Switchable longitudinal voltage source, DC transmission system with longitudinal voltage source and method for operating a longitudinal voltage source

Description

[0001] The invention relates, inter alia, to a switchable longitudinal voltage source with the features according to the preamble of claim 1. Such a longitudinal voltage source is known from publication WO 2017 / 108073 A1. The previously known longitudinal voltage source has two H-bridge modules, each with four semiconductor switches.

[0002] A charging circuit and a charging method for an electrical energy storage system are known from document WO 2017 / 102414 A1. The charging circuit has a first input and a second input for electrical connection to an energy source, as well as a first output and a second output for electrical connection to an electrical component that is to be supplied with energy during the charging process.

[0003] The German patent application EP 1 035 637 A2 discloses a power supply system comprising switchable connected capacitors. A small number of switches are used. These switches are controlled in such a way as to reduce the resulting switching losses.

[0004] The invention is based on the objective of providing an arrangement with a switchable longitudinal voltage source that requires very few components.

[0005] This problem is solved according to the invention by an arrangement with a switchable longitudinal voltage source having the features according to claim 1. Advantageous embodiments of the longitudinal voltage source according to the invention are specified in the dependent claims.

[0006] According to the invention, a polarity reversal device is arranged downstream of the longitudinal voltage source, which is connected between the two output terminals and two conductor terminal contacts of the longitudinal voltage source, wherein the polarity reversal device connects the first output terminal to the first conductor terminal contact and the second output terminal to the second conductor terminal contact in a first position, and wherein the polarity reversal device connects the first output terminal to the second conductor terminal contact and the second output terminal to the first conductor terminal contact in a second position.

[0007] When a longitudinal voltage source according to the invention is mentioned below, this always refers to a longitudinal voltage source as part of the arrangement according to the invention.

[0008] A significant advantage of the longitudinal voltage source according to the invention is that it requires relatively few switches and can still distribute current or load flow to, for example, two downstream high-voltage direct current transmission lines of a DC transmission system.

[0009] It is advantageous if the second switching device, the first switching unit and the first switch are each formed by an externally controllable valve or have one or more externally controllable valves.

[0010] The valves that can be controlled externally are preferably valves that can be switched on and off, preferably mechanical switches or transistors with a diode connected in antiparallel to them.

[0011] The first and second switches, the first and second switching units and the first and second switching devices are preferably each unipolar switching.

[0012] With a view to using as many identical components as possible, it is advantageous if the first and second switches are identical and / or the first and second switching units are identical and / or the first and second switching devices are identical and / or the switches of the first switching device are identical to the switch(es) of the second switching unit and / or the switches of the first switching device are identical to the second switch and / or the switches of the second switching unit are identical to the second switch.

[0013] In a particularly preferred embodiment, the first and second switching units are each formed by one switch or two or more switches connected in series, the first and second switching devices are each formed by two or more switches connected in series, and the first and second switching devices each have one more switch than the first and second switching units. All of the latter switches are preferably unipolar.

[0014] In another advantageous embodiment, at least one of the switches of the series voltage source is formed by a non-controllable valve, in particular by a diode alone. It is particularly advantageous if at least one of the switches, preferably all switches, of the first switching device is formed by a non-controllable valve, in particular a diode, and at least one of the switches, preferably all switches, of the second switching device is formed by a non-controllable valve, in particular a diode, and / or the second switch is formed by a non-controllable valve, in particular a diode.

[0015] The invention further relates to a longitudinal voltage source cascade. According to the invention, such a longitudinal voltage source cascade comprises two or more longitudinal voltage sources, as described above. Regarding the advantages of the longitudinal voltage source cascade according to the invention, reference is made to the above explanations concerning the longitudinal voltage source according to the invention.

[0016] It is advantageous if a first input of the series voltage source cascade is formed by a first feed terminal of the first series voltage source of the series voltage source cascade, a second input of the series voltage source cascade is formed by a second feed terminal of the first series voltage source of the series voltage source cascade, a first output of the series voltage source cascade is formed by a first output terminal of the last series voltage source of the series voltage source cascade, a second output of the series voltage source cascade is formed by a second output terminal of the last series voltage source of the series voltage source cascade, and each series voltage source except the last one is followed by a series voltage source.wherein the first input terminal of each downstream longitudinal voltage source is connected to the first output terminal of the upstream longitudinal voltage source and the second input terminal of each downstream longitudinal voltage source is connected to the second output terminal of the upstream longitudinal voltage source.

