DEVICE FOR CONNECTING TWO ALTERNATING VOLTAGE NETWORKS AND METHOD FOR OPERATING THE DEVICE

Abstract

A connecting device for connecting two n-phase alternating voltage grids of the same frequency includes n susceptance elements each having continuously variable susceptance values. Through the use of each susceptance element, two connecting conductors, which are associated with one another, of the alternating voltage grids can be connected to one another and the active power exchange between the AC voltage grids can be controlled by varying the susceptance values in a targeted manner. A method for operating the connection device is also provided.

Claims

1-15. (canceled)

16. A connecting device for connecting two n-phase AC voltage grids of the same frequency, the connecting device comprising: n susceptance elements each having respective continuously variable susceptance values; each of said susceptance elements configured to connect two connecting conductors, associated with one another, of the AC voltage grids, to one another and permitting an exchange of active power between the AC voltage grids to be controlled by a targeted variation of the susceptance values.

17. The connecting device according to claim 16, wherein each of said susceptance elements includes a plurality of controllable semiconductor switches and at least one DC-link capacitor.

18. The connecting device according to claim 16, wherein each of said susceptance elements includes a series circuit of switching modules, and each of said switching module includes a plurality of semiconductor switches configured to be switched off and a switching module capacitor.

19. The connecting device according to claim 16, wherein each of said susceptance elements includes a series circuit of full-bridge switching modules and an inductor configured to be connected in series between the connecting conductors that are associated with one another.

20. The connecting device according to claim 16, which further comprises a matching transformer for setting a voltage phase angle.

21. The connecting device according to claim 20, wherein said matching transformer includes an on-load tap changer.

22. The connecting device according to claim 16, which further comprises surge arresters connected in parallel with said susceptance elements.

23. The connecting device according to claim 16, wherein said n susceptance elements include 2*n susceptance elements, and a respective two of said susceptance elements connected in parallel are configured to interconnect the connecting conductors that are associated with one another.

24. The connecting device according to claim 16, which further comprises a transformer including: a first primary winding connected to a first connecting conductor of a first AC voltage grid; a second primary winding connected to a second connecting conductor of the first AC voltage grid; a third primary winding connected to a third connecting conductor of the first AC voltage grid; a first secondary winding connected to a first connecting conductor of a second AC voltage grid by a first of said susceptance elements; a second secondary winding connected to a second connecting conductor of the second AC voltage grid by a second of said susceptance elements; a third secondary winding connected to a third connecting conductor of the second AC voltage grid by a third of said susceptance elements; a first tertiary winding connected to the first connecting conductor of the second AC voltage grid by a fourth of said susceptance elements; a second tertiary winding connected to the second connecting conductor of the second AC voltage grid by a fifth of said susceptance elements; a third tertiary winding connected to the third connecting conductor of the second AC voltage grid by a sixth of said susceptance elements; and said secondary windings and said tertiary windings each being interconnected in star connections generating a phase offset of pi/3 relative to one another and of pi/6 relative to said primary windings.

25. A method for operating a connecting device connecting two n-phase AC voltage grids of the same frequency, the method comprising: using a respective one of n susceptance elements to interconnect two connecting conductors associated with one another, of the AC voltage grids; continuously varying susceptance values of each of the susceptance elements; and controlling a transfer of active power between the AC voltage grids by a targeted variation of the susceptance values of the susceptance elements.

26. The method according to claim 25, which further comprises using the connecting device to actively set a voltage phase angle.

27. The method according to claim 26, which further comprises setting the voltage phase angle to 30°.

28. The method according to claim 26, which further comprises using a matching transformer to set the voltage phase angle.

29. The method according to claim 25, which further comprises using surge arresters connected in parallel with the susceptance elements to limit a voltage across the susceptance elements.

30. A method for operating a connecting device connecting two n-phase AC voltage grids of the same frequency, the method comprising: providing a connecting device according to claim 24; using a respective one of n susceptance elements to interconnect two connecting conductors associated with one another, of the AC voltage grids; continuously varying susceptance values of each of the susceptance elements; controlling a transfer of active power between the AC voltage grids by a targeted variation of the susceptance values of the susceptance elements; using the susceptance elements connected to the secondary side of the transformer to form a first connecting branch; using the susceptance elements connected to the tertiary side of the transformer to form a second connecting branch; and actuating the susceptance elements to compensate for a reactive power requirement of the first and second connecting branches.

Description

[0051] The invention will be explained in more detail below with reference to the exemplary embodiments of FIGS. 1 to 5.

[0052] FIG. 1 shows a first exemplary embodiment of a connecting device, according to the invention, in a schematic illustration;

[0053] FIG. 2 shows an example of a susceptance element in a schematic illustration;

[0054] FIG. 3 shows a second exemplary embodiment of a connecting device, according to the invention, in a schematic illustration;

[0055] FIG. 4 shows a third exemplary embodiment of a connecting device, according to the invention, in a schematic illustration;

[0056] FIG. 5 shows a further example of a susceptance element in a schematic illustration.

[0057] FIG. 1 shows a first, three-phase AC voltage grid 1 that is connected to a second, likewise three-phase AC voltage grid 3 by means of a connecting device 2. The first AC voltage grid 1 comprises a first, second and third connecting conductor L11, L12, L13. The second AC voltage grid 3 correspondingly comprises a first, second and third connecting conductor L21, L22, L23. The frequency in the two AC voltage grids is 50 Hz in each case. The connecting device 2 comprises a first susceptance element 4, by means of which the first connecting conductor L11 of the first AC voltage grid 1 is connected to the first connecting conductor L21 of the second AC voltage grid 3. The remaining connecting conductors L12, L13, L23, L33 are correspondingly connected to one another by means of a second or a third susceptance element 5 or 6.

