A modular multilevel converter

Abstract

The application relates to the technical field of converters and discloses a modular multilevel converter, for each phase, the first ports and the second ports of all double-port submodules are connected in series respectively, the first end of the first port connected in series is taken as the first end of each phase, the second end of the second port connected in series is taken as the second end of each phase, the second end of the first port connected in series and the first end of the second port connected in series are connected to form the third end of each phase; the first end of each phase and the first end of other phases are connected to form a point and lead out the first end of the converter, the second end of each phase and the second end of other phases are connected to form a point and lead out the second end of the converter, and the first end of the converter and the second end of the converter form the direct-current side of the converter; and the third end of each phase forms the alternating-current side of the converter. The upper bridge arm and the lower bridge arm of each phase completely reuse multiple double-port submodules, the number of conventional flexible direct-current submodules is reduced by 50%, and the volume and the cost are reduced.

Description

Technical Field

[0001] This invention relates to the field of converter technology, and more specifically to a modular multilevel converter. Background Technology

[0002] With its advantages of high voltage quality, flexible control, and easy expansion, the modular multilevel converter (MMC) has been widely applied in numerous engineering projects in China, becoming one of the mainstream technologies for converters used in high-voltage direct current (HVDC) transmission. As voltage levels and transmission capacities continue to increase, the number of series-connected submodules required by MMCs also increases, leading to higher equipment costs and larger size. Since submodules in the upper and lower arms of a conventional MMC are not used simultaneously, the submodule utilization rate is low, only 50%. Therefore, by reusing a portion of submodules in both upper and lower arms, the submodule utilization rate can be improved, the number of submodules can be reduced, and thus costs can be decreased. Summary of the Invention

[0003] Therefore, the technical problem to be solved by the present invention is to overcome the defect of low utilization rate of modular multilevel converter submodules in the prior art, thereby providing a modular multilevel converter.

[0004] To achieve the above objectives, the present invention provides the following technical solution:

[0005] This invention provides a modular multilevel converter. Each phase of the converter includes an upper arm and a lower arm. The upper and lower arms simultaneously reuse multiple two-port submodules. For each phase, the first ports of all the two-port submodules are connected in series sequentially, and the second ports of the two-port submodules are connected in series sequentially. For each phase, the first end of the series-connected first ports serves as the first end of each phase, the second end of the series-connected second ports serves as the second end of each phase, and the second end of the series-connected first ports and the first end of the series-connected second ports serve as the third end of each phase. The first end of each phase is connected to the first ends of the other phases at a single point and leads out to the first end of the converter. The second end of each phase is connected to the second ends of the other phases at a single point and leads out to the second end of the converter. The first end and the second end of the converter constitute the DC side of the converter. The third end of each phase constitutes the AC side of the converter.

[0006] In this invention, the upper and lower bridge arms of each phase fully reuse multiple dual-port sub-modules, reducing the number of sub-modules by 50% compared to conventional flexible straight-line modules, thus reducing volume and cost; the sub-module utilization rate is 100%, maximizing the use of sub-modules and improving utilization.

[0007] In one optional implementation, the modular multilevel converter is characterized by further comprising: the second end after the first port is connected in series, and the second end after the second port is connected in series, both being connected through a bridge arm reactor to serve as the third end of each phase.

[0008] In one optional embodiment, the dual-port submodule includes: a first power electronic switch, a second power electronic switch, a third power electronic switch, a fourth power electronic switch, a fifth power electronic switch, a sixth power electronic switch, and a supporting capacitor. The first end of the first power electronic switch is connected to the first end of the second power electronic switch, serving as the first end of the first port. The second end of the first power electronic switch is connected to the first end of the third power electronic switch, serving as the second end of the first port. The first end of the fourth power electronic switch is connected to the second end of the fifth power electronic switch, serving as the first end of the second port. The second end of the fourth power electronic switch is connected to the second end of the sixth power electronic switch, serving as the second end of the second port. The first end of the fifth power electronic switch is connected to the second end of the second power electronic switch and the first end of the supporting capacitor. The first end of the sixth power electronic switch is connected to the second end of the third power electronic switch and the second end of the supporting capacitor.

