EP2810290B1 - Apparatus for switching of direct current in a dc network terminal - Google Patents

Description

The invention relates to a device for switching a direct current in a pole of a DC voltage branch.

The world's rising energy demand and the simultaneous desired reduction of CO ₂ emissions make so-called renewable energy increasingly attractive. Sources of renewable energies are, for example, wind power plants set up at sea or photovoltaic power plants in sunny desert areas. In order to be able to use the energy generated in this way economically, the connection of renewable energy sources to a shore supply network is becoming increasingly important. Against this background, the construction and operation of a meshed DC voltage network is becoming increasingly discussed. The prerequisite for this, however, is that short-circuit currents which can occur in such a meshed DC voltage network can be switched off quickly and reliably. For this purpose, however, DC switches are required, which are currently not available on the market. Different concepts for such a DC voltage switch are known from the prior art.

In the WO 2011/057675 A1 a DC voltage switch is described, which has an operating current path with a mechanical switch and a Abschaltzweig which is connected in parallel to the operating current path. In the Abschaltzweig a series circuit of power semiconductor switches is arranged, each of which a freewheeling diode is connected in parallel in opposite directions. The consisting of power semiconductor switch and freewheeling diode switching units are arranged anti-serial, the turn-off power semiconductor switches are arranged in series and for each power semiconductor switch, a corresponding power semiconductor switch is provided with opposite passage direction. On this way, the current can be interrupted in both directions in Abschaltzweig. In the operating current path, an electronic auxiliary switch is arranged in series with the mechanical switch in addition to the mechanical switch. During normal operation, the current flows through the operating current path and thus via the electronic auxiliary switch and via the closed mechanical switch, since the power semiconductor switches of the turn-off branch represent an increased resistance for the direct current. To interrupt, for example, a short-circuit current of the electronic auxiliary switch is transferred to its disconnected position. As a result, the resistance increases in the operating current path, so that the direct current commutes in the Abschaltzweig. The fast mechanical disconnector can therefore be opened normally. The short-circuit current conducted via the turn-off branch can be interrupted by the power semiconductor switches. For receiving the energy stored in the DC power network and to be reduced during switching, arresters are provided which are connected in parallel to the power semiconductor switches of the turn-off branch.

In the DE 694 08 811 T2 a DC voltage switch is described in which two mechanical switches are connected in series. The series circuit consisting of the two mechanical switches is protected against high overvoltages by a surge arrester and a capacitor. Only one of the mechanical switches is a switched on and off power semiconductor switch connected in parallel. When opening the mechanical switch creates an arc. The voltage drop across the arc ignites the power semiconductor switch, thereby shorting the parallel open mechanical switch. The arc goes out. The guided via the power semiconductor switch current can now be interrupted by appropriate control of the power semiconductor.

In the US 5,999,388 a DC voltage circuit breaker is described, which can be integrated serially in a DC voltage line is. It consists of a series connection of power semiconductor switches which can be switched on and off, to each of which an opposite freewheeling diode is connected in parallel. Furthermore, an arrester, for example a varistor, is connected in parallel with each power semiconductor switch for limiting the voltage. The previously known DC voltage switch is designed purely electronic and thus switches considerably faster compared to commercially available mechanical switches. Within a few microseconds, a short-circuit current flowing via the DC voltage switch can be interrupted. The disadvantage, however, is that the operating current must also be conducted via the power semiconductor switches. This results in high transmission losses.

The WO 2011/141055 discloses a DC voltage switch that can be serially connected to one pole of a high voltage DC network. The DC voltage switch consists of a mechanical switch in series with a power semiconductor switch, which is again connected in parallel with a counter-rotating freewheeling diode. Parallel to the series circuit of power semiconductor switch and mechanical switch, a series circuit of coil and capacitor, ie an LC branch and an arrester, connected, which limits the voltage drop across the LC branch voltage. The power semiconductor switch, a trap is connected in parallel. After opening the mechanical switch, the power semiconductor switch is turned on and off at the natural frequency of the LC branch. As a result, a vibration and finally a current zero crossing is generated in the mechanical switch, so that the resulting arc can be deleted.

The DE 10 2010 008 972 A1 discloses a tap changer for a tapped transformer having two load branches, each of the two load branches having a main mechanical contact which carries a continuous current. Parallel to each of the load branches, a series connection of a mechanical auxiliary contact and a semiconductor switching unit is further provided, by means of which the actual load switching is performed.

The object of the invention is to provide a device of the type mentioned, can be switched off reliably and inexpensively with the fault currents in a DC power system, while at the same time low losses occur in normal operation.

The invention solves this object by a device for switching a direct current in one pole of a direct current network with two terminals for serial connection to the pole, a main current branch extending between the terminals, in which two mechanical switches are arranged, in parallel to the main current branch between The bypass terminals extending sidestream branch, in which also two mechanical switches and / or two power semiconductors are arranged, a central branch, which connects a arranged between the mechanical switches Mittelzweigpotenzialpunkt the main flow path with a arranged between the mechanical switches or power semiconductors Mittelzweigpotenzialpunkt the Nebenstromzweiges and the one Power switching unit having a series circuit of two-pole submodules each having at least one power semiconductor switch and means for reducing a switch n has released energy, and commutation means for commutating the direct current into the center branch, so that the entire is led to direct current via the central branch, wherein the commutation means comprise at least one controllable power semiconductor.

According to the invention, a so-called H-circuit is provided, which has two branches running parallel to one another, namely a main flow branch and a branch flow branch. The two parallel branches each extend between the two terminals, each of said branches having two mechanical switches. The potential point between the mechanical switches of the main current branch is connected to the potential point between the two mechanical switches or between the power semiconductor switches of the bypass branch via a central branch. In the middle branch, a power switching unit is arranged, which in turn comprises a series connection of two-pole submodules. Each submodule has at least one power semiconductor switch which can be switched on and off, ie IGBT, IGCT, GTO or the like, if necessary with each opposite parallel freewheeling diode. Instead, however, reverse conductive power semiconductor switches can be used. The number of submodules depends on the respective requirements. In any case, the submodules of the power switching unit must be able to absorb the applied voltages and also to switch off high short-circuit currents safely and quickly enough. The energy stored in the DC voltage network and released during switching off is reduced by appropriate means for reducing the switching energy. These are, for example, non-linear resistors, for example, arresters, varistors or the like. If the voltage dropped across them exceeds a threshold voltage, these components behave as ohmic resistors, converting the energy released during switching into thermal energy and releasing it to the outside atmosphere. Conveniently, the means for breaking down the switching energy are integrated into the submodules. Notwithstanding this, the non-linear resistors are each connected in parallel to one or more submodules. In addition, the submodules can also have energy storage in the invention. The direct current to be switched can be performed in the context of the invention in normal operation over the main stream branch alone. Alternatively, during normal operation, the direct current is conducted both via the main current branch and the secondary branch. In any case, the H circuit allows the direct current to be commutated into the center branch so that it is always guided in one direction over the central branch, regardless of the direction of the direct current. The power semiconductors of the power switching unit must therefore be designed basically only for switching currents in one direction. Countercurrents in the middle branch may, however, have to be taken into account in the case of possible network oscillations.

