Converter arrangement

Abstract

A converter arrangement comprises first and second modular multilevel converters, Each of the modular multilevel converters comprises two converter branches. Each converter branch comprises a plurality of series-connected converter cells. Each converter cell comprises a cell capacitor and semiconductor switches for connecting and disconnecting the cell capacitor to the converter branch. At least two converter branches of the first modular multilevel converter are connected via first branch connection point and at least two converter branches of the second modular multilevel converter are connected via second branch connection point. The multilevel converters are connected in parallel via a phase connection point for connecting the converter arrangement to a load or a power source, wherein the phase connection point is connected via a first inductance with the first branch connection point and/or via a second inductance with the second branch connection point. At least one of the modular multilevel converters comprises a protection system.

Claims

1. A converter arrangement, wherein the converter arrangement comprises: at least first and second modular multilevel converters; wherein each of the at least first and second modular multilevel converters comprises at least two converter branches; wherein each converter branch comprises a plurality of serious-connected converter cells, each converter cell comprising a cell capacitor and semiconductor switches for connecting the cell capacitor to the converter branch; wherein the at least two converter branches of the first modular multilevel converter are connected via a first branch connection point and the at least two converter branches of the second modular multilevel converter are connected via a second branch connection point; wherein the at least first and second multilevel converters are connected in parallel via a phase connection point for connecting the converter arrangement to a load or a power source, wherein the phase connection point is connected via a first inductance with the first branch connection point and/or via a second inductance with the second branch connection point, wherein at least one of the first and second modular multilevel converters comprises a protection system, which comprises: a fault detection device; a mechanical or paraelectronic switch for interconnecting at least two phases at the branch connection point of the associated modular multilevel converter; and a controller adapted for switching converter cells of the associated modular multilevel converter to zero output voltage and for closing the switch, when receiving a fault signal from the fault detection device, wherein the fault detection device comprises a current sensor for sensing an over-current in the at least two phases at the phase connection point; and/or wherein the current sensor is adapted for measuring a current between the phase connection point and the first and/or second inductance before the branch connection point.

2. The converter arrangement of claim 1, wherein each of the first and second modular multilevel converters comprises converter branches for at least two phases, and either the converter branches of each phase are connected via an inductance for each phase with the phase connection point for each phase or a pair of two converter branches of the first and the second modular multilevel converter share one or more inductances.

3. The converter arrangement of claims 2, wherein each of the at least two converter branches of each of the at least first and second modular multilevel converters is connected via a branch inductance with the respective converter branch connection point.

4. The converter arrangement of claims 2 wherein the first and second modular multilevel converters are direct converters.

5. The converter arrangement of claim 2, wherein the at least first and second modular multilevel converters are galvanically connected via a load-side phase connection point to a load or a power source; wherein the load-side phase connection point is connected via a first load-side inductance with a first load-side branch connection point and via a second load-side inductance with a second load-side branch connection point.

6. The converter arrangement of claim 2, wherein the converter arrangement comprises a transformer and the at least first and second modular multilevel converters are connected via a transformer-side phase connection point with the transformer, such that the at least first and second modular multilevel converters are galvanically connected; wherein the transformer-side phase connection point is connected via a first transformer-side inductance with a first transformer-side branch connection point and via a second transformer-side inductance with a second transformer-side branch connection point.

7. The converter arrangement of claims 1, wherein each of the at least two converter branches of each of the at least first and second modular multilevel converters is connected via a branch inductance with the respective converter branch connection point.

8. The converter arrangement of claim 7, wherein the at least first and second modular multilevel converters are galvanically connected via a load-side phase connection point to a load or a power source; wherein the load-side phase connection point is connected via a first load-side inductance with a first load-side branch connection point and via a second load-side inductance with a second load-side branch connection point.

9. The converter arrangement of claims 7, wherein the first and second modular multilevel converters are direct converters.

10. The converter arrangement claim 1, wherein the at least first and second modular multilevel converters comprise a subconverter for converting an AC current into an DC current.

