Electrical energy transmission arrangement and coupling of two asynchronous three-phase systems

Abstract

The invention relates to an electrical energy transmission arrangement (1) and to a method for coupling two asynchronous three-phase systems (2, 3, 12, 13, 14). The electrical energy transmission arrangement (1) comprises a converter (4), which is designed to generate a second three-phase AC voltage with an adjustable frequency from a first three-phase AC voltage, and has three converter units (15), which each have a three-phase coupling transformer (16) and a modular multilevel converter (17). The multilevel converter (17) of each converter unit (15) is connected on the input side to the three phases (A, B, C) of the first three-phase AC voltage via the three-phase coupling transformer (16) of the converter unit (15), has two output connections (U, V) on the output side, and is designed to generate a single-phase AC voltage between the output connections (U, V). The output connections (U, V) of the three multilevel converters (17) are connected to one another to form output-side phase connections (L1, L2, L3) of the converter (4) for the three phases of the second three-phase AC voltage.

Description

[0001] Description

[0002] Electrical power transmission arrangement and coupling of two asynchronous three-phase systems

[0003] The invention relates to an electrical energy transmission arrangement and a method for coupling two asynchronous three-phase systems.

[0004] Asynchronous three-phase systems can be coupled, for example, with a so-called back-to-back converter or a matrix converter.

[0005] A back-to-back converter has two converters with a common DC link, where the DC link voltage can be a DC voltage or a high-frequency AC voltage. A problem with back-to-back converters is their low current-carrying capacity, which is why this design only works with transformers on both sides of the converters. At low frequencies, this results in very large and expensive transformers on one of the AC sides.

[0006] While a matrix converter is capable of delivering high currents, it requires a very complex and therefore error-prone control system. If parallel converter branches are used to increase the current-carrying capacity of a matrix converter, the signal interfaces of the available control units for modular multi-level converters are often insufficient to operate the matrix converter.

[0007] Furthermore, it is possible to use submodules with higher current-carrying capacity. However, their development is very lengthy and costly.

[0008] The invention is based on the objective of coupling two asynchronous three-phase systems by means of a converter arrangement that has a high current-carrying capacity and is built from available modules and control units.

[0009] The problem is solved according to the invention by an electrical energy transmission arrangement with the features of claim 1 and a method with the features of claim 10.

[0010] Advantageous embodiments of the invention are the subject of the dependent claims.

[0011] An electrical energy transmission arrangement according to the invention comprises

[0012] - a converter configured to generate a second three-phase alternating voltage with an adjustable frequency from a first three-phase alternating voltage, and comprising three converter units, each comprising a three-phase coupling transformer and a modular multilevel converter, wherein

[0013] - the multilevel converter of each converter unit is connected on the input side via the three-phase coupling transformer of the converter unit to the three phases of the first three-phase AC voltage, has two output terminals on the output side and is configured to generate a single-phase AC voltage between the output terminals, and

[0014] - the output terminals of the three multilevel converters are interconnected to the output-side phase terminals of the converter for the three phases of the second three-phase alternating voltage.

[0015] An electrical power transmission arrangement according to the invention thus comprises a converter with three converter units, each of which has a three-phase coupling transformer and a modular multilevel converter and is interconnected on the output side, in particular in a star or delta connection (see below), to generate the output three-phase AC voltage. Surprisingly, it has been found that this connection of the converter units exhibits a very high rated current on the output side and thus a high current-carrying capacity. Furthermore, the modular multilevel converters can each be implemented with modules and control units that have already been developed and tested. The connection of the three converter units according to the invention also avoids problematic parallel connections of converter branches as in matrix converters.

[0016] One design of the electrical power transmission arrangement further includes

[0017] - a first current measuring unit, which is set up to measure phase currents of the first three-phase alternating voltage,

[0018] - a second current measuring unit, which is set up to measure phase currents of the second three-phase alternating voltage,

[0019] - a first voltage measuring unit, which is set up to measure phase voltages of the first three-phase alternating voltage relative to a reference potential,

[0020] - a second voltage measuring unit, which is set up to measure phase voltages of the second three-phase alternating voltage relative to the reference potential,

[0021] - a converter control unit that is configured to generate control signals for controlling the converter depending on setpoints for the second three-phase AC voltage and on measured values ​​from the two current measuring units and the two voltage measuring units, and

[0022] - for each converter unit, a separate control unit which is configured to control the multilevel converter of the converter unit depending on the control signals of the converter control unit.