[0017] It is also advantageous if the voltage resistance of the switching devices and switching units of the longitudinal voltage sources of the longitudinal voltage source cascade increases from longitudinal voltage source to longitudinal voltage source by the blocking voltage of the energy storage of the longitudinal voltage sources.

[0018] Preferably, the number of switches per switching unit and switching device increases by one switch from longitudinal voltage source to longitudinal voltage source.

[0019] The invention further relates to a direct current transmission system. With regard to such a direct current transmission system, the invention provides that it has at least one longitudinal voltage source, as described above, and / or a longitudinal voltage source cascade, as described above, as well as a first and a second high-voltage direct current transmission line, which is connected to the output terminals of the longitudinal voltage source and the outputs of the longitudinal voltage source cascade, respectively.

[0020] Regarding the advantages of the direct current transmission system according to the invention, reference is made to the above statements in connection with the longitudinal voltage source according to the invention.

[0021] The invention further relates to a method for operating a longitudinal voltage source, a longitudinal voltage source cascade or a DC transmission system as described above, wherein the energy storage device is either connected between the first input terminal and the first output terminal of the longitudinal voltage source or longitudinal voltage sources or between the second input terminal and the second output terminal of the longitudinal voltage source or longitudinal voltage sources, or is disconnected from all of these terminals.

[0022] Regarding the advantages of the method according to the invention, reference is made to the above statements in connection with the longitudinal stress source according to the invention.

[0023] The invention is explained in more detail below with reference to exemplary embodiments; these show, by way of example, Figure 1 shows an embodiment of a direct current transmission system according to the invention, which is equipped with an embodiment of a longitudinal voltage source according to the invention; Figure 2 shows a first operating state of the longitudinal voltage source according to Figure 1 Figure 3 shows a second operating state of the longitudinal voltage source according to Figure 1 , Figure 4 a third operating state of the longitudinal voltage source according to Figure 1 Figure 5 shows an embodiment of a direct current transmission system according to Figure 1 A suitable longitudinal voltage source in more detail, Figure 6 shows an embodiment of a DC transmission system in which a polarity reversal device is arranged downstream of a longitudinal voltage source, Figure 7 shows an embodiment of a DC transmission system with a longitudinal voltage source cascade, and Figure 8 shows a variant of the embodiment according to Figure 5 , in which some switches are designed as non-controllable valves in the form of a diode.

[0024] For the sake of clarity, the same reference symbols are always used in the figures for identical or comparable components.

[0025] The ones relating to the Figures 1 to 5 The described embodiments serve to illustrate the structure and function of the respective components. For this reason, the illustration and description of the polarity reversal device are omitted initially. A longitudinal voltage source with a polarity reversal device is, with reference to the Figure 6 described.

[0026] The Figure 1 Figure 1 shows an embodiment of a switchable longitudinal voltage source 10. The longitudinal voltage source 10 comprises a first input terminal E1 for injecting a first current I1, a first output terminal A1 for outputting the first current I1, a second input terminal E2 for injecting a second current I2, and a second output terminal A2 for outputting the second current I2.

[0027] The series voltage source 10 makes it possible to selectively connect an electrical voltage Uc of an electrical energy storage device 11 to the series voltage source 10 either between the first input terminal E1 and the first output terminal A1 or between the second input terminal E2 and the second output terminal A2. For this purpose, the first input terminal E1 can be connected to a first terminal C1 of the energy storage device 11 by means of a first switching device SR1, and the second input terminal E2 can be connected to the second terminal C2 of the energy storage device 11 by means of a second switching device SR2.

[0028] A first series circuit R1 is connected to the first feed-in terminal E1 of the longitudinal voltage source 10, which includes a first switch S1 and a first switching unit SH1 and connects the first feed-in terminal E1 to the second terminal C2 of the energy storage device 11.