11. The converter arrangement of claim 1, wherein each of the at least first and second modular multilevel converters is an indirect converter comprising an active rectifier connected via a DC link with an inverter.

12. The converter arrangement of claim 11, wherein the first modular multilevel converter comprises a first DC link and the second modular multilevel converter comprises a second DC link, which is galvanically separated from the first DC link.

13. The converter arrangement of claim 11, wherein the at least first and second modular multilevel converters comprise a common DC link.

14. The converter arrangement of claim 1 wherein the first and second modular multilevel converters are direct converters.

15. The converter arrangement of claim 1, wherein the converter arrangement comprises a transformer and the at least first and second modular multilevel converters are connected via a transformer-side phase connection point with the transformer, such that the at least first and second modular multilevel converters are galvanically connected; wherein the transformer-side phase connection point is connected via a first transformer-side inductance with a first transformer-side branch connection point and via a second transformer-side inductance with a second transformer-side branch connection point.

16. The converter arrangement of claim 1, wherein the at least first and second modular multilevel converters are galvanically separated on a transformer side; wherein the first modular multilevel converter is connected via a first transformer to a phase connection point and the second modular multilevel converter is connected via a second transformer to the phase connection point; or wherein the first modular multilevel converter is connected to a first secondary winding of a transformer and the second modular multilevel converter is connected to a second secondary winding of the transformer.

17. The converter arrangement of claim 1, wherein the at least first and second modular multilevel converters are galvanically connected via a load-side phase connection point to a load or a power source; wherein the load-side phase connection point is connected via a first load-side inductance with a first load-side branch connection point and via a second load-side inductance with a second load-side branch connection point.

18. The converter arrangement of claim 1, wherein the controller is adapted for closing the switch after a predefined time (ΔT) after the switching the converter cells to zero output voltage; and/or wherein the predefined time does not differ more than 10% from the period of a frequency of a current in the branch connection point.

19. The converter arrangement of claim 18, wherein the converter arrangement comprises at least two modular multilevel converters, which are connected in parallel via a common phase connection point and/or at least two modular multilevel converters, which are connected in series via a DC link; wherein the converter arrangement comprises a protection system for each of the at least two converters; wherein, when receiving a fault signal, one first protection system is adapted for transmitting the fault signal to another second protection system, which is adapted for switching converter cells of an associated converter to zero output voltage and for closing an associated switch, when receiving a fault signal from the first protection system.

20. The converter arrangement of claim 1, wherein the converter arrangement comprises at least two modular multilevel converters, which are connected in parallel via a common phase connection point and/or at least two modular multilevel converters, which are connected in series via a DC link; wherein the converter arrangement comprises a protection system for each of the at least two converters; wherein, when receiving a fault signal, one first protection system is adapted for transmitting the fault signal to another second protection system, which is adapted for switching converter cells of an associated converter to zero output voltage and for closing an associated switch, when receiving a fault signal from the first protection system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The subject matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings.

(2) FIG. 1 schematically shows a converter arrangement according to an embodiment of the invention.

(3) FIG. 2 schematically shows a converter arrangement according to a further embodiment of the invention.

(4) FIG. 3 schematically shows a converter arrangement according to a further embodiment of the invention.

(5) FIG. 4 schematically shows a converter arrangement according to a further embodiment of the invention.

(6) FIG. 5 schematically shows a converter arrangement according to a further embodiment of the invention.

(7) FIG. 6 schematically shows a converter arrangement according to a further embodiment of the invention.

(8) FIG. 7 schematically shows a converter arrangement according to a further embodiment of the invention.

(9) FIG. 8 schematically shows a converter arrangement according to a further embodiment of the invention.

(10) FIG. 9 schematically shows a converter arrangement according to a further embodiment of the invention.

(11) FIG. 10 schematically shows a converter arrangement according to a further embodiment of the invention.

(12) FIG. 11 schematically shows a converter arrangement according to a further embodiment of the invention.

