Modular multilevel cascaded converter and control method thereof

Abstract

The application belongs to the technical field of power electronics, and particularly relates to a modular multilevel series-parallel converter and a control method thereof, comprising a single-phase modular multilevel series-parallel converter and a three-phase modular multilevel series-parallel converter; wherein the single-phase modular multilevel series-parallel converter adopts a single-phase single-bridge arm topological structure or a single-phase double-bridge arm topological structure, the three-phase modular multilevel series-parallel converter adopts a three-phase single-bridge arm topological structure and a three-phase six-bridge arm topological structure, the three-phase single-bridge arm topological structure comprises cascaded bridge arms and a three-phase commutation circuit in parallel with each other; and the three-phase six-bridge arm topological structure comprises six cascaded bridge arms, each phase bridge arm comprises upper and lower cascaded bridge arms in series with each other, and the three-phase bridge arms are in parallel with each other. The application can not only retain many advantages brought by parallel connection of sub-modules, but also realize more level numbers and higher voltage grade outputs.

Description

Technical Field

[0001] This invention belongs to the field of power electronics technology, specifically relating to a modular multilevel series-parallel converter and its control method. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Modular multilevel series-parallel converters, as an extension of traditional modular multilevel converters, allow sub-modules to be connected not only in series but also in parallel. Parallel connection of sub-modules offers numerous advantages, such as sensorless voltage equalization of sub-module capacitors, bridge arm current sharing, reduced sub-module capacitor values, and improved system efficiency.

[0004] According to the inventor, existing technologies such as... Figure 1 The modular multilevel series-parallel converter shown only allows parallel connections between adjacent sub-modules. By cyclically cycling through the parallel states of all adjacent sub-modules within the same bridge arm, sensorless voltage equalization of the capacitors in all sub-modules within the same bridge arm is achieved. This means that the rated value of the capacitor voltage of each sub-module within the same bridge arm needs to remain equal. However, in order to achieve a greater number of levels and higher voltage outputs with a limited number of sub-modules, the rated values ​​of the capacitor voltages of the sub-modules within the same bridge arm need to be set according to a certain ratio, such as a geometric series with a common ratio of 2, or a Fibonacci sequence. Parallel connections between adjacent sub-modules are no longer allowed. The converter needs to have the ability to connect across modules in parallel; otherwise, the converter will lose the advantages of sensorless voltage equalization of the sub-module capacitors, bridge arm current sharing, reduced sub-module capacitor values, and improved system efficiency brought about by parallel sub-module connections. Summary of the Invention

[0005] To address the aforementioned issues, this invention proposes a modular multilevel series-parallel converter and its control method. By optimizing the submodule topology, it enables cross-module parallel connection capability, thus meeting the parallel connection requirements between submodules when the capacitor voltages are set according to a specific ratio. This modular multilevel series-parallel converter with cross-module parallel connection capability retains the numerous advantages of submodule parallel connection while achieving a greater number of voltage levels and higher voltage output levels.

[0006] According to some embodiments, the first aspect of the present invention provides a modular multilevel series-parallel converter, which adopts the following technical solution:

[0007] A modular multilevel series-parallel converter includes a single-phase modular multilevel series-parallel converter and a three-phase modular multilevel series-parallel converter; wherein, the single-phase modular multilevel series-parallel converter adopts a single-phase single-bridge-arm topology or a single-phase double-bridge-arm topology, the single-phase single-bridge-arm topology includes cascaded bridge arms connected in parallel and a single-phase commutator circuit; the single-phase double-bridge-arm topology includes two cascaded bridge arms connected in series and two DC-side capacitors connected in series, the cascaded bridge arms and the DC-side capacitors being connected in parallel;

[0008] The three-phase modular multilevel series-parallel converter adopts a three-phase single-bridge-arm topology and a three-phase six-bridge-arm topology. The three-phase single-bridge-arm topology includes cascaded bridge arms connected in parallel and a three-phase commutator circuit. The three-phase six-bridge-arm topology includes six cascaded bridge arms, each phase bridge arm including an upper cascaded bridge arm and a lower cascaded bridge arm connected in series, and the three phase bridge arms are connected in parallel.

[0009] As a further technical limitation, the cascaded bridge arm includes multiple sub-modules, and the output ports of the sub-modules are connected in sequence; the sub-modules are connected in series or in parallel according to their different working states.

[0010] Furthermore, the submodule includes four IGBT power semiconductor switches and one capacitor, or four IGBT power semiconductor switches and one battery pack.