[0029] One of the terminals of the first switch S1, hereinafter referred to as the first terminal of the first switch S1, forms an outer terminal of the first series circuit R1 and the first power supply terminal E1; one of the terminals of the first switching unit SH1, hereinafter referred to as the first terminal of the first switching unit SH1, forms a second outer terminal of the first series circuit R1 and is connected to the second terminal C2 of the energy storage device 11. Electrically, between the other terminal of the first switch S1, hereinafter referred to as the second terminal of the first switch S1, and the other terminal of the first switching unit SH1, hereinafter referred to as the second terminal of the first switching unit SH1, lies a center terminal M1 of the first series circuit R1, which directly forms the first output terminal A1 of the series voltage source 10.

[0030] A second series circuit R2 is connected to the second feed-in terminal E2. This circuit comprises a second switch S2 and a second switching unit SH2 and connects the second feed-in terminal E2 to the first terminal C1 of the energy storage device 11. One of the terminals of the second switch S2, hereinafter referred to as the first terminal of the second switch S2, forms a first outer terminal of the second series circuit R2 and also the second feed-in terminal E2 of the series voltage source 10. One of the terminals of the second switching unit SH2, hereinafter referred to as the first terminal of the second switching unit SH2, forms a second outer terminal of the second series circuit R2 and is connected to the first terminal C1 of the energy storage device 11.Electrically, between the other terminal of the second switch S2, hereinafter referred to as the second terminal of the second switch S2, and the other terminal of the second switching unit SH2, hereinafter referred to as the second terminal of the second switching unit SH2, there is a center terminal M2 of the second series circuit R2. The center terminal M2 of the second series circuit R2 directly forms the second output terminal A2 of the series voltage source 10.

[0031] In the embodiment according to Figure 1 The switching devices SR1 and SR2 each consist of two switches S; the two switching units SH1 and SH2 each consist of one switch S.

[0032] High-voltage direct current transmission lines 21 and 22 of a direct current transmission system 20 can be connected to the two output terminals A1 and A2 of the longitudinal voltage source 10, and a feed line 23 for supplying an input current Iin can be connected to the two feed terminals E1 and E2 of the longitudinal voltage source 10, as shown by way of example in the Figure 1 shown.

[0033] The input current Iin flowing in the feed line 23 is split into the first current I1 and the second current I2, which flow into the longitudinal voltage source 10 at the first feed connection E1 and at the second feed connection E2.

[0034] In order to achieve a desired current or load distribution of the input current Iin or the currents I1 and I2 fed into the two feed-in terminals E1 and E2 of the longitudinal voltage source 10 to the two output terminals A1 and A2, and thus a corresponding current distribution in the two high-voltage direct current transmission lines 21 and 22, the two switches S1 and S2, the two switching units SH1 and SH2, and the two switching devices SR1 and SR2 can enable three operating states of the longitudinal voltage source 10; this will be explained in more detail below by way of example in connection with the Figures 2 to 4 explained.

[0035] The Figure 2 shows a first operating state of the longitudinal voltage source 10, in which the switches S of the first switching device SR1, the second switch S2 and the first switching unit SH1 are switched on; the first switch S1, the second switching unit SH2 and the second switching device SR2 are switched off.

[0036] In the first operating state according to the Figure 2 The electrical voltage Uc across the energy storage device 11, which is preferably a capacitor C, is switched into the current path P1 between the first input terminal E1 and the first output terminal A1; the second current path P2 between the second input terminal E2 and the second output terminal A2 bypasses the electrical energy storage device 11, so that it does not see the electrical voltage Uc of the energy storage device 11.

[0037] Depending on the sign of the voltage Uc at the energy storage device 11 or the direction of the voltage Uc at the energy storage device 11, the voltage drop between the first input terminal E1 and the first output terminal A1 is increased or reduced by the voltage Uc.

[0038] Depending on the direction of the voltage Uc applied to the energy storage device 11, the first current I1 in the first current path P1 between the first feed-in terminal E1 and the first output terminal A1 is increased or decreased compared to the current I2 in the second current path P2 between the second feed-in terminal E2 and the second output terminal A2, resulting in a corresponding load shift or change in the load current distribution of the input current Iin in the feed-in line 23 onto the two high-voltage direct current transmission lines 21 and 22 of the direct current transmission system 20.

[0039] In a second operating state of the longitudinal voltage source 10, the energy storage device 11 is connected to the second current path P2 between the second input terminal E2 and the second output terminal A2; the first current path P1 bypasses the energy storage device. The second operating state is in the Figure 3 shown.

