Self-voltage-sharing SiC efficient utilization type high-density MMC and control method

Abstract

The invention provides a self-voltage-sharing SiC efficient utilization type high-density MMC and a control method. The MMC comprises a plurality of heterogeneous parallel half-bridge sub-modules which are connected end to end according to branches of the same type. The heterogeneous parallel half-bridge sub-module comprises a SiC high-frequency branch, a Si low-frequency branch and a sub-module capacitor; si low-frequency branches are arranged at two ends of a bridge arm of the MMC, and the SiC high-frequency branches and the Si low-frequency branches between adjacent sub-modules respectively form a SiC high-frequency branch group and a Si low-frequency branch group. According to the MMC provided by the invention, through topological innovation, a collaborative optimization mechanism of'self-voltage-sharing-high efficiency 'is constructed, and a new technical path is provided for engineering application of a middle-high voltage MMC converter.

Description

Technical Field

[0001] This application belongs to the field of voltage converters, and in particular relates to a self-equalizing SiC high-efficiency utilization high-density MMC and its control method. Background Technology

[0002] Modular multilevel converters (MMCs) are widely used in flexible DC transmission and renewable energy grid integration due to their advantages such as low output harmonic content, high modularity, and strong scalability. Parallel MMCs (PMMCs) exhibit significant advantages over traditional series MMCs in terms of capacitor self-balancing characteristics and capacitor ripple suppression: their capacitor voltage is less affected by the load, enabling capacitor self-balancing without the need for additional voltage equalization control strategies. Therefore, parallel MMC topologies are extensively studied.

[0003] Despite the superior performance of parallel MMCs in terms of voltage support and waveform quality, they still face challenges such as structural complexity and high losses. To address these challenges, research primarily focuses on topology improvement and modulation optimization. At the topology level, some researchers have proposed dual H-bridge submodules that support both series and parallel operation of positive and negative transistors, reducing conduction losses through parallel branches. Building upon this, some researchers have replaced a pair of switches with diodes to achieve low-cost unidirectional parallel operation. Other works have proposed simplifying series-parallel submodules, reducing the number of transistors while retaining the ability to output negative levels, thus improving device utilization. Still other schemes utilize diodes and inductors to construct capacitor balancing paths, effectively reducing the number of components and control complexity. Regarding modulation strategies, research focuses on optimizing level synthesis logic and switching sequences, such as minimizing equivalent resistance and conduction losses by evenly distributing the number of parallel modules, or using low-frequency scheduling algorithms to collaboratively optimize switching frequency and losses, balancing conduction and switching losses. However, existing methods are mostly limited to single-dimensional optimization, making it difficult to systematically solve the high switching losses caused by the forced increase in switching frequency to maintain waveform quality in medium-voltage, low-submodule scenarios.

[0004] Furthermore, current research on hybrid devices mainly focuses on traditional series-type MMCs, while systematic exploration of parallel structures remains insufficient, and an integrated solution that takes into account heterogeneous device synergy, capacitor voltage balance, and efficient operation has not yet been formed. Summary of the Invention

[0005] This application provides a self-equalizing SiC high-efficiency utilization type high-density MMC and a control method. To solve the above-mentioned technical problems, this application adopts the following technical method: In a first aspect, this application provides a self-equalizing SiC high-efficiency utilization type high-density MMC, comprising: The MMC includes multiple heterogeneous parallel half-bridge sub-modules connected end to end according to the same type of branches; The heterogeneous parallel half-bridge submodule includes a SiC high-frequency branch, a Si low-frequency branch, and a submodule capacitor. Both ends of the bridge arm of the MMC are Si low-frequency branches, and the SiC high-frequency branches and Si low-frequency branches between adjacent sub-modules form SiC high-frequency branch groups and Si low-frequency branch groups, respectively.