[0023] Controlling a converter or multilevel converter refers to providing control signals for the converter's semiconductor switches. The control signals from the converter's control unit provide specifications for the individual converter units, which are then implemented by the individual control units to control the respective converter unit. In a further configuration of the electrical power transmission arrangement, the first output terminal of each multilevel converter forms a phase connection for one phase of the second three-phase AC voltage, and the second output terminals of the multilevel converters are connected in a star configuration. The neutral point of the star connection of the second output terminals of the multilevel converters can be connected to ground potential.In this configuration of the inverter, the second output terminals of the multilevel inverter are connected in a star configuration on the output side. This makes it possible, in particular, to connect the neutral point of the star configuration to a defined earth potential.

[0024] In an alternative configuration of the electrical power transmission arrangement to the aforementioned design, each output terminal of each multilevel converter is connected to exactly one output terminal of another multilevel converter. The interconnected output terminals form a phase connection for one phase of the second three-phase AC voltage. In this converter configuration, the multilevel converters are thus connected in a delta (triangular) connection on the output side. Compared to the star connection of the second output terminals of the multilevel converters, this allows for higher output currents from the converter.

[0025] In a further embodiment of the electrical power transmission arrangement, the multilevel converter of each converter unit comprises six converter modules. The converter modules are connected on the input side to the secondary windings of the three-phase coupling transformer of the converter unit such that each converter module is connected to exactly one secondary winding and each secondary winding is connected to exactly two converter modules. Furthermore, three converter modules, which are connected on the input side to different secondary windings of the three-phase coupling transformer, are connected to each other in a first star connection on the output side, and the other three converter modules are connected to each other in a second star connection on the output side. The star points of these two star connections each form one of the output terminals of the multilevel converter.

[0026] In a further embodiment of the electrical power transmission arrangement, each converter module comprises several independently controllable submodules and a coupling inductor connected in series. For example, each submodule comprises a power module, a capacitor module, a first submodule terminal, and a second submodule terminal. The power module has a full bridge with four semiconductor switches connected between the poles of an intermediate circuit voltage. The capacitor module has a capacitor connected between the poles of the intermediate circuit voltage and a voltage measuring device connected in parallel to the capacitor. The first submodule terminal is connected to a center tap of a first half-bridge of the full bridge, and the second submodule terminal is connected to a center tap of the second half-bridge of the full bridge. The semiconductor switches are, for example, each an IGBT, IGCT, IEGT, or MOSFET.IGBT is the abbreviation for Insulated-Gate Bipolar Transistor, IGCT is the abbreviation for Integrated Gate-Commutated Thyristor, IEGT is the abbreviation for Injection-Enhanced Gate Transistor, and MOSFET is the abbreviation for Metal-Oxide-Semiconductor Field-Effect Transistor.

[0027] The modular design of the inverter modules, with multiple submodules connected in series, allows the AC voltage generated by a single inverter module to be flexibly adapted to specific requirements. This is achieved by adjusting the number of activated submodules within each inverter module, as the AC voltages generated by these submodules add up to the AC voltage generated by the inverter module itself. The submodules are of a standard design, allowing the use of readily available, commercially available components.

[0028] In the method according to the invention, two asynchronous three-phase systems are coupled by an electrical power transmission arrangement according to the invention. The converter of the electrical power transmission arrangement is connected on the input side to the three phases of a first three-phase system and on the output side to the three phases of the second three-phase system.

[0029] Since the method according to the invention uses an electrical power transmission arrangement according to the invention, the advantages of the method correspond to the advantages of the electrical power transmission arrangement according to the invention mentioned above.

[0030] In one embodiment of the method according to the invention, the first three-phase system is a first three-phase electrical power supply network, and the second three-phase system is a second three-phase electrical power supply network, wherein the converter of the electrical power transmission arrangement is connected on its output side to the three phases of the second power supply network via a line-connection transformer. In this embodiment of the method according to the invention, the electrical power transmission arrangement enables the transfer of power between the two power supply networks. The line-connection transformer serves to provide galvanic isolation between the second power supply network and the converter of the electrical power transmission arrangement.