According to the invention, commutation means are furthermore provided which have at least one controllable power semiconductor. By means of the commutation means, it has become possible within the scope of the invention to control the commutation of the circuit to be switched DC active at least from a portion of the main flow branch in the central branch active to start. To this end, the power semiconductors of the commutation means are controlled by means of a control signal either to increase the resistance in the said section of the main branch and possibly of the bypass branch or to generate a circulating current carried over said section or said sections which coincides with the circuit superimposed switching direct current in about zero. The commutation means support the mechanical switches in the commutation of the direct current in the central branch.

Advantageously, a charging branch is provided which is connected on the one hand to the ground potential or to an opposite polarity of the polar pole pole of the DC network and on the other hand connected to the central branch or connectable, the charging branch has an ohmic resistance. The charging branch is used both for commissioning and for operating the device in normal operation. If the charging branch is connected, for example, to the middle branch potential point of the bypass branch, the main branch is connected via the central branch and the charging branch either to the ground potential or to the opposite pole of the direct current network. Thus, a voltage is applied to the power switching unit, which voltage can be used, for example, for the operation of the electronics of the power semiconductor switches of the power switching unit. If the submodules have energy stores in the middle branch, they can be charged via the charging branch, the height of the charging current being determined by the design of the ohmic resistance of the charging branch. The charging branch is either continuously connected to the central branch in the invention or he has a mechanical switch, with which the connection between the charging branch and central branch can be made and interrupted again. Only because of the ohmic resistance, it is possible to provide the connection between charging branch and central branch in error-free network operation continuously. In the context of the invention, it is also not necessary to disconnect the charging branch from the central branch before switching off a short-circuit current. The flow of current is always limited in the invention by the ohmic resistance of the charging branch.

If the power switching unit is ready for operation, for example, a short-circuit current can be interrupted by the device according to the invention. In normal operation, a direct current flows almost without loss via the main current branch with its two mechanical switches. This applies to symmetrical embodiments of the device according to the invention also for the bypass branch. In the event of a fault, the switch of the main flow branch arranged downstream of the middle branch potential point in the current flow direction and the mechanical switch of the bypass branch possibly upstream of the middle branch potential point opens. By separating the contacts of the switch would, if no further measures would be provided, pulled an arc. The commutation means ideally completely suppress the formation of an arc. By opening the switches, the current commutes from the main branch to the central branch and the lower part of the branch. Subsequently, the power semiconductor switches of the power switching unit can interrupt the short-circuit current. The energy released in this process is reduced by the means for reducing the energy released during switching. Finally, the remaining mechanical switches open the device according to the invention. The short-circuit current is interrupted, the middle branch is galvanically isolated from the lines.

The resistance of the charge branch is to be interpreted as high-impedance so that it is at least temporarily operated at the full DC voltage occurring and at the same time so low that the charging current required for pre-charging and permanent receipt of the charge of energy storage can flow. To charge energy storage units of the power switching unit, a voltage drop of a few kilovolts is sufficient. The resistance of the charging branch can therefore be designed very high impedance become. The maximum power loss and the size of the ohmic resistance are therefore relatively low.

Expediently, the charging branch is connected or connectable to the middle branch potential point of the bypass branch. According to this advantageous embodiment of the invention, the charging current flows from the main current branch over the entire central branch. All arranged in the central branch energy storage can thus be charged when in the current flow direction first mechanical switch of the main flow branch is in its closed position.

Expediently, the charging branch has a mechanical switch in series connection to the ohmic resistance, which is set up to connect the charging branch to the central branch. As already stated, the mechanical switch may be a relatively slow mechanical switch because of the ohmic resistance. The switch is, for example, a simple disconnector that opens almost without power. By the switch, the resistance of the charging branch, which can also be referred to as a pre-charge, thermally relieved.

Advantageously, the submodules of the power switching unit at least partially each have a power semiconductor switch which can be switched on and off and a freewheeling diode connected in parallel thereto in opposite directions. Alternatively, each submodule may also include a single reverse conducting power semiconductor switch. Suitable power semiconductor switches are, for example, IGBTs, IGCTs, GTOs or the like. As a rule, a power semiconductor switch has a plurality of power semiconductor switch chips arranged in a housing. To connect the load terminals of the power semiconductor switch chips serve as bonding wires. Deviating from this, however, pressure-contacted power semiconductor switches can be used in the context of the invention, in which the power semiconductor switch chips on the load connection side via a pressure contact connected to each other. However, such power semiconductor switches are known to the person skilled in the art, so that their design need not be discussed in greater detail here.

The power semiconductor switches of the submodules that can be switched on and off are preferably designed to switch off currents in one direction.

However, according to a deviating from this development, the submodules of the power switching unit form two groups, each with the same oriented transmission directions of their power semiconductor switches, the power semiconductor switches are oriented one group opposite to the power semiconductor switches of the other group. According to this advantageous further development, the current can flow not only in both current directions via the turn-off branch, but also currents can be switched off safely in both directions. If, for example, the current flows in the first direction due to mains fluctuations, the power semiconductor switches of the first group are activated in order to interrupt the current in the said first direction. If the current flows in the opposite second direction, the power semiconductor switches of the second group are used.

In a preferred embodiment of the invention, the submodules of the power switching unit at least partially each have an energy store and a series connected in parallel to the energy storage series circuit of two switched on and off power semiconductor switches each with an oppositely arranged in parallel thereto freewheeling diode, wherein a submodule connection terminal with a potential point between the one and turn-off power semiconductor switches and the other terminal are connected to a pole of the energy storage. Such a submodule topology is also called a half bridge.

Of course, in a submodule instead of a single power semiconductor switch and a synchronously driven series circuit of power semiconductor switches are used. The synchronously driven power semiconductor switches of the series circuit then behave exactly like a single power semiconductor switch. Incidentally, this also applies to the submodules described in more detail below, that is also to the full bridge circuit or the brake controller circuit.

To reduce an energy released during switching, which is stored in the DC voltage network, at least one non-linear resistor, for example in the form of a diverter and / or a varistor, is provided for each submodule of the power switching unit. The non-linear resistor is, for example, connected in parallel with the entire submodule.

Submodules of the power switching unit, which are designed as half bridges, can interrupt the current in only one direction. If the current flow in two directions is interrupted, the formation of two groups of submodules is also required here, wherein the submodules of the one group for the interruption of the current in a first direction and the submodules of the other group for the interruption of the current in a serve the first direction opposite second direction.

According to a preferred embodiment of the invention, however, the submodules of the power switching unit are at least partially formed as a full bridge circuit and therefore have an energy storage and two series connected to the energy storage series circuits with two switched on and off power semiconductor switches each with oppositely parallel freewheeling diodes, wherein a first terminal with the Potential point between the two power semiconductor switches of the first series circuit and a second submodule connection terminal with the potential point between the two power semiconductor switches of the second Series connection is connected. Such a full-bridge circuit is capable of interrupting currents in both directions, in other words switching off.