[0009] In one optional implementation, each power electronic switch is composed of an IGBT and a diode, wherein the collector of the IGBT is connected to the cathode of the diode, and the collector of the IGBT serves as the first terminal of the power electronic switch; the emitter of the IGBT is connected to the anode of the diode, and the emitter of the IGBT serves as the second terminal of the power electronic switch.

[0010] In one alternative implementation, for each phase, during normal operation of the converter, the output voltage of the upper arm and the output voltage of the lower arm satisfy the following relationship:

[0011]

[0012] In the formula, i represents the i-th two-port submodule, i = 1, 2, ..., N, and N is the number of submodules per phase; U um U is the output voltage of the upper arm of phase m; lm U is the output voltage of the lower bridge arm of phase m; m is phase A, phase B, or phase C; ab_i U represents the voltage at the first port of the i-th two-port submodule. cd_i Let be the voltage at the second port of the i-th two-port submodule.

[0013] In one alternative implementation, during converter modulation, the upper and lower arm output voltage reference waveforms of the two-port submodule are:

[0014]

[0015] In the formula, U um_ref U is the reference waveform for the output voltage of the upper bridge arm of phase m; lm_ref The reference waveform for the output voltage of the lower bridge arm of phase m; m is phase A, phase B, or phase C; U dc For the DC side reference of the converter; U ac This represents the amplitude of the AC phase voltage.

[0016] In one optional implementation, the dual-port submodule has two operating states. In the first operating state, the second, third, and fourth power electronic switches are turned on, while the first, fifth, and sixth power electronic switches are turned off. In the second operating state, the first, fifth, and sixth power electronic switches are turned on, while the second, third, and fourth power electronic switches are turned off.

[0017] In one optional implementation, for each phase, the number of dual-port submodules in the first operating state and the second operating state is:

[0018]

[0019] In the formula, n1 represents the number of dual-port submodules in the first operating state; n2 represents the number of dual-port submodules in the second operating state; U c To support the capacitor voltage. Attached Figure Description

[0020] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0021] Figure 1 This is a structural diagram of the modular multilevel converter in this embodiment;

[0022] Figure 2 This is a structural diagram of the dual-port submodule in this embodiment;

[0023] Figures 3(a) and 3(b) show the current flow paths of the submodules in operating state 1 and operating state 2 in this embodiment, respectively.

[0024] Figure 4 The output voltage simulation waveform of the modular multilevel converter in this embodiment; Detailed Implementation

[0025] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0027] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can also refer to the internal connection of two components; and they can refer to a wireless connection or a wired connection. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0028] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0029] This embodiment provides a modular multilevel converter, such as Figure 1 As shown, each phase of the converter includes an upper arm and a lower arm. The upper and lower arms simultaneously reuse multiple two-port submodules (i.e., SM1, SM2, ..., SM...). N ).

[0030] like Figure 1As shown, for each phase, the first ports of all the two-port submodules are connected in series sequentially, and the second ports of the two-port submodules are connected in series sequentially. For each phase, the first end of the first port after being connected in series serves as the first end of each phase, the second end of the second port after being connected in series serves as the second end of each phase, and the second end of the first port after being connected in series with the first end of the second port after being connected in series serves as the third end of each phase. The first end of each phase is connected to the first end of the other phases at a single point and leads out to the first end (P end) of the converter. The second end of each phase is connected to the second end of the other phases at a single point and leads out to the second end (N end) of the converter. The first end and the second end of the converter constitute the DC side of the converter. The third end of each phase constitutes the AC side of the converter.

[0031] Optionally, the modular multilevel converter further includes: the second terminal after the first port is connected in series, and the second terminal after the second port is connected in series, both of which are connected through a bridge arm reactor to serve as the third terminal of each phase.

[0032] Specifically, such as Figure 1 As shown, the converter has three phases: A, B, and C. Each phase has two arms, an upper arm and a lower arm. Each arm is connected in series with an arm reactor L. The midpoint of the arm is used as the AC output port. Each phase consists of N sub-modules connected in series. Each sub-module has output ports on the left and right sides. The left port is connected in series with the upper arm, and the right port is connected in series with the lower arm.