(13) The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(14) FIG. 1 shows a converter arrangement 10a with a modular multilevel converter 12a comprising two modular multilevel subconverters 14a, 14b. For each phase V.sub.U, V.sub.V, V.sub.W, V.sub.R, V.sub.S, V.sub.T, each subconverter 14a, 14b comprises two converter branches 16 that are connected in series between two outputs/inputs DC+ and DC− of a DC link 17, and that provide a branch connection point 18 between them.

(15) Each converter branch 16 comprises a plurality of converter cells 20 that are connected in series. Each converter cell 20 comprises semiconductor switches 22 and a cell capacitor 24. In FIG. 1, a unipolar converter cell 20 that is used with the indirect converter 12a with a snubber circuit 26 is shown. For example, the semiconductor switches 22 may comprise an IGCT with an antiparallel diode (as in FIG. 1), an RC-IGCT (as in FIG. 8 below), an IGBT or other actively switchable semiconductor elements.

(16) The converter branches 16 are connected via an inductance 28 with the branch connection point 18. The inductance 28 may be an inductor or coil on a magnetic core or with an air core. The inductances 28 connected to the branch connection point of one phase are coupled via a common magnetic core and may be seen as a branch reactor for damping and/or controlling circulating currents between the branches 16 of the converter 12a or subconverter 14a, 14b.

(17) The subconverter 14a is connected via a transformer 30 to an electrical grid 32. The subconverter 14b is connected to an electrical machine 34, for example a generator or, as depicted, an electrical motor 34 for driving a pump or turbine 36.

(18) The branch connection points 18 are connected via inductances 38 with the load 34, 36 or power source 30, 32. Also the inductances 38 may be inductors or coils with magnetic cores. However, the inductances 38 are not coupled with each other.

(19) The branch connection points 18 of the grid-side subconverter 14a are connected via grid-side phase connection points 40 with the transformer 30, and the branch connections points 18 of the load-side converter 14b are connected via load-side phase connections points 40 with the motor 34. The phase connection points 40 are used for connecting a further parallel converter (such as depicted in FIG. 2) with the load 34, 36 and power source 30, 32.

(20) The inductances 38 are used for damping and/or controlling circulating currents between the converter 12a and the parallel converter connected via the phase connection points 40. In such a way, the inductances 38 may be seen as paralleling reactors.

(21) Between the transformer 30 and the grid-side phase connection points 40, and/or between the motor 34 and the load-side phase connection points 40, a dv/dt-filter 42 may be connected with the respective phases.

(22) FIG. 2 shows a further embodiment of a converter arrangement 10b with two parallel modular multilevel converters 12a, 12b. The converter arrangement 10b is shown as single-phased on both sides of the converters 12a, 12b. However, it also may be possible that the converter arrangement 12b is multiphased on one or both sides; i.e. the converters 12a, 12b may be designed like the converter 12a shown in FIG. 1.

(23) FIG. 2 also shows a protection system 44 for the converter arrangement 10b that comprises a protection (sub-)system 46 for each subconverter 14a, 14b.

(24) Each protection system 46 comprises a fault detection sensor 48, a mechanical switch 50, and a controller or control hub 52. The fault detection sensor 48 is adapted for measuring currents between the phase connection point 40 and the inductance 38 and/or the branch connection point 18. The mechanical switch 50 is adapted for short-circuiting the phases and/or for grounding one or more phases between the phase connection point 40 and/or the inductance 38 and the branch connection point 18.

(25) In the case of converter faults, the controller 52 turns off the needed power semiconductors 22 within microseconds and brings the whole converter 14a, 14b to a safe state. For example, converter over-currents may be treated in this manner.

(26) Other faults such as arc-faults, double ground faults, and converter internal short-circuits, which may be measured with the sensor 48, may be handled by the controller 52 in the following way: After the detection of a fault, all converter cells 20 may be switched to zero voltage, for example by closing the semiconductor switch 22, which interconnects the two outputs of the unipolar cell as shown in FIG. 1. Simultaneously or after a short time period ΔT, the mechanical switch is used for short-circuiting and/or grounding the phases of the converter 14a, 14b.

(27) The time period ΔT may be controlled and/or may be set to a period of the frequency of the corresponding current, such as 20 ms.