[0031] In another embodiment of the method according to the invention, the first three-phase system is a three-phase electrical power supply network, and the second three-phase system is a three-phase rotor winding of a rotor of a doubly fed asynchronous machine. In this embodiment of the method according to the invention, the electrical power transmission arrangement is used to operate the doubly fed asynchronous machine. Due to the very high output current carrying capacity of the converter of the electrical power transmission arrangement, the doubly fed asynchronous machine can be operated like a synchronous machine in the event of a three-phase network fault; that is, the voltage at the rotor circuit of the doubly fed asynchronous machine can be set to zero in order to provide a high short-circuit current for network protection.

[0032] In another embodiment of the method according to the invention, the first three-phase system is a three-phase electrical power supply network and the second three-phase system is a synchronous machine. In this embodiment of the method according to the invention, the electrical power transmission arrangement enables the synchronous machine to be operated at an adjustable speed.

[0033] In another embodiment of the method according to the invention, the first three-phase system is a three-phase electrical power supply network and the second three-phase system is an electric arc furnace system. In this embodiment of the method according to the invention, the electrical power transmission arrangement enables the electric arc furnace system to be supplied with power from the power supply network.

[0034] The properties, features, and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more readily understandable in connection with the following description of exemplary embodiments, which are explained in more detail in conjunction with the drawings. These show:

[0035] FIG 1 shows a circuit diagram of an exemplary embodiment of an electrical power transmission arrangement that couples two asynchronous power supply networks, FIG 2 shows a circuit diagram of an exemplary embodiment of an electrical power transmission arrangement that couples a doubly fed asynchronous machine to a power supply network,

[0036] FIG 3 shows a circuit diagram of an exemplary embodiment of an electrical power transmission arrangement that couples a synchronous machine to a power supply network ,

[0037] FIG 4 shows a circuit diagram of an exemplary embodiment of an electrical power transmission arrangement that couples an electric arc furnace system to a power supply network.

[0038] FIG 5 shows a circuit diagram of a first embodiment of an inverter for an electrical power transmission arrangement,

[0039] FIG 6 shows a circuit diagram of a second embodiment of an inverter for an electrical power transmission arrangement,

[0040] FIG 7 shows a circuit diagram of an exemplary embodiment of a multilevel converter of an electrical power transmission arrangement,

[0041] FIG 8 shows a circuit diagram of an exemplary embodiment of an inverter module of the multilevel inverter shown in Figure 7 ,

[0042] FIG 9 shows a circuit diagram of an exemplary embodiment of a submodule of the inverter module shown in Figure 8 ,

[0043] FIG 10 shows a circuit diagram of an exemplary embodiment of a power module of the submodule shown in Figure 9 ,

[0044] FIG 11 shows a circuit diagram of an exemplary embodiment of a capacitor module of the submodule shown in Figure 9.

[0045] Corresponding parts are provided with the same reference numerals in the figures. Figure 1 (FIG 1) shows a circuit diagram of an exemplary embodiment of an electrical power transmission arrangement 1, which couples a first three-phase electrical power supply network 2 and a second three-phase electrical power supply network 3. The two power supply networks 2, 3 have different network frequencies.

[0046] The electrical power transmission arrangement 1 comprises a converter 4, a converter control unit 5, a first current measuring unit 6, a second first current measuring unit 7, a first voltage measuring unit 8 and a second voltage measuring unit 9.

[0047] The inverter 4 is configured to generate a second three-phase AC voltage with an adjustable frequency from a first three-phase AC voltage. In this case, the first three-phase AC voltage is the three-phase AC voltage of the first power supply network 2, and the second three-phase AC voltage is the three-phase AC voltage of the second power supply network 3. The three phases A, B, and C of the first three-phase AC voltage are connected to the input side of the inverter 4. The second power supply network 3 is connected to the output-side phase terminals LI, L2, and L3 of the inverter 4 via a line-connection transformer 10. The line-connection transformer 10 is used for galvanic isolation between the second power supply network 3 and the inverter 4.For galvanic isolation from the first power supply network 2, the converter has 4 three-phase coupling transformers, see figures 5 and 6 and their description.

[0048] The first current measuring unit 6 is configured to measure phase currents of the first three-phase AC voltage. The second current measuring unit 7 is configured to measure phase currents of the second three-phase AC voltage. The first voltage measuring unit 8 is configured to measure phase voltages of the first three-phase AC voltage relative to a reference potential. The second voltage measuring unit 9 is configured to measure phase voltages of the second three-phase AC voltage relative to the reference potential.