[0033] In some alternative implementations, such as Figure 2 As shown, the two-port submodule includes: a first power electronic switch, a second power electronic switch, a third power electronic switch, a fourth power electronic switch, a fifth power electronic switch, a sixth power electronic switch, and a supporting capacitor SC. Each power electronic switch consists of an IGBT and a diode. The collector of the IGBT is connected to the cathode of the diode, and the collector of the IGBT serves as the first terminal of the power electronic switch. The emitter of the IGBT is connected to the anode of the diode, and the emitter of the IGBT serves as the second terminal of the power electronic switch.

[0034] like Figure 2As shown, the first end of the first power electronic switch is connected to the first end of the second power electronic switch, serving as the first end of the first port (end A); the second end of the first power electronic switch is connected to the first end of the third power electronic switch, serving as the second end of the first port (end B); the first end of the fourth power electronic switch is connected to the second end of the fifth power electronic switch, serving as the first end of the second port (end C); the second end of the fourth power electronic switch is connected to the second end of the sixth power electronic switch, serving as the second end of the second port (end D); the first end of the fifth power electronic switch is connected to the second end of the second power electronic switch and the first end of the supporting capacitor; the first end of the sixth power electronic switch is connected to the second end of the third power electronic switch and the second end of the supporting capacitor.

[0035] Specifically, the dual-port submodule structure is as follows: Figure 2 As shown, it has two output ports, namely port AB and port CD. Each submodule consists of a capacitor SC and six power electronic switches (T1-T6) that regulate the output voltage of the two ports.

[0036] In some optional implementations, the dual-port submodule has two operating states. In the first operating state, the second, third, and fourth power electronic switches are turned on, while the first, fifth, and sixth power electronic switches are turned off. In the second operating state, the first, fifth, and sixth power electronic switches are turned on, while the second, third, and fourth power electronic switches are turned off.

[0037] Specifically, based on Figure 2 The structure shown has a complementary output voltage at the two ports of a single submodule during normal operation, i.e., when U ab =U c At that time, U cd =0, when U ab When = 0, U cd =U c , among which, U c To support the capacitor voltage, the port output voltage is regulated by the switching states of T1-T6. The output voltage state of the port and the signal of the switching device satisfy the following table.

[0038] Table 1

[0039] Running status 1 0 1 1 1 0 0 Uc 0 Running status 2 1 0 0 0 1 1 0 Uc

[0040] When the module is in running state 1, port U ab The output voltage is the voltage U of the capacitor. c Port Ucd The output voltage is 0; when the module is in operating state 2, port U ab The output voltage is 0, and the port U cd The output voltage is the voltage U of the capacitor. c The current flow paths of the submodules under operating state 1 and operating state 2 are shown in Figure 3(a) and Figure 3(b).

[0041] In some alternative implementations, for each phase, during normal operation of the converter, the upper arm output voltage (i.e., Figure 1 Middle U ua ) represents N submodule ports U ab The sum of the output voltages, while the lower bridge arm output voltage (i.e. Figure 1 Middle U la ) represents N submodule ports U cd The sum of the output voltages, i.e., the output voltage of the upper bridge arm and the output voltage of the lower bridge arm, satisfies the following relationship:

[0042]

[0043] In the formula, i represents the i-th two-port submodule, i = 1, 2, ..., N, and N is the number of submodules per phase; U um U is the output voltage of the upper arm of phase m; lm U is the output voltage of the lower bridge arm of phase m; m is phase A, phase B, or phase C; ab_i U represents the voltage at the first port of the i-th two-port submodule. cd_i Let be the voltage at the second port of the i-th two-port submodule.

[0044] In some optional implementations, to ensure that the invented multiplexed converter can output stable AC / DC voltage, the upper and lower bridge arm output voltages should be modulated according to a specific reference waveform. Therefore, during the converter modulation process, the reference waveform for the upper and lower bridge arm output voltages of the two-port submodule is:

[0045]

[0046] In the formula, U um_ref U is the reference waveform for the output voltage of the upper bridge arm of phase m; lm_ref The reference waveform for the output voltage of the lower bridge arm of phase m; m is phase A, phase B, or phase C; U dc For the DC side reference of the converter; U ac This represents the amplitude of the AC phase voltage.

[0047] To modulate the aforementioned voltage, the number of two-port submodules in the first and second operating states for each phase is as follows:

[0048]

[0049] In the formula, n1 represents the number of dual-port submodules in the first operating state; n2 represents the number of dual-port submodules in the second operating state; U c To support the capacitor voltage.