[0011] Furthermore, the submodule includes three IGBT power semiconductor switches, four diodes, and one capacitor, or three IGBT power semiconductor switches, four diodes, and one battery pack.

[0012] As a further technical limitation, the single-phase commutator circuit includes four power semiconductor switch groups, each of which includes at least one power semiconductor switch.

[0013] Furthermore, the four power semiconductor switch groups are connected in an H-bridge configuration.

[0014] As a further technical limitation, the three-phase converter circuit includes six power semiconductor switch groups, each of which includes at least one power semiconductor switch.

[0015] According to some embodiments, the second aspect of the present invention provides a control method for a modular multilevel series-parallel converter, which adopts the modular multilevel series-parallel converter in the first aspect, and employs the following technical solution:

[0016] A control method for a modular multilevel series-parallel converter includes:

[0017] Determine whether the modular multilevel series-parallel converter contains a single-phase commutator circuit or a three-phase commutator circuit.

[0018] If a single-phase or three-phase converter circuit exists, the turn-on and turn-off logic sequence of the power semiconductor switch group in the converter circuit is determined based on the AC output port voltage in the modular multilevel series-parallel converter, thereby realizing the control of the modular multilevel series-parallel converter.

[0019] As a further technical limitation, based on Kirchhoff's voltage law, the relationship between the AC output port voltage and the cascaded bridge arm voltage is analyzed, and the cascaded bridge arm voltage is solved based on the AC output port voltage to obtain the modulation signal of the cascaded bridge arm.

[0020] As a further technical limitation, the real-time voltage level of the cascaded bridge arm is determined by using the nearest level approximation or multi-carrier modulation method. Combined with the setting ratio of capacitor voltage between sub-modules in the cascaded bridge arm, the connection status between sub-modules is determined, and the trigger pulse of the power semiconductor switch in each sub-module is determined.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] 1. The modular multilevel series-parallel converter with cross-module parallel connection capability in this invention expands the parallel connection range of sub-modules in traditional modular multilevel series-parallel converters. It can not only realize parallel connection between adjacent sub-modules, but also realize parallel connection between sub-modules across one or more sub-modules. This means that the voltage of the sub-module capacitors does not have to be kept equal, but can be set according to a specific ratio to achieve more levels and higher voltage output with a limited number of sub-modules. At the same time, it retains the advantages of sensorless voltage equalization of sub-module capacitors, bridge arm current sharing, reduced sub-module capacitor value and improved system efficiency brought about by the parallel connection of sub-modules.

[0023] 2. The control method for modular multilevel series-parallel converters with cross-module parallel connection capability in this invention only requires the detection of electrical quantities such as voltage and current at the AC output port, with less closed-loop control and simpler implementation. At the same time, it can meet the control requirements of different topologies of modular multilevel series-parallel converters with cross-module parallel connection capability. Attached Figure Description

[0024] The accompanying drawings, which form part of this embodiment, are used to provide a further understanding of this embodiment. The illustrative embodiments and their descriptions are used to explain this embodiment and do not constitute an improper limitation of this embodiment.

[0025] Figure 1 This is a schematic diagram of the traditional half-bridge submodule topology in this invention;

[0026] Figure 2 This is a schematic diagram of the double half-bridge submodule topology in Embodiment 1 of the present invention, which only allows parallel connections between adjacent submodules.

[0027] Figure 3 This is a schematic diagram of the submodule port connection of a bidirectional power electronic switch using reverse series power semiconductor switches in Embodiment 1 of the present invention.

[0028] Figure 4 This is a schematic diagram of the port connection of a submodule using a diode bridge bidirectional power electronic switch in Embodiment 1 of the present invention;

[0029] Figure 5 This is a schematic diagram of the submodule port connection of a bidirectional power electronic switch using reverse series power semiconductor switches in Embodiment 1 of the present invention.

[0030] Figure 6 This is a schematic diagram of the port connection of a submodule using a diode bridge bidirectional power electronic switch in Embodiment 1 of the present invention;

[0031] Figure 7 This is a schematic diagram of the current flow path when adjacent sub-modules are connected in parallel with the current flowing in the positive direction (from left to right) in Embodiment 1 of the present invention.

[0032] Figure 8 This is a schematic diagram of the current flow path when adjacent sub-modules are connected in parallel with negative current (flowing out from right to left) in Embodiment 1 of the present invention.

[0033] Figure 9 This is a schematic diagram of the current flow path when the current flows in the positive direction (from left to right) between sub-modules in Embodiment 1 of the present invention.