As has already been stated, each submodule of the power switching unit expediently has an arrester and / or a varistor connected in parallel either to the single power semiconductor switch which can be switched on and off or in parallel connection to the energy store of the submodule.

Advantageously, the submodules of the power switching unit have a series circuit of a switchable and disconnectable power semiconductor switch with opposite freewheeling diode and a diode having the same forward direction as the freewheeling diode, the series circuit is connected in parallel to an energy storage and a first submodule connection terminal with the potential point between the one - And disconnectable power semiconductor switch and the diode and the other submodule connection terminal connected to a pole of the energy storage and the on and off power semiconductor switch between the submodule connection terminals is arranged. Such a submodule can be referred to as a half bridge with only one controllable power semiconductor. A corresponding embodiment of the full bridge circuit may also be advantageous within the scope of the invention. Such a full bridge circuit would then have two controllable power semiconductor switches.

Notwithstanding this, the submodules of the power switching unit are at least partially designed as a brake actuator modules. Such brake actuator modules have an energy store, to which a first series circuit is connected in parallel. The first series circuit consists of a power semiconductor switch which can be switched on and off with a freewheeling diode in parallel and a diode oriented in the same direction as the freewheeling diode. In addition, a second series circuit is provided, which is also connected in parallel to the energy store. The second series circuit consists of a switched on and off power semiconductor switch with oppositely parallel freewheeling diode and another oriented in the same direction to the freewheeling diode. The diode of the second series circuit bridges an ohmic resistance. The first submodule connection terminal is connected to one pole of the energy store and the second submodule connection terminal is connected to the potential point between the turn-off power semiconductor switch and the diode of the first series circuit. Such brake actuator modules can convert the energy stored in the network and degraded during switching particularly well into thermal energy and dissipate it to the outside atmosphere.

According to a preferred embodiment, the commutation means are arranged in the central branch in series with the power switching unit and arranged to generate a circulating current flowing via at least one of the mechanical switches of the main branch which is opposite to the direct current to be switched. Such a circular flow may be generated in both meshes to the left and right of the central branch, each mesh being formed by the central branch, a portion of the skin and a portion of the secondary flow path. In one mesh, it is opposite to the direct current to be switched in the main current branch. Both currents ideally add up to zero so that subsequently the mechanical switch in the said section of the main flow branch can open normally. In the other loop, however, the circulating current and the direct current to be disconnected in the main current branch flow in the same direction and therefore increase. In the case of a symmetrical embodiment of the device according to the invention, the same applies to the sidestream branch, so that a mechanical switch also opens normally in the sidestream branch. The two normally-open switches are arranged in the direction of the direct current in the bypass branch and in the main branch after the respective middle branch potential point. An external current, that is to say a current flowing outside the DC voltage switch according to the invention, can not be influenced until at least one mechanical switch of the main current branch is open and a flux of the circuit to be switched is opened Direct current via the bypass branch is prevented either by an also open mechanical switch or a diode. In this case, the commutation means are designed such that they generate such a high countervoltage in the required time window in the said loop, that the current flow in the main branch can be suppressed and the middle branch potential point of the main branch in the direction of flow of the direct current downstream mechanical switch can be opened without current.

Expediently, the commutation means have a current sensor which is arranged in the main flow branch. The current sensor is connected to a control unit of the device according to the invention. The current sensor senses the current flowing through the main current branch and provides current readings to the control unit. The control unit checks the received current measured values for the presence of an intervention criterion. Such an intervention criterion is, for example, an excessively high current increase, di / dt, or occurs when the measured current values exceed a current threshold value over a predetermined time window. In principle, however, any linkages with further measured values of protective devices or the like or further criteria are possible within the scope of the invention. If such an intervention criterion is present, the current is commutated into the central branch and the mechanical switch (s) is opened. As soon as the mechanical switches are able to absorb voltage, the current flowing across the central branch is limited or switched off. If the power in the middle branch only limited but not turned off, only some sub-modules of the power switching unit are turned off. If a non-linear resistor, such as an arrester, is connected in parallel with the switched-off submodules, this unfolds its effect in that the electrical resistance of the middle branch is heard. The current flowing over the central branch is therefore limited. If the limitation, for example, after the rapid elimination of a fault, become superfluous, can the mechanical switches of the main current branches are closed again, so that the current flows almost loss-free again over the main branch and, if necessary, bypass branch.

In an expedient further development, the charging branch is connected to the potential point between the power switching unit and the commutation means. Such a connection of the charge branch between the commutation means and the power switching unit makes it possible to use parts of the bypass current branch for the charging current for charging the energy stores of the commutation means. This is particularly advantageous if the commutation means have submodules which are designed as a so-called half-bridge.

According to a preferred embodiment of the invention, the commutation form a series connection of two-pole submodules, each submodule has an energy storage and a power semiconductor switch in parallel with the energy storage. With the aid of the power semiconductor circuit, the voltage drop of the two-pole submodule at the submodule connection terminals can be adjusted. Either the voltage dropping across the energy store is applied to the submodule connection terminals or a zero voltage, ie no voltage. Due to the series connection, therefore, the voltage drop across the entire series connection of the submodules of the commutation means can be set stepwise, the height of the stages corresponding to the voltage drop across the energy store of a submodule. The higher the voltage generated in the said loop by the commutation means, the higher is the circulating current driven by this voltage.

The design of the power semiconductor circuit of the commutation can, as already described in connection with the submodules of the power switching unit, be either a half-bridge or a full-bridge circuit. is the power semiconductor circuit is a half-bridge circuit, only a series connection of two turn-off power semiconductor switches is provided, each with opposite parallel freewheeling diode, wherein a first submodule connection terminal is connected to the potential point between the on and off power semiconductor switches and another submodule connection terminal to a pole of the energy storage. The submodules of the commutation means designed as a half-bridge circuit must be oriented in such a way that a countervoltage with the desired polarity can be generated in the operating current path. This is usually the case when the half-bridge circuits of the commutation are oriented opposite to the half-bridge circuits of the submodules of the power switching unit.

Deviating from this, the power semiconductor circuit of the submodules of the commutation means is formed together with the energy store as a full bridge circuit, wherein, as already described above, two series circuits are provided. The two series circuits are connected in parallel to the energy storage and each have two switched on and off power semiconductor switches, each with opposite directions parallel freewheeling diode. Instead of power semiconductor switch with freewheeling diode and backward conductive power semiconductor switches can be used. The potential point between the two power semiconductor switches is in each case connected to a submodule connection terminal, so that either the voltage dropping across the energy store, a zero voltage or the inverse energy storage voltage can be generated at the submodule connection terminals. The full bridge circuit can thus generate voltages having different polarities. These are particularly advantageous when reverse voltages for currents in both directions to be generated.

As energy storage of the submodules of both the commutation and the power switching unit, for example, a capacitor is provided.