(28) A controlled timing of the short-circuiting of the phases with respect to the switching of the converter cells to zero output may have the following advantages: A fast reaction by switching the converter cells is possible that is faster than the short-circuiting of the phases. Such an overall applied zero voltage switching on cell level may short-circuit all converter potentials and may extinguish fast any arc fault. This may be called semiconductor based protection firing. During faults, diodes of the converter cells 20 are not excessively used, which may result in a more efficient utilization of reversely conducting semiconductors such as RC-IGCTs.

(29) Simultaneously, the other protection systems 46 are informed about the fault, which also synchronously switch the converter cells 20 and short-circuit the phases correspondingly. The time periods ΔT on different sides of the converter arrangement (corresponding to different frequencies) may differ from each other.

(30) A synchronized protection may have the following advantages: A synchronous semiconductor based protection firing in the converter cells 20 may result in a homogenous distribution of the short-circuit stress over the converter branches 16. A synchronous mechanical switch firing may result in a homogenous distribution of the short-circuit stress over the converter arrangement. A synchronous grid-side and machine-side firing may protect the full converter arrangement after the first fault detection at grid or machine side.

(31) As shown in FIG. 2, every subconverter 14a, 14b is provided with an inductance 38 or paralleling reactor 38 at its AC side. When each converter arm is provided with an inductance 38 (as well as a mechanical switch 50), this may enhance the modularity of the converter arrangement. However, also a centralized inductance instead of distributed inductances 38 may be used.

(32) The inductances 38 or paralleling reactors 38 also may be used as the reactors required for bypass operation.

(33) FIG. 3 shows a further embodiment of a controller arrangement 10c, in which the subconverters 14a, 14b are only shown as abstract rectangles but also may have the components as shown in FIGS. 1 and 2. A controller 52 is used for both the active rectifiers 14a and both the inverters 14b after the DC link.

(34) FIG. 4 shows a further embodiment of a controller arrangement 10d, in which the protection systems 46 are depicted in a more abstract way. In FIGS. 4 to 7, possible paralleling configurations with indirect modular multilevel converters 12a, 12b for further embodiments of converter arrangements 10d, 10e, 10f, 10g are shown.

(35) In FIGS. 4 and 7, the converters 12a, 12b are galvanically connected via the secondary winding of a two-winding transformer 30 on the grid side. Therefore, inductances 38 are provided on the grid-side.

(36) In FIGS. 5 and 6, the converters 12a, 12b are galvanically separated either by the two-winding transformers 30 connected in parallel or by a 3-winding transformer 30′. The inductances 38 on the grid side are avoided.

(37) In FIGS. 4, 5 and 6, the converters 12a, 12b have separated DC links 17. In FIG. 7, the parallel converters 12a, 12b share a common DC link 17′. In FIG. 7, with common DC link 17′, there may be two separate ground potentials for the grid- and machine-side subconverters 14a, 14b. The embodiments of FIGS. 1 to 6 may have a single ground potential.

(38) In all embodiments shown in FIGS. 4 to 7, the converters 12a, 12b are galvanically connected at the load-side or machine side. Therefore, always inductances 38 are provided at this side.

(39) The following table shows a comparison of the configurations described above:

(40) TABLE-US-00001 Embodiments 10a to 10d Embodiment 10e Embodiment 10f Embodiment 10g Simple transformer Two transformers 30 More complex Simple transformer 30 (easier (may be preferable transformer 30′ 30 (easier production for high for very high (expectedly) production for high powers) powers) powers) Grid-side paralleling Grid-side paralleling Grid-side paralleling Grid-side paralleling inductances 38 may inductances 38 not inductances 38 not inductances 38 may be needed needed needed be needed Two DC-link lines Two DC-link lines Two DC-link lines Single DC-link line 17 for remote grid- 17 for remote grid- 17 for remote grid- 17′ for remote grid- and machine side and machine side and machine side and machine side sub converters 14a, sub converters 14a, sub converters 14a, converters 14a, 14b 14b 14b 14b No extra busbars No extra busbars No extra busbars Extra busbars between paralleled between paralleled between paralleled between paralleled converters 12a, 12b converters 12a, 12b converters 12a, 12b converters 12a, 12b for close grid- and for close grid- and for close grid- and for close grid- and machine side machine side machine side machine side converters 14a, 14b converters 14a, 14b converters 14a, 14b converters 14a, 14b