[0034] Figure 10 This is a schematic diagram of the current flow path when sub-modules are connected in series with the current flowing in the negative direction (from right to left) in Embodiment 1 of the present invention.

[0035] Figure 11 This is a schematic diagram of the current flow path when the current flows in parallel across a submodule in the positive direction (flowing in from left to right) as shown in Embodiment 1 of the present invention.

[0036] Figure 12 This is a schematic diagram of the current flow path when the current flows in the negative direction (from right to left) across a sub-module in parallel connection according to Embodiment 1 of the present invention;

[0037] Figure 13 This is a schematic diagram of the basic cascaded bridge arm topology in Embodiment 1 of the present invention;

[0038] Figure 14 This is a schematic diagram of a single-bridge arm topology with a commutation circuit in Embodiment 1 of the present invention;

[0039] Figure 15 This is a schematic diagram of a dual-bridge topology without a commutation circuit in Embodiment 1 of the present invention;

[0040] Figure 16 This is a schematic diagram of a single-bridge arm topology with a commutation circuit in Embodiment 1 of the present invention;

[0041] Figure 17 This is a schematic diagram of the six-arm bridge topology without commutation circuit in Embodiment 1 of the present invention;

[0042] Figure 18 This is a schematic diagram of the single-phase AC output port voltage waveform in Embodiment 2 of the present invention;

[0043] Figure 19 This is a schematic diagram of the line voltage waveform at the three-phase AC output port in Embodiment 2 of the present invention;

[0044] Figure 20 This is a voltage step waveform diagram of the cascaded bridge arm of a single-phase modular multilevel series-parallel converter with cross-module parallel connection capability in Embodiment 2 of the present invention.

[0045] Figure 21 This is a waveform diagram of the capacitor of a submodule of a single-phase modular multilevel series-parallel converter with cross-module parallel connection capability in Embodiment 2 of the present invention.

[0046] Figure 22 This is an AC output voltage waveform diagram of a single-phase modular multilevel series-parallel converter with cross-module parallel connection capability in Embodiment 2 of the present invention.

[0047] Figure 23 This is a current waveform diagram of a single-phase modular multilevel series-parallel converter with cross-module parallel connection capability in Embodiment 2 of the present invention. Detailed Implementation

[0048] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0049] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0050] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0051] In this invention, terms such as "upper," "lower," "left," "right," "front," "back," "vertical," "horizontal," "side," and "bottom" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are used only to facilitate the description of the structural relationships of the various components or elements of this invention and do not specifically refer to any component or element in this invention. They should not be construed as limiting the invention.

[0052] In this invention, terms such as "fixed connection," "connected," and "linked" should be interpreted broadly, indicating a fixed connection, an integral connection, or a detachable connection; a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can determine the specific meaning of these terms in this invention based on the specific circumstances, and they should not be construed as limitations on the invention.

[0053] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0054] Example

[0055] Embodiment 1 of this invention introduces a modular multilevel series-parallel converter.

[0056] like Figure 14 The single-phase modular multilevel series-parallel converter shown has cross-module parallel connection capability, including one cascaded bridge arm (600) and a single-phase commutator circuit; as shown Figure 13 As shown, each cascaded bridge arm consists of multiple sub-modules (300 or 310); as Figure 3 and Figure 5 As shown, each submodule (300) consists of four IGBT power semiconductor switches (301, 302, 303, 304) and one capacitor (305) or battery pack (306); Figure 4 and Figure 6 As shown, the submodule (310) consists of three IGBT power semiconductor switches (311, 312, 313), four diodes (314), and one capacitor (315) or battery pack (316). The submodule (300 or 310) is connected in parallel with a conventional dual half-bridge submodule (such as...). Figure 2Compared to the previous model, the required number of power semiconductor switching devices is similar, but it has the ability to be connected in parallel across modules. The single-phase commutator circuit consists of four power semiconductor switch groups (701, 702, 703, 704), and each power semiconductor switch group consists of at least one power semiconductor switch connected in series (705) to meet the voltage level requirements. The four power semiconductor switch groups are connected in an H-bridge structure and lead out AC output ports (706, 707).

[0057] Figure 7 and Figure 8 These represent the current flow paths of the sub-modules when the current is in the positive and negative directions, respectively, and when adjacent sub-modules are connected in parallel. Figure 9 and Figure 10 These represent the current flow paths of the submodules when they are connected in series, indicating the positive and negative directions of the current, respectively. Figure 11 and Figure 12 These represent the current flow paths of a submodule when the current is connected in parallel across a submodule, indicating the positive and negative directions, respectively.