According to a deviating embodiment of the invention, the commutation means are formed as a commutation semiconductor switch, which are arranged in the main flow branch. The commutation semiconductor switches, like the other power semiconductor switches, can be switched on and off and, if necessary, have a freewheeling diode which runs in parallel in opposite directions. Instead of the parallel connection of power semiconductor circuit and freewheeling diode backward conductive power semiconductor switches can be used. To reduce an energy released during switching, the power semiconductor switches can be connected in parallel with an arrester, a varistor or another non-linear resistor. Energy storage such as capacitors or the like can be used to reduce energy. In this case, the commutation semiconductor switches are arranged between the potential point of the central branch, that is to say the middle branch potential point of the main current branch, and one of the mechanical switches of the main current branch. Thus, two commutation semiconductor switches with opposite free-wheeling diode and, if necessary, arresters are provided as a means for reducing an energy released during switching. However, a non-linear resistance is not always mandatory and can be omitted within the scope of the invention depending on the requirement. In order to commutate a current flowing through the main current branch into the central branch, the commutation switch located downstream of the middle branch potential point in the direction of the direct current flow is switched into its interruption position, in which a current flow via the commutation switch is interrupted. If the resistance across the main current branch becomes too large, the direct current commutates into the central branch and can be selectively interrupted there after opening the mechanical switch. Simultaneously with the activation of the commutation semiconductor switch, the mechanical switch arranged downstream of the commutation semiconductor switch in the direction of current flow is also opened. The forward direction of the commutation semiconductor switch is selected such that one of the central branch potential point to the respective associated mechanical Switch flowing current from the commutation semiconductor switch can be interrupted. In order to be able to interrupt both current directions, the two commutation semiconductor switches according to this embodiment are oriented in opposite directions to one another.

If the bypass branch has no commutation semiconductor switch, the mechanical switches are normally open, since otherwise the current flow would flow via the bypass branch due to the increased resistance in the main branch. The bypass branch must therefore have fast closing mechanical switches to ensure commutation in the central branch.

According to a preferred embodiment of the invention, therefore, the bypass branch also has two commutation semiconductor switch, which are arranged and oriented as the commutation of the main branch. During normal operation, the current can thus flow both via the main current branch and via the bypass branch. For commutation of the current in the central branch, therefore, the commutation switch arranged downstream of the central branch potential point of the main branch in the main branch and the commutation switch upstream of the branch branch potential point of the branch branch are transferred to their respective disconnected position.

The bypass branch can also have power semiconductors instead of two mechanical switches, the middle branch potential point being arranged between the said power semiconductors. The said power semiconductors have an oppositely oriented forward direction and are designed, for example, as diodes or thyristors. They prevent a flow of current through the bypass branch during normal operation.

Conveniently, the mechanical switches of the main current branch are designed as fast switches and open set up within 1 ms to 10 ms. However, the mechanical switches of the bypass branch are, for example, comparatively slow mechanical switches which open in a time range of 10 ms to 60 ms. Such fast switches have a low switching mass, which must be moved when switching. In addition, fast-responding drives, such as electro-dynamic drives, are required. Currently commercially available power semiconductor switches usually turn off in the order of 10 ms to 50 ms. Such commercial switches are arranged for example in the bypass branch. They are opened before the occurrence of a fault, the direction of the direct current to be switched is known. However, mechanical switches are also known which open within a few milliseconds.

According to a further variant of the invention, it is expedient that the device according to the invention is also used in a modular manner and is thus used as a bipolar or bipolar component in a series connection.

Finally, it should be noted that switched on and off power semiconductor switches are always disclosed here in each case in connection with each one opposite direction parallel freewheeling diodes or as reverse conducting power semiconductors. However, this is primarily due to the fact that turn-off power semiconductors such as IGBTs, IGCTs, GTOs or the like are generally always marketed with an in parallel parallel freewheeling diode in the market. Such an opposite freewheeling diode, serves to protect the power semiconductor switch, which is extremely sensitive to a voltage opposite to its forward direction. However, said freewheeling diode is not mandatory in all cases shown here. These cases are obvious to the person skilled in the art, so that reference will not be made separately in individual cases. Realizations of the invention, in which the opposite direction parallel to the line semiconductor switch arranged freewheeling diode functionally eliminated, but should be included in the scope of protection.

Figur 1

ein erstes Ausführungsbeispiel der erfindungsgemäßen Vorrichtung,

Figuren 2 und 3

das Ausführungsbeispiel gemäß Figur 1 in unterschiedlichen Stellungen, um das Aufladen der Vorrichtung gemäß Figur 1 schrittweise zu verdeutlichen,

Figur 4

ein weiteres Ausführungsbeispiel der erfindungsgemäßen Vorrichtung,

Figuren 5 bis 8

Ausführungsbeispiele der Submodule für einen Einsatz in der erfindungsgemäßen Vorrichtung und

Figuren 9 bis 14

weitere Ausführungsbeispiele der erfindungsgemäßen Vorrichtung zeigen.

Further expedient embodiments and advantages of the invention are the subject of the following description of embodiments with reference to the figures of the drawing, wherein like reference numerals refer to like-acting components and wherein

FIG. 1

a first embodiment of the device according to the invention,

FIGS. 2 and 3

the embodiment according to FIG. 1 in different positions to charge the device according to FIG. 1 gradually to clarify

FIG. 4

a further embodiment of the device according to the invention,

FIGS. 5 to 8

Embodiments of the submodules for use in the device according to the invention and

FIGS. 9 to 14

show further embodiments of the device according to the invention.

FIG. 1 shows an embodiment of the device 1 according to the invention, which has two terminals 2 and 3, with which the device 1 can be connected in series in a pole, so a line of a DC voltage network. The device 1 is used to interrupt a current flow in the pole of the DC voltage network and can thus be referred to as a DC voltage switch.

The device 1 has a main flow branch 4 and a bypass branch 5. In the in FIG. 1 In the embodiment shown, both current paths are equal and, in other words, have approximately the same electrical resistance. A DC current flowing between the connection terminals 2 and 3 thus flows both via the main current branch 4 and via the branch current branch 5. Both the main current branch 4 and the branch current branch 5 each have two mechanical switches 6, 7, 8 and 9. Between the mechanical switches 6 and 7 of the main current branch 4, a middle branch potential point 10 is formed. The central branch potential point 10 is connected via a central branch 11 to a middle branch potential point 12 of the bypass branch 5. The middle branch 11 has a power switching unit 13, which has a series connection of submodules 14, the configuration of which will be discussed in more detail later. It should already be mentioned here that each of the submodules 14 has at least one power semiconductor switch which can be switched on and off, whose forward direction is oriented from the middle branch potential point 10 of the main branch 4 to the middle branch potential point 12 of the branch branch 5. Thus, currents flowing through the power switching unit 13 in this direction can be selectively interrupted by the power semiconductor unit 13 being switched on and off. For receiving the switching energy released in this case, means are used for receiving the energy released during switching, that is, for example, nonlinear resistors such as arresters or varistors. These arresters are either part of the submodules, as shown, or are connected in parallel to one or more submodules.