[0058] A current iA flows through the first susceptance element 4. The voltage that can be generated at the susceptance element 4 is denoted by uA. The line-to-line voltages in the first AC voltage grid 1 are denoted as ULL(1) and those in the second AC voltage grid 3 are denoted as ULL(2). The susceptance of the susceptance elements 4-6 is denoted by B. The voltages of the two AC voltage grids 1, 3 have a voltage difference of phi relative to one another. The active power Ptrans exchanged between the two AC voltage grids results from

[0059] Ptrans=3*B*ULL(1)*ULL(2)*sin(phi). In this equation, active power losses occurring within the susceptance elements are ignored. The active power transferred between the two AC voltage grids can therefore be varied continuously by varying the susceptance values B. Since the susceptance value can assume both positive and negative values, the direction of the transfer of power can additionally also be controlled (bidirectional transport of active power).

[0060] At the same time, the first AC voltage grid 1 outputs a reactive power Q1, and the second AC voltage grid 3 outputs a reactive power Q2, in accordance with the following equations:

Q1=3*B*(ULL(1)*ULL(2)cos(phi)−ULL(1){circumflex over ( )}2),

Q2=3*B*(ULL(1)*ULL(2)cos(phi)−ULL(2){circumflex over ( )}2).

[0061] The reactive power output of the two AC voltage grids 1, 3 is likewise dependent on the phase difference phi. Overall, the two AC voltage grids 1, 3 cover the reactive power requirement of the susceptance elements.

[0062] FIG. 2 shows a susceptance element S that can be used, for example, as one of the susceptance elements 4-6 of FIG. 1. The susceptance element S comprises a first and a second connection X1, X2. A series circuit of full-bridge switching modules V1 . . . Vn is arranged between the connections X1, 2. The number of full-bridge switching modules V1, Vn connected in series is fundamentally arbitrary and can be adapted to the respective application, which is indicated in FIG. 2 by the dotted line 7. A sum voltage uA can be generated at the full-bridge switching modules V1 . . . Vn. This occurs by means of suitable actuation of the semiconductor switches H of the full-bridge switching modules V1 . . . Vn. Each full-bridge switching module V1 . . . Vn also comprises a switching module energy store in the form of a switching module capacitor CM that can be bypassed by means of the semiconductor switches H or connected into the current path. An inductor LA is connected in series with the full-bridge switching modules V1 . . . Vn.

[0063] FIG. 3 shows a further connecting device 8. In contrast to the connecting device 2 of FIG. 1, the connecting device 8 comprises a matching transformer 9. The primary windings 10 of the matching transformer 9 are arranged in a delta connection and are connected to the connecting lines of the first AC voltage grid 21. The secondary windings 11 of the matching transformer 9 are interconnected in a star point connection and are connected to three susceptance elements 12, 13, 14. The voltage phase shift phi in the example shown is set to 30° by means of the matching transformer. In this case, the second AC voltage grid 23 leads the first AC voltage grid 21 by 30° (=pi/6). In the first AC voltage grid 21, the voltage in the example shown is 8 kV. The voltage in the second AC voltage grid 23 is 20 kV. The frequency is 50 Hz in both cases. The active power transferred between the AC voltage grids 21, 23 can be approximately 30 MW with a current iA of 850A.

[0064] FIG. 4 shows a connecting device 30 that connects the first AC voltage grid 21 to the second AC voltage grid 23. The connecting device 30 comprises a transformer 31. The transformer 31 comprises primary windings 32 that are connected in a delta connection and are connected to associated connecting conductors of the first AC voltage grid 21. The transformer 31 also comprises secondary windings 33 that are interconnected in a star connection and are connected to a first (three-phase) parallel branch 35, and tertiary windings 36 that are likewise interconnected in a star connection and are connected to a second parallel branch 37.

[0065] Three susceptance elements 12-14 are arranged in the first parallel branch 35, and three further susceptance elements 38-40 are arranged in the second parallel branch 37. The three susceptance elements connect the secondary windings 33 to the associated connecting conductors of the second AC voltage grid 23. The further susceptance elements 38-40 correspondingly connect the tertiary windings 34 to the correspondingly associated connecting elements of the second AC voltage grid 23.

[0066] The secondary windings 33 and the tertiary windings 34 are each interconnected in star connections that generate a phase offset of pi/3 relative to one another and respectively pi/6 relative to the primary windings. The susceptance elements 12-14, 38-40 are in each case operated in such a way that the susceptance in the first parallel branch 35 and the susceptance in the second parallel branch 37 each have a different arithmetic sign. In this case, the first parallel branch 35 behaves like a capacitor and the second parallel branch 37 behaves like an inductor. If both grid voltages are the same and the parallel branches are actuated antisymmetrically, the reactive power requirement of the two parallel branches is compensated for and, overall, no reactive power has to be provided by means of the two AC voltage grids. Asymmetrical actuation of the susceptance elements in the two parallel branches 35, 37 furthermore makes it possible (with approximately the same voltage) to ensure that reactive power is generated in the two AC voltage grids 21, 23.

[0067] FIG. 5 shows a further susceptance element S2 that in particular can be used in all the connecting devices shown above. In contrast to the susceptance element S of FIG. 2 (all identical and similar components and elements of FIGS. 2 and 5 are provided with the same reference signs), in this case a surge arrester 15 is provided that is arranged in a branch in parallel with the series circuit of the switching modules V1 . . . Vn. In the case of a fault, the capacitors CM and the semiconductors H can in particular be protected by means of the surge arrester 15 until the connecting device is separated from the two AC voltage grids by mechanical circuit breakers (not shown in FIG. 5).