[0058] like Figure 15 The single-phase modular multilevel series-parallel converter shown has the capability of cross-module parallel connection, including two cascaded bridge arms (600) and two DC-side capacitors (711, 712); AC output ports (713, 714) are led out from the DC-side capacitor connection point and the cascaded bridge arm connection point, respectively; the DC-side capacitors are connected in parallel with the cascaded bridge arms and lead out DC ports (715, 716).

[0059] like Figure 16 The diagram shows a three-phase modular multilevel series-parallel converter with cross-module parallel connection capability, including one cascaded bridge arm (600) and a three-phase converter circuit; the three-phase converter circuit includes six power semiconductor switch groups (801, 802, 803, 804, 805, 806), each power semiconductor group is composed of at least one power semiconductor switch connected in series (807), and the three-phase AC output ports (808, 809, 8010) are led out from the three-phase converter circuit.

[0060] like Figure 17 The three-phase modular multilevel series-parallel converter shown has cross-module parallel connection capability, including 6 cascaded bridge arms (600); each phase consists of upper and lower cascaded bridge arms; the three-phase AC output ports (813, 814, 815) are respectively led out from the connection points of the upper and lower cascaded bridge arms of each phase; and the three phases are connected in parallel and then led out to DC ports (811, 812).

[0061] Example 2

[0062] Embodiment 2 of the present invention introduces a control method for a modular multilevel series-parallel converter based on the modular multilevel series-parallel converter described in Embodiment 1.

[0063] A control method for a modular multilevel series-parallel converter includes the following steps:

[0064] For a modular series-parallel converter containing a commutation circuit, the turn-on and turn-off logic sequence of the semiconductor switching groups in the commutation circuit is first determined based on the AC output port voltage. For example... Figure 18 As shown, during the positive half-cycle of the AC output voltage, the power semiconductor switch group (701, 704) is turned on, and the power semiconductor switch group (702, 703) is turned off; during the negative half-cycle of the AC output voltage, the power semiconductor switch group (702, 703) is turned on, and the power semiconductor switch group (701, 704) is turned off; as Figure 19 As shown, in AC output voltage sector I, power semiconductor switch groups (801, 804) are turned on, and the remaining power semiconductor switch groups (802, 803, 805, 806) are turned off; in AC output voltage sector II, power semiconductor switch groups (801, 806) are turned on, and the remaining power semiconductor switch groups (802, 803, 804, 805) are turned off; in AC output voltage sector III, power semiconductor switch groups (803, 806) are turned on, and the remaining power semiconductor switch groups (801, 802, 804, 805) are turned off. In AC output voltage sector IV, power semiconductor switch groups (802, 803) are turned on, and the remaining power semiconductor switch groups (801, 804, 805, 806) are turned off; in AC output voltage sector V, power semiconductor switch groups (802, 805) are turned on, and the remaining power semiconductor switch groups (801, 803, 804, 806) are turned off; in AC output voltage sector VI, power semiconductor switch groups (804, 805) are turned on, and the remaining power semiconductor switch groups (801, 802, 803, 806) are turned off.

[0065] According to Kirchhoff's voltage law, the voltage of each cascaded bridge arm is calculated from the AC output port voltage. This normalized voltage is then used as the modulation signal for the cascaded bridge arm. Controlling the voltage of the cascaded bridge arm controls the AC output port voltage. Assume the AC output voltage of a single-phase modular series-parallel converter with cross-module parallel connection capability is:

[0066] u o =U o sin(wt)

[0067] Then for Figure 14 The cascaded bridge arm voltages in the bridge can be expressed as:

[0068] u arm =U o|sin(wt)|

[0069] for Figure 15 The voltages of the upper and lower cascaded bridge arms in the above can be expressed as:

[0070]

[0071]

[0072] In the formula U dc This represents the DC-side voltage. Assume the AC output voltage of a three-phase modular series-parallel converter with cross-module parallel connection capability is:

[0073] u a =U o sin(wt)

[0074]

[0075]

[0076] Then for Figure 16 The cascaded bridge arm voltages in the bridge can be expressed as:

[0077] u arm =max{u a ,u b ,u c}-min{u a ,u b ,u c}

[0078] for Figure 17 The voltages of the upper and lower cascaded bridge arms of phase A can be expressed as:

[0079]

[0080]

[0081] After obtaining the modulation signal of the cascaded bridge arm, the real-time voltage level of the cascaded bridge arm is determined using the nearest-level approximation or multi-carrier modulation method. Combined with the proportional setting of the capacitor voltages between sub-modules in the cascaded bridge arm, the connection status between sub-modules is determined, and thus the specific trigger pulse for the power semiconductor switch in each sub-module is determined. Figure 14 Taking the single-phase modular multilevel series-parallel converter as an example, the number of sub-modules is set to 4, and the capacitor voltages between the sub-modules are set according to the Fibonacci sequence, which are 1:1:2:3 respectively. Figure 20 , Figure 21 , Figure 22 and Figure 23 The waveforms of the cascaded bridge arm voltage steps, the submodule capacitor voltage, and the AC output voltage and current are shown respectively.