[0049] The inverter control unit 5 is set up to generate control signals for controlling the inverter 4 depending on setpoints 11 for the second three-phase AC voltage and on measured values ​​from the two current measuring units 6, 7 and the two voltage measuring units 8, 9.

[0050] Figure 2 ( FIG 2 ) shows a circuit diagram of an exemplary embodiment of an electrical power transmission arrangement 1, which couples a doubly fed asynchronous machine 12 to a three-phase electrical power supply network 2 .

[0051] The electrical power transmission arrangement 1 is configured like the electrical power transmission arrangement shown in Figure 1. In this case, the inverter 4 generates a second three-phase AC voltage for a rotor of the asynchronous machine 12 from the three-phase AC voltage of the power supply network 2 (the first three-phase AC voltage). The three phases A, B, C of the first three-phase AC voltage are connected to the input side of the inverter 4. The rotor-side AC terminals of the asynchronous machine 12 are connected to the output-side terminals LI, L2, L3 of the inverter 4. The stator-side AC terminals of the asynchronous machine 12 are connected to the phases of the power supply network 2 via the network connection transformer 10. The network connection transformer 10 is used for galvanic isolation between the power supply network 2 and the stator of the asynchronous machine 12.

[0052] Figure 3 (FIG 3) shows a circuit diagram of an exemplary embodiment of an electrical power transmission arrangement 1, which couples a synchronous machine 13 to a three-phase electrical power supply network 2. The electrical power transmission arrangement 1 is designed like the electrical power transmission arrangement shown in Figure 1. In this case, the inverter 4 generates a second three-phase AC voltage for the synchronous machine 13 from the three-phase AC voltage of the power supply network 2. The three phases A, B, C of the first three-phase AC voltage are connected to the input side of the inverter 4. The AC terminals of the synchronous machine 13 are connected to the output terminals LI, L2, L3 of the inverter 4.

[0053] Figure 4 ( FIG 4 ) shows a circuit diagram of an exemplary embodiment of an electrical power transmission arrangement 1, which couples an electric arc furnace system 14 to a three-phase electrical power supply network 2 .

[0054] The electric arc furnace system 14 comprises a furnace transformer 14.1, a current measuring device 14.2 for measuring furnace currents, a voltage measuring device 14.3 for measuring furnace voltages, three electrodes 14.4, an electric arc furnace 14.5, an adjustable electrode mast 14.6, and an electrode control unit 14.7.

[0055] The electrical power transmission arrangement 1 is configured like the electrical power transmission arrangement shown in Figure 1. In this case, the inverter 4 generates a second three-phase AC voltage for the electric arc furnace 14 from the three-phase AC voltage of the power supply network 2 (the first three-phase AC voltage). The three phases A, B, and 0 of the first three-phase AC voltage are connected to the input side of the inverter 4. The AC terminals of the electric arc furnace 14 are connected to the output terminals LI, L2, and L3 of the inverter 4.

[0056] Figure 5 ( FIG 5 ) shows a circuit diagram of a first embodiment of the converter 4 of an electrical power transmission arrangement 1. The converter 4 has three converter units 15, each comprising a three-phase coupling transformer 16 and a modular multilevel converter 17.

[0057] Each three-phase coupling transformer 16 has primary windings that are each connected to a phase A, B, C of the first three-phase AC voltage, and secondary windings that are each connected to an input terminal A ' , B ' , C ' (see Figure 7) of each multilevel converter 17.

[0058] The multilevel converter 17 of each converter unit 15 is connected on the input side via the three-phase coupling transformer 16 of the converter unit 15 to the three phases A, B, C of the first three-phase AC voltage. On the output side, the multilevel converter 17 of each converter unit 15 has two output terminals U, V and is configured to generate a single-phase AC voltage between the output terminals U, V. The output terminals U, V of the three multilevel converters 17 are connected to output terminals LI, L2, L3 of the converter 4, each of which forms a phase terminal for one phase of the second three-phase AC voltage.

[0059] In the embodiment shown in Figure 5, a first output terminal U of each multilevel converter 17 forms one of the phase terminals LI, L2, L3, and the second output terminals V of the multilevel converters 17 are connected to each other in a star connection. A star point N of the star connection of the second output terminals V of the multilevel converters 17 can be connected to earth potential.