[0006] Optionally, the SiC high-frequency branch includes a first SiC MOSFET and a second SiC MOSFET, and the Si low-frequency branch includes a first Si IGBT and a second Si IGBT. The source of the first SiC MOSFET is connected to the drain of the second SiC MOSFET, the emitter of the first SiIGBT is connected to the collector of the second Si IGBT, the emitter of the second Si IGBT and the source of the second SiC MOSFET are both connected to the negative terminal of the submodule capacitor, and the drain of the first SiC MOSFET and the collector of the first Si IGBT are both connected to the positive terminal of the submodule capacitor.

[0007] Secondly, this application also provides a control method for self-equalizing SiC-efficient high-density MMC, applied to the MMC described in the first aspect, comprising: The output reference voltage is obtained by adopting dual closed-loop feedforward decoupling control; Based on the output reference voltage, determine the number of stepped voltage levels and the compensation PWM voltage; The operating mode of the bridge arm is determined by the number of stepped voltage levels and the compensation PWM voltage input rotating pulse distribution circuit, and corresponding switching signals are generated to drive the bridge arm to output the target reference voltage.

[0008] Optionally, determining the number of stepped voltage levels and the compensation PWM voltage based on the output reference voltage includes: Based on the output reference voltage, the upper bridge arm output reference voltage and the lower bridge arm output reference voltage are determined; The upper bridge arm output reference voltage and the lower bridge arm output reference voltage are modulated by the nearest level pulse width modulation to generate bridge arm output stepped wave voltage and shaping voltage. Based on the stepped wave voltage output by the bridge arm, determine the number of stepped voltage levels; The shaped voltage is compared with the carrier wave to obtain the compensated PWM voltage.

[0009] Optionally, the step of determining the operating mode of the bridge arm and generating a corresponding switching signal by allocating the stepped voltage level number and the compensated PWM voltage input rotating pulse distribution circuit to drive the bridge arm to output the target reference voltage includes: Based on the number of stepped voltage levels, determine whether the bridge arm operating state contains a transition state; If so, determine whether the signal of the compensation PWM voltage is high. If so, increment the current mode number by 1; Based on the adjusted current mode number, the operating mode of the bridge arm is determined and a corresponding switching signal is generated to drive the bridge arm to output the target reference voltage.

[0010] Optionally, the transition state includes a first transition mode and a second transition mode; the first transition mode specifically includes: the first Si branch operates in bypass mode 1, the last Si branch operates in bypass mode 2, and all Si branch groups operate in parallel mode; a first number of SiC branch groups operate in series mode, and the remaining SiC branch groups operate in parallel mode. The second transition mode is as follows: the first Si branch operates in bypass mode 2, the last Si branch operates in bypass mode 1, the Si branch group remains in parallel and does not operate, the first number of SiC branch groups operate in series, and the remaining SiC branch groups operate in parallel.

[0011] Optionally, the process for determining the first quantity is as follows: Determine the first quantity based on the current mode number and the number of step voltage levels.

[0012] This application has the following beneficial effects: The MMC proposed in this application constructs a collaborative optimization mechanism of "self-equalizing voltage and high efficiency" through topological innovation, providing a new technical path for the engineering application of medium and high voltage MMC converters. Attached Figure Description