In FIG. 1 Further, commutation means 34 are recognizable, the configuration of which will also be discussed in more detail later. In each case, the commutation means 34 have at least one power semiconductor switch that can be switched on and off, which is shown in FIG FIG. 1 not shown. The commutation 34 effect with appropriate control of the power semiconductor switch or a commutation of the disconnected direct current in the central branch eleventh

To boot the electronics and optionally to charge the energy storage of the power switching unit 13 and optionally the Kommutierungsmittel a charge branch 15 is provided which has a resistor 16 as a pre-charge and a mechanical switch 17, which is designed here as a circuit breaker. The charge branch 15 is connected in the embodiment shown in Figure 1 to the ground potential. Deviating from this, however, the charging branch 15 is connected to the opposite pole, that is to say, for example, to a negative pole of a DC voltage network, whereas the terminals 2 and 3 are connected to the positive pole of the DC voltage network. In both variants, the supply terminal 2 would be subject to the pole voltage and the closing of the switch 17 with closed mechanical switch 6 the opportunity to tap off a voltage drop across the switchable power semiconductor switches voltage for powering the electronics of the power semiconductor switch and thus the power switching unit 13 is ready close. In addition, there is the possibility of controlled charge in the submodules 14 energy storage, wherein the height of the charging current is determined by the size of the ohmic resistance 16. The ohmic resistance is designed so that it can be operated, at least temporarily, at the full voltage, which drops between the pole and the ground potential or the opposite pole. In principle, the switch 17 can remain permanently closed with a correspondingly high design of the resistor 16. Deviating from this, it can serve to relieve the ohmic resistance 16 which occurs when the switch 17 is opened.

In the Figures 2 and 3 is the charging of energy storage of submodules 14 of the power switching unit 13 and possibly the submodules of the commutating means 34, not shown, illustrated. For this purpose, initially only the mechanical Switches 6 and 17 closed. The charging current I now flows from the terminal 2 via the switch 6 through the central branch 11 and the charging branch 15 to the ground from. First, energy buffers of the electronics of the power semiconductor switches are charged, then the high voltage energy storage of the submodules 14. If the power switching unit 13 is ready for operation, the switch 7 is closed. The main current now flows through the mechanical switches 6 and 7 to the terminal 3. However, the charging current flowing across the central branch 11 is maintained as long as the switches 8 and 9 of the bypass branch 5 are open.

FIG. 4 shows a further embodiment of the device 1 according to the invention, as far as possible according to the embodiment FIG. 1 However, where the switches 8 and 9 between terminal 2 and a branch point 18 between main branch 4 and branch branch 5 and between the branch point 19 and the terminal have migrated. In their place in the bypass branch power semiconductors 20 and 21 are provided in the form of diodes, which prevent a current flowing from the terminal 2 and 3 via the bypass branch 5 directly into the charging branch 15, without being guided over the central branch 11. The switches 8 and 9 between the terminals 2 and 3 and the branch points 18 and 19 can be omitted in principle. However, they allow the controlled turning on a DC power supply section and the galvanic isolation of the unit from the DC power grid.

Examples of possible submodules 14 for the device 1 according to the invention are shown in FIGS FIGS. 5, 6, 7 and 8th shown. This in FIG. 5 shown submodule 14 has only a single on and off power semiconductor switch 22 with opposite parallel freewheeling diode 23. Such a submodule 14 may in the embodiments of the invention according to the FIGS. 1 to 4 only as part of the power switching unit 13, but not as part of Commutation serve 34, since they must have an energy storage in an arrangement in the central branch 11 to produce a circular current. The parallel connection of power semiconductor switch 23 and freewheeling diode is connected in parallel to an arrester 24, which receives the switching energy released during switching. The arrester 24 is thus a means for receiving the energy released during switching. Instead of a single power semiconductor which can be switched on and off, it is also possible within the scope of the invention to use a series connection of simultaneously controllable power semiconductors which can be switched on and off, with a single arrester being connected in parallel across the entire series circuit.

The number of submodules 14 in the power switching unit 13 depends on the blocking capability of the power semiconductor switches 22, in this case IGBTs. This is currently in the range of up to 6.5 kV. The voltage in high-voltage direct current networks, which are currently designed almost exclusively as a point-to-point connections, is usually between 300 and 500 kV. Also 800kV transmission lines are known. The arrester 24 are dimensioned so that they lock together with applied operating voltage, so are not conductive. However, when the voltage drop across them exceeds a maximum voltage, it becomes conductive so that a controlled flow of current is allowed, causing the arresters 24 to heat up and deliver the electrical energy as thermal energy to the outside atmosphere. The number of series-connected arresters 24 corresponds to the number of non-conductive, ie interrupted, submodules. If, therefore, submodules 14 are not transferred to their interruption position, a current flow of controlled size can be determined via the arresters. This serves, for example, for the controlled connection of a network section.

FIG. 6 shows a sub-module 14, which forms a so-called half-bridge. The half-bridge consists of an energy storage 25, here a high-voltage capacitor, and a Series circuit 26, which is connected in parallel to the energy storage 25. The series circuit 26 has two series-connected switched on and off power semiconductor switches in the form of IGBTs 22, which in each case a freewheeling diode 23 is connected in parallel in opposite directions. A first terminal 27 is connected to the potential point between the two power semiconductor switches 22 of the series circuit 26. The second terminal 28 is at the potential of one of the poles of the energy store 25. Between the terminals 27 and 28, a bypass switch 29 is provided, with which the submodule 14 can be bridged in the event of a fault. In the event of failure of a single submodule, the entire power switching unit 13 remains functional. For receiving high short-circuit currents, a diode 30 is arranged between the submodule connection terminals 27 and 28. This supports the freewheeling diode 23, which is likewise arranged between the submodule connection terminals 27 and 28, in the case of high currents which flow via the submodule 14. Instead of a diode 30, a thyristor can also be used. A bypass switch 29 and a diode 30 or a thyristor 30 between the terminals, however, is not always mandatory. If the submodule 14 is used, for example, in a power switching unit 13 for switching off the current flowing via the center branch 11, the diode 30 or a thyristor used instead of the diode can be omitted without substitution. In addition, can be used in the context of the invention, pressure-contacted power semiconductors with a so-called "Conduct on Fail" property, which thus become conductive in the event of a fault. This would usually make the diode 30 dispensable. Finally, it should be noted that, instead of two individual power semiconductor switches, in each case one total of two series circuits of power semiconductor switches can be used. In addition, the half-bridge again has an arrester 24, which is connected in parallel to the energy store 25. This arrester 24 is again used to receive energy released during switching. The arrester 24 is at one of FIG. 6 deviating embodiment between the submodule terminals 27 and 28 arranged. However, the half-bridge circuit according to FIG. 6 interrupt the flow of current only from the first submodule connection terminal 27 in the direction of the second submodule connection terminal 28. In the opposite direction, the current flows unhindered and uncontrolled through the freewheeling diode 23 and optionally via the short-circuit diode 30. By chosen in the invention H circuit, the current to be disconnected flows but in principle only in one direction over the central branch 11, so the one Current switching half-bridge as a sub-module 14 in the central branch 11 is particularly preferred. It should be noted that for switching or limiting the current of the upper in Figure 6 power semiconductor switch 22, which is therefore not disposed between the submodule connection terminals 28, 29, may also be omitted. The series circuit 26 according to FIG. 6 would then correspond to the series circuit 26 according to FIG. 8 , However, such a half-bridge submodule 14 is not suitable for the generation of circulating currents and thus not as part of commutation means 34 in the central branch 11, which will be discussed in more detail later.