(41) FIG. 8 shows a further embodiment of a converter arrangement 10h, analogously to FIG. 1, with the main differences that the modular multilevel converter 12a is a direct converter. The direct converter 12a interconnects directly the phases V.sub.U, V.sub.V, V.sub.W of the grid-side with the phases V.sub.R, V.sub.S, V.sub.T of the load-side, wherein each phase of the grid-side is connected via a converter branch 16 with the load side. The converter cells 20′ of the direct converter 12a are bipolar cells with four semiconductor switches 22.

(42) The branch inductors 28 of the indirect converter 12a are not coupled; therefore, they may take over the function of the paralleling inductors 38. Thus, it is possible that the configurations shown in FIG. 8 to FIG. 11 only have paralleling inductances 38 on the side without the branching inductances 28.

(43) FIG. 9 to FIG. 11 show embodiments of converter arrangements 10i to 10k analogously to the converter arrangements 10d to 10f, respectively. Only the indirect converters are replaced with direct converters 12a, 12b as shown in FIG. 8.

(44) The transformer 30, 30′ and the electrical machine 34 may have different options for their windings in a multiphase configuration. In the following, possible options are listed:

(45) The high-voltage-side (primary) windings of the transformer 30, 30′ may be Y-connected and may have a neutral point connection.

(46) The winding configuration for the two-winding transformer 30 may be YN yn or YN d.

(47) The winding configuration for the three-winding transformer 30′ may be YN yn yn, YN yn d or YN d d. The winding configuration of the electrical machine may be Yn, Y or D.

(48) The following table shows a comparison for two-winding transformer 30 winding types

(49) TABLE-US-00002 YN yn YN d Worse core utilization for Better core utilization for the same step-up the same step-up ratio ratio Preferred for large Preferred for large generating unit inter-tie transformers transformers Secondary-(converter-) The converter arrangement ground potential side neutral point can should be supplied at the machine side (or serve as the converter somewhere, somehow inside the converter) system ground potential

(50) The following table shows a comparison for three-winding transformer winding types

(51) TABLE-US-00003 YN yn yn YN yn d YN d d No phase-shift Phase shifts between the No phase-shift between the between the secondary windings cancel secondary windings secondary certain harmonic current windings

(52) The following table shows a comparison for machine winding types

(53) TABLE-US-00004 Yn Y D Neutral point is available to Neutral point is unavailable. Neutral point is unavailable. serve the system ground The system ground should The converter arrangement potential be supplied by the ground should be supplied transformer (or somewhere, by the transformer (or somehow inside the somewhere, somehow inside converter the converter

(54) Based on the available winding options, there are certain possible configurations considering that the ground potential should be provided at least at the transformer side and/or the machine side.

(55) The following configurations are the favored ones for the following reasons: Transformer windings YN d, YN d d: Transformer delta connection at the MV (secondary) side is usually better for transformer core utilization in voltage step-up. Machine windings Yn: Because of the delta-connected transformer, the machine needs to serve the neutral point. Transformer windings YN d d: Since the converter currents are very high quality sinusoidal, there may be no need for enhancing them via transformer winding arrangement.

(56) While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

LIST OF REFERENCE SYMBOLS

(57) 10a to 10k converter arrangement 12a, 12b modular multilevel converters 14a, 14b subconverters 16 converter branch 17, 17′ DC link 18 branch connection point 20, 20′ converter cell 22 semiconductor switch 24 cell capacitor 26 snubber circuit 28 branch inductance 30, 30′ transformer 32 electrical grid 34 motor 36 turbine 38 inductance 40 phase connection point 42 filter 44 converter protection system 46 subconverter protection system 48 fault detection device 50 mechanical switch 52 controller