[0013] Figure 1 A topology diagram of a self-equalizing SiC high-efficiency high-density MMC provided in an embodiment of this application; Figure 2 A topology diagram of a heterogeneous parallel half-bridge submodule provided in an embodiment of this application; Figure 3 A flowchart illustrating a self-equalizing SiC high-density MMC control method provided in this application embodiment; Figure 4 This is a schematic diagram illustrating the process of generating bridge arm voltage as provided in an embodiment of this application. Figure 5 A schematic diagram of the bridge arm voltage generation process provided in this application, showing the operation mode of the inter-module branch group and its equivalent circuit diagram. Figure 5 (a) is the equivalent circuit diagram of the parallel mode; Figure 5 (b) is the equivalent circuit diagram in series mode; Figure 5 (c) is the equivalent circuit diagram of bypass mode 1; Figure 5 (d) is the equivalent circuit diagram of bypass mode 2; Figure 6 The embodiments provided in this application are as follows A schematic diagram of the bridge arm's operating modes; Figure 7 The embodiments provided in this application are as follows A schematic diagram of the bridge arm's operating modes; Figure 8 The embodiments provided in this application are as follows A schematic diagram of the bridge arm's operating modes; Figure 9 A schematic diagram of the allocation process in the rotation purchase provided in an embodiment of this application; Figure 10 The bridge arm voltage waveform diagram provided in the embodiments of this application; Figure 11 The output current waveform diagram provided for the embodiments of this application; Figure 12 This is a waveform diagram of the capacitor voltage of a submodule provided in an embodiment of this application; Figure 13 The following is a waveform diagram of the switching signal of the Si / SiC device provided in the embodiments of this application; Figure 13 (a) is a waveform diagram of the switching signal of the SiC device; Figure 13 (b) shows the switching signal waveform of the Si device; Figure 14 The modulation scheme bridge arm voltage waveform diagram provided in the embodiments of this application; Figure 15 The output current waveform diagram of the modulation system provided in the embodiments of this application; Figure 16 The waveform diagram of the capacitor voltage of the modulation system submodule provided in this application embodiment. Detailed Implementation

[0014] To facilitate understanding by those skilled in the art, the present application will be further described below in conjunction with embodiments and accompanying drawings. The content mentioned in the embodiments is not intended to limit the present application.

[0015] To solve the above technical problems, such as Figure 1 As shown, this application proposes a self-voltage-balanced high-efficiency SiC MMC (SHSMMC), comprising: MMC consists of multiple heterogeneous parallel half-bridge sub-modules connected end-to-end according to the same type of branches, namely SM1~SM N .

[0016] Figure 1 middle For bridge arm filter inductance; DC side voltage, , ; The output current is given. In the proposed topology, both ends of the bridge arm are Si low-frequency branches; the SiC high-frequency branches and Si low-frequency branches between adjacent submodules form SiC high-frequency branch groups and Si low-frequency branch groups, respectively.

[0017] like Figure 2 As shown, the heterogeneous parallel half-bridge submodule includes a SiC high-frequency branch, a Si low-frequency branch, and a submodule capacitor C. H .

[0018] The SiC high-frequency branch includes a first SiC MOSFET S3 and a second SiC MOSFET S4, and the Si low-frequency branch includes a first Si IGBT S1 and a second Si IGBT S2. The source of the first SiC MOSFET is connected to the drain of the second SiC MOSFET, the emitter of the first Si IGBT is connected to the collector of the second Si IGBT, the emitter of the second Si IGBT and the source of the second SiC MOSFET are both connected to the negative terminal of the submodule capacitor, and the drain of the first SiC MOSFET and the collector of the first Si IGBT are both connected to the positive terminal of the submodule capacitor.

[0019] Based on the above topology, such as Figure 3 As shown, this application proposes a control method for self-equalizing SiC-efficient high-density MMC, including: Step S201: Use dual closed-loop feedforward decoupling control to obtain the output reference voltage; A dual-closed-loop feedforward decoupling control is employed as the core of power control to obtain the output reference voltage. .

[0020] Step S202: Based on the output reference voltage, determine the number of stepped voltage levels and the compensation PWM voltage; like Figure 4 As shown, for the output reference voltage By performing calculations, the upper bridge arm output reference voltage can be obtained. Lower bridge arm output reference voltage Then the upper bridge arm outputs the reference voltage. Lower bridge arm output reference voltage Nearest Level Pulse Width Modulation (NL-PWM) is used to generate a stepped waveform voltage at the bridge arm output, and simultaneously, a shaping voltage is generated to compensate for the sinusoidal reference voltage of the bridge arm. The shaping voltage is compared with the carrier wave to obtain the compensation PWM voltage. After obtaining the stepped waveform voltage at the bridge arm output, the number of stepped voltage levels can be determined.

[0021] Step S203: Using the number of stepped voltage levels and the compensation PWM voltage input rotating pulse distribution circuit, determine the operating mode of the bridge arm and generate corresponding switching signals to drive the bridge arm to output the target reference voltage.