FIG. 7 illustrates a sub-module 14, which is realized as a full bridge circuit. Also the full bridge circuit according to FIG. 7 In addition, however, a second series circuit 31 is provided, which is also connected in parallel to the energy storage 25 and the two IGBTs 22 in series with each other in opposite directions parallel Free-wheeling diode 23 has. The first submodule connection terminal 27 is connected to the potential point between the IGBTs 22 of the first series circuit, whereas the second submodule connection terminal 28 is connected to the potential point between the IGBTs 22 of the second series circuit 31. The half-bridge circuit according to FIG. 6 Depending on the control of the IGBTs 22, it is capable of either the capacitor voltage Uc dropping across the capacitor 25 or a zero voltage, that is to say a zero voltage, at the submodule connection terminals 27 and 28 produce. Moreover, it is not possible to generate at the submodule connection terminals 27 and 28 the capacitor voltage Uc dropping across the energy store 25 and a zero voltage, but also the inverse capacitor voltage -Uc. Thus, the submodule connection terminals 27, 28 of the full bridge circuit can be polarized differently. It should also be noted here again that with each series circuit 26 and / or 31 of one of the IGBTs 22, for example, the in FIG. 7 respectively IGBT 22 shown above, can be omitted without replacement. Such a full-bridge submodule 14 with a total of two or three power semiconductors 22 which can be switched on and off is indeed suitable for switching or limiting the current in the central branch 11 as part of the power switching unit 13. However, the generation of a circulating current is not possible with such a submodule 14. A full-bridge submodule 14 with two or three power semiconductor switches 22 which can be switched on and off is therefore not suitable as part of commutation means 34 in the middle branch 11, the configuration of which will be discussed in more detail later.

FIG. 8 shows a submodule 14, referred to herein as a brake actuator module. The brake actuator module 14 also has an energy store 25 and a first series circuit 26, which is connected in parallel to the energy store 25. However, the series circuit 26 has only one power semiconductor switch 22 with oppositely parallel freewheeling diode 23. In series with the power semiconductor switch 22, here an IGBT, a further diode 32 is connected, which is oriented in the same direction as the freewheeling diode 23 of the first series circuit 26. In addition, a second series circuit 31 is again provided, which is also connected in parallel to the energy storage 25 and also has only one IGBT 22 with opposite freewheeling diode 23 and in series with another diode 32. Parallel to the diode 32 of the second series circuit 31 is an ohmic resistance 33 provided. The second series circuit 31 serves to limit the voltage drop across the energy store 25. If this is too high, the IGBT 22 is turned on, so that a current flow over the ohmic resistance 33 and a discharge of the energy storage 25 takes place. The first submodule connection terminal 27 is connected to the potential point between the diode 32 and the IGBT 22 of the first series circuit 26, whereas the second submodule connection terminal 28 is at the potential of the pole of the energy storage device 25. Due to this interconnection, the application of the energy storage voltage between the submodule connection terminals 27 and 28 is not possible. Only one current flow from the submodule connection terminal 27 to the submodule connection terminal 28 can be switched. The energy store 25 essentially serves to supply power to the electronics of the IGBT or IGBTs. The capacitor also ensures that no voltage peaks occur during switching, which could destroy the semiconductors. In this context, it should be noted that instead of a second series circuit 31 and an arrester can be used, which is connected in parallel to the energy storage 25. In other words, the brake actuator module would then be in FIG. 6 shown half bridge circuit, wherein the not arranged between the two submodule connection terminals 27 and 28 IGBT 22 is omitted.

It can in the context of the invention both the submodule 14 according to FIG. 7 as well as according to FIG. 8 be advantageous if between the submodule connection terminals 27 and 28, a power semiconductor switch, such as a thyristor, or a mechanical switch 29 is arranged, as in connection with the half-bridge in FIG. 6 is shown. The mechanical switch 29 serves to bridge the submodule 14 as needed.

FIG. 9 shows a further embodiment of the device 1 according to the invention, which largely corresponds to the in FIG. 1 however, the commutation means 34 arranged in the middle branch 11 in series with the power switching unit 13 are shown in greater detail. The commutation means 34 also consist of a series connection of submodules 14, of which in FIG. 9 only one is shown, but with the dot-dash lines of the central branch 11, the series connection of these identical submodules 14 is indicated figuratively. According to the in FIG. 9 In the embodiment shown, the submodules 14 of the commutation means 34 are designed as a full bridge circuit with arrester 24 FIG. 7 educated. The submodules 14 of the commutation means 34 are intended to drive two opposing circulating currents in the mesh formed by the main flow branch, bypass branch and central branch. In the FIG. 9 The device shown is designed symmetrically. In other words, in normal operation, the DC current to be disconnected flows, for example, from the terminal 2 to the terminal 3 both via the main current branch 4 and via the branch current branch 5. Each of the two circulating currents generated by the commutation means 34 is in one of the mechanical switches, here 7 and 8, opposite to the direct current to be switched, so that a resulting current in the respective mechanical switch 7 and 8 of about zero results. The mechanical switches 7 and 8 therefore open normally. The total current commutes to the central branch 11 and flows via the power switching unit 13 and the mechanical switches 6 and 9 to the terminal 3. The submodules 14 of the power switching unit 13 can now switch off or limit the DC current. Subsequently, the remaining mechanical switches 6 and 9 are opened.

As was already described above, two circulating currents are driven through the submodules 14 of the commutation means 34 by the two meshes formed from the middle, secondary flow and main flow branches. One of the circulating currents flows in a clockwise direction, while the other circulating current flows counterclockwise over the respective mesh. In this way it is ensured that, regardless of the direction of the direct current to be disconnected, the direct current to be switched and one of the circulating currents always overlap in one of the mechanical switches 6, 7 of the main current branch and a mechanical switch of the bypass branch 8 or 9 to approximately zero. In this case, the switches, in which a resulting current of approximately zero, are arranged on different sides of the central branch, that is, in the direction of the direct current to be switched, before or after the center branch potential point of their respective branch. The said mechanical switches, eg 7 and 8 can now be opened, so that the current flows through the power switching unit 13, whose submodules 14 can then interrupt or limit the current flow. In the FIG. 9 shown submodules 14 of the commutation means 34 also have an arrester 24, so that they can act as submodules of the power switching unit 13. Are the submodules 14 of the power switching unit 13 also as full bridge circuits according to FIG. 7 formed, can also be spoken of only a single series connection, wherein only the control of the submodules by a control and control unit not shown makes a difference to whether the submodules 14 act as part of the commutation 34 or as part of the power switching unit 13. Of course, a full-bridge submodule 14 can also unfold both effects with a time delay. Of course, submodules 14 of the commutation means 34, which according to FIG. 7 are designed as a full bridge circuit with arrester 24, are also used to switch off or limiting the current.

