compared with  FIG. 4A . 
     Considering the electrical converters of the meshed network  30  of  FIG. 4A , reference is made to the explanations regarding  FIGS. 1 through 3  hereinabove. In doing so, the same types of components are identified with the same reference signs. 
     In the meshed network  30  of  FIG. 4A , the two electrical lines  15 ,  16  of the two electrical circuits  10  are connected to each other by two transverse lines  34 ,  35 . 
     Furthermore, two switching systems  37  are provided, said systems comprising pairs of electrical circuit breakers  39 ,  40 ,  41 ,  42 ,  43 ,  44  with which the electrical lines  15 ,  16  of the two electrical circuits  10 , as well as the two transverse lines  34 ,  35 , can be interrupted. 
     The two power supply networks  31 ,  32  are connected by way of additional electrical circuit breakers  46  to the transformers  13 ,  14  on the alternating-voltage side of the converters  11 ,  12 . 
     Each of the four converters  11 ,  12  shown as examples in  FIG. 4A  can be at a distance of several hundred kilometers from each other, for example 100 km. The two switching systems  37  can also be at a distance of several kilometers from each other, for example 100 km. 
     It is pointed out that, depending on the individual application, potentially not all the circuit breakers  39 ,  40 ,  41 ,  42 ,  43 ,  44  are required. For example, it is possible that the circuit breakers  41 ,  42  provided in the two transverse lines  34 ,  35  are not necessary. 
     The four converters  11 ,  12  of  FIG. 4A  are consecutively numbered with the additional reference signs A, B, C, D. The four currents i dcA , i dcB , i dcC  and i dcD  in  FIG. 4A  are plotted accordingly. Furthermore, another voltage u dcD2  and a current i dcD2  are indicated upstream of the circuit breaker, said circuit breaker connecting the converter D to the DC network. 
     In normal operating mode of the meshed network  30 , all the circuit breakers are closed or switched so as to be conducting. Therefore, referring to the exemplary embodiment depicted in  FIG. 4A , the following applies to the normal operation of the meshed network  30 : i dcA +i dcC =i dcB +i dcD . In doing so, the four converters A, B, C, D of  FIG. 4A  are activated in a clocked manner in accordance with the descriptions of  FIGS. 1 through 3 , and are controlled or regulated in this manner to meet the desired values of the aforementioned equation. 
     If now an error, for example a short circuit, occurs in the electrical lines  15 ,  16  to the converter D of the meshed network  30  of  FIG. 4A  at a time TK, as is indicated for example by an arrow  48 , this results in current and voltage characteristics as shown in  FIG. 4B . 
     In  FIG. 4B  the characteristics of the current i dcD2  and the voltage u dcD2  are plotted over time t. It is assumed that each, the current i dcD2  and the voltage u dcD2 , initially display an essentially constant value. 
     The mentioned short circuit occurs at the time TK. Consequently, the voltage u dcD2  becomes zero. 
     With the aid of the converter D associated with the short circuit and the other converters A, B, C, the currents i dcD2  and i dcD  are now controlled or regulated in such a manner that this current will optionally first increase in order to then decrease to zero, or at least to almost zero. Therefore, essentially the following applies: i dcD =0 and i dcD2 =0. 
     This requires a higher-level control or regulation of the converters, said control or regulation adjusting the set point values for the currents i dcA , i dcB , i dcC  and i dcD  in such a manner that the currents i dcD2  and i dcD  are decreased to approximately zero. The control or regulation of the individual converters converts these higher-level default set point values with the aid of the described modules  21 ,  22 , as well as with the accordingly clocked actuation of the power semiconductor components. The higher-level control or regulation of the converters can be centrally accommodated, e.g., in the circuit system or decentrally in the individual converters. In both cases, communication paths exhibiting sufficient transmission speed are required. 
     After the current i dcD  has become approximately zero, the circuit breakers  44  associated with the short circuit  48  or the converter D are opened. The line section affected by the short circuit was thus selectively switched off and galvanically separated from the meshed network. Furthermore, it is now possible to also open the circuit breaker  46 , unless this has already been initiated earlier by the higher-level control or regulation of the converters. The time-dependency diagram of  FIG. 4B  shows this, for example, at a time TO. Then, the following applies: i dcA +i dcC =i dcB . This means that the operation of the meshed network  30  is continued based on the aforementioned equation. In doing so, the three converters A, B, C are activated in a clocked manner consistent with the explanations regarding  FIGS. 1 through 3  and, in this manner, are controlled or regulated to meet the desired values of the aforementioned equation. 
     After the said circuit breakers  44  have been opened, the voltage u dcD2  can again increase to the initial, approximately constant, value in accordance with  FIG. 4B , provided this is desirable or necessary. Alternatively, the voltage u dcD2  of the converters A, B, C can also be adjusted in a different way. 
     In accordance with the time-dependency diagram of  FIG. 4B , the voltage u dcD2  that has become zero has an effect on the meshed network  30  only starting at time TK, i.e., before the occurrence of the short circuit, up to the time TO, i.e., the opening of the associate circuit breaker  44 . By appropriately fast control or regulation of the converter D, this time segment can be limited to a small value, for example, smaller than 100 milliseconds. Consequently, the short circuit  48  has similar effects on the remaining converters A, B, C and the energy supply networks  31 ,  32  connected to these converters, as would be the case with the occurrence of a short circuit in a conventional three-phase power system and can thus be managed without substantial interruption of the energy transmission. 
     Consequently, following the short circuit  48  in the region of the converter D, the operation of the meshed network  30  is taken over and continued by the remaining converters A, B, C. 
     Described herein is a method for operating an electrical circuit, wherein a modular switch  21  comprising four power semiconductor components and one capacitor is provided. With this method, either both the first and the second power semiconductor components V 1 , V 2  are switched so as to be conducting, and both the third and the fourth power semiconductor components V 3 , V 4  are controlled so as to be blocking, so that a current i flows from the first connection  24  across the first power semiconductor component, across the capacitor C and across the second power semiconductor component to the second connection  25 , or both the third and fourth power semiconductor components V 3 , V 4  are switched so as to be conducting, and both the first and the second power semiconductor components V 1 , V 2  are controlled so as to be blocking, so that the current i flows in reverse direction from the second connection  25  across the fourth power semiconductor component, across the capacitor C and across the third power semiconductor component to the first connection  24 . 
     This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.