[0022] Since this step involves the operating modes of the bridge arm, the operating modes of the bridge arm are explained in detail here for ease of understanding: The operating modes and equivalent circuits of inter-module branch groups are as follows: Figure 5 As shown. Figure 5 (a) shows a branch group with the following characteristics: Figure 5 (a) shows the parallel connection mode, such as Figure 5 (b) shows the cascading mode, such as Figure 5 (c) shows bypass mode 1 and as shown in the figure Figure 5 (d) shows four typical operating states of bypass mode 2. Among them, the parallel mode achieves self-balancing of capacitor voltage by connecting the capacitors on both sides of the parallel branch group; the series mode connects the capacitors on both sides of the branch group in series to the bridge arm to achieve voltage superposition output; bypass mode 1 and bypass mode 2 achieve submodule capacitor bypass by connecting the positive and negative sides of the capacitors, respectively.

[0023] In a parallel MMC, all submodule capacitors are always operational during operation, and the arm voltage depends on the operating state of each branch group. Therefore, based on the proposed topology, the arm voltage is characterized by the operating state of the SiC branch group and the Si branch group.

[0024] For an SHSMMC with N submodule capacitors, the bridge arm contains N / 2 SiC branch groups, N / 2-1 Si branch groups, and one Si branch at each end of the bridge arm. Define the first Si branch in the bridge arm... indivual( The state function of the SiC branch group ≤N / 2) is: (1) Define the first arm of the bridge indivual( The state function of the Si branch group (≤N / 2-1) is: (2) Define the state functions of the Si branches at both ends of the bridge arm as follows: (3) Its bridge arm output voltage can be expressed as: (4) In the formula, This represents the capacitor voltage of the submodule.

[0025] In the proposed heterogeneous parallel submodule, half of the branches use Si IGBTs and the other half use SiC MOSFETs. To fully utilize the low switching losses of SiC devices, this application proposes a specific heterogeneous branch group operating mode selection scheme. By concentrating most of the switching operations on the SiC branches without changing the output voltage of the bridge arm, a comprehensive optimization of efficiency and cost is achieved. Therefore, the state function in equation (4) is set... , The values ​​of are shown in Table 1.

[0026] Table 1 State Functions , The value of ; As shown in Table 1, the Si branch only operates twice in one cycle. Specifically, the first and last branches of the bridge arm only operate twice. When the Si branch group operates, it only operates when... Actions occur at that time.

[0027] When the bridge arm output voltage At this time, the number of SiC branch groups in series in the bridge arm for: (5) To minimize the number of switching operations in the SiC branch group, the mode selection schemes for the proposed SHSMMC topology include the following: 1) When At this time, the first branch operates in bypass mode 1, the last branch operates in bypass mode 2, and all Si branch groups operate in parallel mode, that is... , ,like Figure 6 As shown. When the bridge arm output voltage is At that time, the first arm of the bridge Modality Defined as the first One to the first One (i.e., the first number) SiC branch groups operate in series, while the remaining SiC branch groups operate in parallel.

[0028] 2) When At this time, the bridge arm is in the first transitional mode (Mode 1): the first branch operates in bypass mode 2, the last branch operates in bypass mode 1, and the Si branch group remains in parallel and does not operate. , ,like Figure 7 As shown. When the bridge arm output voltage is At that time, the first arm of the bridge Modality Defined as the first One to the first The first number of SiC branch groups operate in series, while the remaining SiC branch groups operate in parallel. Furthermore, to avoid the first and last Si branches operating at high frequencies, the bridge arms can be switched via SiC branch switching. and Switching between them.

[0029] 3) When At this time, the bridge arm is in the second transition mode of the transition state. The first branch operates in bypass mode 2, the last branch operates in bypass mode 1, and all Si branch groups change from parallel mode to series mode, i.e. , ,like Figure 8 As shown. When the output At level, it is obtained from equation (5) When the bridge arm output voltage is At that time, the first arm of the bridge Modality Defined as the first One to the first The first number of SiC branch groups operate in series, while the remaining SiC branch groups operate in parallel. Similarly, to avoid the SiC branch groups operating at high frequencies, the bridge arms can switch voltage levels via SiC branch switching. and Switching between them.