It should be noted that within the scope of the invention, the submodules 14 of the power switching unit 13 need not always be configured identically. Thus, a part of the submodules 14, for example, according to FIG. 5 , another part according to FIG. 6 , another part according to FIG. 7 and a last part according to FIG. 8 be designed. However, the submodules 14 of the commutation means 34 must have an energy store 25, with which only the generation of a circulating current in the mesh is made possible. In addition, the submodule 14 must be able to generate the voltage drop across the energy store 25 at the submodule connection terminals 27, 28.

FIG. 10 shows an embodiment according to FIG. 9 However, wherein the commutation 34 as half-bridge circuits according to FIG. 6 are configured, but no diodes 30 and bridging switches are provided. Also in FIG. 10 several submodules 14 of the commutation means 34 are connected in series, whereby here too this series connection is illustrated by the dot-dash line. In contrast to the full bridge circuit according to FIG. 9 can the half-bridge circuit according to FIG. 10 only generate a voltage polarity at the submodule connection terminals 27 and 28. However, since the current flows across the center branch 11 only in one direction, this one voltage polarity is perfectly sufficient to switch or limit a direct current in both directions. Moreover, a current flowing from the main current branch 4 to the bypass branch 5 via the central branch 11 can not be interrupted by the submodules 14 of the commutation means 34, since this flows via the free-wheeling diodes 23 between the submodule connection terminals 27 and 28. After the commutation of the current in the central branch 11, it may therefore be expedient that the submodules 14 of the commutation means 34 bridging switch 29 which in the Figures 9 and 10 not shown, close. If the submodules 14 of the power switching unit 13 also have a half-bridge circuit, the IGBTs or the IGBTs of the submodules 14 of the commutation means 34 have an opposite orientation. In the in FIG. 10 shown embodiment of the device 1, the commutation 34 generate in opposite directions, ie clockwise or counterclockwise, each other flowing circular currents, so that two of the mechanical switch can be opened normally and the current commutes in the central branch 11. Due to the different polarization of the power semiconductor switches 22 of the submodules 14 of the commutation means 34 compared to the submodules 14 of the power switching unit 13, the charge branch 15 is no longer connected to the middle branch potential point of the bypass branch 5, but is connected to the potential point between the power switching unit 13 and the commutation means 34. The charging current for charging the energy storage 25 of the submodules 14 of the commutation 34 then flows from the terminal 3 via the switch 9, via the commutation means 34 and finally via the charging branch 15 to earth or to the opposite pole from.

FIG. 11 shows a further embodiment of the invention, wherein not a single device, but a plurality of bipolar devices 1 are connected in series with each other. The mode of operation of the individual devices corresponds to the mode of operation which has been explained in connection with the previous figures. Also, the device 1 may be constructed in accordance with the other embodiments of the invention illustrated or set forth above. The number of devices 1 connected in series is quite arbitrary. The series connection has the advantage that the existing total DC voltage switch for interrupting the current is better scalable and can be better designed for different voltage levels. The comparatively smaller devices can be produced and handled less expensively. The voltage drop across the individual switches is smaller, so that the switching speed of the mechanical switches is accelerated. However, the disadvantage is the necessary synchronization of the individual devices. In addition, it is possible, as in FIG. 1 already indicated, equip the device 1 with an inductance in the form of a coil or throttle. Such a coil or throttle is also in the embodiment according to FIG. 11 possible, wherein the throttle is arranged distributed on the individual devices. Thus, the throttle is more easily scalable.

The FIG. 12 shows a further embodiment of the device 1 according to the invention, wherein commutation 34 are no longer arranged in the central branch 11. Rather, between the central branch potential point of the main current branch 4 and each mechanical switch 6 and 7 of the main current branch 4, a commutation semiconductor switch 36 and 37 are arranged, each with opposite parallel freewheeling diode 23. Parallel to each commutation semiconductor switch 36 and thus also to each freewheeling diode 23, a trap 24 is connected, which serves as a means for reducing an energy released during switching. The commutation means 34 therefore comprise the commutation semiconductor switches 36, 37, the respective freewheeling diode 23 and the respective non-linear resistor 24. The commutation semiconductor switches 36 and 37 are oriented in opposite directions to each other, so that a current flow can be interrupted or limited in both directions. Compared to FIG. 1 support the commutation semiconductor switches 36 and 37, the mechanical switches 6 and 7 to commutate the current in the central branch 11. If, for example, the current flows from the terminal 2 to the terminal 3 via the main current branch 4 with the mechanical switch 8 open, the commutation semiconductor switch 37 and simultaneously the mechanical switch 7 are actuated to switch off the current. Due to the resistance rising so rapidly, the current flow commutes into the middle branch 11, so that the power switching unit 13 can interrupt it. Subsequently, all mechanical switches are opened.

FIG. 13 shows a further embodiment of the device 1 according to the invention, the in FIG. 12 however, two commutation semiconductor switches 38 and 39 in the form of IGBTs in the bypass branch 5 are also arranged. According to this advantageous development, all mechanical switches 6, 7, 8 and 9 are closed during normal operation. The commutation semiconductor switches 36, 37, 38 and 39 are transferred to their forward position, so that a current from the terminal 2 can flow through both the main branch 4 and the bypass branch 5 to the terminal 3 through. To switch off the current, the commutation semiconductor switches 37 and 38 and the mechanical switches 7 and 8 are simultaneously transferred to their disconnected position or opened. The current then flows from the terminal 2 only on the main stream branch 4, the mechanical Switch 6, the freewheeling diode 23 in the central branch 11 and then via the freewheeling diode 23, the mechanical closed switch 9 to the terminal 3. The power switching unit 13 can interrupt the current now.

FIG. 14 illustrates a further embodiment of the switch according to the invention, in which the commutation semiconductor switch 38 and 39 of the bypass branch 5 with respect to the embodiment according to FIG. 13 omitted. Instead, as in the embodiment according to FIG. 4 , two diodes 20 and 21 arranged in the bypass branch 5. The diodes 20 and 21 prevent a flow of current through the bypass branch 5, without this being previously passed over the central branch 11. The switches 8 and 9 support the diodes 20 and 21, but can also be omitted with a corresponding design of the diodes 20 and 21.