[0040] The Figure 3 This shows that for the second operating state, the first switch S1, the second switching device SR2, and the second switching unit SH2 are switched on, while the remaining switching components are switched off. Depending on the polarity or direction of the voltage Uc at the energy storage device 11, the current I2 in the second current path P2 is thus increased or decreased relative to the current I1 in the first current path P1, resulting in a corresponding load current shift of the input current Iin in the feed line 23 to the two high-voltage direct current transmission lines 21 and 22 of the direct current transmission system 20.

[0041] The Figure 4 This shows a third operating state of the longitudinal voltage source 10. In this third operating state, the first switch S1 and the second switch S2 are switched on; the remaining switching components are switched off. In this third operating state, according to... Figure 4The energy storage device 11, or rather its voltage Uc, is separated from the two current paths P1 and P2, so that the voltage Uc has no influence on the current distribution.

[0042] The Figure 5 shows an exemplary embodiment for the construction of a longitudinal voltage source 10, which is used in the direct current transmission system 20 according to the Figures 1 to 4 can be used. Switches S1 and S2, the switches of switching devices SR1 and SR2, and the switches of switching units SH1 and SH2 are externally switchable, unipolar semiconductor switches. It can be seen that each switch consists of a transistor T, controlled by a control device 12, and a diode D connected antiparallel to it. Each switch can therefore only be turned on and off by the control device 12 in a unipolar direction; in the opposite current direction, the diode D acts as a freewheeling resistor.

[0043] Through the in the Figure 5 The circuit shown for the diodes D and the transistors T shows that load flow control is only possible in one direction, i.e., unidirectionally; because the energy storage device 11 should always be supplied with a positive voltage Uc in the Figure 5 The voltage direction shown should be used to avoid a short circuit of the energy storage device 11 via the freewheeling diodes D in the first and second operating states.

[0044] To prepare for the operation of the longitudinal voltage source 10, the control unit 12 will first set the voltage Uc at the energy storage device 11 to a desired output value of, for example, + 2 kV.

[0045] If, during the operation of the DC transmission system 20, the situation arises that the current distribution towards the first output terminal A1 and the second output terminal A2 is asymmetrical and the load flow in the first current path P1 is always greater than in the second current path P2, the energy storage device 11 can be switched in the positive voltage direction into the first current path P1 or in the negative voltage direction into the second current path P2 by temporarily switching on the first and second operating states alternately (for example, within the framework of pulse width modulation), thereby shifting the load flow from the first current path towards the second current path in both cases.Since the energy storage device 11 is charged in the first operating state and discharged in the second, the state of charge of the energy storage device 21 can be kept constant within the framework of pulse width modulation, provided that the durations for the first and second operating states are the same or at least approximately the same.

[0046] The length of the first and second operating states (as well as the length of the third operating state in between) and the operating voltage level of the energy storage device 11 determine the extent of the load flow change or the load flow shift in favor of the first current path.

[0047] The Figure 6Figure 1 shows an embodiment of a DC transmission system 20 in which a polarity reversal device 30 is connected downstream of a longitudinal voltage source 10. The polarity reversal device 30 is electrically located between the two output terminals A1 and A2 of the longitudinal voltage source 10 and the two high-voltage DC transmission lines 21 and 22 of the DC transmission system 20.

[0048] The reversing device 30 according to Figure 6 has four switches 31, 32, 33 and 34, which in a first position connect the first output terminal A1 of the longitudinal voltage source 10 with the first conductor terminal contact 30a of the reversing device 30 and thus with the first high-voltage direct current transmission line 21 and the second output terminal A2 of the longitudinal voltage source 10 with a second conductor terminal contact 30b of the reversing device 30 and thus with the second high-voltage direct current transmission line 22 of the direct current transmission system 20.

[0049] In an inverse or second position of the reversing device 30, the first output terminal A1 of the longitudinal voltage source 10 is connected to the second conductor terminal 30b of the reversing device 30 and thus to the second high-voltage direct current transmission line 22 of the direct current transmission system 20, and the second output terminal A2 of the longitudinal voltage source 10 is connected to the first conductor terminal 30a of the reversing device 30 or to the first high-voltage direct current transmission line 21.