[0030] The above operating mode allows each level to correspond to N / 2 modes. Each mode can effectively suppress voltage deviation between capacitors by rotating and connecting different capacitors in parallel.

[0031] The number of stepped voltage levels determined in the above steps is combined with the compensation PWM voltage input rotating pulse distribution stage to determine the operating mode of the bridge arm and generate corresponding switching signals to drive the bridge arm to output the target reference voltage. The specific process is as follows: The control process of the alternating pulse distribution stage is as follows: Figure 9As shown, the system first determines whether the bridge arm's operating state still has transitional states (i.e., the first transitional mode Mode1 and the second transitional mode ModeN / 2) based on the number of stepped voltage levels M. If the bridge arm's operating state contains transitional modes Mode1 and ModeN / 2, the mode number k is incremented by 1 when the PWM signal is high, and returns to 1 when k exceeds the total number of modes N / 2. If the bridge arm operates without transitional states, the mode number k is incremented by 1 when the PWM signal is low, and returns to 1 when k exceeds the total number of modes N / 2. The generated bridge arm mode transition sequence under these two conditions determines the bridge arm's operating state.

[0032] Simulation analysis: To better demonstrate the effectiveness of the topology and voltage equalization control method proposed in this patent application, the following verification is conducted using simulation examples. Figure 1 The topology shown is used to build a simulation platform in MATLAB / Simulink, and the simulation parameters are shown in Table 2.

[0033] Table 2 Main Simulation Parameters ; The MMC bridge arm normally outputs an 11-level PWM waveform, as shown below. Figure 10 As shown. The AC side output current is as follows. Figure 11 As shown, the feasibility of the proposed self-equalizing SiC high-efficiency MMC topology is demonstrated. Figure 12 The waveform of the capacitor voltage shows that multiple capacitor voltages are momentarily equal. Without the need for real-time capacitor voltage information, the difference in capacitor voltage can be avoided, thus verifying the self-balancing characteristic of capacitor voltage. Figure 13 This is the switch signal diagram for the branch group. Figure 13 (a) is a diagram of the switching signals for SiC. Figure 13 (b) shows the switching signal diagram for the Si device. From... Figure 13 The comparison shows that the proposed control method enables high-frequency switching actions to be concentrated on the SiC device, while the Si switch group only operates twice in one cycle.

[0034] To verify the stability of the proposed topology, this application further conducted transient experiments with the modulation index varying from 0.9 to 0.5. The bridge arm output voltage and AC side output current are as follows: Figure 14 , 15 As shown. Both the bridge arm voltage and output current achieved self-stabilization after the modulation index was changed. The sub-mode capacitor voltage is as follows. Figure 16 As shown in the figure, the voltage between capacitors still exhibits self-balancing characteristics after the modulation index changes. It is worth noting that, due to the reduced output voltage of the bridge arms under lower modulation indexes, the bridge arms operate in the mode of multiple capacitors in parallel for a longer period, resulting in a better self-balancing effect in the voltage between capacitors.

[0035] In summary, the MMC proposed in this application integrates the voltage self-balancing characteristics of a parallel structure with the loss optimization advantages of a heterogeneous hybrid strategy. While achieving autonomous voltage balancing of submodules without requiring additional voltage equalization control strategies, it reduces the switching losses of the entire topology through the synergistic operation modes of heterogeneous devices. This application, through topological innovation, constructs a collaborative optimization mechanism of "self-voltage equalization - high efficiency," providing a new technical path for the engineering application of medium- and high-voltage MMC converters.

[0036] In some embodiments, this application also provides a computer system including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps in the above-described method embodiments.

[0037] This application also provides a computer-readable storage medium for storing a computer program. This computer-readable storage medium can be applied to a computer device, and the computer program causes the computer device to execute the corresponding processes in the methods described above in the embodiments of this application; for brevity, further details are omitted here.