[0050] Based on equations (1) to (3), the converter is simulated. The converter can output a stable AC / DC voltage. The simulated output voltage waveform is as follows: Figure 4 As shown.

[0051] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A modular multilevel converter, characterized in that, Each phase of the converter includes an upper arm and a lower arm, wherein the upper arm and the lower arm simultaneously multiplex multiple two-port submodules, wherein, For each phase, the first ports of all the dual-port submodules are connected in series sequentially, and the second ports of the dual-port submodules are connected in series sequentially. For each phase, the first end of the first port after being connected in series serves as the first end of each phase, the second end of the second port after being connected in series serves as the second end of each phase, and the second end of the first port after being connected in series with the first end of the second port after being connected in series serves as the third end of each phase. The first end of each phase is connected to the first end of the other phases at a point and leads out the first end of the converter. The second end of each phase is connected to the second end of the other phases at a point and leads out the second end of the converter. The first end and the second end of the converter constitute the DC side of the converter. The third terminal of each phase constitutes the AC side of the converter; Also includes: The second terminal after the first port is connected in series, and the second terminal after the second port is connected in series, are both connected through a bridge arm reactor and then used as the third terminal of each phase. The dual-port submodule includes: a first power electronic switch, a second power electronic switch, a third power electronic switch, a fourth power electronic switch, a fifth power electronic switch, a sixth power electronic switch, and a supporting capacitor, wherein... The first end of the first power electronic switch is connected to the first end of the second power electronic switch, and the first end of the first power electronic switch serves as the first end of the first port. The second end of the first power electronic switch is connected to the first end of the third power electronic switch, and the second end of the first power electronic switch serves as the second end of the first port. The first end of the fourth power electronic switch is connected to the second end of the fifth power electronic switch, and the first end of the fourth power electronic switch serves as the first end of the second port. The second end of the fourth power electronic switch is connected to the second end of the sixth power electronic switch, and the second end of the fourth power electronic switch serves as the second end of the second port. The first end of the fifth power electronic switch is connected to the second end of the second power electronic switch and the first end of the supporting capacitor; The first end of the sixth power electronic switch is connected to the second end of the third power electronic switch and the second end of the supporting capacitor; During converter modulation, the reference waveforms for the upper and lower bridge arm output voltages of the dual-port submodule are: In the formula, U um_ref This is the reference waveform for the output voltage of the upper bridge arm of phase m; U lm_ref This is the reference waveform for the output voltage of the lower bridge arm of phase m; m It can be phase A, phase B, or phase C; U dc For reference on the DC side of the converter; U ac This represents the amplitude of the AC phase voltage.

2. The modular multilevel converter according to claim 1, characterized in that, Each power electronic switch consists of IGBTs and diodes, among which... The collector of the IGBT is connected to the cathode of the diode, and the collector of the IGBT serves as the first terminal of the power electronic switch. The emitter of the IGBT is connected to the anode of the diode, and the emitter of the IGBT serves as the second terminal of the power electronic switch.

3. The modular multilevel converter according to claim 1, characterized in that, For each phase, when the converter is operating normally, the output voltage of the upper arm and the output voltage of the lower arm satisfy the following relationship: In the formula, i For the first i A dual-port submodule, i =1,2,…,N, where N is the number of submodules per phase; U um For the first m Phase upper bridge arm output voltage; U lm For the first m Lower bridge arm output voltage; m It can be phase A, phase B, or phase C; U ab_i For the first i Voltage of the first port of each dual-port submodule; U cd_i For the first i The voltage at the second port of the dual-port submodule.

4. The modular multilevel converter according to claim 1, characterized in that, The dual-port submodule has two operating states, among which, In the first operating state, the second, third, and fourth power electronic switches are turned on, while the first, fifth, and sixth power electronic switches are turned off. In the second operating state, the first, fifth, and sixth power electronic switches are turned on, while the second, third, and fourth power electronic switches are turned off.

5. The modular multilevel converter according to claim 4, characterized in that, For each phase, the number of dual-port submodules in the first operating state and the second operating state is: In the formula, n 1 represents the number of dual-port submodules in the first operating state; n 2 represents the number of dual-port submodules in the second operating state; U c To support the capacitor voltage.

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