[0060] Figure 6 (FIG 6) shows a circuit diagram of a second embodiment of the converter 4 of an electrical power transmission arrangement 1. This embodiment differs from the embodiment shown in Figure 5 only in the connection of the output terminals U, V of the multilevel converters 17. In the embodiment shown in Figure 6, the first output terminal U of each multilevel converter 17 is connected to the second output terminal V of another multilevel converter 17, and the interconnected output terminals U, V each form a phase terminal LI, L2, L3 for one phase of the second three-phase AC voltage.

[0061] Figure 7 (FIG 7) shows a circuit diagram of an embodiment of the multilevel converter 17 of a converter unit 15 of the converter 4. The multilevel converter 17 has six converter modules 18, a single control unit 25, three input terminals A', B', C', and two output terminals U, V. Each input terminal A', B', C' is connected to a secondary winding of the three-phase coupling transformer 16 of the converter unit 15. The converter modules 18 are connected on the input side to the input terminals A', B', C' such that each converter module 18 is connected to exactly one of the input terminals A', B', C', and each input terminal A', B', C' is connected to exactly two converter modules 18. On the output side, three converter modules 18, each connected on the input side to different input terminals A', B', C', are connected to each other in a star connection.The star points of these two star connections each form one of the output terminals U, V.

[0062] The individual control unit 25 of a multilevel converter 17 generates the individual switching commands for the semiconductor switches (see Figure 10 and its description) of this multilevel converter 17 depending on the control signals of the converter control unit 5 for the purpose of controlling the branch energies and the power flow in the multilevel converter 17.

[0063] Figure 8 (FIG 8) shows a circuit diagram of an embodiment of an inverter module 18 of the multilevel inverter 17 shown in Figure 7. The inverter module 18 has several independently controllable submodules 19 connected in series and a coupling inductor 20. Figure 9 (FIG 9) shows a circuit diagram of an embodiment of a submodule 19 of an inverter module 18. Each submodule 19 has a DC link voltage U ZKa power module 21 with semiconductor switches and a capacitor module 22 connected to the intermediate circuit of the power module 21.

[0064] Figure 10 (FIG 10) shows a circuit diagram of an embodiment of a power module 21 of a submodule 19. The power module 21 comprises a connection between the poles of the intermediate circuit voltage U ZKThe power module 21 comprises a switched full bridge FB with four semiconductor switches S1 to S4, DC link connections DC1 and DC2, a first connection AC1 connected to a center tap of a first half-bridge HB1 of the full bridge FB, and a second connection AC2 connected to a center tap of a second half-bridge HB2 of the full bridge FB. Furthermore, the power module 21 can include a diode D1 to D4 connected antiparallel to each semiconductor switch S1 to S4. The diode D1 to D4 can be omitted if the semiconductor switch S1 to S4 already has an intrinsic inverse diode (body diode), which is particularly the case if the semiconductor switch S1 to S4 is designed as a MOSFET. The semiconductor switches S1 to S4 are, for example, each an IGBT, IGCT, IEGT, or MOSFET.

[0065] Figure 11 (FIG 11) shows a circuit diagram of an embodiment of a capacitor module 22 of a submodule 19. The capacitor module 22 has two DC link terminals DC3, DC4, a capacitor 23 connected to the two DC link terminals DCS, DC4, and a voltage measuring device 24 connected in parallel to the capacitor 23. Each DC link terminal DC3, DC4 of the capacitor module 22 of a submodule 19 is connected to a DC link terminal DC1, DC2 of the power module 21 of this submodule 19. The capacitor 23 serves to buffer the DC link voltage U. ZK Although the invention has been further illustrated and described in detail by 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.

Claims

Patent claims 1. Electrical power transmission arrangement (1) , comprising - a converter (4) configured to generate a second three-phase alternating voltage with an adjustable frequency from a first three-phase alternating voltage, and comprising three converter units (15), each comprising a three-phase coupling transformer (16) and a modular multi-level converter (17), wherein - the multilevel converter (17) of each converter unit (15) is connected on the input side via the three-phase coupling transformer (16) of the converter unit (15) to the three phases (A, B, C) of the first three-phase AC voltage, has two output terminals (U, V) on the output side and is configured to generate a single-phase AC voltage between the output terminals (U, V), and - the output terminals (U, V) of the three multilevel converters (17) are interconnected to the output phase terminals (LI, L2, L3) of the converter (4) for the three phases of the second three-phase alternating voltage.