[0050] The polarity reversal device 30 enables bidirectional load flow control with only one unidirectionally operating longitudinal voltage source 10, whereby the load flow direction is set by selecting the first or second position of the polarity reversal device 30.

[0051] With a view to minimizing electrical losses, it is considered advantageous if the four switches 31, 32, 33 and 34 of the reversing device 30 are mechanical switches, since these cause very low electrical losses in the switched-on state, at least less than semiconductor switches, such as those used in connection with the Figure 5 shown.

[0052] The Figure 7 Figure 1 shows an embodiment of a direct current transmission system 20 in which a plurality of longitudinal voltage sources, preferably those according to the Figures 1 to 6 , a longitudinal voltage source cascade LQK is formed. For the sake of clarity, only two of the longitudinal voltage sources of the longitudinal voltage source cascade LQK are shown, namely the first, which is marked with reference numeral 10, and the last, which is marked with reference numeral 10'.

[0053] A first input Ke1 of the LQK series voltage source cascade is formed by a first feed-in terminal E1 of the first series voltage source 10 of the LQK series voltage source cascade. A second input Ke2 of the LQK series voltage source cascade is formed by a second feed-in terminal E2 of the first series voltage source 10 of the LQK series voltage source cascade. A first output Ka1 of the LQK series voltage source cascade is formed by a first output terminal A1 of the last series voltage source 10' of the LQK series voltage source cascade, and a second output Ka2 of the LQK series voltage source cascade is formed by a second output terminal A2 of the last series voltage source 10' of the LQK series voltage source cascade.

[0054] Each longitudinal voltage source of the longitudinal voltage source cascade LQK is followed by a longitudinal voltage source, with the exception of the last one, wherein the first feed-in terminal E1 of each subsequent longitudinal voltage source is connected to the first output terminal A1 of the upstream longitudinal voltage source and the second feed-in terminal E2 of each subsequent longitudinal voltage source is connected to the second output terminal A2 of the upstream longitudinal voltage source.

[0055] The Figure 7It can be seen that the number of switches S per switching unit SH1 and SH2 or switching device SR1 and SR2 increases by one switch S from one longitudinal voltage source to the next. This measure ensures that the dielectric strength of the switching devices and switching units of the longitudinal voltage sources of the longitudinal voltage source cascade LQK increases by the blocking voltage of the energy storage device 11 of the longitudinal voltage sources 10 from one longitudinal voltage source to the next.

[0056] The direct current transmission system 20 according to Figure 7 It also features a longitudinal voltage source 40, as is known from the prior art.

[0057] Furthermore, the above statements apply in connection with the Figures 1 to 6 for the direct current transmission system 20 according to Figure 7 accordingly.

[0058] The Figure 8 shows a design variant for the longitudinal voltage source 10 according to Figure 5 It can be seen that in the case of the longitudinal voltage source 10 according to Figure 8 The switches S of the first switching device SR1, the switch S of the second switching unit SH2 and the second switch S2 are formed solely by a non-controllable valve in the form of a diode D; unlike the embodiment according to [reference missing], there is no transistor connected in parallel. Figure 5 The parallel-connected transistor can be connected to the first switching device SR1, the second switching unit SH2 and the second switch S2 at the series voltage source 10 according to Figure 5 This is unnecessary, since the respective current flow in all switching states of the series voltage source is always in the forward direction of the diodes.

[0059] Furthermore, the explanations relating to the Figure 5 for the variant according to Figure 8 accordingly.

[0060] Although the invention has been further illustrated and described in detail by means of preferred embodiments, the invention is not limited by the disclosed examples and other variations can be derived from them by the person skilled in the art without leaving the scope of protection of the invention. Reference symbol list

[0061] 10 Longitudinal voltage source 10' Longitudinal voltage source 11 Energy storage 12 Control unit 20 DC transmission system 21 High-voltage DC transmission line 22 High-voltage DC transmission line 23 Feed-in line 30 Polarity reversal device 30a Conductor connection contact 30b Conductor connection contact 31 Switch 32 Switch 33 Switch 34 Switch 40 Longitudinal voltage source A1 Output connection A2 Output connection C Capacitor C1 Connection C2 Connection D Diode E1 Feed-in connection E2 Feed-in connection I1 Current I2 Current Iin Input current Ka1 Output Ka2 Output Ke1 Input Ke2 Input LQK Longitudinal voltage source cascade M1 Center connection M2 Center connection P1 Current path P2 Current path R1 Series connection R2 Series connection S Switch S1 Switch S2 Switch SH1 Switching unit SH2 Switching unit SR1 Switching device SR2 Switching device TTransistor UcVoltage