[0038] The above embodiments are preferred implementations of this application. In addition, this application can be implemented in other ways. Any obvious substitutions without departing from the concept of this technical solution are within the protection scope of this application.

[0039] To facilitate understanding by those skilled in the art of the improvements made by this application compared to the prior art, some of the accompanying drawings and descriptions have been simplified, and for clarity, some other elements have been omitted from this application. Those skilled in the art should realize that these omitted elements may also constitute the content of this application.

Claims

1. A self-distributed SiC high-efficiency utilization type high-density MMC, characterized by, include: The MMC includes multiple heterogeneous parallel half-bridge sub-modules connected end to end according to the same type of branches; The heterogeneous parallel half-bridge submodule includes a SiC high-frequency branch, a Si low-frequency branch, and a submodule capacitor. Both ends of the bridge arm of the MMC are Si low-frequency branches, and the SiC high-frequency branches and Si low-frequency branches between adjacent sub-modules form SiC high-frequency branch groups and Si low-frequency branch groups, respectively.

2. The MMC of claim 1, wherein, The SiC high-frequency branch includes a first SiC MOSFET and a second SiC MOSFET, and the Si low-frequency branch includes a first Si IGBT and a second Si IGBT. The source of the first SiC MOSFET is connected to the drain of the second SiC MOSFET, the emitter of the first SiIGBT is connected to the collector of the second Si IGBT, the emitter of the second Si IGBT and the source of the second SiC MOSFET are both connected to the negative terminal of the submodule capacitor, and the drain of the first SiC MOSFET and the collector of the first Si IGBT are both connected to the positive terminal of the submodule capacitor.

3. A control method of a self-voltage-distributing SiC high-efficiency utilization type high-density MMC, characterized by, Applied to the MMC as described in claims 1-2, comprising: The output reference voltage is obtained by adopting dual closed-loop feedforward decoupling control; Based on the output reference voltage, determine the number of stepped voltage levels and the compensation PWM voltage; The operating mode of the bridge arm is determined by the number of stepped voltage levels and the compensation PWM voltage input rotating pulse distribution circuit, and corresponding switching signals are generated to drive the bridge arm to output the target reference voltage.

4. The method of claim 3, wherein, The step of determining the number of stepped voltage levels and the compensation PWM voltage based on the output reference voltage includes: Based on the output reference voltage, the upper bridge arm output reference voltage and the lower bridge arm output reference voltage are determined; The upper bridge arm output reference voltage and the lower bridge arm output reference voltage are modulated by the nearest level pulse width modulation to generate bridge arm output stepped wave voltage and shaping voltage. Based on the stepped wave voltage output by the bridge arm, determine the number of stepped voltage levels; The shaped voltage is compared with the carrier wave to obtain the compensated PWM voltage.

5. The method of claim 3, wherein, The step of allocating the stepped voltage level number and the compensated PWM voltage input rotating pulse to determine the operating mode of the bridge arm and generate corresponding switching signals to drive the bridge arm to output the target reference voltage includes: Based on the number of stepped voltage levels, determine whether the bridge arm operating state contains a transition state; If so, determine whether the signal of the compensation PWM voltage is high. If so, increment the current mode number by 1; Based on the adjusted current mode number, the operating mode of the bridge arm is determined and a corresponding switching signal is generated to drive the bridge arm to output the target reference voltage.

6. The method of claim 5, wherein, The transition state includes a first transition mode and a second transition mode; the first transition mode specifically is: the first Si branch operates in bypass mode 1, the last Si branch operates in bypass mode 2, and all Si branch groups operate in parallel mode; the first number of SiC branch groups operate in series mode, and the remaining SiC branch groups operate in parallel mode. The second transition mode is as follows: the first Si branch operates in bypass mode 2, the last Si branch operates in bypass mode 1, the Si branch group remains in parallel and does not operate, the first number of SiC branch groups operate in series, and the remaining SiC branch groups operate in parallel.

7. The method of claim 6, wherein, The process for determining the first quantity is as follows: Determine the first quantity based on the current mode number and the number of step voltage levels.

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