2. Electrical power transmission arrangement (1) according to claim 1, further comprising - a first current measuring unit (6) which is set up to measure phase currents of the first three-phase alternating voltage, - a second current measuring unit (7) which is set up to measure phase currents of the second three-phase alternating voltage, - a first voltage measuring unit (8) which is set up to measure phase voltages of the first three-phase alternating voltage relative to a reference potential, - a second voltage measuring unit (9) which is set up to measure phase voltages of the second three-phase alternating voltage relative to the reference potential, - a converter control unit (5) which is configured to generate control signals for controlling the converter (4) depending on setpoints (11) for the second three-phase AC voltage and on measured values ​​from the two current measuring units (6, 7) and the two voltage measuring units (8, 9), and - for each converter unit (15) a single control unit (19) which is configured to control the multilevel converter (17) of the converter unit (15) depending on the control signals of the converter control unit (5).

3. Electrical power transmission arrangement (1) according to claim 1 or 2, wherein a first output terminal (U) of each multilevel converter (17) forms a phase terminal (LI, L2, L3) for one phase of the second three-phase alternating voltage and the second output terminals (V) of the multilevel converters (17) are connected to each other in a star connection.

4. Electrical power transmission arrangement (1) according to claim 3, wherein a star point (N) of the star connection of the second output terminals (V) of the multilevel converters (17) is connected to earth potential.

5. Electrical power transmission arrangement (1) according to claim 1 or 2, wherein each output terminal (U, V) of each multilevel converter (17) is connected to exactly one output terminal (U, V) of another multilevel converter (17) and the interconnected output terminals (U, V) form a phase terminal (LI, L2, L3) for one phase of the second three-phase AC voltage.

6. Electrical power transmission arrangement (1) according to one of the preceding claims, wherein the multilevel converter (17) of each converter unit (15) comprises six converter modules (18), the converter modules (18) being connected on the input side to the secondary windings of the three-phase coupling transformer (16) of the converter unit (15) such that each converter module (18) is connected to exactly one secondary winding and each secondary winding is connected to exactly two converter modules (18), and three converter modules (18) being connected on the input side to different secondary windings of the three-phase coupling transformer (16) being connected on the output side in a first star connection. are bound, the other three converter modules (18) are connected to each other on the output side in a second star connection and the star points of these two star connections each form one of the output terminals (U, V) of the multilevel converter (17).

7. Electrical power transmission arrangement (1) according to claim 6, wherein each converter module (18) comprises a series connection of several independently controllable submodules (19) and a coupling inductor (20).

8. Electrical power transmission arrangement (1) according to claim 7, wherein each submodule (19) comprises a power module (21), a capacitor module (22), a first submodule connection (AC1) and a second submodule connection (AC2), wherein the power module (21) provides a voltage between the poles of an intermediate circuit voltage (U) Z K) switched full bridge (FB) with four semiconductor switches (S1 to S4), the capacitor module (22) has a connection between the poles of the intermediate circuit voltage (U ZK) connected capacitor (23) and a voltage measuring device (24) connected in parallel to the capacitor (23), the first submodule connection (AC1) is connected to a center tap of a first half-bridge (HB1) of the full bridge (FB) and the second submodule connection (AC2) is connected to a center tap of the second half-bridge (HB2) of the full bridge (FB).

9. Electrical power transmission arrangement (1) according to claim 8, wherein the semiconductor switches (S1 to S4) are each an IGBT, IGCT, IEGT or MOSFET.

10. Method for coupling two asynchronous three-phase systems (2, 3, 12, 13, 14) , wherein the converter (4) of an electrical power transmission arrangement (1) designed according to one of the preceding claims is connected on the input side to the three phases (A, B, C) of a first three-phase system (2) and on the output side to the three phases of the second three-phase system (3, 12, 13, 14).

11. Method according to claim 10, wherein the first three-phase system is a first three-phase electrical power supply network (2), the second three-phase system is a second three-phase electrical power supply network (3), and the converter (4) is connected on the output side to the three phases of the second power supply network (3) via a network connection transformer (10).

12. Method according to claim 10, wherein the first three-phase system is a three-phase electrical power supply network (2) and the second three-phase system is a three-phase rotor winding of a rotor of a doubly fed asynchronous machine (12).

13. Method according to claim 10, wherein the first three-phase system is a three-phase electrical power supply network (2) and the second three-phase system is a synchronous machine (13).

14. Method according to claim 10, wherein the first three-phase system is a three-phase electrical power supply network (2) and the second three-phase system is an electric arc furnace system (14).

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