Claims

1. An arrangement with a switchable longitudinal voltage source (10, 10'), wherein the longitudinal voltage source (10, 10') has a first supply terminal (E1) for supplying a first current (I1), a first output terminal (A1) for outputting the first current (I1), a second supply terminal (E2) for supplying a second current (I2) and a second output terminal (A2) for outputting the second current (I2), wherein the longitudinal voltage source is suitable for selectively switching an electrical voltage (Uc) of an electrical energy store (11) of the longitudinal voltage source between the first supply terminal and the first output terminal or between the second supply terminal and the second output terminal, wherein - the first supply terminal (E1) is connectable to a first terminal (C1) of the energy store (11) by means of a first switching device (SR1), - the second supply terminal (E2) is connectable to a second terminal (C2) of the energy store (11) by means of a second switching device (SR2), - a first series circuit (R1), which comprises a first switch (S1) and a first switching unit (SH1) and connects the first supply terminal to the second terminal (C2) of the energy store (11), is coupled to the first supply terminal (E1), - a first terminal of the first switch (S1) forms an outer terminal of the first series circuit (R1) and the first supply terminal, and a first terminal of the first switching unit (SH1) forms a second outer terminal of the first series circuit (R1) and is connected to the second terminal (C2) of the energy store (11), and a centre terminal (M1) of the first series circuit (R1) lies electrically between the second terminal of the first switch (S1) and the second terminal of the first switching unit (SH1), - a second series circuit (R2), which comprises a second switch (S2) and a second switching unit (SH2) and connects the second supply terminal (E2) to the first terminal (C1) of the energy store (11), is coupled to the second supply terminal, - a first terminal of the second switch (S2) forms an outer terminal of the second series circuit (R2) and the second supply terminal (E2), and a first terminal of the second switching unit (SH2) forms a second outer terminal of the second series circuit (R2) and is connected to the first terminal (C1) of the energy store (11), and a centre terminal (M2) of the second series circuit (R2) lies electrically between the second terminal of the second switch (S2) and the second terminal of the second switching unit (SH2), and wherein - the centre terminal (M1) of the first series circuit (R1) directly forms the first output terminal (A1), and the centre terminal (M2) of the second series circuit (R2) directly forms the second output terminal (A2), characterised in that a polarity reversal device (30) is arranged downstream of the longitudinal voltage source and is switched between the two output terminals (A1, A2) and two conductor terminal contacts (30a, 30b) of the longitudinal voltage source (10), - wherein, in a first position, the polarity reversal device (30) connects the first output terminal (A1) to the first conductor terminal contact (30a) and the second output terminal (A2) to the second conductor terminal contact (30b), and - wherein, in a second position, the polarity reversal device (30) connects the first output terminal (A1) to the second conductor terminal contact (30b) and the second output terminal (A2) to the first conductor terminal contact (30a).

2. The arrangement with a switchable longitudinal voltage source (10, 10') according to claim 1, characterised in that the second switching device (SR2), the first switching unit (SH1) and the first switch (S1) are each formed by a controllable valve or have one or more controllable valves.

3. The arrangement with a switchable longitudinal voltage source (10, 10') according to any one of the preceding claims, characterised in that the first (S1) and second switches (S2), the first and second switching units (SH1, SH2) and the first and second switching devices (SR1, SR2) are each unipolarly switching.

4. The arrangement with a switchable longitudinal voltage source (10, 10') according to any one of the preceding claims, characterised in that - switches (S) of the first switching device (SR1) are structurally identical to the one or more switches (S) of the second switching unit (SH2), or - switches (S) of the first switching device (SR1) are structurally identical to the second switch (S2), or - switches (S) of the second switching unit (SH2) are structurally identical to the second switch (S2).

5. The arrangement with a switchable longitudinal voltage source (10, 10') according to any one of the preceding claims, characterised in that - the first and second switching units (SH1, SH2) are each formed by a switch or two or more switches lying in series, - the first and second switching devices (SR1, SR2) are each formed by two or more switches lying in series, - the first and second switching devices (SR1, SR2) each have one switch more than the first and second switching units (SH1, SH2), and - all switches are unipolarly switching.