Claims (22)

1. Device (1) for switching a direct current in a pole of a DC voltage grid comprising

   - two connection terminals (2, 3) for serial connection to the pole,

   - a main current branch (4) for extending between the connection terminals (2, 3), two mechanical switches (6, 7) being arranged in said main current branch,

   - an auxiliary current branch (5) extending in parallel connection to the main current branch (4) between the connection terminals (2, 3), likewise two mechanical switches (8, 9) and/or two power semiconductors (20, 21) being arranged in said auxiliary current branch,

   - a central branch (11), which interconnects a central branch potential point (10) of the main current branch (4), said central branch potential point being arranged between the mechanical switches (6, 7), and a central branch potential point (12) of the auxiliary current branch (5), said central branch potential point being arranged between the mechanical switches (8, 9) or the power semiconductors (20, 21),
   the device (1) being characterized in that the central branch (11) has a power switching unit (13), which has a series connection of two-pole submodules (14) each comprising at least one power semiconductor switch (22), and means for dissipating an energy (24) released during switching, and the device (1) has

   - commutation means (34) for commutating the direct current into the central branch (11), such that the entire direct current is passed via the central branch (11), wherein the commutation means (34) have at least one controllable power semiconductor (22, 36).
2. Device (1) according to Claim 1,
   characterized in that
   a charging branch (15) is provided, which at one end is connected to the earth potential or to an opposite pole of the DC voltage grid polarized oppositely to the pole and at the other end is connected or connectable to the central branch (11), wherein the charging branch (15) has an ohmic resistor (16).
3. Device (1) according to Claim 2,
   characterized in that
   the charging branch (15) is connected or connectable to the central branch potential point (12) of the auxiliary current branch (5).
4. Device (1) according to Claim 2 or 3,
   characterized in that
   the charging branch (15) has a mechanical switch (17) in series connection to the ohmic resistor (16), said mechanical switch being designed for connecting the charging branch (15) to the central branch (11).
5. Device (1) according to any of the preceding claims,
   characterized in that
   the submodules (14) of the power switching unit (13) at least in part have in each case a power semiconductor switch (22) which can be turned on and off and a freewheeling diode (23) connected in parallel therewith in the opposite direction.
6. Device (1) according to any of the preceding claims,
   characterized in that
   the power switching unit (13) is designed for switching off currents in only one direction.
7. Device (1) according to any of the preceding claims,
   characterized in that
   the submodules (14) of the power switching unit (13) at least in part have in each case an energy store (25) and a series connection (26) - connected in parallel with the energy store (25) - comprising two power semiconductor switches (22) which can be turned on and off with in each case a freewheeling diode (23) in parallel in the opposite direction, wherein one submodule connection terminal (27) is connected to a potential point between the power semiconductor switches (22) which can be turned on and off and the other connection terminal (28) is connected to a pole of the energy store (25).
8. Device (1) according to any of the preceding claims,
   characterized in that
   the submodules (14) of the power switching unit (13) at least in part have an energy store (25) and two series connections (26, 31) - connected in parallel with the energy store (25) - comprising in each case two power semiconductor switches which can be turned on and off with a freewheeling diode in parallel in the opposite direction, wherein a first connection terminal (27) is connected to a potential point between the two power semiconductor switches (22) of the first series connection (26) and a second submodule connection terminal (28) is connected to the potential point between the two power semiconductor switches (22) of the second series connection (31).
9. Device (1) according to any of the preceding claims,
   characterized in that
   the means for dissipating the energy released during switching are varistors and/or arresters (24).
10. Device (1) according to Claim 9,
    characterized in that
    the varistors and/or arresters at least in part are connected in parallel with an energy store (25).
11. Device (1) according to any of the preceding claims,
    characterized in that
    the submodules (14) of the power switching unit (13) at least in part are embodied as a braking controller module and have an energy store (25), with which there are connected in parallel a first series connection (26) comprising a power semiconductor switch (22) which can be turned on and off with a freewheeling diode (23) in parallel in the opposite direction and a diode (32) oriented in the same direction as the freewheeling diode (23) and a second series connection (31) comprising a power semiconductor switch (28) which can be turned on and off with a freewheeling diode (23) in parallel in the opposite direction and a further diode (32) oriented in the same direction as the freewheeling diode (23), wherein the diode (32) of the second series connection (31) bridges an ohmic resistor (33), the first submodule connection terminal (27) is connected to a pole of the energy store (25) and the second connection terminal (28) is connected to the potential point between the switchable power semiconductor switches (22) and the diode (32) of the first series connection (26).
12. Device (1) according to any of the preceding claims,
    characterized in that
    the commutation means (34) are arranged in the central branch (11) in series with the power switching unit (13) and are designed for generating a circulating current that is opposite to the direct current to be switched.
13. Device (1) according to Claim 12,
    characterized in that
    the charging branch (15) is connected or connectable to the potential point between the power switching unit (13) and the commutation means (34).
14. Device (1) according to Claim 12 or 13,
    characterized in that
    the commutation means (34) form a series connection of two-pole submodules (14), wherein each submodule (14) has an energy store (25) and a power semiconductor switch (26, 31) in parallel connection to the energy store (25).
15. Device (1) according to Claim 14,
    characterized in that
    the power semiconductor circuit forms a series connection (26) comprising two power semiconductor switches (22) which can be turned off with in each case a freewheeling diode (23) in parallel in the opposite direction, wherein a first submodule connection terminal (27) is connected to the potential point between the power semiconductor switches (22) which can be turned off and a further submodule connection terminal (28) is connected to a pole of the energy store (25).
16. Device (1) according to Claim 14,
    characterized in that
    the power semiconductor circuit forms two series connections (26, 31) comprising in each case two power semiconductor switches (22) which can be turned on and off with in each case a freewheeling diode (23) in parallel in the opposite direction, wherein the potential point between the power semiconductor switches (22) which can be turned on and off of the first series connection (26) is connected to the first submodule connection terminal (27) and the potential point between the power semiconductor switches (22) which can be turned on and off of the second series connection (31) is connected to the second submodule connection terminal (28).
17. Device (1) according to any of Claims 1 to 11,
    characterized in that
    the commutation means (34) are arranged in the main current branch (4) and have commutation semiconductor switches (31, 37), with which means for dissipating an energy (24) released during switching are connected in parallel, wherein each commutation semiconductor switch (36, 37) is arranged between the central branch potential point (10) of the main current branch (4) and one of the mechanical switches (6, 7) of the main current branch (4).
18. Device (1) according to Claim 17,
    characterized in that
    a commutation semiconductor switch is arranged between each mechanical switch (6, 7) and the central branch potential point (10) of the main current branch (4), wherein the two commutation semiconductor switches (36, 37) of the main current branch (4) are oriented in opposite directions with respect to one another.
19. Device (1) according to either of Claims 17 and 18,
    characterized in that
    the auxiliary current branch (5) has commutation semiconductor switches (38, 39) and means for carrying away an energy (24) released during switching are provided in parallel connection to the commutation semiconductor switch(es) (38, 39).
20. Device (1) according to Claim 19,
    characterized in that
    a commutation semiconductor switch (38, 39) is in each case arranged between each mechanical switch (8, 9) or between each power semiconductor (20, 21) of the auxiliary current branch (5) and the central branch potential point (12) of the auxiliary current branch (5), wherein the two commutation semiconductor switches (38, 39) are oriented in opposite directions with respect to one another.
21. Device (1) according to any of the preceding claims,
    characterized in that
    the mechanical switches (6, 7) of the main current branch (4) are fast switches and designed for opening within 1 ms to 10 ms, wherein the mechanical switches (8, 9) of the auxiliary current branch (5) are comparatively slow mechanical switches which open in a time range of 10 to 50 ms.
22. Apparatus for switching direct currents in a pole of a DC voltage grid comprising a series connection of devices (1) according to any of the preceding claims.

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