[0082] This embodiment proposes a control method for a modular multilevel series-parallel converter. By optimizing the submodule topology, it enables cross-module parallel connection capability, thus meeting the parallel connection requirements between submodules when the capacitor voltages of the submodules are set according to a specific ratio. Compared with traditional modular series-parallel converters, this invention retains the many advantages of submodule parallel connection while achieving a greater number of voltage levels and higher voltage output levels.

[0083] The above description is merely a preferred embodiment of this practice and is not intended to limit the scope of this practice. Various modifications and variations can be made to this practice by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this practice should be included within the protection scope of this practice.

Claims

1. A modular multilevel series-parallel converter, characterized in that, The system includes single-phase modular multilevel series-parallel converters and three-phase modular multilevel series-parallel converters. The single-phase modular multilevel series-parallel converter adopts a single-phase single-bridge-arm topology or a single-phase double-bridge-arm topology. The single-phase single-bridge-arm topology includes cascaded bridge arms connected in parallel and a single-phase commutator circuit. The single-phase double-bridge-arm topology includes two cascaded bridge arms connected in series and two DC-side capacitors connected in series, with the cascaded bridge arms and the DC-side capacitors connected in parallel. The three-phase modular multilevel series-parallel converter adopts a three-phase single-bridge-arm topology and a three-phase six-bridge-arm topology. The three-phase single-bridge-arm topology includes cascaded bridge arms connected in parallel and a three-phase commutator circuit. The three-phase six-bridge-arm topology includes six cascaded bridge arms. Each phase bridge arm includes an upper cascaded bridge arm and a lower cascaded bridge arm connected in series. The three phase bridge arms are connected in parallel. The cascaded bridge arm includes multiple sub-modules, and the output ports of the sub-modules are connected sequentially. The sub-modules are connected in series or in parallel according to their different operating states. Furthermore, each sub-module includes four IGBT power semiconductor switches and one capacitor, or four IGBT power semiconductor switches and one battery pack.

2. A modular multilevel series-parallel converter as described in claim 1, characterized in that, The submodule includes three IGBT power semiconductor switches, four diodes and one capacitor, or three IGBT power semiconductor switches, four diodes and one battery pack.

3. A modular multilevel series-parallel converter as described in claim 1, characterized in that, The single-phase commutator circuit includes four power semiconductor switch groups, and each power semiconductor switch group includes at least one power semiconductor switch.

4. A modular multilevel series-parallel converter as described in claim 3, characterized in that, The four power semiconductor switch groups are connected in an H-bridge configuration.

5. A modular multilevel series-parallel converter as described in claim 1, characterized in that, The three-phase converter circuit includes six power semiconductor switch groups, each of which includes at least one power semiconductor switch.

6. A control method for a modular multilevel series-parallel converter, employing the modular multilevel series-parallel converter as described in any one of claims 1-5, characterized in that, include: Determine whether the modular multilevel series-parallel converter contains a single-phase commutator circuit or a three-phase commutator circuit. If a single-phase or three-phase converter circuit exists, the turn-on and turn-off logic sequence of the power semiconductor switch group in the converter circuit is determined based on the AC output port voltage in the modular multilevel series-parallel converter, thereby realizing the control of the modular multilevel series-parallel converter.

7. The control method for a modular multilevel series-parallel converter as described in claim 6, characterized in that, Based on Kirchhoff's voltage law, the relationship between the AC output port voltage and the cascaded bridge arm voltage is analyzed. The cascaded bridge arm voltage is then calculated based on the AC output port voltage to obtain the modulation signal of the cascaded bridge arm.

8. The control method for a modular multilevel series-parallel converter as described in claim 6, characterized in that, By employing the nearest-level approximation or multi-carrier modulation method, the real-time voltage levels of the cascaded bridge arms are determined. Combined with the setting ratio of capacitor voltages between sub-modules in the cascaded bridge arms, the connection status between sub-modules is determined, and the trigger pulse of the power semiconductor switch in each sub-module is determined.

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