6. The arrangement with a switchable longitudinal voltage source (10, 10') according to any one of the preceding claims, characterised in that at least one of the switches (S) of the longitudinal voltage source (10, 10') is formed by a non-controllable valve, in particular solely by a diode (D).

7. The arrangement with a switchable longitudinal voltage source (10, 10') according to any one of the preceding claims, characterised in that - at least one of the switches, preferably all switches, of the first switching device (SR1) is formed by a non-controllable valve, in particular a diode (D), - at least one of the switches, preferably all switches, of the second switching unit (SH2) is formed by a non-controllable valve, in particular a diode (D), and / or - the second switch (S2) is formed by a non-controllable valve, in particular a diode (D).

8. A longitudinal voltage source cascade (LQK), characterised in that it has two or more longitudinal voltage sources according to any one of the preceding claims.

9. The longitudinal voltage source cascade (LQK) according to claim 8, characterised in that - a first input (Ke1) of the longitudinal voltage source cascade (LQK) is formed by a first supply terminal (E1) of the first longitudinal voltage source (10) of the longitudinal voltage source cascade (LQK), - a second input (Ke2) of the longitudinal voltage source cascade (LQK) is formed by a second supply terminal (E2) of the first longitudinal voltage source (10) of the longitudinal voltage source cascade (LQK), - a first output (Ka1) of the longitudinal voltage source cascade (LQK) is formed by a first output terminal (A1) of the last longitudinal voltage source (10') of the longitudinal voltage source cascade (LQK), - a second output (Ka2) of the longitudinal voltage source cascade (LQK) is formed by a second output terminal (A2) of the last longitudinal voltage source (10') of the longitudinal voltage source cascade (LQK), - downstream of each longitudinal voltage source, with the exception of the last one, one longitudinal voltage source is arranged in each case, wherein the first supply terminal (E1) of each downstream longitudinal voltage source is connected to the first output terminal (A1) of the upstream longitudinal voltage source and the second supply terminal (E2) of each downstream longitudinal voltage source is connected to the second output terminal (A2) of the upstream longitudinal voltage source.

10. The longitudinal voltage source cascade (LQK) according to claim 8 or 9, characterised in that the withstand voltage of the switching devices and switching units of the longitudinal voltage sources of the longitudinal voltage source cascade (LQK) increases from each longitudinal voltage source to the downstream longitudinal voltage source of the longitudinal voltage source cascade (LQK) by the blocking voltage of the energy store (11) of the longitudinal voltage sources.

11. The longitudinal voltage source cascade (LQK) according to any one of the preceding claims 8 to 10, characterised in that the number of the switches (S) per switching unit and switching device increases from each longitudinal voltage source to the downstream longitudinal voltage source of the longitudinal voltage source cascade (LQK) by one switch.

12. A DC transmission system (20), characterised in that it is equipped at least with a longitudinal voltage source according to any one of the preceding claims and has first and second high-voltage DC transmission lines (21, 22) which are coupled to the output terminals of the longitudinal voltage source or the outputs of the longitudinal voltage source cascade (LQK), wherein a polarity reversal device (30) is arranged downstream of the longitudinal voltage source and is switched between the two output terminals (A1, A2) and two conductor terminal contacts (30a, 30b) of the longitudinal voltage source, - wherein, in a first position, the polarity reversal device (30) connects the first output terminal (A1) to the first conductor terminal contact (30a) and the second output terminal (A2) to the second conductor terminal contact (30b), and - wherein, in a second position, the polarity reversal device (30) connects the first output terminal (A1) to the second conductor terminal contact (30b) and the second output terminal (A2) to the first conductor terminal contact (30a).

13. A method for operating a longitudinal voltage source (10), a longitudinal voltage source cascade (LQK) or a DC transmission system (20) according to any one of the preceding claims, wherein the energy store (11) is switched either between the first supply terminal (E1) and the first output terminal (A1) of the longitudinal voltage source or longitudinal voltage sources or between the second supply terminal (E2) and the second output terminal (A2) of the longitudinal voltage source or longitudinal voltage sources or is disconnected from all these terminals.